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A thermo-hydraulic approach to conceptual design of an internally heat-integrated distillation column (i-HIDiC)
Computers and Chemical Engineering 31 (2007) 1346–1354
Review
A thermo-hydraulic approach to conceptual design of an internally heat-integrated distillation column (i-HIDiC) M. Gadalla a,∗ , L. Jim´enez a , Z. Olujic b , P.J. Jansens b a
University Rovira i Virgili, Department of Chemical Engineering, Paisos Catalans 26, Campus Sescelades, 43007 Tarragona, Spain b Delft University of Technology, Laboratory for Process Equipment, Leeghwaterstraat 44, 2628 CA Delft, Netherlands Received 7 January 2005; received in revised form 8 November 2006; accepted 12 November 2006 Available online 18 December 2006
Abstract This paper introduces an effective method for the conceptual design of internally heat-integrated distillation columns (i-HIDiC). In i-HIDiC, the two column sections are operated at different pressures. The column configuration uses internal heat transfer on column trays between the different sections for heat integration, i.e. they are heat-integrated pressure swing distillation. The new method is based on stage temperature profiles and hydraulic calculations. The paper proposes a tool to assess the feasibility of design and operation of columns combining both the thermodynamic and hydraulic (physical space) capacities. An algorithm is suggested by which the stage temperature profiles are obtained and the pinched stages are identified. Thus, various feasible alternatives for i-HIDiC can be achieved. The work also introduces new terms which are useful for i-HIDiC designs and evaluation of column configurations. Finally, a model, based on hydraulic calculations and geometry analysis, is developed in order to quantify the capability of a column design for heat transfer areas. The model equations can be used in conjunction with commercial software, such as ASPEN+, in order to optimise basic designs for i-HIDiC. © 2006 Elsevier Ltd. All rights reserved. Keywords: i-HIDiC; Heat integration; Distillation; Process intensification; Feasibility
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stage temperature profiles based approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Pinched and limiting stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Thermodynamic feasibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distillation heat integration hydraulic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Configurations of rectifying and stripping sections/columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Hydraulic feasibility indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction There is no doubt that distillation is the most mature and widely used separation process in the chemical and process industries. It is a reliable technology, however it uses huge
∗
Abbreviation: i-HIDiC, heat-integrated distillation column Corresponding author. E-mail address: [email protected] (M. Gadalla).
0098-1354/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compchemeng.2006.11.006
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amounts of energy with a rather inefficiency. According to various energy estimates, energy requirement of most refining and chemical processes is utilised in distillation columns: nearly 4% of the total energy requirement in the USA in 1988 is directed to distillation processes (Humphrey & Keller, 1997). The low energy efficiency shortcoming of distillation urged the interest of research, management and economics towards implementing advanced distillation technologies with higher efficiencies. As a result, extensive research efforts and studies
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Nomenclature AD.C. AH.P. Astr drec dstr L L1 L2 NH.P. N H.P. Qstage SAH.P. TAH.P. TA H.P.
area of downcomer (m2 ) area of heat panel (m2 ) cross sectional area of stripping column (m2 ) diameter of rectifying column (m) diameter of stripping column (m) length of heat panels (m) perimeter of stripping column (m) perimeter of rectifying column (m) number of heat panels (Fig. 9) (−) number of extra heat panels (Fig. 10) (−) heat transfer rate on stage (kW) summation of total areas of heat panels (m2 ) total area of heat panels (m2 ) total area of extra heat panels (m2 )
Greek letters σ distance between stripping and rectifying columns (m) ε clearance between heat panels and rectifying column shell (m) δ width of downcomer area (m) Subscripts D.C. downcomer H.P. heat panel rec rectifying column str stripping column
have been devoted, over decades, to reduce energy consumption through process optimisation and overall system integration, or by investigating new column designs with high capital and energy-efficient capabilities. Thermal coupling, heat integration, vapour recompression (heat pump) were investigated and recommended as techniques to increase the energy efficiency of distillation (Freshwater, 1951; Linnhoff, Dunford, & Smith, 1983; Stupin & Lockhart, 1972; Sulzer, 2006). In direct vapour recompression designs, which are effective but capital intensive solutions for stand-alone close boiling separation applications, vapours leaving the top of the distillation column are compressed to a certain pressure and then condensed in the reboiler of the same column, providing the heat needed for vapour generation at the bottom of the column. Recently, internally heat-integrated distillation columns (i-HIDiC) were introduced and showed an economical and energy saving potentials over conventional alternatives (Nakaiwa et al., 1997, 2000; Olujic, Fakhri, de Rijke, de Graauw, & Jansens, 2003). These columns are representatives of heat-integrated two-pressure distillation, sometimes called pressure-swing distillation. i-HIDiC column configuration consists of two separate distillation columns, unlike the conventional alternative which has only one column shell with two sections. In i-HIDiC configuration, the two columns are the stripping and rectifying sections of the conventional design. There is a pressure difference between
Fig. 1. Schematic representation of the internally heat integrated distillation column (i-HIDiC).
the two columns; the top vapours of the stripping columns are compressed and then enter the bottom of the rectifying column. The bottom liquid of the rectifying column is fed into the top of the stripping column, as is the feed to the stripping column. The pressure of the recycled stream from the rectifying column is equalised with that of the stripping column through a throttling valve. The distillate of the rectifying column is the light product, while the heavy product is the bottom stream of the stripping column. The energy of the hot rectifying column is exploited to heat the stripping column by integrating the hot vapour with cold liquid streams of the rectifying and stripping columns, respectively, in a heating medium, such as heat panels. Fig. 1 shows a diagram of an internally heat-integrated distillation column. Heat is transferred from the rectifying section to the stripping section through the indirect contact of the hot vapour and the cold liquid streams of the rectifying and stripping sections, respectively. Overall, a continuous condensation of the vapours occurs along the rectifying column and on the other hand, a continuous evaporation, i.e. vapour generation, takes place in the stripping column. This heat transfer is achieved in an external medium, such as heating panels. Various layouts exist for transferring heat, and consequently the configuration of the stripping and rectifying sections and heating panels can vary widely. i-HIDiC was conceptually introduced and evaluated by Mah and co-workers (Fitzmorris and Mah, 1980; Mah, Nicholas, & Wodnik, 1977) under the name ‘Secondary Reflux and Vapourisation’ (SRV). Seader (1978/1980), and Glenchur and Govind (1987) introduced different column configurations for implementing i-HIDiC. A shell and tube-type packed column was then introduced for i-HIDiC by Aso, Matsuo, Noda, Takada, & Kobayashi (1996/1998). Recently, a group of Japanese researchers (Nakaiwa et al., 1997, 2000, 2001; Naito et al., 2000) studied i-HIDiC, concentrating on the theoretical evaluations and pilot plant testing. These results indicated energy saving up to a 60% with respect to conventional columns. Among the research efforts on i-HIDiC designs, few design aspects have been reported. The research rather concentrated on simulation and experimental studies and control aspect. Neither the conceptual design of i-HIDiC nor the design and operation feasibility were addressed. A more recent work has been published propos-
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ing a design method based on the insights of pinch analysis. This method was applied to a number of applications not only for performance improvement but also for better understanding (Gadalla, Olujic, de Rijke, & Jansens, 2005). In this work, more attention is directed to the conceptual design of i-HIDiC and the evaluation of design feasibility with respect to thermal and physical space capacities.
stages. A reasonable temperature difference needs to be specified together with either the duty of the exchanger or the streams flow rates. This modelling technique provides the calculations of the heat transfer area simultaneously with the column model calculations; although it shows some difficulties to converge.
