Reactive dividing wall columns: A comprehensive review

Accepted Manuscript Title: Reactive Dividing Wall Columns: A Comprehensive Review Authors: Jeffrey A. Weinfeld, Scott A. Owens, R.Bruce Eldridge PII: DOI: Reference:

S0255-2701(17)30916-9 https://doi.org/10.1016/j.cep.2017.10.019 CEP 7103

To appear in:

Chemical Engineering and Processing

Received date: Revised date: Accepted date:

10-9-2017 24-10-2017 24-10-2017

Please cite this article as: Jeffrey A.Weinfeld, Scott A.Owens, R.Bruce Eldridge, Reactive Dividing Wall Columns: A Comprehensive Review, Chemical Engineering and Processing https://doi.org/10.1016/j.cep.2017.10.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Reactive Dividing Wall Columns: A Comprehensive Review

Jeffrey A. Weinfelda, Scott A. Owensb, R. Bruce Eldridgea,* a

Process Science Technology Center, The University of Texas at Austin, Austin, TX 78681, United States

b

Eastman Chemical Company, Kingsport, TN 37660, United States

Graphical abstract

A

A B C

A B

Convention Distillation Column

A

AB B

A

A

A B

A B+C

B

BC

B

C Dividing Wall Column

C

B Reactive Distillation Column

Reactive Dividing Wall Column

Highlights:  



A comprehensive review of experimental, modeling, and dynamic control studies on reactive dividing wall distillation columns Current simulation literature leads to the conclusion that reactive dividing wall columns are industrially feasible with the potential to save between 15 percent and 75 percent energy and at least 20 percent capital cost compared to conventional processes Research gaps preventing commercialization of these columns are highlighted

Abstract Reactive dividing wall columns (RDWCs) are highly integrated systems that can simultaneously perform chemical reactions and multicomponent separations within the same vessel. While reactive distillation columns (RDCs) are well established and dividing wall columns (DWCs) are becoming increasingly popular in chemical processes due to their ability to present significant capital and operating costs 1

savings, RDWCs have not been commercially adopted. RDWCs have been an active area of research only over the past decade, with studies concluding the technology is industrially feasible. This review presents experimental, modeling, and dynamic control studies with the goal of illustrating gaps in research which need to be resolved to bring the cost saving advantages of RDWC commercialization to industry.

Keywords Reactive dividing wall column; dividing wall column; reactive distillation; process intensification; thermally coupled distillation

1. Introduction Distillation is the most common method of chemical separation. As of 2010, there were reportedly 40,000 distillation columns operating around the world 1. However, it is an energy inefficient operation and it has been estimated that together these columns consume approximately 3 percent of all energy worldwide 2, and up to 80 percent of energy in chemical plants 3. As a result, a focus of distillation research has been to save energy, and therefore money in chemical processes by increasing the efficiency of columns. A promising method of saving energy in distillation processes is through process intensification, which combines different unit operations into a single piece of equipment for costs and emissions savings. This research area has attracted funding from government organizations such as the United States Department of Energy 4, and the European Union 5.

2

The dividing wall column (DWC) is a non-conventional distillation column that has become a popular example of process intensification. Introduced by Wright in 1949 6, the DWC performs multicomponent separations that normally require multiple distillation columns, in a single piece of equipment. This is achieved through the unique feature of a DWC, a partial vertical partition that exists inside of the column. On the feed side of the dividing wall, the high and low boiling components are separated, while the middle boiling components are split to both the top and bottom of the wall. On the product side of the dividing wall, the middle boiling components are then separated from both the high and low boiling components and withdrawn from the column as a side product. Therefore, the DWC essentially combines the separation ability of two different distillations columns into a single unit. Since two separation steps are being combined into one, it can be referred to as first level process integration. A conventional distillation column for a sample binary separation, and a DWC for a sample ternary separation are shown in figures 1a and 1b. 1a

1b

A

Convention Distillation Column

B

1d

A

A

A

AB

A B C

A B

1c

A B

A B+C

B

BC

B

C DWC

C

B RDC

RDWC

Figure 1. (a) Conventional, (b) dividing wall, (c) reactive, and (d) reactive dividing wall distillation columns shown in order of increasing integration for sample separation and reaction processes. Components A, B, and C are generic compounds in order of decreasing volatility. The shaded region represents a reactive zone involving either a heterogeneous or homogeneous catalyst.

3

There are many advantages of integrating two distillation columns into a DWC. Since one column is now required instead of two, there are significant capital cost savings, despite the slightly larger diameter typical of DWCs. The DWC has a higher thermodynamic efficiency than multiple column processes since the middle boiling components are no longer forced to remix with other components. In chemical plants with limited space, the reduction of plant area required is another advantage. Despite its advantages, the DWC does have some limitations. The column can only operate at one given pressure, whereas a two-column setup can operate each column at a different pressure to minimize boiling temperature and enhance relative volatility. Also, there is an inability to control internal flows and lack of surge tanks between columns that decreases the operational degrees of freedom compared to a multiple column sequence. However, studies indicate the advantages of DWCs outweigh its drawbacks, and it can produce significant capital and operating costs savings. DWCs have been studied extensively in literature, with papers covering a wide range of subjects and chemical systems. Topics covered include shortcut design methods, rigorous steady state simulations, dynamic control simulations, experimental operation at both pilot and industrial scale, and column hardware. A number of reviews of DWCs have been published 7–11. These works conclude that DWC modeling, design, and control is feasible with 25-40 percent energy savings and 30 percent lower capital costs compared to traditional processes. As a result, DWCs have received considerable attention from industry. Due to varying definitions and lack of industrial disclosure, the total number of DWCs used in industry is unknown. However, it has been estimated that there are at least 125 operating DWCs, with over 70 units belonging to BASF 10. DWCs clearly represent a successfully first level process integration. Another first level process integration scheme that fits within the guidelines of process intensification is reactive distillation (RD). First patented in 1922 by Backhauss 12, reactive distillation columns (RDCs) are traditional distillation columns in which a reaction occurs, therefore combining 4

reaction and distillation into a single unit operation. By doing this, the products of the reaction are formed and separated simultaneously. The reaction can be carried out using either a homogeneous or heterogeneous catalyst. Backhauss’ patent was for homogeneous systems, and it was not until 1980 that the first heterogeneous RD process was patented 13. In the case of a heterogeneous catalyst, fixed bed catalytic packing must be installed in the column. The advantage of using a heterogeneous catalyst is that the catalyst location can be precisely controlled. However, a drawback to this approach is that the catalyst can deactivate due to contact with feed components and operating conditions. This deactivation results in lost conversion and, ultimately, the need to shut the column down and replace the catalyst. A RDC for a sample reaction is shown in figure 1c. Like DWCs, RDCs have many advantages associated with the integration of unit operations, and a few disadvantages. Capital costs are reduced since a reactor vessel is no longer needed. Because products of the chemical reaction are distillated away as they are evolved, RDCs can shift equilibrium limited reactions toward additional product formation, and increase conversion and selectivity. Additionally, in an exothermic reaction, heat released from the reaction will reduce the amount of required reboiler duty. Further advantages are the stabilization of reaction temperature by elimination of catalytic hot spots, and the breaking of azeotropes in the chemical system through reaction. The main disadvantage of RDCs is that the amount of catalyst, residence time, and reaction conditions all must be compatible with the separation requirements. Similar to DWCs, further integration can make control more difficult. However, the advantages of RDCs can outweigh the disadvantages and this technology has been successfully implemented in industry. While it was first patented nearly a century ago, significant commercialization of RDCs did not occur until the 1980s 13. Design of RDCs is complex, as modeling the reaction location and properties can be difficult, and since parameters such as catalyst loading, liquid holdup, and feed location can have a major impact. Researchers have studied a wide range of systems dealing with various areas such as 5

modeling, shortcut and rigorous simulation, experimental operation, and control. The current body of work on RDCs totals over 1100 articles and 800 US patents covering over 235 reaction systems 10. Detailed reviews of RD processes can be found in published literature 10,13,14. Conclusions are that modeling and experimental operation are feasible with typically over 20 percent savings in energy requirements and capital costs when compared to the traditional process of a reactor followed by a distillation column. It is difficult to estimate the number of RDCs in industry, but it is worth noting that CDTECH, the major commercial RD technology provider, has licensed over 200 commercial scale processes worldwide as of 2010 10. Process intensification presents an interesting question – how many unit operations can be integrated into a single piece of equipment while still achieving operational and control objectives? Taking integration one step further, reaction and multicomponent separation can be combined into a single piece of equipment, known as the reactive dividing wall column (RDWC). In a reactive dividing wall column, a reaction as well as two unit operations of distillation take place, therefore making it a second level process integration. This column can be thought of as a reactor integrated with a DWC. Figure 1d depicts the RDWC, alongside a conventional distillation column (1a), DWC (1b), and RDC (1c), showing the progression of complexity. While Kaibel first mentioned the opportunity to carry out a reaction in a DWC in his 1984 patent 15, RDWCs were not modeled until 2004 16, and experimented until 2007 17. To date, there are over around 60 published articles on RDWCs covering topics such as experimental operation, shortcut modeling, rigorous steady state modeling, and dynamic control. Despite this body of literature, gaps in knowledge required for industrial operation are limiting RDWCs’ commercial adoption. RDWC research is currently very active with over a third of these studies being published in just the past two years. The primary conclusion is reactive dividing wall distillation can present significant capital and operating costs savings and does have an industrial future.

