Design of reactive distillation process for fatty acid esterification

Design of reactive distillation process for fatty acid esterification

European Symposium on Computer Aided Process Engineering - 11 R. Gani and S.B. J~rgensen (Editors) 9 2001 Elsevier Science B.V. All rights reserved. ...

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European Symposium on Computer Aided Process Engineering - 11 R. Gani and S.B. J~rgensen (Editors) 9 2001 Elsevier Science B.V. All rights reserved.

463

Design of Reactive Distillation Process for Fatty Acid Esterification Florin Omota, Alexandre C. Dimian a and Alfred Bliek Department of Chemical Engineering, University of Amsterdam Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands Esters of fatty acids are produced nowadays in batch reactors. In this study we present a innovative continuous process based on reactive distillation that can be used as a multipurpose configuration allowing the synthesis of a variety of fatty esters. The approach consists in using a chemical and phase equilibrium analysis to identify the feasible design region, and computer simulation to generate process alternatives. Simulation enables also to define the experimental work. Results are presented for the esterification of lauric acid with methanol and 2-ethylhexanol, the lightest and the heaviest in the CI-C8 alcohol series. Two process alternatives can be accommodated in the same hardware, but with different operation procedures: one with alcohol reflux, other with acid reflux. The first is feasible for heavy alcohols, forming heterogeneous azeotrope with water. The second is suited for both light and heavy alcohols, and may be seen as a generic esterification process of fatty acids.

1. INTRODUCTION Esterification of fatty acids is industrially important because fatty esters serve as feed stock for detergents and surfactants. Nowadays these are produced commercially in rather expensive processes consisting of batch reaction followed by distillation for water removal, recovery of excess alcohol and product purification. In this study we present the conceptual design of a multipurpose continuous process based on reactive distillation. The remarkable feature is that this can produce in the same hardware a variety of esters, of both light and heavy alcohols. Because of selective catalyst and mild reaction conditions the only reaction to consider is: CH3-(CH2)Io-C00H + R-OH ~

CH3-(CH2)lo-C00R + H20

(1)

The alcohol can be either methanol or 2-ethylhexanol. An ester purity higher than 99.8 % is desirable. The selected catalyst, based on sulphated zirconia, has a good activity and selectivity at least up to 200 ~ It may be coated on structured packing or other ceramic material that can be placed on the reactive plates in a kind of 'tee-bag' arrangement. Because the esterification is equilibrium limited the main problem is in-situ water removal. Light alcohols, such methanol, do not form azeotropes, and tend to distil preferentially in top. At the first sight, a second column for alcohol recovery is necessary. On the other hand, heavier alcohols may form azeotropes with water. In this case the heterogeneity could be exploited to remove water and shift the reactive process in a feasible region. In addition, the synthesis of fatty esters must take into account high boiling points of long chain components, which impose vacuum or other means to limit excessive temperature.

aCorresponding author, e-mail [email protected]

464 These severe thermodynamic and physical constraints might give the impression that a generic approach would be impossible. However, in this paper we demonstrate that a generic process can be developed, and adapted to produce various esters. Here we present results regarding the esterification of laurie acid with methanol and 2-ethyl hexanol, the first and the heaviest component in the C1-C 8 alcohol series. Innovative ideas and feasibility of the alternative designs has been investigated by computer simulation with ASPEN Plus TM version 10.1. This served also to guide the experimental research. 2. THERMODYNAMIC ANALYSIS

2.1 Pure component properties Table 1 presents the boiling points of key components at normal pressure, and at 240 mmHg, a value to consider in a vacuum process. Vapour pressure data for 2-ethylhexyl laurate is missing. Estimation of nbp by ASPEN Plus TM leads to controversial values: 441 ~ by Joback, 332 ~ by Gani et al., and 332.6 ~ by Chien et al. Because Pv-T data affects significantly multiphase chemical equilibrium calculations, the accuracy of vapour pressure data for all components was tested and completed with own measurementsb. Table 1 - Boiling points for key components in ~ Laurie acid 2-ethylhexanol 2-ethylhexyl laurate 760 mmHg 298.6 184.6 334.5 240 mmHg 254.4 147.5 289.2

A

300

B

250 200 150 E I- 100

250 200

Q.

50

300 0 o

Taz=70.1~ Yaz=0"9806

o

El.