2. Stage temperature profiles based approach
For a given system, the i-HIDiC configuration is simulated. The simulation starts by specifying the feed conditions and composition; then the two columns are built for the given number of stages, i.e. stripping and rectifying columns. The vapour leaving the stripping column is compressed to the specified pressure and then fed into the bottom of the rectifying column. The liquid stream leaving the bottom of the rectifying column is recycled into the top stage of the stripping column. The product specifications (i.e. product recoveries) are specified. Hence, on simulation convergence, the compressor load, reboiler and condenser duties are obtained. With no heat integration between the two columns (Qstage = 0, Fig. 1), the reboiler and compressor will operate on maximum loads. To transfer any heat from the rectification column to stripping column, there should be a thermal (temperature) driving force, i.e. the rectifying stages must be hotter than the corresponding stages in the stripping column. The temperature of the rectifying column is determined by the pressure ratio of the compressor, there is a minimum pressure that enables the heat transfer between the two columns. From simulation results, the temperature for each stage in the rectifying and stripping columns is obtained. The temperatures of the stages are plotted against the stages number for both rectifying and stripping columns. Hence, two temperature profiles are obtained, i.e. hot and cold temperature profiles. For a possible heat transfer between the two columns, the temperature profile of the rectifying column must be above the stripping temperature profile, i.e. rectifying column is hotter. In order to demonstrate the features of the design approach proposed for i-HIDiC, a simple example, separating an equimolar mixture of benzene and toluene with the same product specification on both ends, is considered. Fig. 2(a) shows the stage temperature profiles for the separation of benzene–toluene mixture in a i-HIDiC configuration. As shown, there is a positive temperature driving force between all the rectifying and
Ahead of carrying out design studies for i-HIDiC or performing experimental setups, the designer should be able to predict the thermal performance and determine the feasibility of i-HIDiC designs or operation. This incentive guides the designer to, firstly, evaluate the potential of heat integration and quantify the opportunities for maximum energy savings. It also helps find different design alternatives with various heat integration schemes, and hence, the optimisation of i-HIDiC can be achieved. This can be done by obtaining the temperatures of the two column stages by using a simulation package (e.g. ASPEN Plus), before performing any heat transfer. The performance of internal heat-integrated distillation columns is simulated using commercial flowsheeting packages (e.g. ASPEN Plus, HYSYS). The design task includes the calculations of the compressor, condenser and reboiler duties, stage heat rates, and the heat transfer area required. Existing commercial simulation softwares have no modelling facilities for i-HIDiC configurations. Therefore, modelling of i-HIDiC is carried out by using two separate columns as shown in Fig. 1; the heat integration is performed through adding heat loads to stripping stages and removing the same amounts of heat from the rectifying stages. Then, the calculations of heat transfer area are done separately when the column simulation has converged. Such a modelling technique is a two-step procedure; however, the modelling can be performed differently and in a more robust manner, adjusting the heat exchanger model to the column configuration. This has been implemented in HYSYS commercial software. The heat exchanger represents the heating transfer medium through which the hot vapours and cold liquids are exchanging heat. The hot vapours enter the heat exchanger, transferring heat to the cold liquid streams and then return again to the rectifying column on the same stage. Similarly, the cold liquid streams are heated and returned to the same stripping
2.1. Pinched and limiting stages
Fig. 2. Distillation stage temperature profiles for benzene–toluene separation (100 kmol/h equimolar feed; 20 stages; 2.0 pressure ratio; 99.5% and 0.5% distillate and bottoms-benzene purities, respectively) (a) stage temperature profiles, (b) pinch location and limiting stages.
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stripping column stages. Therefore, heat transfer is possible for benzene–toluene system throughout all column stages, the temperature profiles indicate a variation in the driving forces along the columns. At the top and bottom stages, there is a large temperature driving force; whereas, this driving force is very small in the middle section of the i-HIDiC column. This indicates that considerable amounts of heat can be transferred between the top and bottom stages of the two columns; while smaller amounts are available in the middle section. Some particular stages show limiting temperature differences; stage number 6 has the smallest driving force (T). This stage is a design key in the detailed design stage of i-HIDiC and identifies the bottleneck of transferring heat from the source (rectifying column) to the sink (stripping column). Both stage temperature profiles can be moved vertically against each other till a touch point or region is observed. This point (region) identifies the condition of maximum heat integration of i-HIDiC for specific conditions and the stages with the limiting driving force, limiting stages or pinched stages (Fig. 2b). Maximum heat integration leads to a minimum reboiler duty. Pinched stages have the minimum temperature driving force which is set by the designer ahead of design, while limiting stages show the smallest temperature driving force. The pinch location is equivalent to the maximum total amount of heat that can be internally transferred for a given system with specific conditions and a given minimum temperature difference and of a specific pressure ratio. The condition of zero temperature difference on pinched stage sets the upper boundaries of the constant heat that can be integrated between the two columns. No heat integration can further be achieved constantly beyond the pinch point; this leads to two types (degrees) of i-HIDiC designs and operations, partial and ideal i-HIDiC. In the partial i-HIDiC, the reboiler duty can be reduced to a certain level which is still larger than zero; this is affected by whether some stages are bottlenecked (pinched) or not. When there are no pinched stages, the reboiler duty can be further reduced to zero (i.e. ideal iHIDiC); this indicates that the reboiler is not needed. To achieve any level of heat integration, large amounts of heat transfer area need to be installed on the limiting stages, while infinite amount of heat transfer area is required on the pinched stage. Fig. 2b indicates that the pinch occurs in the middle of the column; however these results are much dependent on the system and its problem specifications. For example, for other systems, such as propylene–propane splitter (Sun, Olujic, de Rijke, & Jansens, 2003), the pinch location is found at the bottom of the column. Stage temperature profiles with a simulation can determine simultaneously the maximum heat recovery and heat load per stage, and the minimum reboiler duty for a given system and a specific pressure ratio. Typical results for the separation of benzene–toluene mixture are: pinched stage: #6; limiting stages: #5 and 7; maximum total heat recovery: 900 kW; maximum stage heat load: 90 kW; minimum reboiler duty: 808 kW. The results indicate that a fixed load of 90 kW/stage is the upper limit for heat transfer for benzene–toluene separation. However, this scheme can be considered as one of various heat integration schemes that can be achieved; others can be obtained by varying the heat load on every stage. Theoretically, an infinite amount
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Fig. 3. Heat transfer area required for maximum heat recovery (benzene–toluene system).
of area needs to be installed on the pinched stage to achieve the maximum heat recovery. Fig. 3 shows the heat transfer area required for the maximum heat recovery conditions. The heat transfer area is calculated for each stage by knowing the amount of heat to be transferred across and the temperature difference. Also, the overall heat transfer coefficient which can be assumed with a reasonable value. In our calculation, this coefficient is set to 1 kW/m2 ◦ C which is a reasonably satisfactory. However, a further experimental work in the Laboratory for Process Equipment at Delft University of Technology is underway in order to measure the actual values for heat transfer coefficients and their changes. In this work, a pilot i-HIDiC has been built, consisting of a concentric column with rectification section inside the stripping section. The stripping section contains three annular sieve trays equipped with rectangular heat panels. The hot vapour from rectification section enters the panels and condenses releasing the heat which is transferred through the panel walls to the outer surface covered by liquid film causing partial evaporation of the liquid, i.e. generating certain amount of boil-up in the stripping section. The heat transfer coefficient is calculated for both sides and thus the overall coefficient is obtained. Preliminary experimental evaluations showed that the value used in our calculations is realistic. As shown in Fig. 3, the required amount of heat transfer area depends on the number of stage for a constant heat load per stage. Some stages require large amounts of heat transfer area, while others only need small amounts to transfer the same amount of energy. This could be impractical solution due to a limited physical space inside the columns. Thus, the i-HIDiC can be redesigned for a fixed heat transfer area installed per stage. 2.2. Thermodynamic feasibility Fig. 4 illustrates the algorithm for thermodynamic feasibility identification. Design and operation of a i-HIDiC is thermodynamically feasible when the rectifying section is hotter than the stripping section; this can be assessed from the stage temperature profiles. Stage profiles can help to find feasibility regions in which both, the design and operation, are possible. The stage vapour and liquid heat contents determine the potential of heat integration. The quantity and quality of heat are essential for
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Fig. 4. An algorithm for thermodynamic calculations.
