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Optimal design and performance analyses of the glycerol ether production process using a reactive distillation column
Journal of Industrial and Engineering Chemistry 43 (2016) 93–105
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Optimal design and performance analyses of the glycerol ether production process using a reactive distillation column Lida Simasatitkula , Punjawat Kaewwisetkulb , Wisitsree Wiyaratnc, Suttichai Assabumrungratd, Amornchai Arpornwichanopb,* a
Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand Computational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand c Faculty of Industrial Education and Technology, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand d Center of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand b
A R T I C L E I N F O
Article history: Received 17 April 2016 Received in revised form 8 July 2016 Accepted 30 July 2016 Available online 9 August 2016 Keywords: Reactive distillation Glycerol ether Design Parametric analysis Total annual cost
A B S T R A C T
The use of glycerol from biodiesel industries to produce high value-added products would increase the competitive potential of biodiesel producers. The present work investigates the production of glycerol ethers, a diesel and biodiesel additive. A single reactive distillation is proposed to overcome the low glycerol conversion and product yield that limit a conventional process. The theoretical performance of the reactive distillation is analyzed taking glycerol conversion and the glycerol ether yield into consideration. The simulation results show that the glycerol and mono-tert-butyl ether of glycerol are completely used in the reactive stages and thus, the recycle section by which glycerol and the mono-tertbutyl ether of glycerol are recovered is unessential. The parametric analysis for the reactive distillation is reported. Based on the process economic analysis from a total annual cost, the optimal reactive distillation configuration for glycerol ether production consists of two rectifying stages, eight reactive stages and one stripping stage. ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Using biomass to produce biodiesel provides a possible alternative to fossil fuels, particularly given that diesel and biodiesel are important fuels for both industrial and agricultural product transportation. Biodiesel can be used instead of diesel fuel without requiring modification to diesel engines because the properties of biodiesel are similar to those of conventional diesel fuel [1,2]. Transesterification is the most common process used to produce biodiesel from methanol and vegetable oil, such as palm oil, soybean oil, rapeseed oil, mustard oil, canola oil, waste cooking oil or animal fat [3]. However, this process converts 10% of the vegetable oil used to crude glycerol. Because of the large amount of biodiesel production in many countries as a result of increasing energy demands and limiting reserves of conventional fossil fuel, a
Abbreviations: DTBG, Di-tert-butyl ether of glycerol; G, Glycerol; MTBG, Monotert-butyl ether of glycerol; TTBG, Tri-tert-butyl ether of glycerol. * Corresponding author. Fax: +66 2 218 6877. E-mail address: [email protected] (A. Arpornwichanop).
large quantity of crude glycerol is generated world-wide. The current demand for glycerol is substantially less than the excess of glycerol generated via biodiesel production, and the trend toward glycerol production is expected to increased continually in future years [4]. This oversupply of glycerol has already resulted in decreased glycerol prices and rising environmental concerns with regard to contaminated glycerol disposal, prompting the search for new glycerol uses, such as converting glycerol into value-added products. Some simple value-added products produced using glycerol have been discovered, including acrolein (used in acrylic acid ester and acylic acid synthesis) and 1,3-propanediol (used to produce polymers, cosmetics, foods and lubricants) [5,6]. Moreover, using glycerol to produce high value-added products should reduce biodiesel production costs and increase the competitiveness of biodiesel producers [2,7]. Glycerol is known to be a versatile chemical that can be converted into numerous chemicals or biobased products, such as drugs, personal products, oral care products, cosmetics, emulsifiers, skin protectants, asphalt, ceramics, adhesives, food and beverages, polyether, alkyd resins, and
http://dx.doi.org/10.1016/j.jiec.2016.07.052 1226-086X/ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Nomenclature
ai ki Keq,i ri T xD
Activity of species i () Rate constant of reaction I (mol s1 kg1) Equilibrium constant of reaction I () Reaction rate of reaction i (mol s1 kg1) Temperature (K) Purity of product ()
triacetin [5]. Alternatively, glycerol can be used to produce valueadded products like fuel additives, such as the di- and tri-tert-butyl ethers of glycerol [8]. Such fuel additives increase the cetane numbers of diesel fuels and decrease pollutants emissions, including smoke, dust, carbon dioxide, hydrocarbon compounds and aldehydes. Moreover, these additives can reduce the cloud points of the fuels to which they are added and increase the performances and volumes of diesel or bio-diesel products [9]. Researchers have previously used isobutylene and glycerol to produce fuel additives; however, isobutylene is costly and requires additional purification procedures, i.e., sulfuric acid extraction or molecular sieves, to separate the fuel additive from the by-product (C8–C16) produced by an oligomerization reaction between glycerol and isobutylene [10]. A few studies have focused on using butylene instead of isobutylene; however, butylene use is challenging because of the unstable catalyst required by the process. Bio-alcohols, such as ethanol and butanol, are also used to synthesize fuel additives. The main disadvantage to using these two chemicals is the greater production of the mono-tert-butyl ether of glycerol (by-product) compared to the di- and tri-tertbutyl ethers of glycerol (the desired fuel additives). Presently, tertbutyl alcohol and glycerol are preferred for producing fuel additives, particularly given the lower cost of tert-butyl alcohol compared to isobutylene. Furthermore, tert-butyl alcohol can be produced from biomass, such as starch, lignocellulosic and straw, but isobutylene is produced by catalytic cracking and steam cracking fractions of refining petroleum. In addition, tert-butyl alcohol is a by-product of propylene oxide production. Furthermore, the use of tert-butyl alcohol can prevent disadvantageous oligomerization reactions (C8–C16 hydrocarbon), and tert-butyl alcohol can be dissolved in glycerol, whereas isobutylene requires solvents or must be used at high pressures. However, because the catalyst is deactivated by water as a by-product, the etherification reaction of glycerol with tert-butyl and alcohol exhibits lower conversion and selectivity compared to the etherification reaction between glycerol and isobutylene [11]. Many studies have been published regarding the synthesis of fuel additives from glycerol and tert-butyl alcohol. Ozbay et al. [12] compared the use of various solid acid catalysts (in the etherification reaction), such as Amberlyst-15, Amberlyst-16, Amberlyst-35, Nafion-SAC-13 and gamma-alumina. The results indicated that Amberlyst-15 had the highest activity at 110 C, but A-16 showed higher selectivity for the di-tert-butyl ether of glycerol. Frusteri et al. [13] found that water, a by-product, can inhibit the etherification reaction; thus, separating water from the reactor can improve the conversion of glycerol. Vlad et al. [14] developed a fuel-additive productionprocess using a plug flow reactor, and separation and recycling were included in this process. However, a high amount (up to 60% of total product) of the mono-tert-butyl ether of glycerol, a by-product of the etherification reaction between tert-butyl alcohol and glycerol, was generated. Thus, high energy and a large distillation column were required to redirect/recycle this mono-tert-butyl ether to the
