Intensification of Reaction and VL Separation in ... - ACS Publications

Chapter 24

Intensification of Reaction and V-L Separation in Batch Systems Downloaded by OHIO STATE UNIV LIBRARIES on June 9, 2012 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch024

Jae W. Lee, Mudassir Ghufran, James Chin, and Zhe Guo The Department of Chemical Engineering, The City College of New York, New York, NY 10031

This article addresses the feasibility requirement for producing pure products when intensifying reaction and V-L separation in batch systems. With one unstable node (UN) product in a residue curve map that is reachable from all distillation regions or a part of reaction equilibrium manifolds, a batch rectifier can produce pure products. The symmetric result is conserved for a batch stripper with one stable node (SN) product. With the UN and S N products that share a distillation region with a reaction equilibrium manifold, a middle vessel column ( M V C ) is feasible to produce those pure products. With all saddle (S) products, a batch reactive extractive (BRED) column can be used to produce pure products. In this case, we should use a suitable entrainer.

© 2005 American Chemical Society

In Microreactor Technology and Process Intensification; Wang, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Downloaded by OHIO STATE UNIV LIBRARIES on June 9, 2012 | http://pubs.acs.org Publication Date: August 9, 2005 | doi: 10.1021/bk-2005-0914.ch024

1. Introduction The combination of reaction and distillation is not a recent idea. The earliest official record about reactive distillation dates back to Backhaus's patent on methyl aceate production in the 1920's (7). However, six decades had passed before Eastman Chemcials reported their commercially running process in the early eighties (2). This significant delay in commercializing the methyl acetate process reflects the lack of design methods for reactive separation. Since Eastman's methyl acetate production system came out, academia as well as industry has actively started to develop design methods for reactive separation. Representative design methods are static analysis (3, 4), the transformed space approach (5, 6, 7), the fixed-point method (8, 0), the mathematical programming approach (10, 11, 12), and the difference point method (13, 14, 15, 16). For more rigorous literature survey, refer to the recent two review articles (17, 18). Despite the large volume of publications on reactive distillation, it's surprising that we cannot understand under what conditions we can combine reaction and V - L separation to produce pure products. This is the feasibility question of whether or not pure products can be produced with the integration of reaction and distillation. Producing pure products within their specifications is very important, otherwise, we still need additional separation units to purify products in addition to a reactive distillation column. In the past, to explore the feasibility of reactive distillation, a lot of time and money had to be spent to perform trial-and-error experiments and rigorous calculations. In this work, we will present a feasibility guideline using the relatively simple information of reaction and phase equilibrium data. Here phase equilibrium is represented by residue curve maps (19, 20). In these maps, the reactants and products in the reaction will be categorized according to their dynamic properties as nodes and saddles. Then, reaction equilibrium manifolds will be superimposed onto these maps. Feasibility criteria for combining reaction and V - L separation will be developed for several batch configurations that are prevalent in specialty chemical and pharmarceutical processes. A l l of the feasibility criteria will be confirmed using dynamic simulations.

2. Motivation To c o n ç a r e between one batch reactive distillation (BRD) unit and a conventional two-step process consisting of reaction followed by separation, we take the production of terf-amyl methyl ether as an example. A conventional batch process consists of a reactor followed by a separation column as shown in Figure 1. A n intensified B R D column is given in Figure 2. The dynamic simulation results for producing T A M E in these two flowsheets show the huge advantages of the intensified unit over the conventional unit: for the same T A M E production purity (99.9 mol%), 1) the conventional process has an 80-tray


395 column with a reflux ratio of 40 while the B R D column with a reactive overhead drum has only 10 trays and a reflux ratio of 10, 2) the conventional process has a cycle time of 1.4 times the cycle time of the intensified process to recover an identical amount of T A M E , and 3) the number of operation units reduces from 2 to 1 by integrating reaction and V - L separation in one column. In addition to these advantages, i f we consider the time required for cleanup and transfer of process streams between the two units in the conventional flowsheet, the intensified unit has even more merit than the conventional process.


Reaction in the

Figure 1. Conventionalflowsheetfor TAME production (MT+2M2B TAME). TAME: tert-amyl methyl ether, MT: methanol 2M2B: 2-methyl-2-butene.

Figure 2. Intensified batch reactive distillation unit for TAME production. Now, naturally, we can ask the following question: how can we combine reaction with V - L separation to produce pure products? In other words, we want to develop a systematic view to analyze the feasibility of B R D . In the T A M E reaction system, there are two minimum boiling azeotropes and one distillation


396


boundary. If reaction equilibrium allows the reactive overhead drum composition to cross the distillation boundary as shown in Figure 2, then T A M E can be produced at the bottom of the column since it is the least volatile in the residue curve map. Thus, using the simple information of reaction and phase equilibrium, it is possible to determine the feasibility of reactive separation systems before doing many experiments and rigorous simulations. To analyze the feasibility of B R D , we take three basic configurations: batch rectifier, batch stripper and middle vessel column (21) as shown in Figure 3. We allow reactions to occur in the charge drums, but not on the distillation trays, to simplify our analysis.

1

• B» x (c) Middle vessel column B

(a) Rectifier

(b) Stripper

Figure 3. Three basic batch configurations with reactive charge drums (22, Reproduced with permission of the American Institute of Chemical Engineers. Copyright ©2003 AIChE).