9991'_ -

150

E

01ases0,iI.io0

, 0

0.2

0.4 0.6 Xl, Yl

Methyl laurate 267.2 221.7

0.8

1

100

rwo,,0u,00

50

0

0204000

1

xl, Yl

Fig. 1. VLLE at 240 mmHg: A-water(1)+2-ethyhexanol(2) and B-water(1)+laurie acid(2)

2.2 Phase equilibrium The esterification must take place exclusively in the organic phase in order to preserve the catalyst activity. The formation of a second water phase in the reactive zone must be prevented. On the other hand, two-phase separation would be of help for water removal as top product. Water gives heterogeneous azeotropes with laurie acid and 2-ethylhexanol. Figure 1 presents T-xy diagram calculated with UNIQUAC, with interaction parameters regressed from own experimental data. It is worthy to note a very low solubility of both organics in water, bDetails over experimental data available at [email protected][.

465 although the reciprocal solubility of water is significant. Accordingly, a good yield of water removal could be achieved when the top distillate would consist only of these two binaries. Conversely, a non-negligible water amount returns in the column with the reflux. Table 1 indicates that the lauric ester is obtained always as a bottom product, but the temperature would be excessive, even under vacuum. Therefore, it is rational to get the bottom product as a mixture with a certain amount of alcohol. Note that the vapour-liquid equilibrium of binaries lauric ester/alcohols is not a thermodynamic constraint. However, if the lauric acid is not entirely converted, its separation from ester would be very difficult.

2.3 Chemical and phase equilibrium Inside the reactive zone, chemical and phase equilibria occur simultaneously. The composition can by found of Gibbs free energy minimisation [1-2], as expressed by the relation: AG(T) = RTlnKa(T)=RTIn(KrK~) (2) 2-ethylhe~l laurate

A

w~, 0 . 8 1 . \ ~ , ~ ~ ~ '

+

0.6

'"

~

o.4 "---.~..

X

"~

B

0.8 co

q~,

lauric acid

methyl laurate 1

~cid

0.6

+ "O O

L v

l

\

rLE ._.L.

0.4

k..

/

iVLL

0.2

\

\ |

0 0.2 0.4 0.6 0.8 1 2-ethylhexanol Xl (water+acid) Water

0.2 methanol

i

0.4 0.6 0.8 1 Xl (water+acid) water

Fig. 2. CPE diagrams for lauric acid esterification with 2-ethylhexanol (A) and methanol (B) Good estimation of liquid activity coefficients is essential. This is an issue in itself, and it will be handled in a separate publication. We mention only that an experimental cell has been built to measure phase and chemical equilibrium up to 10 bars and 200 ~ It was found that the difference between experimental and predicted values by Aspen Plus T M is significant, but a major improvement was obtained by replacing vapour pressure estimations with experimental data. Another significant factor was the regression of binary interaction parameters for heterogeneous azeotropes from experimental data over mutual solubility. As an order of magnitude, at the temperatures between 140 and 160 ~ the equilibrium conversion can exceed 80 % starting with an initial equimolar reactants' ratio. The thermal effect of reaction is small, of 3 kcal/mol. Figure 2 presents a generalised representation of chemical and phase equilibrium in suitable co-ordinates [2-4]. It can be observed that a boundary separates homogeneous and heterogeneous distillation regions. The reaction should take place in the homogeneous (left) domain, where the 'residue curves' converge to the ester node. The lines in the heterogeneous region are 'tie lines' linking the composition of liquid phases at equilibrium. The aspect of the

466 heterogeneous domain depends on the solubility in water of the examined alcohol. The boundary sets also the minimum temperature of the reaction zone. Roughly speaking, a temperature above 100 ~ in the reaction zone is sufficient to place the reaction in the homogeneous domain. Contrary, the condensation in the top must occur at a temperature for which the phase equilibrium falls inside the heterogeneous domain.