reactor. Moreover, the process developed in this study generated a substantial amount of water, used a high TBA-to-glycerol ratio and required high energy consumption and many distillation columns to separate water and TBA from the final product. Furthermore, the conversion of glycerol was low so that a process development to to improve the productivity is required. Due to its advantages over the conventional process, reactive distillation is considered a potential means of enhancing the production of fuel additives [15]. For example, both the reaction and separation tasks can be performed in a single unit, so that capital and operating costs can be reduced. Moreover, reactive distillation provides high conversion, improves product yields, and reduces energy consumption [16,17]. However, optimizing the design for reactive distillation is difficult and involves many design variables, such as the number of rectifying stages, the number of reactive stages, and the number of stripping stages. A reactive distillation for etherification of glycerol with tert-butyl alcohol was investigated by Vlad and Bildea [18]. However, in their system design, the reactants were fed into the reactive distillation as the vapor phase below the reactive zone, resulting in low liquid accumulation in the reactive stages. Moreover, there is no detail in the steady-stage design of a reactive distillation for glycerol etherification in term of the economic evaluation. The present study is to design a reactive distillation for the etherification reaction between glycerol and tert-butyl alcohol to produce glycerol ether. The total annual cost is the criterion used to determine a suitable design for the reactive distillation. Simulations of the reactive distillation are performed using an Aspen Plus simulator. The effects of primary design operating parameters, such as the numbers of reactive, rectifying and stripping stages, the molar reflux ratio, the reboiler duty, and the molar ratio of reactant, on fuel additive production are investigated. Kinetic model of the etherification reaction between tert-butyl alcohol and glycerol This study is focused on the production of glycerol ether (the diand tri-tert-butyl ethers of glycerol). Di- and tri-tert-butyl ethers of glycerol are produced via an etherification reaction between tertbutyl alcohol and glycerol that consists of the following three steps (Eqs. (1)–(3)): k1
Glycerol þ TBA $ MTBG þ H2 O
ð1Þ
k1
k2
MTBG þ TBA $ DTBG þ H2 O
ð2Þ
k2
Table 1 Equilibrium constants and rate constants for the etherification reaction between tert-butyl alcohol and glycerol [19]. Equilibrium constant
Rate constant (mol1 kg1)
Keq1 = exp(2.534 1005.5/T) Keq2 = exp(2.087 1051.1/T) Keq3 = exp(0.978 2212/T)
k1 = exp(16.114 6641.5/T) k2 = exp(26.511 10,305/T) k3 = exp(5.462 3715/T)
Table 2 Standard conditions for reactive distillation. Condition of feed Temperature (K) Feed flow rate (kmol/h) Glycerol tert-Butyl alcohol Feed stage Glycerol tert-Butyl alcohol
Column specification 298 2 10 7 11
Rectifying stages Reaction stages Stripping stages Reboiler duty (kW) Reflux ratio
5 5 0 80 1
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Fig. 1. Comparison of simulated results this work and experimental result from Kiatkittipong et al. [19].
k3
DTBG þ TBA $ TTBG þ H2 O k3
ð3Þ
In the above reactions, the di-tert-butyl ether of glycerol (DTBD) and the tri-tert-butyl ether of glycerol (TTBG) are the major products (fuel additives) that can be added to diesel and biodiesel fuels, while the mono-tert-butyl ether of glycerol (MTBG) and
Fig. 2. Effects of (a) molar feed ratios of tert-butyl alcohol to glycerol and (b) reboiler duty on the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol.
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water are by-products. All of the reactions are reversible and can be catalyzed by amberlyst 15 (acidic cation-exchange resin), and the three reversible reactions are exothermic. The kinetic models of each reaction provide important information for process simulation and have been studied by Kiatkittipong et al. [19]. The rate constants and equilibrium constants for etherification reactions of glycerol with tert-butyl alcohol are given in Table 1. The rate expressions for each reaction step are as follows (Eqs. (4)–(6)): 1 aMTBG aH2 O ð4Þ r1 ¼ k1 aG aTBA Keq1 1 aDTBG aH2 O r2 ¼ k2 aMTBG aTBA Keq2
ð5Þ
1 aTTBG aH2 O r3 ¼ k3 aDTBG aTBA Keq3
ð6Þ
The NRTL-Redich Kwong model is used to predict the thermodynamic properties of substances in the system. The interaction parameters are taken from the Aspen Plus database or estimated using the UNIFAC method. Reactive distillation for glycerol ether production The reactive distillation column is utilized under standard conditions, and it consists of 12 stages, which include a partial reboiler and a total condenser, for which the numbering of stages is top downward. The column pressure is 1 bar. The feed conditions under standard conditions are as follows: 2 kmol/h glycerol and 10 kmol/h tert-butyl alcohol, a reflux ratio of 1, a reboiler duty at 80 kW and a feed location of glycerol and tert-butyl alcohol at stage 1 and at the bottom of the reactive stages, respectively. Stages 7– 11 constitute the reactive stages. The standard conditions for reactive distillation are given in Table 2. Simulation of the production of glycerol ether in the reactive distillation is carried out by using the RADFRAC model in Aspen
(a)
(b)
Fig. 3. Effects of tert-butyl alcohol feed stages locations on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol at various glycerol feed stage locations.
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Plus simulation package. All system performances pertaining to reactive distillation for glycerol ether production are considered in terms of the resulting conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol. The conversion of glycerol and yields of di and tri-tert-butyl ethers of glycerol are defined according to the equations below (Eqs. (7) and (8)).
% Conversion ¼
%Yield ¼
mentioned in the previous section is tested by comparing the simulated results obtained from Aspen Plus with the experimental data from Kiatkittipong et al. [19]. For comparison, the reactive distillation column consisting of six rectifying stages and six reactive stages is operated at the pressure of 1 bar with reflux ratio at 2. Feed streams of tert-butyl alcohol and glycerol at temperature
Difference of glycerol in molar flow rate of inlet and outlet 100% Molar feed flow rate of glycerol to reactive distillation
Molar flow rate of outlet of di and tritertbutyl ethers of glycerol 100% Difference in molar flow rate of inlet and outlet of glycerol
Model validation In order to validate the simulation model of the reactive distillation for glycerol ether production, the reliability of the reactive distillation model coupled with the kinetic model as
97
ð7Þ
ð8Þ
298 C and feed ratio of four by volume are fed to the bottom and first stage of reactive stages in reactive distillation. The bottom products of the reactive distillation obtained from the model prediction and experiment are compared as shown in Fig. 1 and the simulated results are in good agreement with the experimental data.
(a)
(b)
Fig. 4. Effects of reactive stages on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol.
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reboiler duty increases both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol.
Results and discussion Effects of feed ratio of tert-butyl alcohol to glycerol
Effects of feed stage location The molar feed ratio of tert-butyl alcohol to glycerol is an important parameter related to reactive distillation performance. The molar ratios of tert-butyl alcohol to glycerol studied herein are 2:1 to 8:1. Fig. 2(a) shows the effects of the feed molar ratio of tertbutyl alcohol to glycerol on the conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol. The simulation results show that the conversion of glycerol and yields of di- and tri-tertbutyl ethers of glycerol increase as the molar feed ratio of tert-butyl alcohol to glycerol increases. This is because tert-butyl alcohol is the reactant of three etherification reactions of glycerol and tertbutyl alcohol, such that tert-butyl alcohol forces etherification forward. Effects of reboiler heat duty Fig. 2(b) shows the effects of reboiler duty on the conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol within a range of 60–160 kW. As the heat duty increases, more unreacted reactants, such as glycerol and the mono-tert-butyl ether of glycerol, react in the reactive stages. For this reason, increasing the
The two feed locations also constitute an important design variable. The feed stage locations are adjusted to optimize the performance of the reactive distillation in terms of the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol. Feeding tert-butyl alcohol (lighter reactant) at the bottom of the reactive stages and feeding glycerol (heavier reactant) at the top of the reactive stages results in the greatest reactive reaction performance. The amount of glycerol in the reactive stages decreases as the feed location of glycerol is changed from the top to the bottom of the reactive stages. On the other hand, the amount of tert-butyl alcohol decreases when the feed location of methanol is changed from the bottom to the top of the reactive stages. For this reason, tert-butyl alcohol (lighter reactant) and glycerol (heavier reactant) must be fed at the bottom and the top of the reaction stages, respectively. The effects of the feed stage locations of glycerol and tert-butyl alcohol on the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol resulting from the reactive distillation are shown in Fig. 3.