heat integration. The quality of heat is represented in the temperature of the heat source and the heat sink, i.e. rectifying and stripping sections; the better the quality the more heat recovery. On the other hand, the quantity of heat is represented through the heat content of the vapour streams of the rectifying section; the vapour streams need to have higher capacities than those of the liquid streams of the stripping section. As mentioned in the benzene–toluene system, the maximum stage heat load is 90 kW; this corresponds to a minimum reboiler duty of 808 kW. However, the reboiler duty can be reduced further by adopting different heat integration schemes and improving the energy efficiency of the distillation column. It is clear from the system temperature profiles (Fig. 2a) that the top and bottom stages have larger driving forces. Therefore, heat can be transferred in larger amounts on these stages, while the load on the middle stages can be relaxed. This is carried out using a simulation package and together with the stage temperature profiles in order to maintain the design feasibility. Fig. 5 shows the regions of feasible thermodynamic designs for i-HIDiC applications in the separation of benzene–toluene system. Each area represents a feasible thermodynamic design for i-HIDiC with a specific heat integration scheme. At no heat transfer between the columns, the heat requirement is 1347 kW (reboiler duty). As heat is transferred from rectifying column to the stripping column, the reboiler duty is reduced, as shown on the Y-axis (Fig. 5). Fig. 5 reveals that ideal i-HIDiC design for
benzene–toluene separation is not possible for a constant heat transfer rate. In order to transfer more heat between columns, the load on middle stages (#5, 6, 7) need to be relaxed, i.e. no heat transfer. This suggests that any heat integration scheme leading to a reboiler duty lower than 808 kW should not have any heat transfer throughout the limiting stages; this conclusion agrees with that drawn from Fig. 2(b). As a result, the next scheme of fixed heat load per stage is that transferring a 170 kW on stages #1, 2, 3, 4, 8, 9, 10, and the corresponding reboiler duty is 665 kW. The table in Fig. 5 summarises the results of the different designs and shows also the total annual costs of each design. The total cost includes the capital expenses for the column, condensers, reboilers, compressors and heat transfer areas and those for the utilities. As shown in Fig. 5, several feasible design alternatives are obtained. Each alternative shows a variable heat load distribution throughout the column stages and consequently, each design has a different total cost. The best energy saving-alternative is that exploiting the large temperature driving forces of the top and bottom stages and reducing the reboiler duty to a minimum of a 51 kW of energy which is close to an ideal i-HIDiC design. On the other hand, the optimum design in overall is that design which requires the lowest total annual cost of 390 k$/year compared with a cost of 521 k$/year. Therefore, design #4 is the optimum alternative. The selection of the best i-HIDiC design or optimisation can be done for the cost of energy or the total cost. In the first option, the reboiler duty and compressor electricity consumption will be considered as the main criteria, whereas for the second case, the cost of heat transfer areas will be included. The calculations of Fig. 5 were obtained for a pressure ratio of 2. Increasing pressure ratio increases the temperature driving force, and hence the driving force of the pinched stages increased. At the same time, it increases the cost of both electricity in the compressor and the capital cost of the compression unit. On the other hand, more heat can be transferred from the rectifying to the stripping; this may lead probably to an ideal i-HIDiC design and obviously the cost of heat transfer area is reduced and so as for the reboiler costs. Using the stage profiles for benzene–toluene separation, we found that the minimum pressure ratio for ideal i-HIDiC designs is 2.2. Thus, below a pressure ratio of 2.2, partial i-HIDiC designs are expected. Design #5 is the best overall design from the point of view of energy and economic perspectives. This reveals that when heat
Fig. 5. Feasibility regions of i-HIDiC for benzene–toluene separation (pressure ratio = 2).
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Fig. 6. Various column configurations for i-HIDiC designs.
transfer rate is allowed to vary throughout the column stages, better energy performance may be obtained. 3. Distillation heat integration hydraulic analysis After reaching a reasonable (optimum) design from the point of view of heat integration, the next step is to design the heat transfer medium that provides the designed heat transfer rates. The design typically includes the calculations of the device heat transfer area required for a given transfer rate. However, other issues, such as the configuration of the rectifying and stripping sections and the layout of the heat transfer device, have also to be addressed at this stage. The heat transfer area on a particular stage can be calculated from the information on the temperature driving force and the heat transfer rate, for a given heat transfer coefficient. 3.1. Configurations of rectifying and stripping sections/columns In i-HIDiC columns, the heat transfer is conducted in an external device. The shape and layout of this medium vary from one design to another and also depend on the column configuration. The rectifying and stripping sections can be configured in different ways. Fig. 6 shows four possible configurations. In the ‘Multi-tube’ configuration, the rectification section is arranged as a number of small diameter columns placed in parallel inside the stripping section (shell), and the surface area of the wall should be enough to transfer the required heat duty. Since the diameter of rectification columns is rather a small and the configuration of the shell side stripping section complex, this configuration is only suitable for packed column applications, as proposed by Aso et al. (1996/1998). Other configurations are applicable for tray distillation columns. In the ‘Split’ layout, the rectifying and stripping sections are placed adjacently, divided by a partition wall. The most conventional configuration is a concentric column, in which the rectifying section is placed concentrically in the stripping section. The ‘Multi-concentric’ configuration is a couple of concentric columns arranged axially. These configurations differ from each other in the degree of utilising the physical area available inside the column. In a typical concentric configuration with panels placed in the stripping column, the hot vapour enters the panel, while the cold liquid flows across the outer surface (Fig. 7). Due to temperature difference, the vapours condense inside the panel and the liquids vapourise on the surface. The heat transfer medium employed in this research is a heat transfer panel; it is a rectangular thin plate with a corrugated
surface to increase the heat transfer contact. The panel has an opening and exit tubes. The hot vapour of the rectifying section passes through the heat panel, while the cold liquid of the stripping section flows across the outer surface. Due to temperature difference, the vapour condenses inside the panel and the liquid vapourises on the surface. This heat panel provides the area for heat transfer required by the design calculations. The diameter of the column, which is calculated from the hydraulic flooding limits, determines the physical space in which the heat panels can be placed. Therefore, the available space for heat panels must allow the required heat rate to be transferred, i.e. the total physical space area on a particular stage should be larger than that required by the heat integration design on that stage. A new term is defined: the hydraulic (physical) design feasibility. i-HIDiC design is hydraulically feasible when the total area of heat panels that can be physically placed on stages is large enough to provide the heat transfer area calculated by the design simulations. If the heat transfer area is larger than that available by heat panels or the physical space, the heat cannot be transferred with the required amount; thus, the design is impossible at this level. So, the heat transfer rate may be reduced. This concept can be used to assess the hydraulic feasibility of a given i-HIDiC design or to redesign a i-HIDiC to achieve a feasible hydraulic performance. In order to determine the hydraulic feasibility of a i-HIDiC design, a hydraulic model is proposed to determine the heat panels transfer area available by the physical space available on stages. This model is based on the hydraulic diameter obtained from the flooding limits of the column, configuration of the rectifying and stripping sections, and the area of the heat panels. The calculation model for the heat panel is a function of the layout of the panel placed on the column stages and its dimensions. The geometry of the column configuration and the panel layout need to be analysed in order to calculate all the physical space areas that can be used to place the heat panels. Fig. 8 illustrates the geometrical analysis of a concentric i-HIDiC configuration. In this case, the heat panels are placed in the stripping side and in the
Fig. 7. Sketch of heat transfer panel with column sections.
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Fig. 9. Layout of extra heat panels in a concentric i-HIDiC.
