Process Retrofitting via Intensification: A Heuristic Methodology and Its

Article pubs.acs.org/IECR

Process Retrofitting via Intensification: A Heuristic Methodology and Its Application to Isopropyl Alcohol Process Mark Wendou Niu and G.P. Rangaiah* Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore, 117585 ABSTRACT: Although process retrofitting is essential for existing chemical plants to remain sustainable, studies on systematic retrofitting methodologies for chemical processes are limited and also do not focus on the possibility of replacing a few unit operations with an intensified unit. Hence, in this study, a heuristic-based systematic methodology is developed for process retrofitting via intensification/integration. Compared with the conventional method, it focuses on exploring integrated retrofit solutions. Fast screening with heuristics, process knowledge, and process simulation is used to avoid infeasible solutions in the early stage. The proposed methodology is applied to retrofitting the conventional isopropyl alcohol (IPA) process. An improved solution without capital investment is obtained by operation optimization; it results in reducing the manufacturing cost per unit product by 5.5%. Then, two retrofit solutions, both involving reactive distillation, are developed. One of them reduces the manufacturing cost per unit product by 23% and results in a reduction in the number of equipment in the process.

1. INTRODUCTION The chemical industry, comprising a number of distinctive sectors such as petroleum refining, petrochemicals, polymers, agrochemicals, biotechnology products and more, is one of the most energy intensive industries. Most of the chemical plants were built at a time when energy costs and environmental concerns were not on the agenda. Further, process and computational technologies continue to be developed, thanks to ongoing research in various parts of the world. Hence, the design of existing plants may not be the most efficient and optimal for the present situation. All these things call for modifying and/or replacing the relevant process equipment in the plant with more efficient units having lower operating cost and/or emissions. Thus, process retrofitting is essential for existing chemical plants to remain sustainable. Many studies on chemical process retrofitting have been reported in the literature. These can be classified into two categories: individual case studies and systematic retrofitting methodologies. Recent studies in the former category are briefly reviewed below. Heat exchanger network retrofitting is a specialized area not considered in this study. The review of techniques for heat exchanger networks and their applications is available in Sreepathi and Rangaiah.1 Premkumar and Rangaiah2 investigated dividing wall columns (DWC) for retrofitting conventional 2-column systems. Their results on a number of applications suggest 25% to 48% reduction in total annual cost. Ngyuen and Demirel3 used column grand composite curves (CGCCs) and exergy loss profiles to generate retrofit solutions for columns in a biodiesel plant. Exergy loss profiles help to examine energy degradation on all internal trays, and CGCCs help to identify possible retrofit © XXXX American Chemical Society

schemes including feed location, reflux ratio, feed conditioning and side condensing/reboiling. It was found that total exergy loss can be reduced from 7492 kW to 3628 kW for the three columns in the biodiesel process. Long et al.4 explored DWC to increase the throughput of an existing acetic acid purification process. Several column arrangements were analyzed, and results showed that DWC requires less energy cost than conventional columns. Long and Lee5 converted an extractive distillation (ED) to a thermally coupled ED sequence, where solvent recovery column was used as the side rectifier column. In their first case study on separation of acetone and methanol with water as the extractive solvent, retrofit design achieved 13% energy savings and 10% additional capacity. For the second case study on separation of heptane and toluene with aniline as the extractive solvent, results show a 32% energy savings. Park et al.6 proposed a retrofit design for a boil-off gas (BOG) handling process, and the retrofit design was optimized using the SQP program in MATLAB. Optimal results show 23% energy savings and a payback period of only 2.2 months. Development of new technologies motivates and accelerates retrofitting of conventional chemical processes. For example, with a better performance, improved life, and lower cost, membrane separation is increasingly studied for process retrofitting. Motelica et al.7 investigated the potential of adding a membrane unit for butadiene separation from the C4-fraction in an olefin plant. Their simulation results show that, with a Received: July 23, 2015 Revised: February 14, 2016 Accepted: March 2, 2016

A

DOI: 10.1021/acs.iecr.5b02707 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Studies on systematic retrofitting methodologies are limited compared to individual case studies. Moreover, most of the current retrofitting methodologies are limited to operation optimization of a single unit operation, restructure/repiping within a few unit operations, or adding a few unit operation(s) in the upstream/downstream of the energy intensive unit. They do not consider the possibility of replacing a series of unit operations with an intensified unit operation. Hence, they may not achieve attractive improvements for complicated chemical processes such as complex reactions followed by azeotropic distillation. Therefore, a new retrofitting methodology, which can achieve substantial improvements, needs to be developed. If a new concept and/or technology is brought into process retrofitting, significant improvements are possible. In this context, process intensification (PI) is attractive. Defined as “any chemical engineering development that leads to a substantially smaller, cleaner, safer, and more energy efficient technology”,18 it can bring significant benefits in terms of process capital/operating costs, safety, product quality, etc.19 PI covers both process equipment types and methods, which respectively refer to novel intensified equipment (e.g., rotating packed bed/HiGee and DWC), and to integration of unit operations, that is, process integration.20 According to these authors, process integration includes multifunctional reactors (e.g., reactive distillation, reactive extraction) and hybrid separations (e.g., membrane absorption, membrane distillation). When PI is introduced into process retrofitting, it usually implies process integration rather than novel intensified equipment. Hence, PI and process integration are used synonymously in the rest of this paper. If introduced into retrofitting methodology, PI-based solution(s) will be able to cross the boundary of single unit operation, and that is exactly the focus of this study. This is probably the first study that specifically includes PI into process retrofitting methodology. Furthermore, a complicated chemical process, namely, the isopropyl alcohol (IPA) process, will be improved by applying the methodology. So far, retrofit of the IPA process has not been reported in the open literature. The rest of this paper is organized as follows. Section 2 presents the new retrofitting methodology. Section 3.1 analyses the conventional IPA process by this methodology. Section 3.2 discusses modifications within each unit operation (i.e., local optimal solution). Sections 3.3 and 3.4 develop the two PI-based retrofit solutions. Section 3.5 compares all the retrofit solutions to choose the best one. Conclusions of this study are presented in section 4.