(a)
(b)
Fig. 5. Effects of rectifying stages on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol.
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Effects of the number of reactive stages Fig. 4 demonstrates the effects of changing the number of reactive stages in terms of the resulting conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol under standard conditions. The number of reactive stages is varied from 1 to 8. The conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol increase as the number of reactive stages increases from 1 to 8. This is because increasing the number of reactive stages can improve the space time of the reactant during the reactive stages. Fig. 4 shows that both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol increase as the number of reactive stages increases. This is because increasing the number of reactive stages can increase the space time of the reactant (i.e., tert-butyl alcohol) and the intermediate (i.e., the mono-tert-butyl ether of glycerol) during the reactive stages. For this reason, tert-butyl alcohol and the mono-tert-butyl ether of glycerol are reacted and converted to the di- and tri-tert-butyl ethers of glycerol. Effects of the number of rectifying stages Fig. 5 demonstrates how the number of rectifying stages affects the performance of the reactive distillation under standard
99
conditions. The number of rectifying stages is varied from 1 to 10. Increasing the number of rectifying stages can help separate water, which inhibits the etherification reaction of tert-butyl alcohol and glycerol during the reactive stages, so the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol increase accordingly. Effects of the number of stripping stages Fig. 6 shows how the number of stripping stages affects the performance of the reactive distillation. The number of stripping stages is varied from 0 to 8. As the number of stages in the stripping section increases, more unreacted glycerol and mono-tert-butyl ether of glycerol cannot separate from the bottom of the reactive distillation and, therefore, cannot return to the reactive stages. For this reason, the conversion of glycerol and the yields of di- and tritert-butyl ethers of glycerol decrease. Effects of reflux ratio Fig. 7 shows how the reflux ratio affects the reactive distillation in terms of the conversion of glycerol and the yields of di- and tritert-butyl ethers of glycerol. The results demonstrate that the conversion of glycerol and the yields of di- and tri-tert-butyl ethers
(a)
(b)
Fig. 6. Effects of stripping stages on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol.
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(a)
(b)
Fig. 7. Effects of reflux ratio on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol.
of glycerol decrease as the reflux ratio increases. This is because increasing the reflux ratio increases the amount of water in the reactive stages. Because water can inhibit the etherification reaction of tert-butyl alcohol and glycerol, the reflux ratio should be minimized as much as possible.
between trays is maintained at 2 ft. The annualized capital cost is calculated using a payback period of 3 years and the cost data from Appendix B of Douglas’s book [20]. The total annual cost (TAC) is defined as shown in Eq. (9) below. TAC ¼ TACope þ
Economic assessment of reactive distillation for glycerol ether production This section investigates the steady-state economics of reactive distillation for glycerol ether (di- and tri-tert-butyl ethers of glycerol) production and optimizes the single reactive distillation requirements for glycerol ether production based on total annual costs, including operating cost and annualized capital cost. The tray sizing function in the Aspen Plus simulation is used to estimate the diameter of the reactive distillation. The distance
TACcap Payback Period
ð9Þ
The optimal design configuration of a reactive distillation is determined by minimizing the total annual cost (Eq. (10)). Minimize
TAC
Subject to :
Conversion ¼ 99% Yield ¼ 99% Production rate ¼ 2:12 kmol=h xD ¼ 98%
x
ð10Þ
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Fig. 8. Design procedure for reactive distillation for glycerol ether production.
where x is the vector of design variables (i.e., the number of reactive stages, the number of rectifying stages and the number of stripping stages). The capital cost calculation includes the cost of reactive distillation column, trays, reboiler and condenser. The operating cost calculation includes costs associated with water cooling for condenser and steam for reboiler. In all cases in the present study, the molar ratio of tert-butyl alcohol to glycerol is 6:1. Assumptions and specifications for the glycerol ether stream and the reactive distillation design The annualized capital cost is calculated with a payback period of 3 years and using the cost data from Appendix B of Douglas’s book [20]. The assumptions and specifications of the reactive distillation design and glycerol ether stream are as follows:
1. The design objective is to obtain a 99 mol% conversion of glycerol. 2. The design objective is to obtain a 99 mol% yield of fuel additive (di- and tri-tert-butyl ethers of glycerol). 3. The design specification is to obtain 2.124 kmol/h of fuel additive (di- and tri-tert-butyl ethers of glycerol) at the glycerol ether stream. 4. The purity of the product stream is 98 mol%. 5. The feed stages locations of glycerol and tert-butyl alcohol are fixed at the top and bottom of the reactive stages, respectively. 6. The molar ratio of tert-butyl alcohol to glycerol is fixed at 6:1. 7. There is no pressure drop and the pressure column is fixed at 9 bar. 8. A plant availability of 8150 h/year is assumed. Based on these assumptions and specifications, the following three variables require optimization: (1) number of rectifying
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Fig. 9. Effects of reactive stages on total annual costs.