Fig. 8. Geometrical analysis of concentric i-HIDiC configuration.
annular space outside the rectifying hot column. The placement of panels is determined by the larger space available inside the column. The panel layout of Fig. 8 is favoured in the top sections of the i-HIDiC since the diameter of the stripping column is relatively larger than that of the rectifying column. This allows more space for the heat panels. However, in the bottom sections, the rectifying diameter is relatively larger than the stripping diameter. In this case, it is recommended to place the heat panels in the rectifying side to exploit the larger physical space. The heat panel has a fixed thickness and height. The panel length determines the panel area and it is determined by the space available between the two column sections, and considering some space allowances. It can be calculated from the geometry analysis of the column sections. The perimeter of the inner surface of the stripping section (L1) determines the number of heat panels that can be placed in parallel. This perimeter takes into account the downcomer area required. Given the diameter of the rectifying and stripping sections, as well as the downcomer area and for specific panel dimensions, the following model equations are obtained from the geometry analysis of the concentric column (variables are defined in Fig. 8): σ=
dstr − drec 2
AD.C. = 10 − 20% (Astr ) δ=
2 0.10πdstr 2(dstr − drec )
(1) (2) (3)
L=σ−ε
(4)
L1 = πdrec − δ
(5)
L2 = πdstr − δ
(6)
AH.P. = 0.30L
(7)
L1 0.03 = NH.P. AH.P.
NH.P. =
(8)
TAH.P.
(9)
The area of each heat panel is calculated in Eq. (7), while the number of panels is given by Eq. (8). The total area of heat panels is the sum of all individual panel areas (Eq. (9)). In order to exploit all physical space available between the two columns, heat panels can be placed in the two triangle parts as shown in Fig. 9. This aspect increases the total heat panel area as calculated in Eqs. (10)–(12): L2 − L1 NH.P. = (10) 2(0.03) TAH.P. = NH.P. AH.P.
(11)
SAH.P. = TAH.P. + TAH.P.
(12)
Note that both sides of the heat panel are available for heat transfer; therefore, the total area calculated in Eq. (12) is doubled. In a similar manner, the geometry of all other i-HIDiC configurations shown in Fig. 6 can be analysed, and hence the corresponding model equations can be obtained. All model equations have been programmed using FORTRAN. A copy of the program code and details may directly be requested from the corresponding author. Table 1 summarises the number of heat panels and heat transfer area required per stage for various Table 1 Heat panel calculations for different i-HIDiC configurations Configuration type
Panels layout side
Available area (m2 )
# of panels
Concentric Concentric Multi-concentric Multi-concentric Split Multi-tube
Stripping Rectifying Stripping and rectifying Walls Stripping Walls
360 394 621 51 322 88
473 159 839 – 94 50a
a
Tubes, heat panel dimensions: 30 cm × 6 mm.
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Fig. 10. Panels arrangements in a concentric i-HIDiC design for propylene–propane splitter.
i-HIDiC configurations. The model calculations were carried out for a state of the art, heat pump assisted propylene-propane splitter (PP-splitter) processing a 110 t/h feed (Sun et al., 2003). The i-HIDiC configurations considered in this study have 182 stages in rectification section, while the stripping section contains 57 stages. For a concentric i-HIDiC, the internal column (rectifying) diameter is 8.7 m, while the external column (stripping) diameter is 11.3 m. As shown in Table 1, more heat transfer area can be installed on trays in rectification section (394 m2 ) than in the stripping section (360 m2 ). Note that for integrated stages, the heat panels can be placed in the rectifying side or in the stripping side. Since in the present case, a heat transfer area of 350 m2 is required per stage, both sections provide enough space for placing heat panels. In addition, the walls of the columns are also used for heat transfer, but this is pronounced only in case of multi-concentric configuration. The corresponding heat transfer area per stage (51 m2 ) is calculated as the outer surface area of the column walls. The complex multi-concentric configuration with double columns has a maximum heat panel area of 621 m2 , which is nearly double of that (322 m2 ) available in case of the split arrangement. The split arrangement is based on a PPsplitter with equivalent columns diameters of 8.7 m and 7.2 m for rectifying and stripping sections, respectively. A rather small increase in the diameter of stripping is needed to meet the heat transfer area requirement. 3.2. Hydraulic feasibility indicator The physical area obtained from the hydraulic and geometrical analysis represents the maximum area that can be utilised by heat panels for heat integration. Therefore, this area is a feasibility limit for i-HIDiC design. Hence, a hydraulic feasibility indicator is proposed to determine whether the i-HIDiC design is practical according to the physical area limitations or not. This indicator is defined as the ratio between the physical space area available by trays and the area required for heat integration. For a practical design, the value of the hydraulic feasibility indicator must be at least one. Otherwise, there is no enough space inside the column to provide a sufficient heat transfer area for the required application. The indicator can be calculated for all column stages of any given design. The indicator values can then be plotted against the column stage numbers; a design with an indicator of unity indicates that the physical space of a stage is exploited maximally. Hence, regions of design unfeasibility can be determined and therefore, the heat transfer rates across these regions can be reduced.
On the other hand, the feasibility indicator of a given impractical design can be used to redesign the i-HIDiC to achieve a feasible condition. In this case, the indicator is assumed to have a reasonable value (greater than 1.0); then, the heat transfer rate is calculated from the known heat transfer area that can be exploited. Next, the column is resimulated to adjust the energy requirements of the i-HIDiC. In addition, the indicator can help to determine whether the panels should be placed on the stripping side or on the rectifying side. Typical results for a propylene–propane splitter are shown in Fig. 10. The results show that the heat panels should be placed on the stripping side from stages #1 to 27, counting from the top of the stripping section, and on the rectifying side from stages #34 to 57, while the middle section should have no panels (or should operate with lower heat transfer rates). 4. Summary and conclusions A new approach has been presented for the conceptual design of i-HIDiC configurations. This approach is based on the stage temperature profiles and the physical area available by the column hydraulic capacity. Stage temperatures profiles have been shown to be a valuable tool in the conceptual design stage. Profiles are obtained ahead of heat integration design; they assist the designer in finding a potential for heat integration, evaluating design feasibility and proposing different design alternatives with various heat integration schemes. They can guide the designer to determine the optimum operating condition. The results show that a variable heat transfer rate design leads to better designs with minimum total cost compared with constant heat transfer rate schemes. The proposed hydraulic model is a key tool to assess the hydraulic feasibility of i-HIDiC and to redesign a given impractical design. Both stage profiles and hydraulic indicator can guide the designer to thermally optimum and practical i-HIDiC designs. References Aso, K., Matsuo, H., Noda, H., Takada, T., & Kobayashi, N. (1996/1998). Heat integrated distillation column, US Patent 5,783,047. Fitzmorris, R. E., & Mah, R. S. H. (1980). Improving distillation column design using thermodynamic availability analysis. AIChE Journal, 26(2), 265–273. Freshwater, D. C. (1951). Thermal economy in distillation. Transactions IChemE, 29, 149–160. Gadalla, M., Olujic, Z., de Rijke, A., & Jansens, P. J. (2005). Pinch analysisbased approach to conceptual design of internally heat-integrated distillation columns. Chemical Engineering Research and Design (ChERD), 83(A8), 987–993.