3. HEAVY ALCOHOL PROCESS (Flowsheet A) Firstly, we investigated the feasibility of the esterification process with a heavy alcohol, namely 2-ethyl-hexanol. As an assumption we considered that the reaction reaches equilibrium on each stage. Figure 3 depicts the flowsheet and indicates some operation parameters. The reactive distillation (RD) column operates at 240 mmHg, and has five reactive stages. Lauric acid (liquid) preheated at 110 ~ is introduced on the high position of the reaction zone, while the alcohol (vapour) is fed at 146 ~ on the low position of the reaction zone. The bottom product with approximately 10% alcohol goes to an evaporator, from which the ester is recovered and the alcohol recycled. The vapour leaving the top of the reaction zone is condensed and cooled to about 70 ~ where two-phase separation occurs. Water is removed quantitatively from the process with only a small amount of alcohol. The organic phase is refluxed to the RD column. Figure 4 presents temperature and concentrations profiles. The reaction is almost completed in three equilibrium stages. The maximum reaction rate is located on the top tray. The energy balance can control the recycle of alcohol. In this way a high internal alcohol/acid ratio can be achieved, such to increase the local reaction rate, while the reactants' ratio in the initial feed is always 1:1. Because of higher local concentration of alcohol on the low position stages, an undesired etherification reaction might become possible. This aspect deserves a special attention in catalyst development. Note that a stripping section is not necessary. At most, one or two non-reactive stages in the top zone may be provided for a safer separation of ester with respect of alcohol and acid. We simulated also a reactive distillation column with the same configuration, but using a kinetic model. Several rate equations were tested, as for the similar esterification of n-butanol with oleic acid, as well as based on own laboratory measurements. The results differ only in the number of reactive stages, between 4 and 10, depending on catalyst activity.

Fig. 3. Flowsheet for lauric acid esterification with 2-ethylhexanol under vacuum

467

Fig. 4. Temperature (A) and concentration profiles (B) for 2-ethylhexanol esterification

4. LIGHT ALCOHOL PROCESS (Flowsheet B) When methanol is used, a similar process with that described above leads to a two column arrangement: RD column and methanol recovery. The investment and energy consumption only for the second column are higher than for RD column, but always below a batch process. However, another strategy is possible based on the observation that the water removal is easy from an heterogeneous azeotrope with lauric acid. Hence, the design must ensure a complete consumption of the alcohol in top. Only lauric acid and water are allowed in the top vapour stream, from which water can be easy removed after condensation and decantation. This principle leads to a second flowsheet alternative presented in Figure 5. A higher number of stages is necessary, in this case approximately 20. Profles of temperature and concentrations are presented in Figure 6. Note that in this case the maximum reaction rate is located somewhere to the middle of the reaction zone. The position depends on the reflux ratio. A middle location corresponds to an optimum reflux, where both acid and alcohol are completely converted. A lower position corresponds to an incomplete consumption of acid, found as impurity in the bottom product. A higher position corresponds to a an incomplete consumption of alcohol, and lost with the top product. Hence, contrary to the normal distillation, when purity increases with reflux, here purity and yield reach maximum at an optimum reflux rate.

Fig. 5. Flowsheet for lauric acid esterification with methanol at normal pressure

468

Fig. 6. Temperature (A) and concentration profiles (B) for methanol esterification 5. GENERALISED PROCESS

From the above discussion it may be observed that the second alternative can be generalised both for lights and heavy alcohols. The acid recycle via water decanting leads to a total consumption of alcohol on the top reactive stage. As a consequence, the process becomes independent on the type of alcohol used. 6. ON-GOING RESEARCH We proceed researches to improve activity and selectivity of zirconia sulphate catalyst in the operation range determined by simulation. Kinetic measurements are tested by simulation with Aspen Plus TM. A laboratory set-up is currently built-up to prove the feasibility of this new process. 7. CONCLUSIONS The paper presents an innovative process for the synthesis of fatty acids esters by reactive distillation. Accurate simulation of simultaneous chemical and phase equilibria combined with laboratory experiments is used to identify the design space. On this basis two alternative designs are proposed, with alcohol and acid recycle, respectively. The last appears to be generic for any type of alcohol. Purity and yield are maximum at an optimum reflux ratio. REFERENCES

1. Castier, M, Rasmussen P., Fredenslund A., 'Calculation of Simultaneous Chemical and Phase Equilibria in Nonideal Systems', Chem. Eng. Sci., 44 (1989) 237. 2. Perez Cisneros, E. S., Gani, R., Michelsen, M. L., 'Reactive separation systems. Computation of physical and chemical equilibrium', Chem. Eng. Sci., 52 (1997) 527. 3. Barbosa, D., Doherty, M.F., 'Design and minimum-reflux calculations for single-feed multicomponent reactive distillation columns', Chem. Eng. Sci., 43 (1988) 1523. 4. Ung, S., Doherty M.F., 'Synthesis of reactive distillation systems with multiple equilibrium chemical reactions', Ind. Eng. Chem. Res., 34 (1995) 2555.