stages, (2) number of stripping stages, and (3) number of reactive stages. The reactive distillation design procedure is shown in Fig. 8. The range of design variables is chosen depending the assumptions and product specifications. Fig. 9 demonstrates the importance of the number of reactive stages as a parameter. Increasing the number of reactive stages provides many benefits. First, increasing the number of reactive stages correspondingly increases the space time of the reactant during the reactive stages, thereby increasing conversion of glycerol and fuel additive yields. Furthermore, the liquid holdup is increased with an increase in the number of reactive stages and thus, the reaction rates are increased. Second, increasing the number of reactive stages results in better separation performance, such as the separation of a heavy product from a light unreacted reactant and by-product. Thus, the operating and capital costs associated with the reboiler can be reduced by increasing the number of reactive stages. However, increasing the number of reactive stages to more than eight stages results in higher capital costs and it is insignificant in the production yield, leading us to conclude that 8 is the optimum number of reactive stages for the conditions utilized in the present study. The effects of reactive stages, the rectifying stage and the stripping stage on the total annual cost of reactive distillation are demonstrated in Fig. 10. Because glycerol and the mono-tert-butyl ether of glycerol are used completely during the reactive stages, all of the components remaining during the rectifying and stripping stages are fuel additives (di- and tri-tert-butyl ethers of glycerol), tert-butyl alcohol and water. The boiling point of glycerol ether is higher than those of tert-butyl alcohol and water, so separating fuel additives from tert-butyl alcohol and water requires a few rectifying and stripping stages to achieve purity specifications. Increasing the number of rectifying stage provides many benefits. First, increasing the number of rectifying stages can help to separate water from the reactive stages, such that the reboiler duty is reduced. Secondly, increasing the number of rectifying stages increases both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol. However, increasing the number of rectifying stages increases capital costs. Increasing the number of stripping stages can help to separate tert-butyl alcohol from glycerol ether to meet the purity specifications of the product. Fig. 10 shows that the number of stripping stages is more sensitive than the number of rectifying stages. However, increasing the number of stripping stages
enhances already high capital costs due to increased trays and heat exchanger area. Furthermore, both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol are reduced, leading to a high accumulation of tert- butyl alcohol in the stripping stages. However, product purity cannot increase to 99% without stripping stages. For this reason, stripping stages are among the most essential considerations regarding reactive distillation (due to specifications related to product contamination). Fig. 10 shows that the optimum number of rectifying stages is 2 and the optimum number of stripping stages is 1. Regarding the boiling point ranking of the components in this system, tert-butyl alcohol is the lightest component and thus, a few number rectifying stages and stripping stages are required. With regard to the reboiler duty, increasing the reboiler duty can increase conversion and selectivity for di- and tri-tert-butyl alcohol. This is because unreacted glycerol is available and reacts during the reactive stages. The reboiler duty must be high enough to separate all of the unreacted glycerol in the bottom of the reactive distillation apparatus, such that this glycerol can be utilized during the reactive stages. In addition, the reboiler duty separates all of the water (as by product) as well as the excess tertbutyl alcohol from the product (di- and tri-tert-butyl ethers of alcohol). However, a higher reboiler duty means higher operating costs. Thus, the optimal operating reboiler duty determined herein is 260.1 kW. Increasing the reflux ratio reduces both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol. This is because increasing the reflux ratio increases the amount of water that can inhibit the etherification reaction of tert-butyl alcohol and glycerol during the reactive stages. Furthermore, increasing the reflux ratio results in higher total annual costs. For these reasons, the reflux ratio must be kept as low as possible. Considering the minimization of the total annual cost, the optimal design and operating parameters of reactive distillation are summarized in Table 3. The optimal configuration of reactive distillation is shown in Fig. 11. Conclusions The present study investigates using reactive distillation to produce glycerol ether from the etherification reaction of tertbutyl alcohol and glycerol. Reactive distillation is used to improve
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(a)
(b)
(c)
Fig. 10. Effects of rectifying and stripping stages on the total annual cost of reactive distillation, when the number of reactive stages is fixed at (a) 7, (b) 8 and (c) 9.
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L. Simasatitkul et al. / Journal of Industrial and Engineering Chemistry 43 (2016) 93–105 Table 3 Optimal operating parameters for reactive distillation for glycerol ether production. Condition of feed Temperature (K) Feed flow rate (kmol/h) Glycerol tert-Butyl alcohol Feed stage Glycerol tert-Butyl alcohol
Column specification 298 2.15 12.9
Rectifying stages Reaction stages Stripping stages Reboiler duty (kW) Reflux ratio
2 8 1 260.1 0.05
4 11
Fig. 11. Optimal configuration of a reactive distillation for glycerol etherification.
the performance of fuel additive production in terms of the resulting conversion of glycerol and the yields of di- and tri-tertbutyl ethers of glycerol. RADFRAC module in Aspen Plus is used to study the parameters of reactive distillation, including the reflux ratio, the reboiler duty, the number of stages on performance of reactive distillation in terms of conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol. The optimal reactive distillation configurations can also be found by optimizing the total annual cost. The glycerol and mono-tert-butyl ether of glycerol are completely used in the reactive stages. Therefore, the recycle section by which glycerol and the mono-tert-butyl ether of glycerol are recovered is unessential. In conclusion, reactive distillation can reduce energy requirements. The simulation results herein show that increasing the feed ratio of tert-butyl alcohol to glycerol, the number of reactive stages, the number of rectifying stages, and the reboiler duty improves both the conversion of glycerol and the yields of di- and tri-tertbutyl ethers of glycerol. However, increasing the number of stripping stages and the reflux ratio causes an opposite trend, although increasing the number of stripping stages benefits separation performance. Suitable feed stage locations of tert-butyl alcohol and glycerol are the bottom and the top, respectively, of the reactive stages. The optimal reactive distillation configuration for
glycerol ether production consists of two rectifying stages, eight reactive stages and one stripping stage. Acknowledgments Support from the Ratchadaphiseksomphot Endowment Fund, Chulalongkorn University, and the Thailand Research Fund (DPG5880003) is gratefully acknowledged. References [1] W. Liu, P. Yin, X. Liu, S. Zhang, R. Qu, J. Ind. Eng. Chem. 21 (2015) 893. [2] A. Singhabhandhu, T. Tezuka, Energy 35 (6) (2010) 2493. [3] K. Neumann, K. Werth, A. Martín, A. Górak, Chem. Eng. Res. Des. 107 (2016) 52– 62. [4] M. Ayoub, A.Z. Abdullah, Renew. Sustainable Energy Rev. 16 (5) (2012) 2671. [5] H.W. Tan, A.R. Abdul Aziz, M.K. Aroua, Renew. Sustainable Energy Rev. 27 (2013) 118. [6] L. Bueno, C. Toro, M. Martín, Chem. Eng. Res. Des. 93 (2015) 432. [7] H. Rastegari, H.S. Ghaziaskar, J. Ind. Eng. Chem. 21 (2015) 856. [8] Z. Gholami, A.H. Abdullah, K. Lee, Renew. Sustainable Energy Rev. 39 (2014) 327. [9] N. Rahmat, A.Z. Abdullah, A.R. Mohamed, Renew. Sustainable Energy Rev. 14 ( 2010) 987. [10] O.D. Bozkurt, F.M. Tunc, N. Baglar, S. Celebi, D. Gunbas, A. Uzun, Fuel Process. Technol. 138 (2015) 780. [11] N. Viswanadham, S.K. Saxena, Fuel 103 (2013) 980.
L. Simasatitkul et al. / Journal of Industrial and Engineering Chemistry 43 (2016) 93–105 [12] N. Ozbay, N. Oktar, G. Dogu, T. Dogu, Int. J. Chem. React. Eng. 8 (1) (2010), doi: http://dx.doi.org/10.2202/1542-6580.2149. [13] F. Frusteri, F. Arena, G. Bonura, C. Cannilla, L. Spadaro, O. Di Blasi, Appl. Catal. A: Gen. 367 (1–2) (2009) 77. [14] E. Vlad, C.S. Bildea, G. Bozga, Sci. World J. 2012 (2012). [15] P. Lutze, A. Gorak, Chem. Eng. Res. Des. 91 (2013) 1978. [16] A. Arpornwichanop, K. Koomsup, S. Assabumrungrat, J. Ind. Eng. Chem. 14 (6) (2008) 796.
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[17] L. Zhang, H. Chen, Y. Yuan, J. Yu, S. Wang, K. Huang, Chem. Eng. Res. Des. 100 (2015) 311. [18] E. Vlad, C.S. Bildea, Chem. Eng. Trans. 29 (2012) 589. [19] W. Kiatkittipong, P. Intaracharoen, N. Laosiripojana, C. Chaisuk, P. Praserthdam, S. Assabumrungrat, Comput. Chem. Eng. 35 (10) (2011) 2034. [20] J.M. Douglas, Conceptual Design of Chemical Processes, McGraw-Hill, New York, USA, 1988.