6

The lack of recent studies can be attributed to the complexity associated with RDWCs in addition to its recent emergence as an area of distillation research. RDWCs are more complex than RDCs or DWCs due to their large number of design and operational variables. When a wall is inserted into a conventional distillation column to yield a DWC, the height and axial location of the wall must be specified. From an operational view, the liquid split to each side of the column from above the wall can have a significant impact on column performance. The vapor split from the section below the wall can also impact column operation, but is difficult to control in practice. When a conventional column is transformed to carry out a chemical reaction, either a heterogeneous or homogeneous catalyst must be chosen. If heterogeneous, the catalytic packing, as well as its placement in the column must be specified. Therefore, if a heterogeneous reaction is carried out in a RDWC, both the catalyst and packing arrangement, as well as the wall location must be specified. The introduction of catalytic packing into one side of the column can lead to further complexity since the different packing configuration will lead to a difference in pressure drop between both sides of the column. This impacts the vapor split and causes it to not be proportional to the ratio of the cross-sectional areas on each side of the wall. Figure 2 shows design and operational variables to illustrate the high level of complexity in a RDWC.

7

Cooling Water Temperature? Catalyst Type? Catalytic Packing?

Liquid Split?

Wall Height and Location? Stage? Flow?

Stage? Flow? Composition? Reactive Zone Placement?

Reflux Ratio?

Vapor Split? Duty? Flow?

Figure 2. Design and operational variables involved with running a RDWC.

Currently, there is not a comprehensive review summarizing RDWC research efforts. This paper presents current RDWC research and identifies gaps in knowledge that need to be filled for industrial commercialization of RDWCs. The review presents studied RDWC topics from least to most complex: The first topic covered will be an overview of all experimental studies of RDWCs. Next will be a discussion of how RDWCs relate to DWCs, RDCs, and thermally coupled distillation sequences (TCDS) in the context of modeling, and how integration impact RDWCs. After that, steady state modeling will be reviewed, first focusing on shortcut and then rigorous modeling. Then control studies will be discussed. Finally, concluding remarks will be made with suggestions for important areas for future research.

2. Experimental Operation of a RDWC

8

Experimental studies implementing reactive distillation into a dividing wall column are scarce, with only a total of four columns detailed in the open literature. While the results of these studies have been promising, there is not enough information to make any comprehensive conclusions. The lack of experimental data available makes experimental studies the area of RDWC research in greatest need. The first set of experiments were reported by Sander et al. in 2007. For the system of methyl acetate hydrolysis (Equation 1), a lab scale RDWC was designed at a BASF laboratory. MeAc + H2O ↔ HAc + MeOH

(1)

The RDWC was approximated using a 4-column sequence, with upper and lower columns representing the rectifying and stripping sections having a diameter of 55 mm and a height of 1.5 mm, and parallel columns representing the feed and product side of the wall having diameters of 50 mm and 40 mm, respectively. The non-reactive sections were equipped with Kuhni Rombopak in the stripping section, and Sulzer CY in all other non-reactive sections. The reactive zone was packed with Sulzer Katapak-SP 11 filled with Amberlyst 48 catalyst. The study reports that 23 successful experiments were run using an azeotropic mixture of methyl acetate and methanol as a feed, and provides sample results of feed and product weight percentage and flow rate. The reaction obtained 49.5 percent conversion, with compositions of the distillate as 81.7 weight percent methyl acetate, side product as 98.6 weight percent methanol, and bottom product as 62.9 percent water 17. Results of Sander’s pilot scale experiments were used to scale up to a 220 mm diameter industrial scale column at a Sulzer Chemtech facility with a total packing height of 14.3 m, a dividing wall in the middle, and an external tank used to manipulate the liquid split to either side of the wall. MellapakPlus 752.Y and Katapak-SP 11 were used for the non-reactive and reactive sections, respectively. The study reports that several tests were performed during 2 weeks of column operation to examine liquid split to either side of the dividing wall, feed compositions, and feed and distillate rates. 9

Results are reported, with the maximum methanol purity in the side draw 92.1 weight percent, and maximum methyl acetate conversion of 82.2 percent. While the target methanol purity and desired methyl acetate conversion was not reached, the authors believed the study served as a good basis for further investigations and that improvements would be expected 5,17. The only other reported column with a physical dividing wall used for a RDWC study was designed by Hernández et al. at the University of Guanajuato to produce ethyl acetate (Equation 2) using sulfuric acid as a homogeneous catalyst. HAc + EtOH ↔ EtAc + H2O

(2)

The height of the column is reported as 2.5 m, and the diameter as 0.17 m 18. The column used a side tank to control liquid split, and contained sections of Teflon raschig super-rings as well as a combination of collectors and distributors used to promote uniform mixtures above and below the dividing wall and at the side product withdrawal point. The wall of the column was implemented so that it can be moved to three positions to manipulate the split of the vapor stream. Additionally, a reflux valve was also installed in the column so that it could be controlled to manipulate the reflux to the distillation column. The orientation of collectors and distributers and other column internals will be of particular interest to those investigating the design and installation of a physical reactive dividing wall column 19. In the experimental study, acetic acid and ethanol are fed to reboiler, which is then turned on. While limited data is reported, the study does detail the experimental start-up procedure of the column, and the authors conclude that their tests proved that combining chemical reaction with a dividing wall column is feasible 20. In the same experimental column, Hernández et al. studied the esterification of oleic acid with methanol to produce methyl oleate and water in the presence of homogeneous catalyst sulfuric acid. The reaction is shown in Equation 3. 10

R-COOH + MeOH ↔ H2O + R-COOCH3 where R = C17H33

(3)

Again, limited data is reported, but it took 1.25 h to achieve steady state, and analyzed samples from the reboiler indicated a conversion of 56 percent for methyl oleate. The authors stated that this test proves the concept of RDWCs, but more tests were required to obtain optimal operational conditions 18. To date, the most detailed experimental studies were performed at the Institute of Process and Plant Engineering at Hamburg University of Technology and study the transesterification of n-butyl acetate with n-hexanol to form n-butanol and n-hexyl acetate, shown in Equation 4. CH2COOC4H9 + C6H13OH ↔ C4H9OH + CH3COOC6H13

(4)

The first of these two studies was published by Ehlers, Egger, and Fieg to examine the process using an Amberlyst 35 as a solid acid catalyst. In their setup, the feed and product sides of the wall were represented by two separated columns that are placed in parallel. These columns are joined by ushaped pieces at the top and bottom to mimic the joining of both sides of the column above and below the dividing wall. The advantage of this design is that it eliminates heat transfer between the wall, which can impact dividing wall columns at the pilot scale. Unlike other experimental studies, the liquid split in this column is handled using a funnel swinging between sides with a programmed cycle time. The feed side of the column contains Katapak-SP Labor catalytic packing with two beds of height 1.00 m each, while the remaining parts of the column contain B1-500 packing with four beds of height 0.98 m each. The diameter of the column is 68 mm above and below the wall section, while the wall section pieces are 54 mm each. The column is also equipped with several Coriolis mass flow meters, and two capacitive pressure sensors. In addition to RDWC experiments, the authors determined pressure drops through relevant structured packing, boiling points and vapor-liquid equilibria of the system, and the reaction kinetics. Five RDWC experiments were performed, with reboiler duty and reflux ratio varied, 11

yielding a conversion of up to 66 percent. Detailed results reporting purities and temperature profiles are included. The experimental results were used for model validation. While temperature sensors could not be installed in the catalytic packing section of the column without damaging the catalyst, temperature and composition profiles for the rest of the column were successfully predicted by the model. The model, and specific simulation results of this study will be discussed in the steady state modeling section of this review 21. The second study was published by Egger and Fieg and is the only existing study on an enzymatic RDWC. Using the same experimental column, the system was studied using the immobilized lipase CALB, commercially available as Novozym 435. Before running tests on the physical column, the authors experimentally determined the physical properties and kinetics in a manner similar to the acid-catalyzed system, but a continuous fixed bed reactor was used to test enzyme deactivation and determine reaction kinetics. A total of eight experiments were performed, each running at steady state for at least two hours, to test reflux flow rate, pressure, feed composition, liquid split, and reboiler duty over a wide range for the purpose of model validation. PI temperature control is chosen as the control strategy for the column. The reflux flow is used to control the temperature above the feed stage of the column, and the side stream is used to control the temperature above the vapor split, while distillate and bottoms streams are used to control the level of the distillate tank and reboiler. Results show that the steady state operation of an enzymatic RDWC is feasible and as with the acid-catalyzed system will be discussed further in the steady state modeling section of the review 22.