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Glenchur, Th., & Govind, R. (1987). Study on a continuous heat integrated distillation column. Separation Science and Technology, 22, 2323–2328. Humphrey, J. L., & Keller, I. I. G. E. (1997). Separation Process Technology. New York: McGraw-Hill. Linnhoff, B., Dunford, H., & Smith, R. (1983). Heat integration of distillation columns into overall processes. Chemical Engineering Science, 38(8), 1175–1188. Mah, R. S. H., Nicholas, J. J., & Wodnik, R. B. (1977). Distillation with secondary reflux and vaporization: a comparative evaluation. AIChE Journal, 23, 651–658. Naito, K., Nakaiwa, M., Huang, K., Endo, A., Aso, T., Nakanishi, T., Nakamura, T., Noda, H., & Takamatsu, T. (2000). Operation of bench-scale HIDiC: an experimental study. Computers and Chemical Engineering, 24, 495–499. Nakaiwa, M., Huang, K., Naito, K., Endo, A., Owa, M., Akiya, T., Nakane, T., & Takamatsu, T. (2000). A new configuration of ideal heat integrated distillation columns (HIDiC). Computers and Chemical Engineering, 24, 239–245. Nakaiwa, M., Huang, K., Naito, K., Endo, A., Akya, T., Nakane, T., & Takamatsu, T. (2001). Parameter analysis and optimization of ideal heat
integrated distillation columns. Computers and Chemical Engineering, 25, 737–744. Nakaiwa, M., Huang, K., Owa, M., Akiya, T., Nakane, T., Sato, M., & Takamatsu, T. (1997). Energy savings in heat-integrated distillation columns. Energy, 22, 621–625. Olujic, Z., Fakhri, F., de Rijke, A., de Graauw, J., & Jansens, P. J. (2003). Internal heat integration – the key to an energy-conserving distillation column. Journal of Chemical Technology and Biotechnology, 78, 241–248. Seader, J. D. (1978/1980). Continuous distillation apparatus and method, US Patent, 4,234,391. Stupin, W. J., & Lockhart, F. J. (1972). Thermally coupled distillation–a case history. Chemical Engineering Progress, 68(10), 71–72. Sulzer, C. (2006). Distillation and heat pump technology, Brochure 22.47.06.40V.91-100. Sun, L., Olujic, Z., de Rijke, A., & Jansens, P. J. (2003). Industrially viable configurations for a heat-integrated distillation column, Better processes for bigger profits. In M. Gough (Ed.), Proceedings of the fifth International Conference on Process Intensification for Chemical Industry (pp. 151–66). Maastricht, The Netherlands: BHR Group 2003, 13–15 October 2003.
Review
A thermo-hydraulic approach to conceptual design of an internally heat-integrated distillation column (i-HIDiC) M. Gadalla a,∗ , L. Jim´enez a , Z. Olujic b , P.J. Jansens b a
University Rovira i Virgili, Department of Chemical Engineering, Paisos Catalans 26, Campus Sescelades, 43007 Tarragona, Spain b Delft University of Technology, Laboratory for Process Equipment, Leeghwaterstraat 44, 2628 CA Delft, Netherlands Received 7 January 2005; received in revised form 8 November 2006; accepted 12 November 2006 Available online 18 December 2006
Abstract This paper introduces an effective method for the conceptual design of internally heat-integrated distillation columns (i-HIDiC). In i-HIDiC, the two column sections are operated at different pressures. The column configuration uses internal heat transfer on column trays between the different sections for heat integration, i.e. they are heat-integrated pressure swing distillation. The new method is based on stage temperature profiles and hydraulic calculations. The paper proposes a tool to assess the feasibility of design and operation of columns combining both the thermodynamic and hydraulic (physical space) capacities. An algorithm is suggested by which the stage temperature profiles are obtained and the pinched stages are identified. Thus, various feasible alternatives for i-HIDiC can be achieved. The work also introduces new terms which are useful for i-HIDiC designs and evaluation of column configurations. Finally, a model, based on hydraulic calculations and geometry analysis, is developed in order to quantify the capability of a column design for heat transfer areas. The model equations can be used in conjunction with commercial software, such as ASPEN+, in order to optimise basic designs for i-HIDiC. © 2006 Elsevier Ltd. All rights reserved. Keywords: i-HIDiC; Heat integration; Distillation; Process intensification; Feasibility
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stage temperature profiles based approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Pinched and limiting stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Thermodynamic feasibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Distillation heat integration hydraulic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Configurations of rectifying and stripping sections/columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Hydraulic feasibility indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction There is no doubt that distillation is the most mature and widely used separation process in the chemical and process industries. It is a reliable technology, however it uses huge
∗
Abbreviation: i-HIDiC, heat-integrated distillation column Corresponding author. E-mail address: [email protected] (M. Gadalla).
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amounts of energy with a rather inefficiency. According to various energy estimates, energy requirement of most refining and chemical processes is utilised in distillation columns: nearly 4% of the total energy requirement in the USA in 1988 is directed to distillation processes (Humphrey & Keller, 1997). The low energy efficiency shortcoming of distillation urged the interest of research, management and economics towards implementing advanced distillation technologies with higher efficiencies. As a result, extensive research efforts and studies
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Nomenclature AD.C. AH.P. Astr drec dstr L L1 L2 NH.P. N H.P. Qstage SAH.P. TAH.P. TA H.P.
area of downcomer (m2 ) area of heat panel (m2 ) cross sectional area of stripping column (m2 ) diameter of rectifying column (m) diameter of stripping column (m) length of heat panels (m) perimeter of stripping column (m) perimeter of rectifying column (m) number of heat panels (Fig. 9) (−) number of extra heat panels (Fig. 10) (−) heat transfer rate on stage (kW) summation of total areas of heat panels (m2 ) total area of heat panels (m2 ) total area of extra heat panels (m2 )
Greek letters σ distance between stripping and rectifying columns (m) ε clearance between heat panels and rectifying column shell (m) δ width of downcomer area (m) Subscripts D.C. downcomer H.P. heat panel rec rectifying column str stripping column
have been devoted, over decades, to reduce energy consumption through process optimisation and overall system integration, or by investigating new column designs with high capital and energy-efficient capabilities. Thermal coupling, heat integration, vapour recompression (heat pump) were investigated and recommended as techniques to increase the energy efficiency of distillation (Freshwater, 1951; Linnhoff, Dunford, & Smith, 1983; Stupin & Lockhart, 1972; Sulzer, 2006). In direct vapour recompression designs, which are effective but capital intensive solutions for stand-alone close boiling separation applications, vapours leaving the top of the distillation column are compressed to a certain pressure and then condensed in the reboiler of the same column, providing the heat needed for vapour generation at the bottom of the column. Recently, internally heat-integrated distillation columns (i-HIDiC) were introduced and showed an economical and energy saving potentials over conventional alternatives (Nakaiwa et al., 1997, 2000; Olujic, Fakhri, de Rijke, de Graauw, & Jansens, 2003). These columns are representatives of heat-integrated two-pressure distillation, sometimes called pressure-swing distillation. i-HIDiC column configuration consists of two separate distillation columns, unlike the conventional alternative which has only one column shell with two sections. In i-HIDiC configuration, the two columns are the stripping and rectifying sections of the conventional design. There is a pressure difference between
Fig. 1. Schematic representation of the internally heat integrated distillation column (i-HIDiC).
the two columns; the top vapours of the stripping columns are compressed and then enter the bottom of the rectifying column. The bottom liquid of the rectifying column is fed into the top of the stripping column, as is the feed to the stripping column. The pressure of the recycled stream from the rectifying column is equalised with that of the stripping column through a throttling valve. The distillate of the rectifying column is the light product, while the heavy product is the bottom stream of the stripping column. The energy of the hot rectifying column is exploited to heat the stripping column by integrating the hot vapour with cold liquid streams of the rectifying and stripping columns, respectively, in a heating medium, such as heat panels. Fig. 1 shows a diagram of an internally heat-integrated distillation column. Heat is transferred from the rectifying section to the stripping section through the indirect contact of the hot vapour and the cold liquid streams of the rectifying and stripping sections, respectively. Overall, a continuous condensation of the vapours occurs along the rectifying column and on the other hand, a continuous evaporation, i.e. vapour generation, takes place in the stripping column. This heat transfer is achieved in an external medium, such as heating panels. Various layouts exist for transferring heat, and consequently the configuration of the stripping and rectifying sections and heating panels can vary widely. i-HIDiC was conceptually introduced and evaluated by Mah and co-workers (Fitzmorris and Mah, 1980; Mah, Nicholas, & Wodnik, 1977) under the name ‘Secondary Reflux and Vapourisation’ (SRV). Seader (1978/1980), and Glenchur and Govind (1987) introduced different column configurations for implementing i-HIDiC. A shell and tube-type packed column was then introduced for i-HIDiC by Aso, Matsuo, Noda, Takada, & Kobayashi (1996/1998). Recently, a group of Japanese researchers (Nakaiwa et al., 1997, 2000, 2001; Naito et al., 2000) studied i-HIDiC, concentrating on the theoretical evaluations and pilot plant testing. These results indicated energy saving up to a 60% with respect to conventional columns. Among the research efforts on i-HIDiC designs, few design aspects have been reported. The research rather concentrated on simulation and experimental studies and control aspect. Neither the conceptual design of i-HIDiC nor the design and operation feasibility were addressed. A more recent work has been published propos-
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ing a design method based on the insights of pinch analysis. This method was applied to a number of applications not only for performance improvement but also for better understanding (Gadalla, Olujic, de Rijke, & Jansens, 2005). In this work, more attention is directed to the conceptual design of i-HIDiC and the evaluation of design feasibility with respect to thermal and physical space capacities.
stages. A reasonable temperature difference needs to be specified together with either the duty of the exchanger or the streams flow rates. This modelling technique provides the calculations of the heat transfer area simultaneously with the column model calculations; although it shows some difficulties to converge.