membrane selectivity of 7.5 for butadiene/mono-olefins, 23% reduction in energy required can be achieved. Ploegmakers et al.8 studied the addition of a membrane unit to an ethylene/ethane fractionation column, in different configurations. Their results show that with a minimum ethylene/ethane membrane selectivity of 30, cost savings can reach as much as 16% for the membrane unit in series. Although achieving attractive cost savings and/or capacity increases, studies based on individual applications is casespecific, and their procedure is not general and difficult to extend to other applications. Therefore, systematic retrofitting methodologies are required. Fischer et al.9 presented the first systematic procedure for developing and screening retrofit solutions. Their procedure includes an analysis step to identify “bottlenecking” equipment, and a retrofitting step that considers modifications in both structure and equipment. The case study of retrofitting the hydro-dealkylation (HDA) process indicated that retrofitting by this methodology could achieve significant savings. Another early study was by Dantus and High,10 who developed a superstructure-based methodology for waste minimization and cost savings. It starts with sensitivity and hierarchical analysis to identify waste minimization options and modified configurations, respectively. Then, the alternatives identified are used to construct a superstructure for formulating and solving a mixed integer nonlinear programming problem. Dantus and High10 applied their proposed methodology to the manufacture of methyl chloride. Kralj and Glavic11 developed a mathematical model based method to sequentially optimize superstructure, material, and energy flow rates; subsequently, Kralj et al.12 extended it to a stepwise simultaneous optimization scheme. A methanol plant was retrofitted by both the earlier and the latter methods, and the results show that the former leads to extra profit of 2.8 million USD/year compared 5.17 million USD/year by the latter method.12 Researchers from Gani’s group developed an indicatorbased methodology for assessing the cost-saving potential of continuous chemical processes.13 The methodology consists of decomposition analysis, retrofit solution generation, and rough economic evaluation, and it was tested by retrofitting the HDA process. The methodology was later extended to include environmental indicators so as to achieve sustainable retrofit targets.14 Compared with its earlier version in Uerdingen et al.,13 one major improvement was the use of an industrial simulator (PRO-II) in both process analysis and optimization. Uerdingen et al.15 improved the methodology further by distinguishing retrofit options that require new investment with those options without any investment. They implemented the improved methodology for a fine chemical process. Carvalho et al.16 developed an Excel-based software (Sustain-Pro), which eliminated manual calculation of selected indicators. Moreover, an indicator-based sensitivity analysis algorithm was added into the original methodology to accelerate generation of retrofit solutions. Some other researchers studied retrofitting methodology for a specific type of processes. For example, Simon et al.17 developed a systematic framework for retrofitting batch processes; it consists of top-down analysis to cover three levels: the plant, the process, and the unit operation. Investigating interaction between different levels helps to get a deeper understanding of the bottlenecks in the original process. Simon et al.17 successfully applied their methodology to a fine chemical batch plant.

2. METHODOLOGY In this section, a heuristic-based methodology for retrofitting via process intensification/integration is discussed. Unlike the previous methodologies in the literature, the new methodology focuses on generating integrated retrofit solutions. As part of the methodology, optimization of operating conditions (referred to as ‘improved solution without capital investment’ in this study) is also covered and compared to the integrated solution(s). This is because the former does not require any new investment, and so it may be attractive in some applications. The proposed methodology consists of four steps: (1) base case analysis, retrofit target, and contributing units; (2) generation of an improved solution without capital investment; (3) generation of integrated solution(s); and (4) comparison of retrofit solutions and decision making. B

DOI: 10.1021/acs.iecr.5b02707 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Step 1: Base Case Analysis, Retrofit Target, and Contributing Units. In the first step, the original process is simulated using a commercial simulator (e.g., ASPEN Plus), process data from the simulation are collected, and the performance of each unit operation is evaluated by selected metrics. Table 1 gives an example of a retrofit target and the

Note that the metrics eventually contribute to the chosen objective function. For example, if the cost is the objective function, each of these metrics will have to be multiplied by relevant unit cost (such as of utility and component lost). Although contribution of a unit operation to the objective function (such as cost) can be used, metrics allow analysis to be focused on a fundamental quantity characterizing the performance of the unit operation, which is useful in developing integrated retrofit solutions. The retrofit of heat exchanger networks (HENs) is generally solved as a separate problem; therefore, heat exchangers between processes streams are not evaluated in Table 2. After evaluating the performance of each unit operation, it is essential to find the main contributors for each metric. For this, taking metric A as an example, the maximum contributor is identified as follows:

Table 1. Metrics for the Retrofit Target with an Example for TAC target metric metric metric metric

A B C D

minimize total annual cost ($/yr) utility cost ($/yr) component loss ($/yr) conversion of reactants selectivity of products

metrics used for evaluating the process performance. As can be seen, the retrofit target is to minimize the manufacturing cost per unit product ($/kg), which is the total annual cost (TAC, $/yr) divided by annual production rate. The TAC consists of amortized capital cost (ACC) and annual production cost (APC).21 ACC is not involved in evaluating the original process. Therefore, metrics are generally developed for APC. Three metrics A and B (such as utility cost and component loss), directly related to annual production cost, are used to evaluate the process performance. Metrics C and D are related to reactor performance, and they indirectly influence TAC; these metrics are important because reactions usually play an essential role in chemical processes. Next is to use the metrics from Table 1 to evaluate the process performance. Unlike the classical operating cost diagram,9,22 the entire process is decomposed into reaction and nonreaction parts (e.g., separation units, compressors, etc.). This classification of unit operations is vital for exploring integrated retrofit solutions in the later steps. In Table 2, nonreactive unit operations (e.g., distillation columns, absorber, and heater) are evaluated while reactors 1 to s are evaluated in Table 3. Note that each type of

dmax, A = max{d1A , d 2A , ..., dm A }

A less important contributor could be neglected if it satisf ies: diA < 0.2dmax,A

metric A

D1 ⋮ Dm AB1 ⋮ ABn ⋮ COM1 ⋮ COMk

d1A ⋮ dmA

⋮ com1A ⋮ comkA

(i = 1, 2, ..., m , and i ≠ max)