3. From TCDS to RDWC The large number of variables associated with RDWCs makes accurate model development challenging. In commercial modeling software, such as Aspen Plus or CHEMCAD, there does not exist a 12

model for DWCs or RDWCs. Simulations are typically constructed by decomposing the RDWC into multiple columns with interconnecting streams to form a system thermally and hydraulically equivalent to the RDWC. A very common method of decomposing DWCs is using the Petlyuk configuration. This two-column arrangement uses a prefractionator to simulate the feed side of the wall and a mainfractionator to simulate the product side of the wall, the rectifying, and stripping sections. The prefractionator and mainfractionator are connected through liquid and vapor streams representing the liquid split above the wall and vapor split below the wall. Both columns are thermally driven by a reboiler and condenser on the mainfractionator. The size of the two columns and location of the interconnecting streams determines the location of the wall and its height. There exists a second type of two column configuration where the feed stream connects to a column that simulates the feed side of the wall, rectifying and stripping sections, while the second column simulates the product side of the wall. In this configuration, the reboiler and condenser are on the feed column. Known as the prefractionator feed and mainfractionator feed configurations, these are shown in Figure 3a and 3b, respectively. In the case that the wall is located at the top or bottom of the wall, then the DWC can be simulated by using a column with a side rectifier or side stripper. These arrangements as well as their equivalent dividing wall column are shown in Figure 4. When adding a reaction to the DWC to make it a RDWC, the reaction kinetics, zone, and packing if heterogeneous, must also be included in the model. Commercial modeling software with built in reaction simulators can be used. RDWCs can further be decomposed into arrangements using more than two columns. Other configurations are also possible. Figure 5 shows three, four, and five column arrangements that are all thermodynamically equivalent to the RDWC in figure 1d. This procedure of decomposing columns leads to an important question. How well do multiple column configurations that are thermally linked represent a single column? This has been discussed in studies as Kenig et al. 23, who extended the DWC decomposition method of Triantaffylou and Smith 24, which is used extensively in literature for reactive systems 25. It was shown

13

that these decomposed columns are in fact valid representations of integrated columns. Additionally, Schröder, Ehlers, and Fieg further proved this by comparing minimum vapor demands of different distillation sequences to show that the RDWC can be decomposed into multiple column arrangements that are approximately thermally equivalent. Though there were slight differences in vapor demand, the authors concluded that the feed side of the wall in a RDWC acts as a RDC 25.

3a

3b

Figure 3. Petlyuk configuration. Fig 3a shows the prefractionator feed arrangement, and fig 3b shows the mainfractionator feed arrangement.

14

4a

4c

4b

4d

Figure 4. Two column configurations to model DWCs with the wall connected to the top or bottom of the column. 4a shows a DWC with the wall connected to the top, and 4b shows its equivalent side stripper configuration. 4c shows a DWC with the wall connected to the bottom, and 4d shows its equivalent side rectifier configuration.

15

5a

5b

5c

Figure 5. Three (a), four (b), and five (c) column arrangements that are all thermodynamically equivalent to the RDWC in figure 1.

Another question that researchers have addressed is whether a reactive TCDS will always save energy compared to its non-thermally coupled equivalent process. As with DWCs, the influence of a RDWC is very system dependent. Each new system will have a different number of feed components and reaction products with differing relative volatilities, as well as different reaction kinetics. The varying number of influential parameters associated with reactions makes it difficult to compare any two reactions to each other. Therefore, generic systems have been used to test this concept. Using the common quaternary reactive system, A + B = C + D, Wang et al. assigned the generic compounds relative volatilities that corresponded to the least efficient and most expensive process possible. Comparing four different configurations with differing levels of thermal coupling to their equivalent base uncoupled cases using the rigorous distillation model in ChemCad, it was found that the more thermal coupling in a system, the higher the energy savings. The study also found that even the lowest level of thermal 16

coupling had energy savings compared to the uncoupled case 26. Since even the slightest amount of thermal coupling for the least favorable quaternary reaction system saves energy, it can be concluded that for quaternary reaction systems, a RDWC would save energy compared to a non-coupled process. Schröder, Ehlers, and Fieg also attempted to resolve this matter by comparing the minimum vapor demand of RDWCs to processes with just an RDC connected to a standard distillation column for also a quaternary reactions system, but this time with a middle boiling trace component, E. Using a shortcut method that will be detailed later this review, it was found that no matter the relative volatility of E, the minimum vapor demand and therefore energy consumption of the RDWC process will always be lower than the base case process 25. While there is much more analysis to be done to quantify the possible energy savings, the conclusions from these authors imply that if a RDWC can deliver the required product purities, it will save energy compared to a non-intensified process. While all RDWCs can be decomposed into a TCDS, not all TCDS can be composed into a RDWC. Based on the height and number of stages of each column and the location of the interconnecting streams, some TCDSs are only able to be constructed into a dividing wall column with both vertical and horizontal walls. Figure 6 shows such a case, where the RDWC equivalent to the TCDS results in a horizontal wall. While these columns can be simulated, there is no discussion in the open literature about a practical installation of a horizontal wall. When a horizontal wall is added, the degrees of freedom of operation increase, making it easier to control. Therefore, in operation these columns are not representative of RDWCs with a vertical wall. The scope of this paper is to review RDWCs that have a vertical wall, and this paper will not comment on these studies.

17

10

15 stages 25 25

Figure 6. Thermally coupled two column sequence and its thermally equivalent RDWC. 5a depicts a side stripper with 15 stages connected to a distillation column at stage 25. 5b shows the thermally equivalent RDWC containing a horizontal wall at stage 10, and a vertical wall that continues to stage 25.

4. Shortcut Methods Several shortcut design methods have been presented in open literature with the goal of being able to predict system feasibility and produce starting design variable values to use in a more rigorous simulation. These methods can have industrial importance as they serve to quickly estimate whether a process has the potential to work and is worth researching. Since a RDWC behaves as an integrated DWC and RDC, RDWC shortcut methods can be generated from those of DWCs and RDCs. Both mathematical optimization and graphically based conceptual design methods have been used for RDC design. On the other hand, DWC design typically uses the Fenske-Underwood-Gilliland techniques combined with the Kirkbride correlation. Currently semi-rigorous approaches based on the boundary value method are growing in popularity. Daniel et al. were the first to present a shortcut method for the RDWC. In their paper, the RDWC is represented by a standard distillation column whose distillate and bottoms product streams are thermally coupled to a second distillation column, where no reaction is assumed to take place. The first of these columns is 18

designed using the boundary value method presented by Dragomir et al. 27 An initial value for the reaction extent is used for this procedure. The second column is designed by specifying two of the product purities to define a product region for the third, and then calculating composition profiles in the stripping section from a range of boil-up ratios from the specified product. These composition profiles are then used to determine the feed stage, and then further calculate stripping profiles to fit the side draw composition region. An energy balance is used to calculate the reflux and reboil ratios. These values are used to calculate the downward composition of the column, and find an intersection between the rectifying and stripping profiles to finalize the column design. The authors tested the method by comparing shortcut results from a case study of hydrolysis of methyl acetate to rigorous simulation results. While the rigorous results are not presented in their paper, the authors report excellent agreement with their shortcut design 28. The next proposed shortcut method was by Rico-Ramirez et al., who used a design based on the Fenske-Gilliland-Underwood equations to determine the number of stages for non-reactive and reactive sections. The chemical reaction is incorporated by estimating the overall conversion of the reactive system for use in mass balances. The authors used two strategies to minimize heat duty: the first formulated all equations as a nonlinear programming problem to be solved in GAMS, while the second formulated the mass and energy balances as a nonlinear programming problem, and then used the Fenske-Underwood-Gilliland equations to calculate the remaining variables. The tests system for the study was the esterification of oleic acid to produce methyl oleate, shown in Equation 3. The shortcut method performed well compared to the rigorous optimization, with the only difference in design being that the shortcut method under-predicted the number of required reactive stages necessary to obtain high conversion by one 29. Believing the first two methods to be too complex, Sun and Bi proposed a new method that combined the minimum vapor flow method and Vmin diagram based on the Underwood equations by 19

introducing a new variable to eliminate effects of the reaction. This variable, referred to as the reactant conversion or the production yield, is used to formulate an expression for the improved recovery for each component due to simultaneous reaction and distillation so that the vapor demand of the reaction can be ignored. In this design, a computer plots the Vmin diagram, then the minimum number of stages in each section is calculated through the Fenske equation in a similar manner to the non-reactive case. The method was tested for the reactions: synthesis of methyl tert-butyl ether (Equation 5), ethyl tertbutyl ether (Equation 6), and dimethyl ether (equation 7). C4H8 + MeOH ↔ C5H12O

(5)

C4H8 + EtOH ↔ C6H14O

(6)

2MeOH ↔ C2H6O + H2O

(7)

Rigorous Aspen Plus simulations using a flowsheet of a reactive Petlyuk design were optimized for minimum energy requirement and then used to plot Vmin diagrams to compare to the shortcut method. Results show that while the method needs improvement to better account for the reaction, it can be used for preliminary process design 30. A comprehensive graphical shortcut method was created by Lee and Kang, who unlike previous graphical methods, included not only equilibrium but also kinetic data to account for the reaction. The method was applied to a generic ternary isomolar reaction since a constant molar overflow with a constant heat of vaporization and negligible sensible and reaction heat changes can be assumed. The method was successfully applied to both an equilibrium and kinetic model. The compositions trajectory of the column is traced, which leads to the identification of the number of required stages, minimum reflux and boilup ratios, and molar compositions of each stage. When compared to a rigorous simulation for DME production (Equation 7), it was concluded that the shortcut method could work for a feasibility analysis, but more studies would be required to see if the method can be extended to systems 20

besides ternary isomolar reactions with a volatile intermediate reactant since constant molar overflow was assumed for this system. The authors stated that the method will be extended to a general form using concepts such as reaction difference point, and projected composition space 31. A recent method, presented by Schröder, Ehlers, and Fieg, decomposes the RDWC into a thermally equivalent RDC followed by two conventional distillation columns: one connected to the distillate of the RDC and another connected to the bottoms product. The minimum vapor demand of the conventional distillation columns is calculated from the Underwood equations. Since the minimum vapor demand of the RDC is still required, this method was referred to as being a “semi-shortcut” method. Comparisons to rigorous simulation for an ideal four component reaction system with a fifth inert component present show that the semi-shortcut method differs by around 0.4 percent to 1 percent of the total vapor demand of the RDWC. Most of this error is due to the assumptions made in the rigorous calculation of the vapor demand for the decomposed RDC. These results establish the validity of the semi-shortcut method 25. In the final method, Orjuela et al. performed feasibility analysis for triethyl citrate production from citric acid and ethanol with monoethyl citrate and diethyl citrate intermediates, and diethyl ether side product, in a RDWC with a double reboiler configuration. Shown in equations 8-11, the process is carried out by feeding ethanol and citric acid to the RDWC using amberlyst 15 as a heterogeneous ionexchange catalyst. CA + EtOH ↔ H2O + MEC