3. Developing Feasibility Criteria for Basic Configurations In this section, feasibility criteria will be developed with the decomposition reaction of 21 +-* L+H in the three basic B R D columns. Starting with this reaction, we will extend the feasibility criteria to any single reaction with arbitrary reaction stoichiometry. Constant molar overflow, absence of reaction, and V / L phase equilibrium are assumed on each stage. It is also assumed that the V / L holdups on each stage are negligible compared to the holdups of the feed charge drums, that the number of stages is very large, and that the columns operate with infinite reflux ratio (R) or reboil ratio (s).


397 3.1. Material balances The following material balance equations are used to determine the dynamic composition trajectories of the reactive charge drums (or still pot) in the rectifier (23), the stripper, and the middle vessel (22).


^ ας

= (x -XsMDaiv,

-ν ,Χ*

SJ

=

Λ

χ

( X s j

_ _2 ψ ί

£)

+

D a ( V i

_

Μ

· -Zfte) Keq

V T X s t ) ( X s /

(2) (3)

where Da is the Damkôhler number and represents the reaction holdups i n the reactive charge drum. The calculations of composition profiles in the stages above or/and below the reactive charge drums are given as follows for the rectifying and stripping sections, respectively. J? 1 y*v "ΊΓ~Λ «ί VT*** = stags i = ^components () Χ

x

y~~~ n+u

s

+

n

+~~ BJ X

N

= Ï9—*Nstagç

4

î = \,..ccomponents

(5)

s

From the dynamic simulations using the above equations, we will develop and verify the feasibility criteria for the three batch configurations. During the simulations, equations (1) - (3) of reactive drums will be simultaneously solved with the equations of stages in equations (4) and (5).

3.2. Feasibility criteria for reactive batch rectifier and stripper The V / L equilibrium data chosen for the ternary mixture in reaction 21 L+H are given in residue curve map (RCM) 320, as shown in Figure 4(a). Here, reactant I is an intermediate bolier. It forms a minimum boiling azeotrope with heavy product H and a maximum boiling azeotrope with light product L . The details for the classification o f residue curve maps can be found i n previous literature (24, 25). In R C M 320, all of the liquid composition profiles leave the L vertex since it is the most volatile. So, vertex L is an unstable node (UN). A l l liquid composition profiles move into either the vertex H or the maximum boiling azeotrope. Therefore, these two singular points are stable nodes (SNs). A t vertex I and the minimum boiling azeotrope, liquid compositions enter those



398 singular points from one direction and come out of them in a different direction. Those points are saddles (S). Two distillation regions are formed and are separated from each other by one distillation boundary that cannot be crossed by simple batch distillation. The dynamic simulation results of R C M 320 with reaction 2IL+H are shown in Figure 4(b). The still pot composition path starts from the intial feed point and ends at the H vertex. The most important point is that, from any location (distillation region I or II) of the still pot path, the U N product (L) is reachable by the non-reactive rectification of column trays above the still. Since product L is continuously removed from the still and withdrawn at the top, the reactive still path always lies in the forward reaction region. Consequently, reactant I is completely consumed and the still contains only H at the end of the reaction.

(a) R C M 320

(b) Time-dependent still path

Figure 4. Dynamic simulation results of a batch reactive rectifier (22, Reproduced with permission of the American Institute of Chemical Engineers. Copyright ©2003 AIChE).

Here we can obtain one important feasibility criterion: if one of the reaction products is a UN (the most volatile) that is reachable by non-reactive distillation from any distillation region, then the reactive batch rectifier can produce pure products. The second feasibility criterion for the batch rectifier is that even if one of reaction products is a UN and is not reachablefromboth distillation regions, it can still produce pure products when reaction equilibrium allows the still path to cross the distillation boundary as shown in Figure 5. The reaction equilibrium


399


curve lies within the same distillation region as the two products (L and H). Thus, the still path starting from the I vertex can move towards the reaction equilibrium curve and cross the distillation boundary. Once it reaches the distillation region from which the light product (L) is reachable using nonreactive distillation, then simultaneous distillation can begin and pure L can be produced. Again, the contiuous removal of light product L forces die still path to lie in the forward reaction region and reactant I is completely converted, leaving only heavy product H in the still pot.

L(UN)

Max A Z (SN) Figure 5. Dynamic simulation results of a rectifier with RCM 430 (22, Reproduced with permission of the American Institute of Chemical Engineers. Copyright ©2003 AIChE).

Similar arguments can be made for a batch reactive stripper: 1) If one of the products is an SN that is reachablefromall distillation regions, then the batch stripper in Figure 3(b) can be used to produce pure products. 2) Even if the SN product is not reachablefromall distillation regions, the batch stripper can still



400 produce pure products when the reaction equilibrium curve shares the same distillation region as the products. If the stable node product (H) is continuosly removed at the bottom of a batch stripper, then the reactive overhead drum path lies within the forward reaction region. Then, the reactant (I) is completely consumed and, finally, the drum contains only light product L . Figure 6 shows various R C M s whose V / L phase equilibrium can lead to the production of pure products using a batch rectifier with reaction 2I

Intensification of Reaction and VL Separation in ... - ACS Publications

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