(2)

The factor of 0.2 is used here instead of 0.1 in Douglas et al.22 This is mainly to identify retrofit opportunities with greater benefit and hence more potential for implementation. Note that the proposed methodology can be applied with any reasonable factor except that more effort, perhaps without much benefit, will be required if the factor is low. Screened by eq 2, those metric values less than 1/5 of their respective maximum are not considered; in contrast, metric values between 0.2dmax and dmax are not negligible, and their associate units are referred to as contributing units, which are considered for retrofitting. Similarly, abmax,B and commax,A could be calculated. Some of the metrics may have the same units and significance (e.g., $/yr for metrics A and B). Such metrics could be evaluated together to further remove the less contributing units. Equation 3 provides an example for such circumstances; suppose dmax,A satisfies

Table 2. Evaluation of Nonreaction System unit operation

(1)

dmax,A < 0.2 × max{abmax,B , com max,A}

(3)

metric B

Then, Di (i = 1 to m) metrics are no longer considered as the contributing unit operations. If the retrofit target is to maximize the metric, then max in eqs 1 and 3 should be changed to min. Step 2: Generation of an Improved Solution without Capital Investment. As noted by Grossmann et al.,23 the simplest modification is to optimize the operating conditions without adding any new equipment, as it requires no extra capital investment. Further, the typical implementation period (excluding design, analysis, safety study, approval etc., which can be done in advance without affecting the existing process operation) for adding a new unit operation is a few weeks. Similarly, changes in an operating variable can be implemented in a few hours or a day, mainly to bring the process to the new steady state. Therefore, as long as modifying the operating conditions can improve the retrofit targets, other more complicated retrofit designs (such as integrated retrofit solutions mentioned later) should be compared with the former, and the integrated retrofit solution is attractive only when it provides cost savings more than the retrofit solution without capital investment. Therefore, step two focuses on optimizing the operating conditions, generating an improved solution without capital investment, which is later used for evaluating the more complicated retrofit designs.

ab1B ⋮ abnB ⋮

Table 3. Evaluation of Reaction System unit operation

metric C

metric D

R1 ⋮ Rs

r1C ⋮ rsC

r1D ⋮ rsD

unit operation has its own metric; for example, Di is evaluated by metric A (e.g., utility cost), while ABj is evaluated by metric B (e.g., component/solvent loss), and COMk is again evaluated by metric A (utility cost). Reactor R is evaluated by conversion of reactants (metric C) and selectivity of products (metric D). C

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Figure 1. Process scheme with contributing unit operations circled.

savings can be significant. Another important aspect is that reactors usually play an essential role in chemical processes, and integrated units such as reactive distillation, reactive absorption, and the reactor−membrane hybrid system have been well studied.20,24 If no reactor is involved, integration happens among contributing units since the purpose of integration in retrofit design is to simultaneously resolve drawbacks of the involved units. The second rule is straightforward, and a similar statement can be found in Lutze et al.25 The above two rules provide basic guidelines; in the following, when developing root integration, heuristics will be introduced to generate integrated solutions. Step 3 contains several substeps; to better understand these substeps, consider the process flow sheet shown in Figure 1. The first substep is to summarize the contributing units and their associated metric values, and highlight these units in the process flow sheet (Table 4 and Figure 1). The analysis of the reactors is presented in Table 3.

As mentioned in the above section, the retrofit target is to minimize manufacturing cost per unit product ($/kg); when capital investment is not considered, it becomes minimization of production cost per unit product ($/kg), given by Min E = (C U + CO + C M + COPO + C P + C D + CGEN)/P (4)

Here, CU is the cost of utilities (steam, electricity, and cooling agents, $/yr); COP represents cost of operations ($/yr); CM refers to cost of maintenance ($/yr); COPO, CP, and CD are, respectively, cost of operating overhead ($/yr), cost of property, taxes, and insurance ($/yr), and cost of depreciation ($/yr); CGEN represents general expenses ($/yr); and P is the annual production rate of the desired product (kg/yr).21 Decision variables for optimization are selected after analyzing the degrees of freedom for the relevant unit operations as well as their effect on the objective function. Constraints are defined for meeting both product specifications (e.g., product purity) and process requirements (e.g., outlet flow rate should not exceed 10% of its original value). Note that it may not be possible to improve some unit operations through simple modification of operating conditions for one reason or other. If the optimization is successful, a solution without capital investment is achieved, which could later be used as a benchmark for evaluating the integrated retrofit solutions. In case there is no improved solution without capital investment, the integrated retrofit solution will be directly compared with the original process. Step 3: Generation of Integrated Solution(s). Some mathsize="9.5pt"complicated chemical processes tend to have reversible reactions, and require energy intensive separation units (e.g., azeotropic distillation). To address this challenge, it is possible to bring the idea of process intensification into retrofit solutions, which breaks the boundary of each single unit operation and may result in significant savings. Theoretically, any unit operation may have a chance to be integrated with another unit operation. However, to avoid generating too many infeasible solutions, two basic rules are proposed: I. Integration happens among reactors and other contributing units; if no reactor is involved, integrations should happen only among contributing units II. Do not integrate units inhibiting each other’s performance According to the first rule, a reactor has higher priority than other unit operations. One reason is that, although a reactor may or may not directly consume utilities, chemicals generated or reacted during chemical reactions could heavily influence the subsequent separation process. If the reactor is integrated with certain separation unit(s) in a proper way, then the energy