(8)

MEC + EtOH ↔ H2O + DEC

(9)

DEC + EtOH ↔ H2O + TEC

(10)

EtOH + EtOH ↔ H2O + DEE

(11)

21

In the method, reactive residual curve maps are used to choose compositions above and below the dividing wall which are then used as starting compositions to calculate traditional residual curve trajectories. These results are used to design the column, which removes an azeotropic mixture of ethanol and water as the distillate, water from the reboiler on the product side of the dividing wall, and triethyl carbonate with some ethanol from the reboiler on the feed side of the dividing wall. The feasibility of this design as a way to reduce costs for the process was therefore predicted using the shortcut method 32.

5. Rigorous Simulation The majority of published information on RDWCs is based on rigorous simulation. Roughly 20 reaction systems have been studied using a variety of feed compositions, distillation models, thermodynamic models, and optimization methods. The diverse number of reactions studied in literature makes it complicated to systematically analyze the entire body of work. Despite the large number of reaction systems, there are only a few that have been the subject of multiple studies. Within these common systems, there often many different thermodynamic models used. Because of this variety, the review of rigorous steady simulations will focus on the different distillation modeling methods, optimization techniques, and energy and costs savings results, and not a comparison of the different thermodynamic models and specific reaction rates used. Since the strongest determining factor in RDWC performance is the reaction system, review of rigorous simulation will be classified by chemical reaction.

5.1 Biodiesel Production

22

Biodiesel production has emerged as a potential industrial application for RDWCs. To produce biodiesel, a fatty organic acid reacts with methanol to form water and a fatty acid methyl ester (FAME) to be used as biodiesel, as shown in Equation 12. R-COOH + MeOH ↔ H2O + R-COOCH3

(12)

After the equilibrium limited reaction is carried out, a ternary separation of FAME, methanol, and water must be performed. RDCs for biodiesel production are frequently studied in the open literature. These columns typically feed excess methanol to the RDC to shift equilibrium toward product formation, and show significant energy savings compared to the conventional process 33. The success of RDCs for this application has led to interest in RDWCs. The Hernández group has been very productive in this area, publishing several studies on RDWCs for biodiesel production. In their first RDWC biodiesel study, Hernández et al. modeled three cases: a RDC, a RDC coupled with a side rectifier, and a reactive Petlyuk column. Using the equilibrium stage RadFrac model in Aspen Plus, biodiesel production reactions using with oleic acid, linoleic acid, and n-dodecanoic acid as the fatty organic acid were simulated to form methyl oleate, methyl linoleate, and methyl dodecanoate as the FAMEs. The fatty organic acids were fed at the top of the reactive column, while the methanol was fed at the bottom. Sulfuric acid was used in the reaction as a homogeneous catalyst. The system in this study is unique, since none of the configurations had a reboiler. Methanol was injected directly into the bottom of the column in the vapor phase, therefore supplying energy to the column. Results found that the configuration with a side rectifier removes the lowest heat duty in the condenser while separating all three products with the highest purity, thus making it the best configuration. The temperature profile of the main column for this configuration shows a spike, demonstrating the high efficiency heat integration obtained by simultaneously carrying

23

out reaction and separation. When composed into a RDWC, the system forms a thermally equivalent RDWC where the wall is at the top of the column 34. In a similar study, Hernández et al. used a fatty organic acid feed stock consisting of oleic, linoleic, and palmitic acids in a molar composition similar to the Jatropha curcas L. seed oil. Two configurations were rigorously simulated in Aspen Plus: a distillation column thermally coupled to a side rectifier, and a reactive Petlyuk column, which were both homogeneously catalyzed by sulfuric acid. Following the typical biodiesel double feed configuration, methanol is fed to the bottom of the column as saturated vapor, and the fatty organic acid feed stock is fed near the top. Using the sequential quadratic programming tool in Aspen, the minimum energy requirement is optimized by changing the interconnecting flow rates. Energy reductions of 28 percent for the side rectifier configuration, and 20 percent for the Petlyuk configuration were found compared to a base case of a RDC. The side rectifier configuration can be composed into a RDWC with a vertical wall attached to the top of the column. These results were consistent with previous findings that integration saves energy, and for biodiesel production a wall attached to the top of the column presents optimal savings 33. Rico-Ramirez et al. performed rigorous simulations using oleic acid to produce methyl oleate to verify their shortcut method. This was done by starting with the designs obtained from the two shortcut methods, then performing sensitivity analysis with respect to the design variables to minimize heat duty. Columns were rigorously modeled in Aspen Plus, with one configuration being a reactive Petlyuk column, and the other being a column thermally coupled with a side rectifier. In the case with a side rectifier, the number of stages in the rectifier is smaller than the number of stages in the main column above the interconnecting stream. Therefore, when composed into a RDWC, this case will have a horizontal wall. But the composed reactive Petlyuk configuration was still equivalent to a RDWC with only a vertical wall. Results showed that the RDWC was sensitive to changes in any of the design

24

variables, and that care should be taken when designing these systems. Also, sensitivity analysis indicated that the side stream should be located within the reactive zone, as expected 29. While all the studies presented so far involve homogeneous catalysts, the heterogeneous solid acid catalyst sulfated zirconia has also been used to study biodiesel production. Nguyen and Demirel studied a TCDS with a fatty organic acid feed of lauric acid. In this case, a side stripper configuration is considered since it is hypothesized by the authors that using the RDC as a side rectifier may lower the reaction rate by changing the reaction zone temperature. This side stripper configuration is equivalent to a RDWC with a vertical wall attached to the bottom of the column. Rigorous simulations in Aspen Plus indicate that, compared to a base case of a RDC followed by a conventional distillation column for methanol recovery, the thermally coupled design with a side stripper can reduce energy consumption by 13.1 percent in the RDC and 50 percent in the recovery column. This is equivalent to stating that the RDWC saves 13.1 percent energy in the feed side of the dividing wall combined with the rectifying and stripping sections, and 50 percent energy in the product side of the wall. However, while this configuration saves energy, a higher pressure is required to achieve a comparable conversion to the base case due to the interconnecting vapor flow from the top of the column 2. Kiss et al. approximated the fatty organic acid feed with lauric acid, and used sulfated zirconia as a catalyst to study a novel biodiesel production process that uses only a slight excess of methanol. The process was studied using the RateSep rate-based model in the RadFrac unit in Aspen Plus. As an optimization strategy, simulated annealing in MATLAB was coupled with Aspen Plus to find the design with the lowest reboiler duty while using Microsoft Excel to communicate between the two programs. Over the course of the optimization, several RDWC designs were recorded and analyzed to determine the impact of the optimization method. All designs had lower energy consumption compared to the optimized uncoupled design, but the further along the optimization progression then the lower energy consumption of the RDWC. The most optimized case had many more total and reactive stages than any 25

other case, and as a result saved 25 percent energy compared to the initial uncoupled design. The authors also did an economic analysis to estimate the total annual cost (TAC) as of March 2011 with a plant lifetime of 5 years at 8400 hours per year, and found that the most optimized case was similar to the base case but had significantly lower CO2 emissions 35. 5.2 Ethyl Acetate Synthesis Ethyl acetate is an industrially relevant solvent used for a variety of products. It is typically produced through an endothermic equilibrium limited reaction between acetic acid and ethanol to form ethyl acetate and water. A reaction and multicomponent separation is required, therefore making it potentially suitable for a RDWC. The reaction is shown for reference in Equation 2. The Hernández group examined three configurations, all modeled rigorously in Aspen through equilibrium stage calculations with a homogeneous sulfuric acid catalyst: a RDC thermally coupled to a side stripper, a RDC thermally coupled to a side rectifier, and a reactive Petlyuk configuration. The interconnecting liquid and vapor flow rates were varied with the goal of minimizing heat duty. The side stripper and Petlyuk configurations reached 80 percent conversion, while the side rectifier case only reached 68 percent, compared to 56 percent conversion in the conventional process of a reactive distillation column followed by two conventional distillation columns. Results indicate that energy savings for ethyl acetate synthesis through using thermally coupled distillation sequences are expected to be between 30 percent and 50 percent 36. Hernández et al. used a reactive Petlyuk configuration to model the reaction, again using sulfuric acid as a homogeneous catalyst. The reaction occurred in the reboiler of the main distillation column. Required purity were obtained by manipulating the reflux rate and reboiler duty. A RadFrac simulation in Aspen Plus was optimized by using a grid search on the flows of the interconnecting vapor and liquid

