Synthesis of separation systems for azeotropic mixtures: Preferred distillation region

Synthesis of separation systems for azeotropic mixtures: Preferred distillation region

16th European Symposiumon Computer Aided Process Engineering and 9th International Symposiumon Process SystemsEngineering W. Marquardt, C. Pantelides ...

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16th European Symposiumon Computer Aided Process Engineering and 9th International Symposiumon Process SystemsEngineering W. Marquardt, C. Pantelides (Editors) © 2006 Published by Elsevier B.V.

Synthesis of Separation Systems for Azeotropic Mixtures: Preferred Distillation Region Stanislaw K. Wasylkiewicz

Aspen Technology, Inc., 900, 125 - 9th Avenue SE, Calgary, Alberta T2G OP6, Canada

Abstract An algorithm for automatic generation of sequences of distillation columns and decanters for separation of azeotropic mixtures has been developed where distillation boundaries can be crossed by moving them with pressure change, by exploring curvatures of distillation boundaries or by liquid-liquid splits in decanters. Based on a broad knowledge of distillation regions and distillation boundaries for the separated mixture, open-loop sequences are generated and primary recycles are automatically detected. Then preferred distillation regions are identified and suitable recycle streams are selected. In this paper, we are focused on internal secondary recycles where species present in the sequence feed are introduced as separating agents. This type of recycles can simplify tremendously the whole sequence and reduce significantly the total cost of separation. In this paper, an example based on an industrial case is presented where the internal secondary recycle was automatically calculated during synthesis of column sequences.

Keywords: azeotropic distillation, synthesis, recycles I. Introduction A fundamental problem in synthesis of systems for separation of azeotropic mixtures is a fast and reliable determination of feasible products that can be attained in individual separators for a given feed composition. Contrary to zeotropic mixtures, not all separation schemes generated based on relative volatilities of components are feasible, as well as not all desired specifications can be met. These facts are usually discovered after extensive and lengthy simulation studies if only process simulators are used to solve design problems. This so far the most popular design-by-simulation approach requires several performance simulations for various design parameters selected usually by trial and error. It can be extremely time-consuming to find out that the specified separation is inherently infeasible. An algorithm for automatic generation of sequences of homogeneous as well as heterogeneous distillation columns for separation of azeotropic mixtures has been developed where distillation boundaries can be crossed by moving them with pressure change (Wasylkiewicz, 2004), by exploring curvatures of distillation boundaries or by liquid-liquid splits in decanters (Wasylkiewicz, 2005). In the systematic synthesis of the azeotropic separation schemes, first we generate open-loop sequences by systematic application of split generator to determine feasible products that can be attained in individual separators for specified feed compositions. Then we identify suitable recycling options. From the synthesis point of view, we distinguish two types of recycles: primary recycles and secondary recycles. The main goal of the primary recycles is to reduce the number of separation steps. Recycling should merge units that perform identical tasks. Our algorithm automatically detects primary recycle streams

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and their destinations, and calculates recycle mass balances. A secondary recycle stream is introduced in order to shift the total feed composition and by doing this change the functionality of a separation unit. In an external secondary recycle an external species is introduced as the separating agent. In an internal secondary recycle a species present in the sequence feed is introduced as the separating agent. It can be a pure component produced somewhere in the sequence or any other intermediate stream. In this paper, we are focused on internal secondary recycles. Based on a broad knowledge about distillation regions and boundaries for the separated mixture, a preferred distillation region can be identified and a suitable recycle stream can be selected. This type of recycles can simplify tremendously the whole sequence and reduce significantly the total cost of separation. In practical separation problems, we often do not need to separate the sequence stream to all pure components because quite frequently some of their mixtures can be recycled e.g. to a reactor. In such cases, internal secondary recycles can tremendously simplify the whole sequence and reduce significantly the total cost of separation. In this paper, we present in details an example based on an industrial case where the internal secondary recycles were automatically calculated during synthesis of column sequences.