Table 4. Contributing Unit Operations (Other than Reactors) and Their Metrics for the Process in Figure 1 unit operation

metric value

D2 D3 AB1 D5

d2A= v1 d3A = v2 ab1B = v3 d5A = v4

The second substep is to develop Level One integration (root integration), which refers to possible integration between two unit operations. The following heuristics help to decide the feasibility of root integration: (1) Reactor is first considered to be integrated with its downstream separation unit. This happens when the main reaction(s) is equilibrium controlled and at least one of the reactants is provided in excess. (2) Root integration should not cross the reactors. (3) Integrated unit operations should preferably be those being used in the industry. The first heuristic provides a guideline for exploring the initial root integration involving a reactor. A typical equilibrium-based reaction is K

α A + β B ↔ γC + δD

(5)

Suppose the reactor is integrated with its downstream separation unit, then the new reactive−separation unit must have the ability to continuously remove products C and/or D, thus pushing the equilibrium toward the products and allowing more reactants to be converted into products. As the conversion of D

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it is inappropriate. One such example is the root integration between D2 and D5 in Figure 1; as can be seen, D2 and D5 are on different sides of reactor R2, if they are integrated, component changes in R2 are likely to be ignored and result in a faulty solution. The purpose of the last heuristic is to eliminate integrated solutions which are not currently in use in the chemical process industries. Finally, root integration R1−D2, for example, follows the basic rules (I and II), and also assumed to satisfy heuristics 1 and 3; and so it is recorded for further consideration. Introducing an integrated unit operation will bring changes to the base case; in most of the applications, some of the original unit operations will be replaced. In some applications, increased flow rate may require additional unit operation(s) (e.g., when flow rate exceeds 10% of its original value). So, the third substep is to figure out these changes to the process scheme. Considering the process example in Figure 1, after R1 → D2 integration, there are two possible new schemes shown in Figure 3, in which definition of each unit/block is the same as

the reactants is increasing, the amount of reactant(s) (in excess) can be closer to reaction stoichiometry. Note that use of excess reactant (or low-conversion reactant) in the current process is essential for integration. Capital cost of introducing a new reactive−separation unit is not negligible, and it should be offset by savings from the operating cost, which usually comes from reducing the separation cost of the excess reactant. The larger is the amount of excess reactant required in the process (or the lower is the conversion of this reactant), more will be the potential improvement through integration. The first heuristic suggests under what circumstances the integrated reactiveseparation unit should be considered. However, the operating window of a reactive−separation unit may be limited since both separation and reaction must occur within the same ranges of temperature and pressure, which must also meet the mechanical design requirements (Figure 2).20

Figure 3. Process retrofit scheme for (a) root R1 → D2 (top) and (b) root R1 → D2 (bottom).

that in Figure 1. In Figure 3a, R1 and D2 are replaced by a new unit operation RD. Suppose the new integrated unit operation satisfies certain operating conditions (e.g., operating temperature and feed ratio), and D2 and D3 are no longer required, allowing RD to be directly connected to AB1 (absorber). All the remaining downstream unit operations after AB1 remain the same as they are in the original process, which are represented by “ORI” (for original). On the other hand, when RD satisfies some other operating conditions, there is a different retrofit process scheme shown in Figure 3b. As can be seen, R1 and D2 are replaced by the same unit operation RD; the difference is that D3 (and not D2) is still required, and absorber AB1 and desorption tower D4 are no longer required, allowing D3 to be connected to the other reactor R2; the remaining unit operations are the same as in the original process. Note that the two ORIs in Figure 3 are different. Both options in Figure 3 obey rules and heuristics developed. Since they are derived from the same root integration, they are represented as

Figure 2. Feasible operating range of a reactive−separation unit.

The first heuristic, however, does not indicate which integrated unit to be used. The following guidelines from the literature will be useful in choosing the integrated unit. Reactive distillation (RD) is considered when (a) volatility of the product and that of the remaining component are different18 to ensure that the product is not contaminated by the remaining component; (b) the reaction and catalyst must be active at the set temperature and pressure; and (c) the downstream separation after the considered reactor contains distillation column(s) and at least one distillation column is the contributing unit. Reactive absorption/extraction: RA (or RE) is appropriate when (a) the absorbent does not absorb components other than the target component; (c) the absorbent is not involved in the reaction and is not toxic to the catalyst (if any); and (d) the downstream separation after the considered reactor contains an absorber and at least one of the desorption columns is the contributing unit. Note that the contributing unit is usually the desorption column. RD and RA/RE are the commonly used reactive−separation units. Hence, the criteria for implementing them are given above. Other systems include reactive−adsorption and reaction−crystallization, and their selection criteria are available in the literature.18,20 The second heuristic helps to reduce infeasible root integrations, which can be explained as follows. Products/reactants are generated/consumed during chemical reactions. If root integration crosses the reactor, it may ignore these reactions and the associate changes of components in the process, and therefore

{(R1 → D2 ): H1‐RD‐AB1‐ORI ∪ H1‐RD‐D3‐R 2‐ORI}

where R1 → D2 indicates unit operations involved for root integration; H1-RD-AB1−ORI and H1-RD-D3-R2-ORI are the two retrofit process schemes. Before they could be added into the retrofit solution pool, fast screening should be implemented to reject the inferior retrofit designs in their early stage. The criterion for fast screening is that value of retrofit target for the integrated solution should be no worse than that of the improved solution without capital investment (if exists) or at least 10% better than that of the original process. The following equation represents this criterion in a mathematical form. Vr ≤ Min{V0 , 1.15V 0OP} E

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Figure 4. Possible root integrations.