26

streams, which simulated the vapor and liquid split in a RDWC. The results found that the energy consumption was highly dependent on these values 19,37. Hernández et al. presented a thermodynamic comparison between a reactive Petlyuk column and a classical reactor followed by a non-reactive Petlyuk column. Studied in Aspen Plus using the rigorous RadFrac model, the interconnecting stream flow rates were varied to minimize reboiler duty. It was found that the reactive Petlyuk configuration, which can be composed into a RDWC, saved 36.8 percent energy compared to the base case 38. In their most recent ethyl acetate study, Hernández et al. used the RadFrac model in Aspen Plus to simulate a RDWC using a reactive Petlyuk configuration with sulfuric acid employed as a homogeneous catalyst. The reaction zone was set as the reboiler of the column. To minimize reboiler duty, sequential quadratic programming was used to optimize the flow rates of the interconnecting liquid and vapor streams. The results give an 86 percent conversion. Differing temperature profiles on each side of the wall indicated that there is heat transfer through the wall. However, previous DWC studies have shown that this is not a cause for concern, since heat transfer through the wall can be beneficial in some parts of the column 20. Recently, Orjuela et al. has proposed a novel configuration for the industrial ethyl acetate production process, which involved a double feed with ethanol at the bottom of the column and acetic acid fed at the top of the reactive zone. The flowsheet contains a recycle stream from the bottoms product back to the ethyl acetate feed. The process was intensified by producing either a DWC, RDC, or RDWC to determine whether intensification can improve the process. Sulfuric acid was used as the homogeneous catalyst and simulations were performed in Aspen plus to compare these three options. The RDWC was simulated in Aspen by coupling an RDC with a conventional distillation column. Sequential quadratic programming with sensitivity analysis in Aspen was used to minimize TAC by

27

examining the design variables of feed stage location, total column stages, stages above and below the wall, and the operational variables of feed flow rate, reflux ratio, bottoms flow rate, liquid and vapor flow rates of interconnecting streams, reboiler duty, and distillate to feed flow ratio. Results showed that the RDWC offered the highest energy savings at 46 percent and the best TAC savings at 26 percent compared to the base process 39.

5.3 Diethyl Carbonate Synthesis The transesterification of dimethyl carbonate with ethanol to produce diethyl carbonate and methanol through an intermediate of ethyl methyl carbonate is an industrially relevant reaction whose products are used as reactants and solvents for numerous chemical processes. The system is given in Equations 13 and 14. DMC + EtOH ↔ EMC + MeOH

(13)

EMC + EtOH ↔ DEC + MeOH

(14)

This equilibrium reaction requires high purity multicomponent separation, therefore making it a viable process for RDWC design. Kenig et al. first studied the system using a custom model. They used a rate-based stage approach to account for kinetic and hydrodynamic phenomena such as multicomponent heat and mass transfer in films, chemical reactions, and thermodynamic non-idealities. Inside each stage, mass transfer was described by a thin film model with multicomponent diffusion within the films that was modeled using the Maxwell-Stefan equations. A unique feature of this model is that it considered all internals of the column including distributers for the structured packing. It accounts for heat transfer at the wall by assuming the wall is covered by one phase only, this method introduces error due to the lack 28

of accurate heat transfer coefficient correlations. Before testing with a reactive system, the model was validated using data from a non-reactive DWC study by implementing the model into Aspen Custom modeler. Results showed average deviations of 2 percent for temperature, and 3 percent for product purities 16. The results were also compared to an equilibrium stage model where it was found that differences in concentration and temperature profiles within the prefractionator were quite significant. Analysis showed that the equilibrium stage model likely overestimated the separation efficiency, leading to this difference. Next, the model was used to study the reactive system of diethyl carbonate synthesis using potassium carbonate as a heterogeneous catalyst. The dividing wall was shifted toward the product side, resulting in a 0.625/0.375 area ratio in favor of the feed side. The actual vapor flow split was less than this number since the catalytic packing on the feed side of the wall had a higher vapor flow resistance. Since the reactants have different boiling points, a double feed was used with higher boiling dimethyl carbonate being fed in the upper part of the reactive section, and lower boiling ethanol feed being fed to the lower part to ensure contact of both reactants through counter-flow. Due modeling difficulties, heat transfer was ignored despite being incorporated into the original custom model. The RDWC led to a significantly large increase from 42 percent to 73 percent in selectivity of dimethyl carbonate to main product diethyl carbonate, while the conversion was nearly unchanged at 87 percent. The temperature difference on each side of the wall was less than 10 degrees Celsius, which implies that heat transfer through the wall had a minimal effect 43. In another study, Kenig et al. used an identical custom model in Aspen Custom Modeler to test the same diethyl carbonate synthesis system and catalyst against two other models: a reactive Petlyuk column using rate-based calculations, and a RDC followed by a conventional column connected to the distillate, and another conventional column connected to the bottoms product of the RDC. Due to the non-linear nature of these highly integrated systems, the optimized values can be dependent on the starting values. Therefore, the Fenske Underwood Gilliland equations with the Kirkbride equation were 29

used to obtain starting values for the rigorous simulation. Results showed that upon transition from the three-column sequence to the reactive Petlyuk column, then to the RDWC, there was no loss of performance and the positive effect of excess ethanol in the feed was not diminished. Compared to the corresponding three column configuration, the RDWC saved 65 percent energy for a stoichiometric feed, and 55 percent energy for an excess ethanol feed. Production costs savings for the three RDWC compared to the three column configuration and Petlyuk configuration for stoichiometric and excess feeds were also significant 23. Wang et al. also studied the identical diethyl carbonate reaction system but using sodium methoxide as a homogeneous catalyst. The RDWC was modeled rigorously in Aspen Plus with equilibrium stage calculations by using a RDC and side stripper with interconnecting streams. Additionally, a RDC with an excessive ethanol feed followed by a recovery distillation column was designed and optimized to minimize TAC. For comparison, the RDWC is designed with the same number of trays, feed locations, and product specifications as the non-integrated base case. The optimization procedure is carried out in a sequential and iterative manner for the RDC, with the number of trays in the two columns being the outer loops, and remaining variables being the inner loops of iteration. In the RDWC, the wall is attached to the column bottom, so there are two reboilers. The design degrees of freedom are the two reboiler duties, reflux ratio, and liquid split ratio, which are adjusted to obtain the product specifications. When comparing costs of RDWCs with conventional columns, correlations were used to assess a penalty on column cost. Design equations were used to obtain the RDWC diameter from the RDC diameter. Results show that the RDWC saves 18.7 percent of total operating costs (TOC), and 13.9 percent of TAC. However, due to the double reboiler, the duty increased by 37.9 percent 40.

5.4 DME Synthesis

30

Dimethyl ether has many industrial applications including use as diesel fuel. DME is required to be in high purity, and can be synthesized from the dehydration of methanol in an equilibrium reaction as shown in Equation 7. To obtain high purity DME, it must be separated from methanol and water. The reaction and multicomponent separation requirement makes the reaction plausible for a RDWC. Kiss and Suszwalak studied the system using a microporous sulfonic acid ion exchange resin as a heterogeneous catalyst. For comparison, a conventional process with a reactor and two distillation columns, and a process with a RDC followed by a distillation column were also modeled and optimized. All columns were designed using the RadFrac model in Aspen Plus with a methanol recycle stream, and all optimization was done using the sequential quadratic programming tool in Aspen Plus to minimize the energy requirement constrained by product purities of DME and water. For RDWC design, the problem was optimized with the discrete variables of total stages, number of reactive stages, location of reactive zone, number of dividing wall stages, location of dividing wall, stage of side-stream withdrawal, and the continuous variables of reflux ratio, boilup rate, flowrate of side stream product, vapor split, and liquid split. When possible, initial guesses for values of these variables are obtained from the RDC followed by distillation column sequence. The results showed less than a 15 degrees Celsius temperature difference on either side of the wall, indicating that little heat transfer is expected. Profiles are similar to the RD system, but with differences around the feed, showing the importance of the feed location. The methanol concentration on the product side remains nearly constant, indicating location of the side draw has little impact. Despite a conversion of only about 50 percent compared to 90 percent with the conventional process, the RDWC is the most energy efficient and can allow savings of 11.6 percent and 58.65 percent compared to the RD and conventional processes, respectively. The RDWC also has significant CO2 emissions reductions and lower total investment cost (TIC), TOC, and TAC compared to the other processes 44.

31

Sun and Bi performed a rigorous Aspen Plus simulation for the system of DME synthesis to produce a Vmin diagram for model validation of their shortcut method. The RDWC was modeled as a reactive Petlyuk configuration, and the system was optimized for minimum energy requirement. Since the models were purely used for validation purposes, there are no energy or costs savings compared to the conventional case reported. However, a Vmin diagram with compositions of column products are presented that showed agreement with the shortcut method 30. Lee and Kang used rigorous Aspen Plus simulations of the same DME synthesis system to validate their graphical shortcut model. This was done using an ion-exchange resin as a heterogeneous catalyst. Within Aspen, a two-column arrangement similar to a Petlyuk configuration was used, but with the feed being connected to the main column instead of the prefractionator as in typical Petlyuk configurations. During the simulation, the flow rate of the connected streams and reflux ratio were specified to be similar to the results found from the graphical method. The liquid and vapor composition trajectories of the rigorous simulation were then compared to the graphical method to successfully validate the shortcut method 31.