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Optimal design and performance analyses of the glycerol ether production process using a reactive distillation column Lida Simasatitkula , Punjawat Kaewwisetkulb , Wisitsree Wiyaratnc, Suttichai Assabumrungratd, Amornchai Arpornwichanopb,* a
Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand Computational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand c Faculty of Industrial Education and Technology, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand d Center of Excellence in Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand b
A R T I C L E I N F O
Article history: Received 17 April 2016 Received in revised form 8 July 2016 Accepted 30 July 2016 Available online 9 August 2016 Keywords: Reactive distillation Glycerol ether Design Parametric analysis Total annual cost
A B S T R A C T
The use of glycerol from biodiesel industries to produce high value-added products would increase the competitive potential of biodiesel producers. The present work investigates the production of glycerol ethers, a diesel and biodiesel additive. A single reactive distillation is proposed to overcome the low glycerol conversion and product yield that limit a conventional process. The theoretical performance of the reactive distillation is analyzed taking glycerol conversion and the glycerol ether yield into consideration. The simulation results show that the glycerol and mono-tert-butyl ether of glycerol are completely used in the reactive stages and thus, the recycle section by which glycerol and the mono-tertbutyl ether of glycerol are recovered is unessential. The parametric analysis for the reactive distillation is reported. Based on the process economic analysis from a total annual cost, the optimal reactive distillation configuration for glycerol ether production consists of two rectifying stages, eight reactive stages and one stripping stage. ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction Using biomass to produce biodiesel provides a possible alternative to fossil fuels, particularly given that diesel and biodiesel are important fuels for both industrial and agricultural product transportation. Biodiesel can be used instead of diesel fuel without requiring modification to diesel engines because the properties of biodiesel are similar to those of conventional diesel fuel [1,2]. Transesterification is the most common process used to produce biodiesel from methanol and vegetable oil, such as palm oil, soybean oil, rapeseed oil, mustard oil, canola oil, waste cooking oil or animal fat [3]. However, this process converts 10% of the vegetable oil used to crude glycerol. Because of the large amount of biodiesel production in many countries as a result of increasing energy demands and limiting reserves of conventional fossil fuel, a
Abbreviations: DTBG, Di-tert-butyl ether of glycerol; G, Glycerol; MTBG, Monotert-butyl ether of glycerol; TTBG, Tri-tert-butyl ether of glycerol. * Corresponding author. Fax: +66 2 218 6877. E-mail address: [email protected] (A. Arpornwichanop).
large quantity of crude glycerol is generated world-wide. The current demand for glycerol is substantially less than the excess of glycerol generated via biodiesel production, and the trend toward glycerol production is expected to increased continually in future years [4]. This oversupply of glycerol has already resulted in decreased glycerol prices and rising environmental concerns with regard to contaminated glycerol disposal, prompting the search for new glycerol uses, such as converting glycerol into value-added products. Some simple value-added products produced using glycerol have been discovered, including acrolein (used in acrylic acid ester and acylic acid synthesis) and 1,3-propanediol (used to produce polymers, cosmetics, foods and lubricants) [5,6]. Moreover, using glycerol to produce high value-added products should reduce biodiesel production costs and increase the competitiveness of biodiesel producers [2,7]. Glycerol is known to be a versatile chemical that can be converted into numerous chemicals or biobased products, such as drugs, personal products, oral care products, cosmetics, emulsifiers, skin protectants, asphalt, ceramics, adhesives, food and beverages, polyether, alkyd resins, and
http://dx.doi.org/10.1016/j.jiec.2016.07.052 1226-086X/ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Nomenclature
ai ki Keq,i ri T xD
Activity of species i () Rate constant of reaction I (mol s1 kg1) Equilibrium constant of reaction I () Reaction rate of reaction i (mol s1 kg1) Temperature (K) Purity of product ()
triacetin [5]. Alternatively, glycerol can be used to produce valueadded products like fuel additives, such as the di- and tri-tert-butyl ethers of glycerol [8]. Such fuel additives increase the cetane numbers of diesel fuels and decrease pollutants emissions, including smoke, dust, carbon dioxide, hydrocarbon compounds and aldehydes. Moreover, these additives can reduce the cloud points of the fuels to which they are added and increase the performances and volumes of diesel or bio-diesel products [9]. Researchers have previously used isobutylene and glycerol to produce fuel additives; however, isobutylene is costly and requires additional purification procedures, i.e., sulfuric acid extraction or molecular sieves, to separate the fuel additive from the by-product (C8–C16) produced by an oligomerization reaction between glycerol and isobutylene [10]. A few studies have focused on using butylene instead of isobutylene; however, butylene use is challenging because of the unstable catalyst required by the process. Bio-alcohols, such as ethanol and butanol, are also used to synthesize fuel additives. The main disadvantage to using these two chemicals is the greater production of the mono-tert-butyl ether of glycerol (by-product) compared to the di- and tri-tertbutyl ethers of glycerol (the desired fuel additives). Presently, tertbutyl alcohol and glycerol are preferred for producing fuel additives, particularly given the lower cost of tert-butyl alcohol compared to isobutylene. Furthermore, tert-butyl alcohol can be produced from biomass, such as starch, lignocellulosic and straw, but isobutylene is produced by catalytic cracking and steam cracking fractions of refining petroleum. In addition, tert-butyl alcohol is a by-product of propylene oxide production. Furthermore, the use of tert-butyl alcohol can prevent disadvantageous oligomerization reactions (C8–C16 hydrocarbon), and tert-butyl alcohol can be dissolved in glycerol, whereas isobutylene requires solvents or must be used at high pressures. However, because the catalyst is deactivated by water as a by-product, the etherification reaction of glycerol with tert-butyl and alcohol exhibits lower conversion and selectivity compared to the etherification reaction between glycerol and isobutylene [11]. Many studies have been published regarding the synthesis of fuel additives from glycerol and tert-butyl alcohol. Ozbay et al. [12] compared the use of various solid acid catalysts (in the etherification reaction), such as Amberlyst-15, Amberlyst-16, Amberlyst-35, Nafion-SAC-13 and gamma-alumina. The results indicated that Amberlyst-15 had the highest activity at 110 C, but A-16 showed higher selectivity for the di-tert-butyl ether of glycerol. Frusteri et al. [13] found that water, a by-product, can inhibit the etherification reaction; thus, separating water from the reactor can improve the conversion of glycerol. Vlad et al. [14] developed a fuel-additive productionprocess using a plug flow reactor, and separation and recycling were included in this process. However, a high amount (up to 60% of total product) of the mono-tert-butyl ether of glycerol, a by-product of the etherification reaction between tert-butyl alcohol and glycerol, was generated. Thus, high energy and a large distillation column were required to redirect/recycle this mono-tert-butyl ether to the