2. Common Industrial Separation Problem In chemical industry, it is a common problem to identify possible routes of separation of a complex mixture into products (often individual pure components), waste streams and reactor recycles. The mixture quite often contains several dozen components, is highly non-ideal, with numerous azeotropes and distillation boundaries. It can take several months of trial and error to solve this problem using only simulator. Distil (2004) can help to solve this problem much faster, usually in a few days by providing the designer with all information about the system necessary to be able to make the best decisions during the separation sequence synthesis. The real industrial case (Wasylkiewicz, 2001) was solved in a few days. Half of the time was devoted to develop appropriate thermodynamic model by creation of pseudo-components, regression or estimation interaction parameters, checking thermodynamic consistency etc. Then several possible routes to separate twelve-component mixture into individual pure products, waste streams and reactor recycles were identified using automatic column sequencing and split generation features in Distil. Since we can not publish any proprietary information about this industrial case, for the example presented in this paper we selected the fivecomponent mixture (C5) of ethanol, chloroform, methanol, acetone and benzene to show how the internal secondary recycle can be automatically calculated in the synthesis of column sequences. The objective of the example is to identify possible routes of separation of the C5 mixture into the following streams: • Pure ethanol and pure benzene streams (complete recovery). • One or a few streams that can be recycled to the reactor (can not contain ethanol or benzene). In the final five-component mixture of the real industrial case, there was one purecomponent product, another pure component was a waste stream (could not be recycled) and the rest of components should be recycled to the reactor.

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3. Preferred Distillation Region Our systematic synthesis of azeotropic separation schemes is based on the rigorous calculation of distillation regions and distillation boundaries in the mixture. For the specified feed composition, we can precisely determine in which distillation region the feed is and find all feasible products that can be attained in individual separators for a feed belonging to a particular distillation region. If any of the products meets the separation goals we call such a region the preferred distillation region. Singular I: 2: 3 : 4: 5: 6: 7: 8: 9: I0: 11: 12: 13: 14: Basic

Points: Chloroform - Methanol Methanol - Acetone Acetone Chloroform - Methanol Chloroform - Methanol Methanol - Benzene Ethanol - Chloroform Chloroform Ethanol - Chloroform Chloroform - Acetone Methanol Ethanol - Benzene Ethanol Benzene Distillation

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Figure 1. Singular points and distillation regions for the C5 mixture at 101 kPa. At atmospheric pressure, Wilson-Ideal model predicts nine azeotropes in the C5 mixture. All singular points (azeotropes and pure components) and basic distillation regions are shown in Figure 1. One quaternary, two ternary and six binary azeotropes have been found. Singular points are displayed in order of increasing boiling temperatures. There are two unstable nodes, two stable nodes and ten saddles in the mixture. This gives rise to four basic distillation regions. The original feed to the separation system is in distillation region 2. Several feasible splits were calculated by Distil. The most interesting one is the indirect split where the heaviest singular point in the region (benzene, No. 14) can be completely separated as the bottom product from the rest of components (top product). A distillation column, which carries out this separation (Column 1 in Figure 2), fulfils one of the objectives of our synthesis problem - completely recovery of benzene from the original feed. That is why we identify region 2 as the preferred distillation region. Top product from Column 1 (Stream 3 in Figure 4) is practically benzene free. For this four-component (C4) sub mixture four distillation regions have been found and several feasible splits for Stream 3 have been calculated by Distil. Unfortunately, none of them fulfills our next objective in the synthesis - complete recovery of ethanol from the C4 mixture. Stream 3 is not in preferred distillation region. However, there are two other preferred distillation regions that contain ethanol as the stable node and a simple distillation column placed in this region can produce ethanol in the bottoms. To obtain the overall feed composition in one of these regions, we have to add a recycle stream to the Stream 3. This calculation intensive process of selecting proper recycle stream, checking feasibility of the split and arranging another column that produces enough

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recycle stream has been done automatically by Distil. The resulting column sequence is shown in Figure 2 and parameters of all the streams in Figure 4. Mass balances for all three columns and the recycle are shown in a projection of the C5 composition space into 3D space (without benzene) in Figure 3. F l o w = 90.01 kgmolefh Ethanol = 0

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Chloroform = 0.4444 Methanol = 0.2777 Acetone = 0.2777 Benzene = 0.0001056

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Acetone = 0 Benzene = 0

Chloroform = 8.215e-005 Methanol = 2.5e-009 Acetone = 2.5e-009 Benzene = 0.9999

Figure 2. Sequence of columns for complete recovery of benzene and ethanol from C5 mixture.