5.5 Ethyl Tert-Butyl Ether (ETBE) Synthesis ETBE is synthesized by reaction of isobutene with ethanol and is usually catalyzed by an ionexchange resin. It has industrial and research applications as a fuel additive than can reduce carbon emissions. For this application, ETBE is required in high purity and must be separated through multicomponent distillation after the reaction takes place. This combination of process requirements makes RDWC a potential way to reduce costs in the process. The reaction is shown in Equation 15 for reference. IB + EtOH ↔ ETBE

(15) 32

Bumbac et al. studied two different sequences with an Amberlyst acidic ion-exchange resin used as a heterogeneous catalyst in Aspen HYSYS: a RDC followed by a conventional distillation column for excess ethanol recovery, and a RDWC. Both processes utilized a recycle line for excess ethanol. The catalytic zones of both schemes were modeled using a series of well mixed reactors, and separation was assumed to occur on all other trays that are non-reactive. Better energy savings occurred when the side product line was near the bottom of the wall. The final results showed a 36 percent energy reduction from the RDWC compared to the RDC base case 3. Recently, Sangal et al. studied the ETBE synthesis system in Aspen Plus using the RadFrac model with a fixed bed catalyst to simulate four sequences: a base case of a reactor followed by two distillation columns, a RDC, a reactor followed by a RDC, and a RDWC. The RDWC was modeled similar to a Petlyuk configuration as a main reactive column thermally coupled with a second column representing the product side of the wall. All four sequences were modeled using the same reflux ratio and similar column specifications. Results showed that the RDWC had a higher purity than the RD configurations, and the reduced reboiler duty by 74 percent compared to the base case, and 43 percent compared to the RD sequence. This result can be partially attributed to the reduced remixing effects in DWCs. Carbon dioxide emissions were studied and showed that the RDWC reduced emissions by 74 percent compared to the conventional process. Economic analysis was done using Aspen Technologies’ ICARUS tool, which showed that the RDWC had a 53 percent TAC reduction and a 41 percent TIC reduction compared to the conventional process 45. Sun and Bi used the ETBE synthesis system to validate their previously presented shortcut method. Simulations were performed by minimizing energy requirements on a reactive Petlyuk column designed to mimic a RDWC in Aspen Plus. Product compositions and Vmin diagrams were presented that successfully validate the shortcut method. No compositions profiles, energy savings, costs savings, or further analysis were presented 30. 33

5.6 Methyl Acetate Hydrolysis Methyl acetate hydrolysis (Equation 1) is a common test system for reactive distillation, so it has been extended to RDWC distillation by several researchers. Sander et al. modeled the column by using a Petlyuk configuration with an un-named process simulator, with interconnecting streams just below and above the wall location. Amberlyst 48 was used as a catalyst. Since there was no kinetic data in literature, the kinetics were experimentally determined using tubular fixed bed reactors, and data was fitted using a pseudo-homogeneous model. No optimization was reported as the model was used to validate pilot plant data previously discussed. Comparison of liquid mole fraction temperature profiles with experimental data presented in section was used to validate the model 17.

5.7 Additional Systems Bumbac, Pleşu, and Valentin studied the esterification of the isoamylenes 2-methyl-butene-1 and 2-methyl-butene-2 with ethanol to produce tert-amyl-ethyl-ether (TAEE) as shown (Equations 1618). EtOH + 2M1B ↔ TAEE

(16)

EtOH + 2M2B ↔ TAEE

(17)

2M1B ↔ 2M2B

(18)

TAEE is an industrially relevant oxygenative additive in light gasoline. The system was simulated using a heterogeneous acidic ion exchange catalyst in Aspen HYSYS by splitting the RDWC into four zones: the reactive zone of the prefractionator, the non-reactive zone of the prefractionator, the rectifying and stripping section, and the product side of the dividing wall. The prefractionator reactive packing was modeled with a back-flow cell model with forward flow of liquid and back flow of vapor. 34

The upper and lower prefractionator separation zones were modeled as absorbers. The rectifying and stripping section, and product side of the wall were modeled as standard distillation columns. There is no mention of optimization. However, it was reported that good separation numbers were obtained with over 80 mole percent purity in each product stream and 80.6 percent TAEE conversion. Additionally, it was recommended by the authors to place the reaction zone as close as possible to the top of the prefractionator and to have the column feed below the reaction zone 46. In a few papers, Kiss, Pragt, and van Strien studied an un-disclosed industrial process that involves a relatively complex and fast chemical equilibrium of 10 species referred to as reactants A through J. The system has a homogeneous catalyst, so it is assumed that reactions take place everywhere in the system. RadFrac distillation modeling was used in Aspen Plus. The base design was a RDC followed by a distillation column, while the RDWC was modeled using two flowsheets: a reactive Petlyuk configuration, and a two column Petlyuk-like configuration but with the feed being connected to the main column. In both flowsheets, the total investment for the RDWC was 35 percent less and energy requirements were 15 percent less compared to the base uncoupled case of a reactor followed by two distillation columns in series. This also implies that in this reaction when using a two-column configuration, it does not matter which column the feed is connected to. The authors note that the key factor in this integration was the similar pressure and temperature conditions in the two standalone columns. The height of the RDWC was the same as the RDC, but with a larger diameter. The missing description of the reaction system prevents analyses of the impact of the reaction on column performance. However, these results demonstrate that a RDWC can perform better than an unintegrated sequence for a complex industrial reaction 47–49. Han et al. use a RDWC to produce high purity lactic acid from a dilute lactic acid stream. High purity lactic acid has research potential as a raw material to produce poly lactic acid, which is a biodegradable polymer. The reaction is heterogeneously catalyzed and consists of two parts shown in 35

Equations 19 and 20: esterification of lactic acid with methanol to produce methyl lactic acid and water, and then the hydrolysis of methyl lactic acid to produce lactic acid and methanol. LA + MeOH ↔ MLA + H2O

(19)

MLA + H2O ↔ LA + MeOH

(20)

This system is unique, the first reaction is fast and exothermic, while the second is slow and endothermic. The RDWC is modeled as a Petlyuk configuration, with the esterification reaction taking place in the prefractionator, and the hydrolysis reaction taking place in the main column. A double feed is used, with dilute lactic acid fed at the top of the reactive zone, and methanol fed at the bottom. Feed molar ratio and methanol composition are varied for investigation. Results show that the feed molar ratio has a large effect on performance, and excess feed molar ratio should be used. However, methanol feed composition is shown to have little impact. When compared to a base case of a CSTR followed by a RDC, the RDWC is shown to reduce energy consumption by about 22 percent. The authors hypothesized this number would increase for an optimized column 50. Using Aspen Plus, Hernández et al. used RadFrac to model a RDWC using a Petlyuk column, as well as a reactor in series with a non-reactive Petlyuk column of the same configuration. The model was used for the reaction of methanol with isobutylene to produce methyl tert-butyl-ether, and the reaction of ethanol with ethylene oxide to produce ethoxyethanol, shown in Equations 21 and 22. MeOH + IByne ↔ MTBE

(21)

EtOH + ETO ↔ EthoxyEtOH

(22)

The flow rates of the interconnecting liquid and vapor streams were varied to find the minimum reboiler duty. Results showed that the methyl tert-butyl-ether synthesis in the RDWC consumed about

36

60.9 percent less energy than the alternative configuration, while ethoxyethanol synthesis consumed about 17.4 percent less energy, indicating the dependence of the savings on the chemical system 38. Sun and Bi also performed a simulation on the production of MTBE (Equation 21) in Aspen Plus using a Petlyuk column optimized for minimum energy requirement. A Vmin diagram was produced that verified the shortcut method. Results also indicated that increasing the amount of isobutylene results in a lower reboiler duty 30. Chien et al. used Aspen Plus to study the esterification of a n-amyl alcohol and n-hexanol mixture with acetic acid to produce n-amyl acetate and n-hexyl acetate using Amberlyst-15 as a heterogeneous catalyst applied using a fixed bed. The reaction is shown in Equation 23. AmOH/HexOH + HAc ↔ AmAc/HexAc + H2O

(23)

Three sequences are studied: a RDC with the bottoms product connected to a conventional distillation column (direct sequence), a RDC with the organic rich phase from the distillate decanter going to a conventional distillation column where the distillate is recycled to the first RDC (indirect sequence), and then a RDWC. The RDWC is modeled with the feed for the process connected to a reactive side rectifier, which is thermally coupled to a conventional distillation column. This configuration can be composed into a column with the dividing wall attached to the top of the wall. For optimization, the interconnecting vapor stream is varied to minimize the TAC, while the reflux ratio and reboiler duty are used to satisfy purity specifications. The results showed that the RDWC had TAC savings of 12.5 percent and 42.8 percent, and energy savings of 14.0 percent and 48.5 percent, compared to the direct and indirect sequences, respectively 51. Ye et al. studied the esterification of propanoic acid and n-propyl alcohol to produce n-propyl propionate and water, shown in Equation 24, using Amberlyst 46 as a heterogeneous catalyst using the rigorous equilibrium stage modeling in Aspen Plus. 37

PROAC + PROOH ↔ PROPRO + H2O

(24)