reactor. Moreover, the process developed in this study generated a substantial amount of water, used a high TBA-to-glycerol ratio and required high energy consumption and many distillation columns to separate water and TBA from the final product. Furthermore, the conversion of glycerol was low so that a process development to to improve the productivity is required. Due to its advantages over the conventional process, reactive distillation is considered a potential means of enhancing the production of fuel additives [15]. For example, both the reaction and separation tasks can be performed in a single unit, so that capital and operating costs can be reduced. Moreover, reactive distillation provides high conversion, improves product yields, and reduces energy consumption [16,17]. However, optimizing the design for reactive distillation is difficult and involves many design variables, such as the number of rectifying stages, the number of reactive stages, and the number of stripping stages. A reactive distillation for etherification of glycerol with tert-butyl alcohol was investigated by Vlad and Bildea [18]. However, in their system design, the reactants were fed into the reactive distillation as the vapor phase below the reactive zone, resulting in low liquid accumulation in the reactive stages. Moreover, there is no detail in the steady-stage design of a reactive distillation for glycerol etherification in term of the economic evaluation. The present study is to design a reactive distillation for the etherification reaction between glycerol and tert-butyl alcohol to produce glycerol ether. The total annual cost is the criterion used to determine a suitable design for the reactive distillation. Simulations of the reactive distillation are performed using an Aspen Plus simulator. The effects of primary design operating parameters, such as the numbers of reactive, rectifying and stripping stages, the molar reflux ratio, the reboiler duty, and the molar ratio of reactant, on fuel additive production are investigated. Kinetic model of the etherification reaction between tert-butyl alcohol and glycerol This study is focused on the production of glycerol ether (the diand tri-tert-butyl ethers of glycerol). Di- and tri-tert-butyl ethers of glycerol are produced via an etherification reaction between tertbutyl alcohol and glycerol that consists of the following three steps (Eqs. (1)–(3)): k1
Glycerol þ TBA $ MTBG þ H2 O
ð1Þ
k1
k2
MTBG þ TBA $ DTBG þ H2 O
ð2Þ
k2
Table 1 Equilibrium constants and rate constants for the etherification reaction between tert-butyl alcohol and glycerol [19]. Equilibrium constant
Rate constant (mol1 kg1)
Keq1 = exp(2.534 1005.5/T) Keq2 = exp(2.087 1051.1/T) Keq3 = exp(0.978 2212/T)
k1 = exp(16.114 6641.5/T) k2 = exp(26.511 10,305/T) k3 = exp(5.462 3715/T)
Table 2 Standard conditions for reactive distillation. Condition of feed Temperature (K) Feed flow rate (kmol/h) Glycerol tert-Butyl alcohol Feed stage Glycerol tert-Butyl alcohol
Column specification 298 2 10 7 11
Rectifying stages Reaction stages Stripping stages Reboiler duty (kW) Reflux ratio
5 5 0 80 1
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Fig. 1. Comparison of simulated results this work and experimental result from Kiatkittipong et al. [19].
k3
DTBG þ TBA $ TTBG þ H2 O k3
ð3Þ
In the above reactions, the di-tert-butyl ether of glycerol (DTBD) and the tri-tert-butyl ether of glycerol (TTBG) are the major products (fuel additives) that can be added to diesel and biodiesel fuels, while the mono-tert-butyl ether of glycerol (MTBG) and
Fig. 2. Effects of (a) molar feed ratios of tert-butyl alcohol to glycerol and (b) reboiler duty on the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol.
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water are by-products. All of the reactions are reversible and can be catalyzed by amberlyst 15 (acidic cation-exchange resin), and the three reversible reactions are exothermic. The kinetic models of each reaction provide important information for process simulation and have been studied by Kiatkittipong et al. [19]. The rate constants and equilibrium constants for etherification reactions of glycerol with tert-butyl alcohol are given in Table 1. The rate expressions for each reaction step are as follows (Eqs. (4)–(6)): 1 aMTBG aH2 O ð4Þ r1 ¼ k1 aG aTBA Keq1 1 aDTBG aH2 O r2 ¼ k2 aMTBG aTBA Keq2
ð5Þ
1 aTTBG aH2 O r3 ¼ k3 aDTBG aTBA Keq3
ð6Þ
The NRTL-Redich Kwong model is used to predict the thermodynamic properties of substances in the system. The interaction parameters are taken from the Aspen Plus database or estimated using the UNIFAC method. Reactive distillation for glycerol ether production The reactive distillation column is utilized under standard conditions, and it consists of 12 stages, which include a partial reboiler and a total condenser, for which the numbering of stages is top downward. The column pressure is 1 bar. The feed conditions under standard conditions are as follows: 2 kmol/h glycerol and 10 kmol/h tert-butyl alcohol, a reflux ratio of 1, a reboiler duty at 80 kW and a feed location of glycerol and tert-butyl alcohol at stage 1 and at the bottom of the reactive stages, respectively. Stages 7– 11 constitute the reactive stages. The standard conditions for reactive distillation are given in Table 2. Simulation of the production of glycerol ether in the reactive distillation is carried out by using the RADFRAC model in Aspen
(a)
(b)
Fig. 3. Effects of tert-butyl alcohol feed stages locations on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol at various glycerol feed stage locations.
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Plus simulation package. All system performances pertaining to reactive distillation for glycerol ether production are considered in terms of the resulting conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol. The conversion of glycerol and yields of di and tri-tert-butyl ethers of glycerol are defined according to the equations below (Eqs. (7) and (8)).
% Conversion ¼
%Yield ¼
mentioned in the previous section is tested by comparing the simulated results obtained from Aspen Plus with the experimental data from Kiatkittipong et al. [19]. For comparison, the reactive distillation column consisting of six rectifying stages and six reactive stages is operated at the pressure of 1 bar with reflux ratio at 2. Feed streams of tert-butyl alcohol and glycerol at temperature
Difference of glycerol in molar flow rate of inlet and outlet 100% Molar feed flow rate of glycerol to reactive distillation
Molar flow rate of outlet of di and tritertbutyl ethers of glycerol 100% Difference in molar flow rate of inlet and outlet of glycerol
Model validation In order to validate the simulation model of the reactive distillation for glycerol ether production, the reliability of the reactive distillation model coupled with the kinetic model as
97
ð7Þ
ð8Þ
298 C and feed ratio of four by volume are fed to the bottom and first stage of reactive stages in reactive distillation. The bottom products of the reactive distillation obtained from the model prediction and experiment are compared as shown in Fig. 1 and the simulated results are in good agreement with the experimental data.
(a)
(b)
Fig. 4. Effects of reactive stages on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol.
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reboiler duty increases both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol.
Results and discussion Effects of feed ratio of tert-butyl alcohol to glycerol
Effects of feed stage location The molar feed ratio of tert-butyl alcohol to glycerol is an important parameter related to reactive distillation performance. The molar ratios of tert-butyl alcohol to glycerol studied herein are 2:1 to 8:1. Fig. 2(a) shows the effects of the feed molar ratio of tertbutyl alcohol to glycerol on the conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol. The simulation results show that the conversion of glycerol and yields of di- and tri-tertbutyl ethers of glycerol increase as the molar feed ratio of tert-butyl alcohol to glycerol increases. This is because tert-butyl alcohol is the reactant of three etherification reactions of glycerol and tertbutyl alcohol, such that tert-butyl alcohol forces etherification forward. Effects of reboiler heat duty Fig. 2(b) shows the effects of reboiler duty on the conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol within a range of 60–160 kW. As the heat duty increases, more unreacted reactants, such as glycerol and the mono-tert-butyl ether of glycerol, react in the reactive stages. For this reason, increasing the
The two feed locations also constitute an important design variable. The feed stage locations are adjusted to optimize the performance of the reactive distillation in terms of the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol. Feeding tert-butyl alcohol (lighter reactant) at the bottom of the reactive stages and feeding glycerol (heavier reactant) at the top of the reactive stages results in the greatest reactive reaction performance. The amount of glycerol in the reactive stages decreases as the feed location of glycerol is changed from the top to the bottom of the reactive stages. On the other hand, the amount of tert-butyl alcohol decreases when the feed location of methanol is changed from the bottom to the top of the reactive stages. For this reason, tert-butyl alcohol (lighter reactant) and glycerol (heavier reactant) must be fed at the bottom and the top of the reaction stages, respectively. The effects of the feed stage locations of glycerol and tert-butyl alcohol on the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol resulting from the reactive distillation are shown in Fig. 3.