Here, Vr, V0, and VOP are the retrofit target values of the 0 integrated solution, original process, and improved solution without capital investment, respectively. Using the above criterion to evaluate H1-RD-AB1-ORI and H1-RD-D3-R2-ORI, assume that both of them are acceptable (i.e., they improve the retrofit target). Up to now, one root (R1 → D2) is considered. The next step is to consider all other root integrations, which are shown in Figure 4 where the definition of each unit/block is the same as that in Figure 1). As can be seen, it starts with exploring the remaining root integrations, in which D2 is involved. Note that D2 → D5 is not considered as it violates the second heuristic. D2 is located in the upstream of R2 (Figure 1), which means the reactant needs to be purified before entering the reactor; however, reactive distillation usually requires the reactor to be integrated with the downstream distillation column, and not the other way. Therefore, it is reasonable to assume that there is no root integration between R2 and D2. In contrast, possible integration between D2 and D3 exists, namely, DWC. Repeating the analysis as for R1 → D2 integration, D2 → D3 integration yields

Up to now, all root integrations have been explored. Before moving ahead, root integrations need to be compared to eliminate the duplicate designs. Two designs are considered duplicate when the process scheme is exactly the same. Using this criterion for comparing all the candidates shows that {(R1-D3): H1-RD-AB1-ORI} and {(R1-D2): H1-RD-AB1-ORI} are duplicates, and one of them should be removed. Finally, the possible root integrations are listed in the retrofit solution pool, as shown in Table 5. Table 5. Possible Retrofit Schemes from Root Integrations root

modified process

R1 → D2

H1-RD-AB1-ORI H1-RD-D3-R2-ORI H1-R1-D1-DWC-AB1-ORI ORI′-D4-RD

D2 → D3 R2 → D5

The next substep is extending the root integration and exploring further integration, which occurs by combing root (Level 1) integrations. Note that this combination is among different roots such as (R1 → D2) + (D2 → D3); (R1 → D2) + (R2 → D5); and (D2 → D3) + (R2 → D5), and it can be referred as Level 2 integration. However, retrofit schemes from the same root are not considered (e.g., H1-RD-AB1-ORI and H1-RD-D3R2-ORI) because they usually contain different subschemes or have different operating conditions and it may not be possible to integrate them further. There are two types of combinations for Level 2 integration. In the first type, one unit operation may be involved in multiple (root) integrations (e.g., (R1 → D2) + (D2 → D3)), where a new integrated unit operation could be introduced to replace all these unit operations in the retrofit scheme. For example, a reactive dividing wall column (RDWC) may be used as the integrated unit operation instead of RD or DWC alone. Note that this RDWC should also satisfy the rules and heuristics given above. The new retrofit scheme is represented as

{(D2 → D3): H1‐R1‐D1‐DWC‐AB1‐ORI}

Further, the root integration D2 → AB1 is rejected based on eq 6. Until now, all the root integrations involving D2 are explored. There are totally two roots, and three possible schemes. After exploring all possible root integrations for D2, it is necessary to move to other contributing unit operations. It can be seen from Figure 4 that, for D3, only D3 → R1 and D3 → AB1 are considered (assuming that they all satisfy the respective rules and heuristics) as D2 → D3 has already been evaluated; D3 → R2 and D3 → D5 are not considered for the reasons mentioned earlier while exploring root integration of D2 → R2 and D2 → D5. Assuming the root integration of D3 → AB1 is not attractive based on eq 6, there is only D3 → R1 left. The analysis as for R1 → D3 integration gives the following retrofit scheme:

{(R1 → D2 → D3): H1‐RDWC‐R 2‐ORI}

{(R1 → D3): H1‐RD‐AB1‐ORI}

In root (R1 → D2) in Table 5, only H1-RD-D3-R2-ORI is considered to be combined with {(D2 → D3): H1-R1-D1-DWCAB1-ORI}. This is because D3 is not present in the retrofit scheme {(R1 → D2): H1-RD-AB1-ORI}, and so it is not possible to explore the integration between a RD and a DWC. Suppose H1-RD-D3-R2-ORI satisfies the rules and heuristics, it is then evaluated by eq 6, after which, this level 2 retrofit scheme can be added into the retrofit solution pool. The second type of combinations for Level 2 integration refers to combining the root integrations with no shared unit operation such as (R1 → D2) + (R2 → D5) and (D2 → D3) + (R2 → D5). This scenario is a simple combination and does not involve any

For AB1, only AB1 → R1 needs to be considered; assuming that it violates heuristics for using reactive distillation, and hence not included in the retrofit solution pool. For D5, following the second heuristic, R2, is the only candidate and assume this root integration satisfies the rules and heuristics. Repeating the analysis for exploring root integration design, the following retrofit scheme is obtained: {(R 2 → D5): ORI′‐D4 ‐RD}

Similar to ORI, ORI′ means unit operations in the upstream of D4 are the same as those in the original process. F

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ing cost per unit product as the retrofit target. For retrofit designs involving new unit operations, both APC and ACC should be considered for calculating the manufacturing cost per unit product (E):

new integrated unit operation. The new solution fulfilling the criterion in eq 6 will be compared to those in the solution pool; if it is not a duplicated one, the new solution will be added into the retrofit solution pool (Table 6).