2. Stage temperature profiles based approach
For a given system, the i-HIDiC configuration is simulated. The simulation starts by specifying the feed conditions and composition; then the two columns are built for the given number of stages, i.e. stripping and rectifying columns. The vapour leaving the stripping column is compressed to the specified pressure and then fed into the bottom of the rectifying column. The liquid stream leaving the bottom of the rectifying column is recycled into the top stage of the stripping column. The product specifications (i.e. product recoveries) are specified. Hence, on simulation convergence, the compressor load, reboiler and condenser duties are obtained. With no heat integration between the two columns (Qstage = 0, Fig. 1), the reboiler and compressor will operate on maximum loads. To transfer any heat from the rectification column to stripping column, there should be a thermal (temperature) driving force, i.e. the rectifying stages must be hotter than the corresponding stages in the stripping column. The temperature of the rectifying column is determined by the pressure ratio of the compressor, there is a minimum pressure that enables the heat transfer between the two columns. From simulation results, the temperature for each stage in the rectifying and stripping columns is obtained. The temperatures of the stages are plotted against the stages number for both rectifying and stripping columns. Hence, two temperature profiles are obtained, i.e. hot and cold temperature profiles. For a possible heat transfer between the two columns, the temperature profile of the rectifying column must be above the stripping temperature profile, i.e. rectifying column is hotter. In order to demonstrate the features of the design approach proposed for i-HIDiC, a simple example, separating an equimolar mixture of benzene and toluene with the same product specification on both ends, is considered. Fig. 2(a) shows the stage temperature profiles for the separation of benzene–toluene mixture in a i-HIDiC configuration. As shown, there is a positive temperature driving force between all the rectifying and
Ahead of carrying out design studies for i-HIDiC or performing experimental setups, the designer should be able to predict the thermal performance and determine the feasibility of i-HIDiC designs or operation. This incentive guides the designer to, firstly, evaluate the potential of heat integration and quantify the opportunities for maximum energy savings. It also helps find different design alternatives with various heat integration schemes, and hence, the optimisation of i-HIDiC can be achieved. This can be done by obtaining the temperatures of the two column stages by using a simulation package (e.g. ASPEN Plus), before performing any heat transfer. The performance of internal heat-integrated distillation columns is simulated using commercial flowsheeting packages (e.g. ASPEN Plus, HYSYS). The design task includes the calculations of the compressor, condenser and reboiler duties, stage heat rates, and the heat transfer area required. Existing commercial simulation softwares have no modelling facilities for i-HIDiC configurations. Therefore, modelling of i-HIDiC is carried out by using two separate columns as shown in Fig. 1; the heat integration is performed through adding heat loads to stripping stages and removing the same amounts of heat from the rectifying stages. Then, the calculations of heat transfer area are done separately when the column simulation has converged. Such a modelling technique is a two-step procedure; however, the modelling can be performed differently and in a more robust manner, adjusting the heat exchanger model to the column configuration. This has been implemented in HYSYS commercial software. The heat exchanger represents the heating transfer medium through which the hot vapours and cold liquids are exchanging heat. The hot vapours enter the heat exchanger, transferring heat to the cold liquid streams and then return again to the rectifying column on the same stage. Similarly, the cold liquid streams are heated and returned to the same stripping
2.1. Pinched and limiting stages
Fig. 2. Distillation stage temperature profiles for benzene–toluene separation (100 kmol/h equimolar feed; 20 stages; 2.0 pressure ratio; 99.5% and 0.5% distillate and bottoms-benzene purities, respectively) (a) stage temperature profiles, (b) pinch location and limiting stages.
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stripping column stages. Therefore, heat transfer is possible for benzene–toluene system throughout all column stages, the temperature profiles indicate a variation in the driving forces along the columns. At the top and bottom stages, there is a large temperature driving force; whereas, this driving force is very small in the middle section of the i-HIDiC column. This indicates that considerable amounts of heat can be transferred between the top and bottom stages of the two columns; while smaller amounts are available in the middle section. Some particular stages show limiting temperature differences; stage number 6 has the smallest driving force (T). This stage is a design key in the detailed design stage of i-HIDiC and identifies the bottleneck of transferring heat from the source (rectifying column) to the sink (stripping column). Both stage temperature profiles can be moved vertically against each other till a touch point or region is observed. This point (region) identifies the condition of maximum heat integration of i-HIDiC for specific conditions and the stages with the limiting driving force, limiting stages or pinched stages (Fig. 2b). Maximum heat integration leads to a minimum reboiler duty. Pinched stages have the minimum temperature driving force which is set by the designer ahead of design, while limiting stages show the smallest temperature driving force. The pinch location is equivalent to the maximum total amount of heat that can be internally transferred for a given system with specific conditions and a given minimum temperature difference and of a specific pressure ratio. The condition of zero temperature difference on pinched stage sets the upper boundaries of the constant heat that can be integrated between the two columns. No heat integration can further be achieved constantly beyond the pinch point; this leads to two types (degrees) of i-HIDiC designs and operations, partial and ideal i-HIDiC. In the partial i-HIDiC, the reboiler duty can be reduced to a certain level which is still larger than zero; this is affected by whether some stages are bottlenecked (pinched) or not. When there are no pinched stages, the reboiler duty can be further reduced to zero (i.e. ideal iHIDiC); this indicates that the reboiler is not needed. To achieve any level of heat integration, large amounts of heat transfer area need to be installed on the limiting stages, while infinite amount of heat transfer area is required on the pinched stage. Fig. 2b indicates that the pinch occurs in the middle of the column; however these results are much dependent on the system and its problem specifications. For example, for other systems, such as propylene–propane splitter (Sun, Olujic, de Rijke, & Jansens, 2003), the pinch location is found at the bottom of the column. Stage temperature profiles with a simulation can determine simultaneously the maximum heat recovery and heat load per stage, and the minimum reboiler duty for a given system and a specific pressure ratio. Typical results for the separation of benzene–toluene mixture are: pinched stage: #6; limiting stages: #5 and 7; maximum total heat recovery: 900 kW; maximum stage heat load: 90 kW; minimum reboiler duty: 808 kW. The results indicate that a fixed load of 90 kW/stage is the upper limit for heat transfer for benzene–toluene separation. However, this scheme can be considered as one of various heat integration schemes that can be achieved; others can be obtained by varying the heat load on every stage. Theoretically, an infinite amount
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Fig. 3. Heat transfer area required for maximum heat recovery (benzene–toluene system).