(a)
(b)
Fig. 5. Effects of rectifying stages on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol.
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Effects of the number of reactive stages Fig. 4 demonstrates the effects of changing the number of reactive stages in terms of the resulting conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol under standard conditions. The number of reactive stages is varied from 1 to 8. The conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol increase as the number of reactive stages increases from 1 to 8. This is because increasing the number of reactive stages can improve the space time of the reactant during the reactive stages. Fig. 4 shows that both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol increase as the number of reactive stages increases. This is because increasing the number of reactive stages can increase the space time of the reactant (i.e., tert-butyl alcohol) and the intermediate (i.e., the mono-tert-butyl ether of glycerol) during the reactive stages. For this reason, tert-butyl alcohol and the mono-tert-butyl ether of glycerol are reacted and converted to the di- and tri-tert-butyl ethers of glycerol. Effects of the number of rectifying stages Fig. 5 demonstrates how the number of rectifying stages affects the performance of the reactive distillation under standard
99
conditions. The number of rectifying stages is varied from 1 to 10. Increasing the number of rectifying stages can help separate water, which inhibits the etherification reaction of tert-butyl alcohol and glycerol during the reactive stages, so the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol increase accordingly. Effects of the number of stripping stages Fig. 6 shows how the number of stripping stages affects the performance of the reactive distillation. The number of stripping stages is varied from 0 to 8. As the number of stages in the stripping section increases, more unreacted glycerol and mono-tert-butyl ether of glycerol cannot separate from the bottom of the reactive distillation and, therefore, cannot return to the reactive stages. For this reason, the conversion of glycerol and the yields of di- and tritert-butyl ethers of glycerol decrease. Effects of reflux ratio Fig. 7 shows how the reflux ratio affects the reactive distillation in terms of the conversion of glycerol and the yields of di- and tritert-butyl ethers of glycerol. The results demonstrate that the conversion of glycerol and the yields of di- and tri-tert-butyl ethers
(a)
(b)
Fig. 6. Effects of stripping stages on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol.
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(a)
(b)
Fig. 7. Effects of reflux ratio on (a) the conversion of glycerol and (b) the yields of di- and tri-tert-butyl ethers of glycerol.
of glycerol decrease as the reflux ratio increases. This is because increasing the reflux ratio increases the amount of water in the reactive stages. Because water can inhibit the etherification reaction of tert-butyl alcohol and glycerol, the reflux ratio should be minimized as much as possible.
between trays is maintained at 2 ft. The annualized capital cost is calculated using a payback period of 3 years and the cost data from Appendix B of Douglas’s book [20]. The total annual cost (TAC) is defined as shown in Eq. (9) below. TAC ¼ TACope þ
Economic assessment of reactive distillation for glycerol ether production This section investigates the steady-state economics of reactive distillation for glycerol ether (di- and tri-tert-butyl ethers of glycerol) production and optimizes the single reactive distillation requirements for glycerol ether production based on total annual costs, including operating cost and annualized capital cost. The tray sizing function in the Aspen Plus simulation is used to estimate the diameter of the reactive distillation. The distance
TACcap Payback Period
ð9Þ
The optimal design configuration of a reactive distillation is determined by minimizing the total annual cost (Eq. (10)). Minimize
TAC
Subject to :
Conversion ¼ 99% Yield ¼ 99% Production rate ¼ 2:12 kmol=h xD ¼ 98%
x
ð10Þ
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Fig. 8. Design procedure for reactive distillation for glycerol ether production.
where x is the vector of design variables (i.e., the number of reactive stages, the number of rectifying stages and the number of stripping stages). The capital cost calculation includes the cost of reactive distillation column, trays, reboiler and condenser. The operating cost calculation includes costs associated with water cooling for condenser and steam for reboiler. In all cases in the present study, the molar ratio of tert-butyl alcohol to glycerol is 6:1. Assumptions and specifications for the glycerol ether stream and the reactive distillation design The annualized capital cost is calculated with a payback period of 3 years and using the cost data from Appendix B of Douglas’s book [20]. The assumptions and specifications of the reactive distillation design and glycerol ether stream are as follows:
1. The design objective is to obtain a 99 mol% conversion of glycerol. 2. The design objective is to obtain a 99 mol% yield of fuel additive (di- and tri-tert-butyl ethers of glycerol). 3. The design specification is to obtain 2.124 kmol/h of fuel additive (di- and tri-tert-butyl ethers of glycerol) at the glycerol ether stream. 4. The purity of the product stream is 98 mol%. 5. The feed stages locations of glycerol and tert-butyl alcohol are fixed at the top and bottom of the reactive stages, respectively. 6. The molar ratio of tert-butyl alcohol to glycerol is fixed at 6:1. 7. There is no pressure drop and the pressure column is fixed at 9 bar. 8. A plant availability of 8150 h/year is assumed. Based on these assumptions and specifications, the following three variables require optimization: (1) number of rectifying
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Fig. 9. Effects of reactive stages on total annual costs.