S

Ethanol

Chloroform

Figure 3. Mass balances for all columns in the sequence and the recycle. Projection of the C5 composition space into 3D space.

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4. Algorithm for Calculation of Internal Secondary Recycles As prerequisites, components that must be fully recovered from the original mixture have to be specified (e.g. benzene and ethanol in the presented example). Then during automatic generation of sequences of distillation columns (Distil, 2004; Wasylkiewicz, 2005), program takes one of the products from recently added distillation column, that is not a sequence product, and tries to find a feasible split that can accomplish any from the specified separation goals. If such split was found a new distillation column is added to the sequence (e.g. Column 1 in the presented example). If not (e.g. for Stream $3), program tries to find internal secondary recycle sequence in the following steps: 1. Find distillation region the stream is in at selected pressure. 2. If this is not a preferred distillation region, look if there is another distillation region (the stream is not in) that could be suitable for complete recovery of any component that must be fully recovered, e.g. ethanol from Stream $3. 3. If the preferred distillation region was found, analyze all singular points of this region for possible recycle streams. In the example, ethanol is the heaviest

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componem in the preferred distillation region (stable node) and as a result a complete recovery of ethanol can be achieved at column bottom. After the recycle candidate was selected, e.g. pure methanol (Stream $7), calculate minimum recycle ratio to accomplish full recovery of the selected componem. First find Stream $6 on composition boundary that will fulfill mass balance around the sequence of Column 2 and Column 3 and provide full recovery of ethanol. Then find Stream $4 that must be in the same distillation region as the stream $5 to make Column 2 feasible. To minimize the recycle ratio the stream $4 is usually chosen close to the distillation boundary but inside the preferred distillation region. Notice also that the overall feed of Column 2 obtained by mixing Stream 7 (recycle stream) with Stream 3 does not need to belong to the preferred distillation region if the distillation boundaries are curved enough. Now look for a pressure, at which a distillation column that separates Stream $4 into Stream $6 and Stream $7 will be feasible by analyzing distillation regions and distillation boundaries for C3 mixture that does not comain the componem already completely recovered in Column 2 (ethanol).

Pressure analysis of azeotropes is an important part of Distil (2004). That is why we used different pressure in Column 3 to produce enough methanol for the recycle. At 101.3 kPa Column 3 would not be able to provide appropriate flow rate of Stream 7 as distillation boundary at this pressure is too close to the feed (Stream 4). By changing operating pressure in Column 3 to 1 kPa we moved the distillation boundary away from the methanol vertex what allowed us to obtain higher flow of bottom product (Stream 7). In fact any type of separation could be used instead of distillation Column 3 (e.g. membrane separation) as long as it produces enough methanol for the recycle.

5. Conclusions A new algorithm has been developed for automatic synthesis of sequences of distillation columns with recycle streams even in cases when intermediate streams are on distillation boundaries. Feasible splits for azeotropic mixtures are rigorously and efficiently calculated based on information about all azeotropes in the mixture, distillation regions and distillation boundaries that are generated using adjacency and reachability matrices. The new algorithm facilitates efficient crossing distillation boundaries and quick finding preferred distillation regions for particular separation tasks. It can be used for mixtures of any number of components. This type of internal secondary recycles can significantly simplify the whole sequence and reduce drastically the total cost of separation.

References Distil, 2004, v 6.2 software, Aspen Technology, Inc., http://www.aspentech.com S.K. Wasylkiewicz, 2001, Case Study, Use of Column Sequencing and Split Generation Features in Distil, Calgary, August 2001 S.K. Wasylkiewicz, 2004, Advances in Synthesis of Continuous Separation Sequences for Azeotropic Mixtures, 54th Canadian Chemical Engineering Conference, paper No. 243, Calgary, Canada, October 2004 S.K. Wasylkiewicz, 2005, Crossing Distillation Boundaries in Synthesis of Separation Sequences for Azeotropic Mixtures, AIChE Spring National Meeting, paper No. 83e, Atlanta, GA, USA, April 2005