of area needs to be installed on the pinched stage to achieve the maximum heat recovery. Fig. 3 shows the heat transfer area required for the maximum heat recovery conditions. The heat transfer area is calculated for each stage by knowing the amount of heat to be transferred across and the temperature difference. Also, the overall heat transfer coefficient which can be assumed with a reasonable value. In our calculation, this coefficient is set to 1 kW/m2 ◦ C which is a reasonably satisfactory. However, a further experimental work in the Laboratory for Process Equipment at Delft University of Technology is underway in order to measure the actual values for heat transfer coefficients and their changes. In this work, a pilot i-HIDiC has been built, consisting of a concentric column with rectification section inside the stripping section. The stripping section contains three annular sieve trays equipped with rectangular heat panels. The hot vapour from rectification section enters the panels and condenses releasing the heat which is transferred through the panel walls to the outer surface covered by liquid film causing partial evaporation of the liquid, i.e. generating certain amount of boil-up in the stripping section. The heat transfer coefficient is calculated for both sides and thus the overall coefficient is obtained. Preliminary experimental evaluations showed that the value used in our calculations is realistic. As shown in Fig. 3, the required amount of heat transfer area depends on the number of stage for a constant heat load per stage. Some stages require large amounts of heat transfer area, while others only need small amounts to transfer the same amount of energy. This could be impractical solution due to a limited physical space inside the columns. Thus, the i-HIDiC can be redesigned for a fixed heat transfer area installed per stage. 2.2. Thermodynamic feasibility Fig. 4 illustrates the algorithm for thermodynamic feasibility identification. Design and operation of a i-HIDiC is thermodynamically feasible when the rectifying section is hotter than the stripping section; this can be assessed from the stage temperature profiles. Stage profiles can help to find feasibility regions in which both, the design and operation, are possible. The stage vapour and liquid heat contents determine the potential of heat integration. The quantity and quality of heat are essential for
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Fig. 4. An algorithm for thermodynamic calculations.
heat integration. The quality of heat is represented in the temperature of the heat source and the heat sink, i.e. rectifying and stripping sections; the better the quality the more heat recovery. On the other hand, the quantity of heat is represented through the heat content of the vapour streams of the rectifying section; the vapour streams need to have higher capacities than those of the liquid streams of the stripping section. As mentioned in the benzene–toluene system, the maximum stage heat load is 90 kW; this corresponds to a minimum reboiler duty of 808 kW. However, the reboiler duty can be reduced further by adopting different heat integration schemes and improving the energy efficiency of the distillation column. It is clear from the system temperature profiles (Fig. 2a) that the top and bottom stages have larger driving forces. Therefore, heat can be transferred in larger amounts on these stages, while the load on the middle stages can be relaxed. This is carried out using a simulation package and together with the stage temperature profiles in order to maintain the design feasibility. Fig. 5 shows the regions of feasible thermodynamic designs for i-HIDiC applications in the separation of benzene–toluene system. Each area represents a feasible thermodynamic design for i-HIDiC with a specific heat integration scheme. At no heat transfer between the columns, the heat requirement is 1347 kW (reboiler duty). As heat is transferred from rectifying column to the stripping column, the reboiler duty is reduced, as shown on the Y-axis (Fig. 5). Fig. 5 reveals that ideal i-HIDiC design for
benzene–toluene separation is not possible for a constant heat transfer rate. In order to transfer more heat between columns, the load on middle stages (#5, 6, 7) need to be relaxed, i.e. no heat transfer. This suggests that any heat integration scheme leading to a reboiler duty lower than 808 kW should not have any heat transfer throughout the limiting stages; this conclusion agrees with that drawn from Fig. 2(b). As a result, the next scheme of fixed heat load per stage is that transferring a 170 kW on stages #1, 2, 3, 4, 8, 9, 10, and the corresponding reboiler duty is 665 kW. The table in Fig. 5 summarises the results of the different designs and shows also the total annual costs of each design. The total cost includes the capital expenses for the column, condensers, reboilers, compressors and heat transfer areas and those for the utilities. As shown in Fig. 5, several feasible design alternatives are obtained. Each alternative shows a variable heat load distribution throughout the column stages and consequently, each design has a different total cost. The best energy saving-alternative is that exploiting the large temperature driving forces of the top and bottom stages and reducing the reboiler duty to a minimum of a 51 kW of energy which is close to an ideal i-HIDiC design. On the other hand, the optimum design in overall is that design which requires the lowest total annual cost of 390 k$/year compared with a cost of 521 k$/year. Therefore, design #4 is the optimum alternative. The selection of the best i-HIDiC design or optimisation can be done for the cost of energy or the total cost. In the first option, the reboiler duty and compressor electricity consumption will be considered as the main criteria, whereas for the second case, the cost of heat transfer areas will be included. The calculations of Fig. 5 were obtained for a pressure ratio of 2. Increasing pressure ratio increases the temperature driving force, and hence the driving force of the pinched stages increased. At the same time, it increases the cost of both electricity in the compressor and the capital cost of the compression unit. On the other hand, more heat can be transferred from the rectifying to the stripping; this may lead probably to an ideal i-HIDiC design and obviously the cost of heat transfer area is reduced and so as for the reboiler costs. Using the stage profiles for benzene–toluene separation, we found that the minimum pressure ratio for ideal i-HIDiC designs is 2.2. Thus, below a pressure ratio of 2.2, partial i-HIDiC designs are expected. Design #5 is the best overall design from the point of view of energy and economic perspectives. This reveals that when heat
Fig. 5. Feasibility regions of i-HIDiC for benzene–toluene separation (pressure ratio = 2).
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Fig. 6. Various column configurations for i-HIDiC designs.
transfer rate is allowed to vary throughout the column stages, better energy performance may be obtained. 3. Distillation heat integration hydraulic analysis After reaching a reasonable (optimum) design from the point of view of heat integration, the next step is to design the heat transfer medium that provides the designed heat transfer rates. The design typically includes the calculations of the device heat transfer area required for a given transfer rate. However, other issues, such as the configuration of the rectifying and stripping sections and the layout of the heat transfer device, have also to be addressed at this stage. The heat transfer area on a particular stage can be calculated from the information on the temperature driving force and the heat transfer rate, for a given heat transfer coefficient. 3.1. Configurations of rectifying and stripping sections/columns In i-HIDiC columns, the heat transfer is conducted in an external device. The shape and layout of this medium vary from one design to another and also depend on the column configuration. The rectifying and stripping sections can be configured in different ways. Fig. 6 shows four possible configurations. In the ‘Multi-tube’ configuration, the rectification section is arranged as a number of small diameter columns placed in parallel inside the stripping section (shell), and the surface area of the wall should be enough to transfer the required heat duty. Since the diameter of rectification columns is rather a small and the configuration of the shell side stripping section complex, this configuration is only suitable for packed column applications, as proposed by Aso et al. (1996/1998). Other configurations are applicable for tray distillation columns. In the ‘Split’ layout, the rectifying and stripping sections are placed adjacently, divided by a partition wall. The most conventional configuration is a concentric column, in which the rectifying section is placed concentrically in the stripping section. The ‘Multi-concentric’ configuration is a couple of concentric columns arranged axially. These configurations differ from each other in the degree of utilising the physical area available inside the column. In a typical concentric configuration with panels placed in the stripping column, the hot vapour enters the panel, while the cold liquid flows across the outer surface (Fig. 7). Due to temperature difference, the vapours condense inside the panel and the liquids vapourise on the surface. The heat transfer medium employed in this research is a heat transfer panel; it is a rectangular thin plate with a corrugated
surface to increase the heat transfer contact. The panel has an opening and exit tubes. The hot vapour of the rectifying section passes through the heat panel, while the cold liquid of the stripping section flows across the outer surface. Due to temperature difference, the vapour condenses inside the panel and the liquid vapourises on the surface. This heat panel provides the area for heat transfer required by the design calculations. The diameter of the column, which is calculated from the hydraulic flooding limits, determines the physical space in which the heat panels can be placed. Therefore, the available space for heat panels must allow the required heat rate to be transferred, i.e. the total physical space area on a particular stage should be larger than that required by the heat integration design on that stage. A new term is defined: the hydraulic (physical) design feasibility. i-HIDiC design is hydraulically feasible when the total area of heat panels that can be physically placed on stages is large enough to provide the heat transfer area calculated by the design simulations. If the heat transfer area is larger than that available by heat panels or the physical space, the heat cannot be transferred with the required amount; thus, the design is impossible at this level. So, the heat transfer rate may be reduced. This concept can be used to assess the hydraulic feasibility of a given i-HIDiC design or to redesign a i-HIDiC to achieve a feasible hydraulic performance. In order to determine the hydraulic feasibility of a i-HIDiC design, a hydraulic model is proposed to determine the heat panels transfer area available by the physical space available on stages. This model is based on the hydraulic diameter obtained from the flooding limits of the column, configuration of the rectifying and stripping sections, and the area of the heat panels. The calculation model for the heat panel is a function of the layout of the panel placed on the column stages and its dimensions. The geometry of the column configuration and the panel layout need to be analysed in order to calculate all the physical space areas that can be used to place the heat panels. Fig. 8 illustrates the geometrical analysis of a concentric i-HIDiC configuration. In this case, the heat panels are placed in the stripping side and in the
Fig. 7. Sketch of heat transfer panel with column sections.