Min E = (CPro + α × CTCI)/P

Table 6. Retrofit Solutions Pool after Level 2 Integration integration root

level 2 combination

R 1 → D2 D2 → D3 R 2 → D5 R 1 → D2 → D3 R1 → D2 + R2 → D5 D2 → D3 + R2 → D5

(7)

CPro represents annual production cost ($/yr), which has the same definition as described in step two; CTCI is the total capital investment ($) and α is the amortization factor (1/yr). Suppose the value of total capital investment is M, plant life is n years, and expected return is i. The amortized capital cost A is given by26

modified process H1-RD-AB1-ORI H1-RD-D3-R2-ORI H1-R1-D1-DWC-AB1-ORI ORI′-D4-RD H1-RDWC-R2-ORI H1-RD1-AB1-D4-RD2 H1-R1-D1-DWC-AB1-D4-RD

i × (1 + i)n A = M (1 + i)n − 1

(8)

For n = 15 years and i = 0.15, this equation gives A/M = 0.171, and so α = 0.171. Evaluate all the integrated solutions in the retrofit solution pool using eq 7, and choose the one with optimal E. Then, compare this E with EO of the original process or EOO of the retrofit solution without capital investment, if it exists. There are several possibilities: if E is better than both EO and EOO, then the integrated solution is the final retrofit solution; if E is better than EO but worse than EOO, then the retrofit solution without capital investment is the final retrofit solution. If E is worse than EO and EOO does not exist, then there is no retrofit solution for the original process.

The next step is to explore Level 3 integration through combining Level 2 retrofit solutions (obtained from combining two root integrations) with another root integration (e.g., hybrid RDWC with membrane separation). One can repeat the procedure to generate Level 3, Level 4 to Level n integrated solutions until there is no further combination available. The solution pool after adding level 2 root integration is shown in Table 6; note that root integrations in Table 5 are also part of this solution pool. In the combined retrofit solution H1-RD1AB1-D4-RD2 (Level 2), RD1 and RD2 are used to distinguish the two different RD columns; otherwise, the design is the same as that in the root integration. Figure 5 summarizes the steps in the development of integrated retrofit solutions. Step 4: Optimization, Cost Estimation, and Solution Comparison. The remaining integrated retrofit solutions are optimized and evaluated to select the best one as the final solution for retrofitting. Generally speaking, the selected retrofit design is the one that brings the largest improvement in the retrofit target, and so the final solution may vary when the retrofit target changes. Assume minimization of the manufactur-

3. CASE STUDY The application and potential of the new methodology described in the previous section are illustrated in this section using a case study, namely, the process for isopropyl alcohol (IPA) production. IPA process is selected because (1) it involves both main and side reactions, which are governed by chemical equilibrium, (2) the separation process includes complicated

Figure 5. Generation of Integrated Solutions. G

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Figure 6. Original IPA process scheme; see Table 7 for data on feed and product streams.

Table 7. Process Data of Key Streams stream

category

component

mole fraction

propylene

feedstock

water IPA

feedstock product

propylene propane water IPA water

0.95 0.05 1 0.9999 0.0001

400 654.2 349.9

azeotropic distillation, (3) the original process is complex involving seven main units (namely, propylene column, reactor, lights column, preconcentrator, ether column, extractive distillation column, and regenerator), and (4) there have been no studies on retrofitting this industrially important process. 3.1. Base Case Analysis, Retrofit Target and Contributing Units. Isopropyl alcohol (IPA), also known as isopropanol or 2-propanol, is a secondary alcohol with the chemical formula of C3H7OH and molecular mass of 60.09502 kg/kmol. It is widely used as a solvent in many industries and finds applications in food processing, coatings to reduce flammability, as thinner and additive in paints, as well as disinfectants such as alcohol wipes in household products and medical applications.27 With water, IPA forms an azeotrope of 87.4 wt % alcohol, having a boiling point of 80.3 °C at atmospheric pressure.28 IPA is manufactured by two major commercial processes: indirect and direct hydration of propylene, of which the latter is common due to less corrosion in unit operations. Direct hydration of propylene is an exothermic, reversible reaction carried out with an acid catalyst, which could be cation-exchange resins such as molybdophosphoric acid, titanium and zinc oxides.28 The main reaction is CH3CH = CH 2 + H 2O ⇌ (CH3)2 CHOH

molar flow (kmol/h)

temp (°C)

pressure (kPa)

−47.6

101.325

25 83.0

101.325 105

The main side product, diisopropyl ether (DIPE) forms after the etherification reaction: 2(CH3)2 CHOH ⇌ H 2O + [(CH3)2 CH]2 O

(10)

The most popular propylene direct hydration is the trickled bed process. This mixed vapor−liquid-phase process uses strongly acidic proton-exchange resin as the catalyst.28 It has good process performance, and sufficient process data are available in the open literature. Hence, it is selected as the original/existing process for the case study. Simulation of the IPA process is conducted using Aspen Plus 8.4. The process flowsheet with selected simulation results is shown in Figure 6 while process data of feedstock, product, and entrainer are given in Table 7. As can be seen, fresh propylene (at −47.6 °C and 101.325 kPa)29 and recycled propylene are mixed in a molar ratio of 4.65:1, pressurized, and preheated before feeding into the trickle bed reactor. Meanwhile, another reactant water (fresh/recycled = 1:7.63) is pressurized to 7500 kPa and also fed into the reactor. The product stream from the bottom of the reactor is depressurized to 902 kPa and sent to the lights column to strip the dissolved propylene and propane. The gaseous mixture leaving the top of the lights column is transferred to the propane column to remove propane (in order to avoid its accumulation in the process) from