stages, (2) number of stripping stages, and (3) number of reactive stages. The reactive distillation design procedure is shown in Fig. 8. The range of design variables is chosen depending the assumptions and product specifications. Fig. 9 demonstrates the importance of the number of reactive stages as a parameter. Increasing the number of reactive stages provides many benefits. First, increasing the number of reactive stages correspondingly increases the space time of the reactant during the reactive stages, thereby increasing conversion of glycerol and fuel additive yields. Furthermore, the liquid holdup is increased with an increase in the number of reactive stages and thus, the reaction rates are increased. Second, increasing the number of reactive stages results in better separation performance, such as the separation of a heavy product from a light unreacted reactant and by-product. Thus, the operating and capital costs associated with the reboiler can be reduced by increasing the number of reactive stages. However, increasing the number of reactive stages to more than eight stages results in higher capital costs and it is insignificant in the production yield, leading us to conclude that 8 is the optimum number of reactive stages for the conditions utilized in the present study. The effects of reactive stages, the rectifying stage and the stripping stage on the total annual cost of reactive distillation are demonstrated in Fig. 10. Because glycerol and the mono-tert-butyl ether of glycerol are used completely during the reactive stages, all of the components remaining during the rectifying and stripping stages are fuel additives (di- and tri-tert-butyl ethers of glycerol), tert-butyl alcohol and water. The boiling point of glycerol ether is higher than those of tert-butyl alcohol and water, so separating fuel additives from tert-butyl alcohol and water requires a few rectifying and stripping stages to achieve purity specifications. Increasing the number of rectifying stage provides many benefits. First, increasing the number of rectifying stages can help to separate water from the reactive stages, such that the reboiler duty is reduced. Secondly, increasing the number of rectifying stages increases both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol. However, increasing the number of rectifying stages increases capital costs. Increasing the number of stripping stages can help to separate tert-butyl alcohol from glycerol ether to meet the purity specifications of the product. Fig. 10 shows that the number of stripping stages is more sensitive than the number of rectifying stages. However, increasing the number of stripping stages
enhances already high capital costs due to increased trays and heat exchanger area. Furthermore, both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol are reduced, leading to a high accumulation of tert- butyl alcohol in the stripping stages. However, product purity cannot increase to 99% without stripping stages. For this reason, stripping stages are among the most essential considerations regarding reactive distillation (due to specifications related to product contamination). Fig. 10 shows that the optimum number of rectifying stages is 2 and the optimum number of stripping stages is 1. Regarding the boiling point ranking of the components in this system, tert-butyl alcohol is the lightest component and thus, a few number rectifying stages and stripping stages are required. With regard to the reboiler duty, increasing the reboiler duty can increase conversion and selectivity for di- and tri-tert-butyl alcohol. This is because unreacted glycerol is available and reacts during the reactive stages. The reboiler duty must be high enough to separate all of the unreacted glycerol in the bottom of the reactive distillation apparatus, such that this glycerol can be utilized during the reactive stages. In addition, the reboiler duty separates all of the water (as by product) as well as the excess tertbutyl alcohol from the product (di- and tri-tert-butyl ethers of alcohol). However, a higher reboiler duty means higher operating costs. Thus, the optimal operating reboiler duty determined herein is 260.1 kW. Increasing the reflux ratio reduces both the conversion of glycerol and the yields of di- and tri-tert-butyl ethers of glycerol. This is because increasing the reflux ratio increases the amount of water that can inhibit the etherification reaction of tert-butyl alcohol and glycerol during the reactive stages. Furthermore, increasing the reflux ratio results in higher total annual costs. For these reasons, the reflux ratio must be kept as low as possible. Considering the minimization of the total annual cost, the optimal design and operating parameters of reactive distillation are summarized in Table 3. The optimal configuration of reactive distillation is shown in Fig. 11. Conclusions The present study investigates using reactive distillation to produce glycerol ether from the etherification reaction of tertbutyl alcohol and glycerol. Reactive distillation is used to improve
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(a)
(b)
(c)
Fig. 10. Effects of rectifying and stripping stages on the total annual cost of reactive distillation, when the number of reactive stages is fixed at (a) 7, (b) 8 and (c) 9.
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L. Simasatitkul et al. / Journal of Industrial and Engineering Chemistry 43 (2016) 93–105 Table 3 Optimal operating parameters for reactive distillation for glycerol ether production. Condition of feed Temperature (K) Feed flow rate (kmol/h) Glycerol tert-Butyl alcohol Feed stage Glycerol tert-Butyl alcohol
Column specification 298 2.15 12.9
Rectifying stages Reaction stages Stripping stages Reboiler duty (kW) Reflux ratio
2 8 1 260.1 0.05
4 11
Fig. 11. Optimal configuration of a reactive distillation for glycerol etherification.
the performance of fuel additive production in terms of the resulting conversion of glycerol and the yields of di- and tri-tertbutyl ethers of glycerol. RADFRAC module in Aspen Plus is used to study the parameters of reactive distillation, including the reflux ratio, the reboiler duty, the number of stages on performance of reactive distillation in terms of conversion of glycerol and yields of di- and tri-tert-butyl ethers of glycerol. The optimal reactive distillation configurations can also be found by optimizing the total annual cost. The glycerol and mono-tert-butyl ether of glycerol are completely used in the reactive stages. Therefore, the recycle section by which glycerol and the mono-tert-butyl ether of glycerol are recovered is unessential. In conclusion, reactive distillation can reduce energy requirements. The simulation results herein show that increasing the feed ratio of tert-butyl alcohol to glycerol, the number of reactive stages, the number of rectifying stages, and the reboiler duty improves both the conversion of glycerol and the yields of di- and tri-tertbutyl ethers of glycerol. However, increasing the number of stripping stages and the reflux ratio causes an opposite trend, although increasing the number of stripping stages benefits separation performance. Suitable feed stage locations of tert-butyl alcohol and glycerol are the bottom and the top, respectively, of the reactive stages. The optimal reactive distillation configuration for
glycerol ether production consists of two rectifying stages, eight reactive stages and one stripping stage. Acknowledgments Support from the Ratchadaphiseksomphot Endowment Fund, Chulalongkorn University, and the Thailand Research Fund (DPG5880003) is gratefully acknowledged. References [1] W. Liu, P. Yin, X. Liu, S. Zhang, R. Qu, J. Ind. Eng. Chem. 21 (2015) 893. [2] A. Singhabhandhu, T. Tezuka, Energy 35 (6) (2010) 2493. [3] K. Neumann, K. Werth, A. Martín, A. Górak, Chem. Eng. Res. Des. 107 (2016) 52– 62. [4] M. Ayoub, A.Z. Abdullah, Renew. Sustainable Energy Rev. 16 (5) (2012) 2671. [5] H.W. Tan, A.R. Abdul Aziz, M.K. Aroua, Renew. Sustainable Energy Rev. 27 (2013) 118. [6] L. Bueno, C. Toro, M. Martín, Chem. Eng. Res. Des. 93 (2015) 432. [7] H. Rastegari, H.S. Ghaziaskar, J. Ind. Eng. Chem. 21 (2015) 856. [8] Z. Gholami, A.H. Abdullah, K. Lee, Renew. Sustainable Energy Rev. 39 (2014) 327. [9] N. Rahmat, A.Z. Abdullah, A.R. Mohamed, Renew. Sustainable Energy Rev. 14 ( 2010) 987. [10] O.D. Bozkurt, F.M. Tunc, N. Baglar, S. Celebi, D. Gunbas, A. Uzun, Fuel Process. Technol. 138 (2015) 780. [11] N. Viswanadham, S.K. Saxena, Fuel 103 (2013) 980.
L. Simasatitkul et al. / Journal of Industrial and Engineering Chemistry 43 (2016) 93–105 [12] N. Ozbay, N. Oktar, G. Dogu, T. Dogu, Int. J. Chem. React. Eng. 8 (1) (2010), doi: http://dx.doi.org/10.2202/1542-6580.2149. [13] F. Frusteri, F. Arena, G. Bonura, C. Cannilla, L. Spadaro, O. Di Blasi, Appl. Catal. A: Gen. 367 (1–2) (2009) 77. [14] E. Vlad, C.S. Bildea, G. Bozga, Sci. World J. 2012 (2012). [15] P. Lutze, A. Gorak, Chem. Eng. Res. Des. 91 (2013) 1978. [16] A. Arpornwichanop, K. Koomsup, S. Assabumrungrat, J. Ind. Eng. Chem. 14 (6) (2008) 796.
105
[17] L. Zhang, H. Chen, Y. Yuan, J. Yu, S. Wang, K. Huang, Chem. Eng. Res. Des. 100 (2015) 311. [18] E. Vlad, C.S. Bildea, Chem. Eng. Trans. 29 (2012) 589. [19] W. Kiatkittipong, P. Intaracharoen, N. Laosiripojana, C. Chaisuk, P. Praserthdam, S. Assabumrungrat, Comput. Chem. Eng. 35 (10) (2011) 2034. [20] J.M. Douglas, Conceptual Design of Chemical Processes, McGraw-Hill, New York, USA, 1988.