The RDWC configuration used a wall that was attached to the top of the column. To model this system, they used a three-column configuration, with one column representing the feed side of the wall, another the product side, and a third the stripping section. The flowsheet was designed based on a previous DWC study for the same system. In the RDWC, results showed that the product purities all reached the required specifications for vapor split ratios between the feed and product sides of the column between 0.3 and 0.7. The TAC was minimized by varying the vapor split, number of stages in the rectifying, stripping section, and reaction zone of the feed column, number of stages in the product side column, and number of stages in the stripping column in an iterative procedure described in the paper. The procedure was also followed for a base case of a RDC followed by a conventional distillation column for comparison. Analysis of the composition profiles of the RDWC compared to the base case showed that the remixing effect exists in the conventional process and is eliminated in the RDWC. Compared to the conventional process, the RDWC was found to save 26.5 percent of total capital costs, 12.4 percent of TOC, and 16.4 percent of TAC 52. Yuan et al. studied hydrogenation and separation of a C3 stream in Aspen Plus. The reaction system is a simplified version of a traditional deethanization process. The feed is approximated as methyl acetylene, propadiene, and propylene. The only desirable product from the hydrogenation of the feed is propylene. Therefore, a selective hydrogenation reactor is typically used to convert methyl acetylene and propadiene into a C3 alkene so that the product propylene is purified. In this study, the selective hydrogenation reactions, shown in Equations 25-27, are approximated as methyl acetylene forming propylene, propadiene forming propylene, and propylene forming propane. CH3C=CH + H2 → C3H6

(25)

CH2=C=CH2 + H2 → C3H6

(26) 38

C3H6 + H2 → C3H8

(27)

The RDWC is modeled as a side stripper with a feed that is thermally coupled to a RDC, therefore representing a RDWC with a wall attached to the bottom of the column and two reboilers. Initially, the DSTWU model is used to obtain initial values for the rigorous simulation. Due to the difficulties of optimizing the RDWC as it is a mixed integer non-linear programming problem, the authors used a particle swarm algorithm (PSO) along with the RadFrac model. PSO is believed to be much more computationally efficient than other techniques for solving similar problems. The algorithm interprets optimization variables to be a particle among others in a feasible solutions space. The optimization method uses the evolution of particle positions in a random search of solution space while evaluating the objective function. The method was used to minimize TAC using nine different process variables. As a comparison, a process with a distillation column followed by a RDC was also designed. Comparing the base case to the optimized RDWC results show that the RDWC can save 27.88 percent of TAC 53. Ehlers, Egger, Fieg modeled the acid-catalyzed transesterification of n-butyl acetate with nhexanol to form n-butanol and n-hexyl acetate (Equation 4). The model results were compared to the test column results at Hamburg University of Technology. They utilized a previously used custom model for DWCs developed at the Institute of Process and Plant Engineering that axially divides the column into a certain number of stages in which equilibrium stages are assumed. Sub-models for the condenser, distributers, and tanks based off conservation of mass without chemical reaction or phase equilibria were used. Chemical reactions occurred in the liquid phase in a pre-determined reaction zone. Heat loss to the environment and between each stage was considered. Correlations were used to calculate the pressure drop, and the relationship between pressure drop and vapor split. The column, modeled with six theoretical stages per packed bed, was simulated with input parameters from the five RDWC experiments performed. Composition and temperature profiles showed good agreement in the main column. In the prefractionator, there was no experimental data since temperature sensors could not be 39

inserted into the column without damaging the catalyst pockets. The composition profiles showed that heavy side products were abundant in the lower portion of the reactive section, which led to nearly 90 percent of the main products being generated in this region. Details of the temperature and composition profiles were validated, such as spikes in temperature and side product formation. The only discrepancies were found in main product conversion, which was over estimated in simulation 21. As previously discussed, Egger and Fieg studied the same system but instead used an enzymatic catalyst. The model applied to the system was the same that was used for the acid-catalyzed process. To test the model, eight different experiments were run using identical column conditions to the eight experiments run in the pilot RDWC. Comparison of pressure shows a small and consistent deviation between experiment and simulation, but generally good agreement. Temperature and concentration of experiments and simulations exhibit very good agreement, with only slight differences near the feed in the middle section of the column. To compare the impact of the chemical reaction, the mass fraction of all components in the combined product stream is analyzed, and shown to be very close in experiments and simulations. The model is therefore proven to be valid for this system. Given the many positive characteristics of this chemical process as a test system, the model serves as a proof of concept and valid starting point for future enzymatic RDWC studies 22. Wang et al. designed a novel process to produce dichlorohydrin by glycerol hydrochlorination using acetic acid as a homogeneous catalyst. The reaction system is shown in Equations 28-31 (Equation 24). GLY + HCl → 1-MCH + H2O

(28)

GLY + HCl → 2-MCH + H2O

(29)

1-MCH + HCl → 1,3-DCH + H2O

(30)

40

2-MCH + HCl → 2,3-DCH + H2O

(31)

Glycerol and hydrogen chloride are fed to a series of two CSTRs to produce dichlorohydrin. The product stream is then fed to a RDWC with a double condenser configuration. Dichlorohydrin is decanted from the distillate on each side of the wall to be obtained in high purity, while water is withdrawn from the side of the column, and a stream containing the acetic acid catalyst is removed as the bottoms product and recycled to the process feed. Optimization is performed iteratively by modifying the vapor split ratio, reboiler duty, feed locations, and sidedraw flow rates until the reboiler duty is minimized. The resulting design is a TCDS which is thermally equivalent to a RDWC. Processes were also designed using two CSTRs and two RDC. The RDWC design had a 42.4 percent lower reboiler duty compared to the most efficient of these designs 54. Orjuela et al. performed rigorous optimization for a RDWC in a triethyl citrate production process (equations 8-11). The initial design was taken from the result of a feasibility analysis and has been previously discussed in this review. The system is optimized by minimizing controllability and profitability functions through changing RDWC design variables with respect to constraints of product purity and maximum operating temperature. The procedures for calculating these functions are typical to studies performed for a less intensified system and are described in detail in literature. The technique employed for the optimization was a multi-objective differential evolution method together with a tabu list algorithm. For comparison, the same design and optimization methods were carried out for a process involving a RDC followed by a conventional distillation column. Surprising results showed that the controllability for both processes was very similar despite the increased intensification of the RDWC compared to the RDC process. Additionally, there was shown to be conflict between the cost and controllability functions, and that one may need to be sacrificed to achieve optimal levels of the other. Overall, the RDWC was shown to save about 11 percent TAC and 40 percent energy compared to the RDC process 32. 41

6. Control of RDWCs Unless it can be successfully operated in practice and controlled in the face of disturbances, then a RDWC is not feasible. Due to their nonlinear nature, RDCs and DWCs are more difficult to control. Further integrating these columns to a RDWC only increases the degree of difficulty. While control of RDWCs has not yet been studied in an experimental setting, several simulations have been performed. Since a rigorous model is required for dynamic simulations, many of the following control studies use the same systems and models to extend the steady state studies previously discussed in this paper. Since the presented studies use a variety of control schemes for a range of systems, the papers will be presented in historical order which generally follows the trend of increasing level of complexity. Wang et al. studied the control of an ideal quaternary reaction system with the least favorable volatility ranking under excess reactant design. The authors identified three keys to controlling the RDWC process. First, to maintain the correct stoichiometric balance between reactant feeds. This is done through feed control, but can have issues if there is measurement bias in feed flow rate. To control this, an internal composition loop was replaced by an internal temperature loop. Second, to maintain product quality, which was done by adjusting the reflux ratio, side draw flow rate, and reboiler duty. The third key was to account for possible changes in the control objective when the throughput rate changes. The PI controller stages were chosen from an RGA procedure, and tuned using a sequential design approach. Dynamic responses to +/- 20 percent changes in reactant feed flow rate show that the controlled stage temperatures settle at their corresponding set point, stoichiometric balance is maintained, and the three product compositions just about return to their operating values 26. Hernández et al. performed dynamic simulations in Aspen Dynamics to test set point changes and load rejection for ethyl acetate synthesis in a reactive Petlyuk column. The steady state model for 42

this system is discussed in section 5.2 of this review. Changes in set point and load rejection were made for both one and two closed temperature loops with PI controllers used to minimize the integral of absolute error. For both one and two closed loops, the Petlyuk column displayed convenient responses for both set point tracking and load rejection 19. Chien et al. studied the control of a RDWC for esterification of a feed with n-amyl alcohol and nhexanol mixture and acetic acid to produce n-amyl acetate and n-hexyl acetate using a fixed bed of Amberlyst-15 catalyst. The steady state model for the system was developed by thermally coupling a side rectifier connected to a double feed with a conventional distillation column. When transferred to a dynamic simulation, there were 12 degrees of freedom, with 6 level and 3 pressure control loops being used for inventory control. The remaining manipulated variables were reflux flow rate, reboiler duty, and feed ratio. The feed ratio and reflux flow rate were used to control temperature, while the reboiler duty is ratioed to the feed. In the study, three types of disturbances were used. For an outer disturbance, changes were made to the mixed alcohol and acetic acid compositions. Throughput disturbances were made with +/- 10 percent changes in the mixed alcohols feed flow rate. For an inner disturbance, the vapor ratio is changed by +/-10 percent. The results from these tests indicate that the dual-temperature control strategy performs satisfactory to maintain high purity of the two products in the face of all disturbances 51. Ye et al. analyzed control of a RDWC for the reaction of propanoic acid and n-propyl alcohol to synthesize n-propyl propionate and water with a heterogeneous catalyst of Amberlyst 46. Aspen plus was used for rigorous steady state modeling of the RDWC using a three-column flowsheet previously described. Aspen Dynamics was used for the dynamic simulation. To select the temperature control trays, sensitivity criterion was used to detect temperature profiles changes with a slight variance in reboiler duty. Once this was done, the PI control strategy was tuned and implemented in the face of disturbance of feed flow rate by +/-20 percent, temperature by +/-10 degrees Celsius, and feed ratio by 43