(9) H

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Industrial & Engineering Chemistry Research propylene as the latter is recycled to the reactor. Note that exchanger E1 for preheating fresh propylene produces chilled water, which can be used in the condenser of the propane column. This is shown in Figure 6 by the dashed line. Alternately, fresh propylene can be directly used as the coolant in the condenser of the propane column. Heavier components from the bottoms of the propane column are depressurized to 120 kPa, mixed with recycled IPA and then sent to the preconcentrator, where more than 90% of water is removed in the bottoms stream; this water stream is recycled to the reactor (Figure 6). The distillate stream of the preconcentrator contains mainly water, IPA, and DIPE. In this mixture, the DIPE mole fraction is only 0.043, and its boiling point is lower than that of water and IPA; therefore, it is removed in the ether column before separation of IPA and water. DIPE leaves the process from the top of the ether column, while the IPA and water mixture is collected from the bottom and fed to the extractive distillation (ED) column. As mentioned above, IPA and water form an azeotrope; to break this azeotrope, dimethyl sulfoxide (DMSO) is introduced as the entrainer.30 After ED, IPA with a molar purity of 0.9999 is collected from the column top as the main product, whereas the bottoms stream is sent to the regenerator to recover the entrainer. DMSO is concentrated to 0.99999 (mole fraction) in the bottoms of the regenerator. After mixing with makeup solvent (in very small quantity, not shown in Figure 6), the entrainer is sent back to the ED column. In Figure 6, E2 is a single heat exchanger with the cold stream coming from P2 and the hot stream is the bottoms of the regenerator. To verify the simulation of the original process, several key results are compared with those in the literature. The water to propylene ratio at the inlet of the reactor in the current simulation is 12 compared to the range of 12−15 stated by Xu et al.31 Conversion of propylene is 0.814 compared to ⩾0.75.31 Selectivity of IPA is 0.962 versus ⩾0.93.31 For the ED column, reboiler duty is 14.66 kW/kmol of column inlet stream (with molar composition: IPA = 0.55 and water = 0.45), while Luyben and Chien30 gave a value of 14.99 kW (for inlet composition: IPA = 0.5 and water = 0.5). Molar composition of the ED bottoms stream is water = 0.328 and DMSO = 0.672, which is same as that in Luyben and Chien.30 Note that solvent loss (DMSO loss) in the water purge is very small (2.6 × 10−7 kmol/h), and hence makeup solvent is not included in Table 7. This loss is similar to 1.1 × 10−8 kmol/h in Arifin and Chien,32 where 100 kmol/h of IPA and water mixture (mole fraction of 0.5 IPA and 0.5 water) is separated through ED and regenerator using DMSO as the entrainer. Thus, the present simulation results are comparable to those in the literature. Following the methodology in the previous section, the next step is to determine the retrofit target and then identify the contributing units. The retrofit target chosen for the case study is to minimize annual manufacturing cost per unit of product ($/kg). Based on this retrofit target and features of the process, “conversion of reactants”, “selectivity of products”, “steam cost”, “cooling cost,” and “electricity cost” are selected as the metrics for evaluating the original process, and their values are listed in Tables 8 and 9. In Table 9, the condenser duty for the propane column is 2105.8 kW, part of which is provided by the chilled water from E1, resulting in the duty of 1362.7 kW to be supplied by an external cold utility. Utilities used are steam (LP: 3.5 bar and HP: 31.0 bar), coolants (cooling water and chilled water) and electricity; their unit prices are $16.9/1000 kg (LP) and $22.3/1000 kg (HP), $0.02/1000 kg

Table 8. Evaluation of Contributing Units: IPA Reactor chemical

conversion

propylene water IPA DIPE

0.814 0.065

selectivity

0.962 0.038

(cooling water) and $4/GJ (chilled water), and $0.06/kWh.21,26 Note that the unit price of steam is the average of that in Seider et al.21 and Turton et al.26 From eq 3 and Table 9, it is clear that the cost of cooling and electricity is much lower than that of steam; therefore, only unit operations with significant steam consumption need to be considered. From eq 1 and eq 2, the contributing units identified are lights column, preconcentrator, ED column, and regenerator. As can be seen from the reactor performance data (Table 8), conversion per pass of water is merely 0.065 (compared to 0.814 for propylene), and hence over 90 mol % of the reactor outlet stream is unreacted water, leading to high separation cost in the subsequent process (e.g., high steam cost in the preconcentrator). Owing to this link between low conversion in the reactor and high utility cost in the separation units, the reactor is also selected as a contributing unit. 3.2. Generation of an Improved Solution without Capital Investment. Following the methodology, the objective function is to minimize manufacturing cost per unit product ($/kg, eq 4) and the decision variables are related to the contributing units identified, namely, reactor, lights column, preconcentrator, ED column, and regenerator. After analyzing their degrees of freedom and the sensitivity of the objective function, reflux ratio of the preconcentrator (Rpre), the reflux ratio of the regenerator (Rreg), and DMSO circulation rate (FDMSO) are selected as the decision variables. Some operating conditions of the contributing units such as reflux ratios of lights and ED columns are not selected because they are used to fulfill the process requirements (e.g., the former is used to limit the concentration of propylene in the bottom outlet, and the latter is used to achieve the IPA purity in the top outlet stream). Constraints in the optimization are based on process knowledge and retrofitting considerations. For example, bounds for decision variables are specified as

R pre ≥ 0

(11)

RReg ≥ 0

(12)

FDMSO ≤ 605 kmol/h

(13)

Equations 11 and 12 ensure a positive reflux ratio. Equation 13 gives the upper bound of DMSO circulation rate, which should not exceed 10% of its original value.36 Besides FDMSO, the flow rate of both top and bottom outlet streams from preconcentrator, ED column, and regenerator should also follow similar constraints as eq 13. Equality constraints which include mass and energy balances for each unit operation in the process are taken care by Aspen Plus simulation, which means these are satisfied once the process simulation converges. The built-in optimizer in Aspen Plus gave the following optimal values: Rpre changes from 1.15 (initial guess) to 0.73, Rreg decreases from 0.45 (initial guess) to 0.39, and FDMSO increases from 515 kmol/h (initial guess) to 545 kmol/h. A cost estimation of both original and the optimized processes I