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Fig. 9. Layout of extra heat panels in a concentric i-HIDiC.
Fig. 8. Geometrical analysis of concentric i-HIDiC configuration.
annular space outside the rectifying hot column. The placement of panels is determined by the larger space available inside the column. The panel layout of Fig. 8 is favoured in the top sections of the i-HIDiC since the diameter of the stripping column is relatively larger than that of the rectifying column. This allows more space for the heat panels. However, in the bottom sections, the rectifying diameter is relatively larger than the stripping diameter. In this case, it is recommended to place the heat panels in the rectifying side to exploit the larger physical space. The heat panel has a fixed thickness and height. The panel length determines the panel area and it is determined by the space available between the two column sections, and considering some space allowances. It can be calculated from the geometry analysis of the column sections. The perimeter of the inner surface of the stripping section (L1) determines the number of heat panels that can be placed in parallel. This perimeter takes into account the downcomer area required. Given the diameter of the rectifying and stripping sections, as well as the downcomer area and for specific panel dimensions, the following model equations are obtained from the geometry analysis of the concentric column (variables are defined in Fig. 8): σ=
dstr − drec 2
AD.C. = 10 − 20% (Astr ) δ=
2 0.10πdstr 2(dstr − drec )
(1) (2) (3)
L=σ−ε
(4)
L1 = πdrec − δ
(5)
L2 = πdstr − δ
(6)
AH.P. = 0.30L
(7)
L1 0.03 = NH.P. AH.P.
NH.P. =
(8)
TAH.P.
(9)
The area of each heat panel is calculated in Eq. (7), while the number of panels is given by Eq. (8). The total area of heat panels is the sum of all individual panel areas (Eq. (9)). In order to exploit all physical space available between the two columns, heat panels can be placed in the two triangle parts as shown in Fig. 9. This aspect increases the total heat panel area as calculated in Eqs. (10)–(12): L2 − L1 NH.P. = (10) 2(0.03) TAH.P. = NH.P. AH.P.
(11)
SAH.P. = TAH.P. + TAH.P.
(12)
Note that both sides of the heat panel are available for heat transfer; therefore, the total area calculated in Eq. (12) is doubled. In a similar manner, the geometry of all other i-HIDiC configurations shown in Fig. 6 can be analysed, and hence the corresponding model equations can be obtained. All model equations have been programmed using FORTRAN. A copy of the program code and details may directly be requested from the corresponding author. Table 1 summarises the number of heat panels and heat transfer area required per stage for various Table 1 Heat panel calculations for different i-HIDiC configurations Configuration type
Panels layout side
Available area (m2 )
# of panels
Concentric Concentric Multi-concentric Multi-concentric Split Multi-tube
Stripping Rectifying Stripping and rectifying Walls Stripping Walls
360 394 621 51 322 88
473 159 839 – 94 50a
a
Tubes, heat panel dimensions: 30 cm × 6 mm.
M. Gadalla et al. / Computers and Chemical Engineering 31 (2007) 1346–1354
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Fig. 10. Panels arrangements in a concentric i-HIDiC design for propylene–propane splitter.
i-HIDiC configurations. The model calculations were carried out for a state of the art, heat pump assisted propylene-propane splitter (PP-splitter) processing a 110 t/h feed (Sun et al., 2003). The i-HIDiC configurations considered in this study have 182 stages in rectification section, while the stripping section contains 57 stages. For a concentric i-HIDiC, the internal column (rectifying) diameter is 8.7 m, while the external column (stripping) diameter is 11.3 m. As shown in Table 1, more heat transfer area can be installed on trays in rectification section (394 m2 ) than in the stripping section (360 m2 ). Note that for integrated stages, the heat panels can be placed in the rectifying side or in the stripping side. Since in the present case, a heat transfer area of 350 m2 is required per stage, both sections provide enough space for placing heat panels. In addition, the walls of the columns are also used for heat transfer, but this is pronounced only in case of multi-concentric configuration. The corresponding heat transfer area per stage (51 m2 ) is calculated as the outer surface area of the column walls. The complex multi-concentric configuration with double columns has a maximum heat panel area of 621 m2 , which is nearly double of that (322 m2 ) available in case of the split arrangement. The split arrangement is based on a PPsplitter with equivalent columns diameters of 8.7 m and 7.2 m for rectifying and stripping sections, respectively. A rather small increase in the diameter of stripping is needed to meet the heat transfer area requirement. 3.2. Hydraulic feasibility indicator The physical area obtained from the hydraulic and geometrical analysis represents the maximum area that can be utilised by heat panels for heat integration. Therefore, this area is a feasibility limit for i-HIDiC design. Hence, a hydraulic feasibility indicator is proposed to determine whether the i-HIDiC design is practical according to the physical area limitations or not. This indicator is defined as the ratio between the physical space area available by trays and the area required for heat integration. For a practical design, the value of the hydraulic feasibility indicator must be at least one. Otherwise, there is no enough space inside the column to provide a sufficient heat transfer area for the required application. The indicator can be calculated for all column stages of any given design. The indicator values can then be plotted against the column stage numbers; a design with an indicator of unity indicates that the physical space of a stage is exploited maximally. Hence, regions of design unfeasibility can be determined and therefore, the heat transfer rates across these regions can be reduced.
On the other hand, the feasibility indicator of a given impractical design can be used to redesign the i-HIDiC to achieve a feasible condition. In this case, the indicator is assumed to have a reasonable value (greater than 1.0); then, the heat transfer rate is calculated from the known heat transfer area that can be exploited. Next, the column is resimulated to adjust the energy requirements of the i-HIDiC. In addition, the indicator can help to determine whether the panels should be placed on the stripping side or on the rectifying side. Typical results for a propylene–propane splitter are shown in Fig. 10. The results show that the heat panels should be placed on the stripping side from stages #1 to 27, counting from the top of the stripping section, and on the rectifying side from stages #34 to 57, while the middle section should have no panels (or should operate with lower heat transfer rates). 4. Summary and conclusions A new approach has been presented for the conceptual design of i-HIDiC configurations. This approach is based on the stage temperature profiles and the physical area available by the column hydraulic capacity. Stage temperatures profiles have been shown to be a valuable tool in the conceptual design stage. Profiles are obtained ahead of heat integration design; they assist the designer in finding a potential for heat integration, evaluating design feasibility and proposing different design alternatives with various heat integration schemes. They can guide the designer to determine the optimum operating condition. The results show that a variable heat transfer rate design leads to better designs with minimum total cost compared with constant heat transfer rate schemes. The proposed hydraulic model is a key tool to assess the hydraulic feasibility of i-HIDiC and to redesign a given impractical design. Both stage profiles and hydraulic indicator can guide the designer to thermally optimum and practical i-HIDiC designs. References Aso, K., Matsuo, H., Noda, H., Takada, T., & Kobayashi, N. (1996/1998). Heat integrated distillation column, US Patent 5,783,047. Fitzmorris, R. E., & Mah, R. S. H. (1980). Improving distillation column design using thermodynamic availability analysis. AIChE Journal, 26(2), 265–273. Freshwater, D. C. (1951). Thermal economy in distillation. Transactions IChemE, 29, 149–160. Gadalla, M., Olujic, Z., de Rijke, A., & Jansens, P. J. (2005). Pinch analysisbased approach to conceptual design of internally heat-integrated distillation columns. Chemical Engineering Research and Design (ChERD), 83(A8), 987–993.
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