+/-2 percent. The result had large transient deviations and settling times in some cases. The basic control structure was improved by varying the pressure control and cascading the reboiler to the feed flow controller. This proved to be effective, as the transient deviations and settling times were reduced, and purity issues in the previous control scheme were resolved 52. Yuan et al. studied a steady state RDWC for the selective hydrogenation and separation of a C3 stream. The steady state model for the system, which consists of a feed into a side stripper thermally connected with a conventional distillation column, was summarized earlier in this review. Next, the authors dynamically modeled the system using Aspen Dynamics. Four different PI control schemes were proposed, two that use composition control, and two that use temperature control. In total, there are eight controlled and manipulated variables. The pairings of controlled and manipulated variables are available in the paper. The control structures were all tested with +/- 20 percent disturbances in fresh feed flow rate and in fresh feed composition by changing the compositions of each feed component by 20 percent. All control schemes could provide effective control of the RDWC. The results demonstrate that for feed flow disturbances, settling time is longer for temperature control than composition control. But the temperature control schemes have a smaller maximum deviation than the composition control schemes for feed composition disturbances. Since the RDWC combines RDCs and DWCs, the concentration of reactant on the feed stage and the liquid split ratio should be considered during the control structure design. In RDCs, the concentration of reactant on the feed stage is controlled by manipulating the feed flow rate. For DWCs, the liquid and vapor split ratios should not be used as manipulated variables, since the actual values may be different than the designed values in practical industrial application. The authors suggest that for a RDWC, the liquid split should not be used as a manipulated variable since the actual liquid split ratio may be different to the designed value due to hydraulic lag. Instead, manipulating the two reboilers of the column to control vapor rate is more practical since it can be done rapidly Considering this, along with the shorter lag time of temperature 44

control, it is recommended that the temperature control schemes are more practical than the composition control schemes 55. Using the same steady state model, Yuan et al. studied model predictive control (MPC) through a joint simulation of the MATLAB Simulink MPC toolbox and Aspen Dynamics. Just as in the PI control study, there are eight controlled and manipulated variables in the model. In this simulation, the liquid split ratio is optimized and then kept constant at the optimized value. The genetic algorithm is used for the multi-objective optimization of MPC tuning to obtain Pareto optimum solutions for the weight of the controlled and manipulated variables’ variations. The controller stability is tested according to Lyapunov stability, and shown to be stable and operable. To compare with previous PI control, +/- 20 percent changes in fresh feed flow rate and fresh feed composition are made. The results show that MPC control outperforms PI and PI with feed forward control since the amplitude and number of oscillations were reduced. This study was also consistent with a previous MPC study for RDCs 56. Rodríguez et al. also investigated MPC’s application for control of RDWCs. They present a methodology that transforms an Aspen dynamics model to a linearized state space model in MATLAB by utilizing Aspen’s Control Design Interface. In this method, a Simulink wrapper of the Aspen dynamics model is used to validate the linear model before the MPC is designed in Simulink using the state space model. This procedure is tested using Sander’s methyl acetate hydrolysis RDWC work as a case study. A disturbance of 5 percent in feed flow rate is applied to test the tuned model. Results show that product compositions return to their set points without difficulty. The strong nature of MPC and these successful results are reason to further investigate MPC of RDWCs for other systems 57. Wang et al. studied the control of a RDWC for the synthesis of diethyl carbonate. The steady state model, which was previously detailed, consisted of a RDC thermally coupled to a side stripper, which represented a RDWC with the wall attached to the bottom of the column. In Aspen dynamics, PI

45

temperature control was used to maintain the molar ratio of the two reactants, as this is very important in RDWC control. Due to the lack of multiple stability phenomenon between ethanol feed flow rate and tray temperatures on the RDC, ethanol flow rate was chosen as the manipulated variable. SVD criterion is used to select the appropriate trays, and a feed forward controller with a ratio of reboiler duty to feed flow rate is introduced to reduce transient deviations and settling time. To test the control scheme, feed flow rate and composition disturbances of +/- 20 percent are made. The results show that all values return to steady state in reasonable time, and can be maintained close to specification 40.

7. Discussion and Conclusions In this work, a comprehensive review of RDWCs is presented. Studies covering experiments, model development, shortcut methods, rigorous simulations, and control have been reviewed. Currently, there are no publicly disclosed commercial RDWCs due to gaps in the knowledge of RDWC processes that are preventing investment in the technology. However, results of modeling and experimental studies so far are encouraging. Ehler, Eggers, and Fieg completed the most detailed of these studies, and were even able to show that data validated a custom simulation for a solid acid catalyst. Later studies experiments showed that data validated the same simulation and system using an enzymatic catalyst. Shortcut design methods have been designed to quickly predict column feasibility by taking a variety of approaches to predict simultaneous reaction and separation. A variety of methods have been shown capable of producing reasonable starting values for a more rigorous simulation as well as model how complex columns can be decomposed into a system of less integrated operations. Steady state rigorous simulations have been published with very useful results. While early studies focused on proof of concept of RDWCs, recent simulations have been focused on optimization, and energy and economic analysis. RDWCs have been simulation for over twenty different systems with

46

results consistently showing that RDWC have value compared to un-integrated processes. Experimental data has been used to verify some of these results. In addition to steady state models, dynamic models have been successfully developed. PI temperature control has found great success. RDWC research studies are showing that the technology is worth investigating and will have an industrial future. There are still challenges to the commercialization of RDWCs. The lack of experimental results is the greatest of these. The current literature is limited to experiments involving just five chemical systems, two lab scale columns, and two industrial scale columns. While these experiments do demonstrate successful startup and operation, most lack complexity. Additional studies showing operation of RDWCs would be useful for many reasons. Data would help validate current steady state models and lead to their improvement. Experiments using a range of chemical systems would be beneficial since models should be compared to data for multiple systems to ensure their accuracy. Uncertainties in RDWC modeling such as the ideal way to decompose a RDWC in modeling software and the best optimization procedure are important issues that need to be resolved. Once steady state models are validated, then control schemes can be experimentally tested for accuracy too. There is little variety of demonstrated control schemes, and further research in the area would lead to the improvement of dynamic models. A second area of RDWC research that can be improved upon is shortcut design. While shortcut methods have been designed and applied toward certain chemical systems, there is no method that has been applied with accuracy to multiple systems. The core of this issue is how to approximate the simultaneous reaction and multicomponent separation in a RDWC, and a method to quickly predict whether a RDWC is feasible and necessary for a given process is an important step in the design of a relevant process. A third area worth researching is complex control schemes for RDWCs. While temperature control has been shown in many studies to handle disturbances in simulation, model predictive control has only been used in two studies with positive results. Few of these control schemes have been tested in a physical unit, and it is expected that the 47

high level of integration in a RDWC will make control difficult. Developing more robust schemes would ensure that operation achieved in pilot scale columns could be translated to a commercial unit. In summary, RDWCs have the potential to add value in chemical processes involving reactions and multicomponent separations compared to traditional process equipment such as reactors in series with distillation columns. In simulation, several industrially relevant systems have proved operable and controllable in the presence of disturbances, resulting in system dependent capital and operating costs savings over conventional process equipment. Due to the high impact of the chemical system on RDWC effectiveness, limited amount literature for any given system, and the range of modeling and optimization methods employed for these systems, a detailed evaluation of the potential savings of using a RDWC will note be made. However, analysis of current simulation literature leads to the conclusion that RDWCs are industrially feasible with the potential to save between 15 percent and 75 percent energy and at least 20 percent capital cost compared to conventional processes. While these results are encouraging, the limited number of experimental demonstrations of RDWCs is preventing their commercial adoption. If future studies using various chemical systems demonstrate steady state operation of a pilot or industrial scale RDWC and validation of a model using column data, then companies would be more inclined to invest in the research and development of RDWCs for their own processes. Along with research in other areas outlined in this review such as shortcut design and complex control, the demonstration of column operation and model validation will cause the advantages of RDWCs to positively impact the chemical industry.

Acknowledgements The financial support provided by Eastman Chemical Company for carrying out this research is appreciated with gratitude. 48

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Figure 1. (a) Conventional, (b) dividing wall, (c) reactive, and (d) reactive dividing wall distillation columns shown in order of increasing integration for sample separation and reaction processes. Components A, B, and C are generic compounds in order of decreasing volatility. The shaded region represents a reactive zone involving either a heterogeneous or homogeneous catalyst. Figure 2. Design and operational variables involved with running a RDWC. Figure 3. Petlyuk configuration. Fig 3a shows the prefractionator feed arrangement, and fig 3b shows the mainfractionator feed arrangement. Figure 4. Two-column configurations to model DWCs with the wall connected to the top or bottom of the column. 4a shows a DWC with the wall connected to the top, and 4b shows its equivalent side stripper configuration. 4c shows a DWC with the wall connected to the bottom, and 4d shows its equivalent side rectifier configuration. Figure 5. Three (a), four (b), and five (c) column arrangements that are all thermodynamically equivalent to the RDWC in figure 1. Figure 6. Thermally coupled two-column sequence and its thermally equivalent RDWC. 5a depicts a side stripper with 15 stages connected to a distillation column at stage 25. 5b shows the thermally equivalent RDWC containing a horizontal wall at stage 10, and a vertical wall that continues to stage 25.

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