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Industrial & Engineering Chemistry Research Table 9. Evaluation of Contributing Units: Unit Operations cost of steam

cost of cooling

unit operation

reboiler duty (kW)

steam type

steam cost ($/h)

propane column lights column pre-concentrator ED column regenerator ether column

1624.6 8175.8 11156.9 9212.9 5906.4 1744.6

LP LP LP LP HP LP

46.1 190.1 359.3 256.0 257.7 54.0

condenser duty (kW)

cooling agent

cooling cost ($/h)

1332.1 8280.4 15303.6 7573.3 4635.1 1444.2 cost of electricity

chilled water cooling water cooling water cooling water cooling water cooling water

19.6 13.5 26.1 13.0 7.9 2.5

unit operation

power consumption (kW)

cost of electricity ($/h)

P1 P2 P3 P4 P5

10.5 120.7 0.24 318.5 0.04

0.6 7.3 0.01 19.1 0.002

stoichiometric ratio. According to Heuristic 1, exploration of root integration will start from integrating reactor with its downstream separation units. In the proposed case study, only distillation columns are involved in separation; therefore, the potential integrated unit is the reactive distillation column. As an RD column is currently used in chemical process industries,33 heuristic 3 is satisfied too. Following the methodology, guidelines for applicability and potential of RD should be considered. In Table 11, the preconcentrator, used for removing the excess water, is the most energy intensive unit (contributing unit). According to simulation results, the reaction temperature is 129 to 135 °C (comparable to the temperature of the IPA reactor28), while the pressure is 1751.4 to 1752.9 kPa (lower than that of the IPA reactor shown in Figure 6). Thus, most of the guidelines for applicability and potential of RD are satisfied; the requirement on relative volatility of components will be later verified when the root integration is analyzed. To develop the initial reaction−separation root integration, the most relevant separation unit should be selected. As mentioned above, the preconcentrator is the most relevant unit for integration; hence, root R → PRC is first considered and this integrated design is shown below:

Table 10. Cost Estimation of both Original and Optimized Processes item

original

cost of utilities ($/yr) cost of operations ($/yr) cost of maintenance ($/yr) cost of operating overhead ($/yr) property taxes and insurance ($/yr) cost of depreciation ($/yr) general expense ($/yr) Summary total manufacturing cost ($/yr) annual IPA production (kg/yr) manufacturing cost per unit product ($/kg)

10,182,496 3,100,937 1,426,194 844,810 310,042 1,240,169 1,204,961

9,224,027 3,100,937 1,426,194 844,810 310,042 1,240,169 1,204,961

optimized

18,309,609 168,207,880 0.109

17,351,140 168,207,880 0.103

is conducted following the steps provided by Seider et al.,21 and the results are shown in Table 10. As can be seen, the objective function value decreases from $0.109/kg to $0.103/kg (by 5.5%), mainly benefitting from less utility consumption (dropped from $10,182,496/yr to $9,224,027/yr). This suggests the existence of an improved solution without capital investment, which will be used as the benchmark to evaluate the integrated retrofit solutions. 3.3. Generation of Integrated Retrofit Solutions. From the previous section, it can be seen that, although there is one retrofit solution without capital investment, the improvement is limited with manufacturing cost per unit product decreasing by only 5.5%. Therefore, it is necessary to explore integrated retrofit solutions, which may bring larger process improvements. From section 3.1, the identified contributing units are listed in Table 11 for exploring possible integrations.

{(R → PRC): PC‐RD‐LC‐ED‐RE}

In the above process notation, PRC is the preconcentrator, PC is the propane column, LC represents the lights column, and RE is the regenerator. This integration does not cross any reactor, and therefore heuristic 2 is fulfilled. Note the ether column is not included; this is because the side reaction is also controlled by the reaction equilibrium, and simulation suggests that DIPE generation is very little when IPA is continuously removed from the system. Similarly, two more roots, which also satisfy heuristic 2, are developed and their integrated retrofit designs are

Table 11. Summary of Contributing Units in the Case Study unit operation

metric

reactor lights column pre-concentrator ED column regenerator

water conversion steam cost steam cost steam cost steam cost

value 0.065 231.6 316.1 261.0 264.3

{(R → LC): RD‐PRC‐ED‐RE} $/h $/h $/h $/h

{(R → ED&RE): PC‐RD}

In the first design, {(R → LC): RD-PRC-ED-RE} assumes that the conversion of propylene is 100%; as a result, the lights and propane columns are no longer required. The inert component (propane) in the propylene feed is removed from the top of RD column, while the water−IPA azeotropic mixture is removed from the bottom. The boiling points of key components (Table 12)

The reactor is identified as one of the contributing units, the main reaction (eq 10) is equilibrium controlled, and the water to propylene ratio at the reactor inlet is 12 times the J

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Industrial & Engineering Chemistry Research Table 12. Boiling Point of Key Components (at 1 atm)28 component

boiling point (K)

propane propylene IPA water

231.15 225.43 355.65 373.13

The physical explanation of this is that, when less water is fed into the RD, water content in the RD outlet stream containing IPA is at a manageable level, this RD outlet stream could be directly fed to the ED column without the need for preconcentrator and lights column. Note that the unreacted propane leaves in the overheads stream of RD, and will not affect the performance of ED. Simulation of this modified process is successful indicating the feasibility of the integrated retrofit solution. Now, a comparison of {(R → LC&PRC): RD-ED-RE} with its root {(R → LC): RD-PRC-ED-RE} shows that the former contains fewer unit operations than the latter, which is achieved by reducing the flow rate of water fed to the RD (operating parameter). Since less water is fed into the RD for the former, the overall utility consumption of {(R → LC&PRC): RD-ED-RE} is expected to be lower than that of {(R → LC): RD-PRC-ED-RE}. Therefore, the latter is dropped. Similarly, comparing {(R → PRC): PC-RD-LC-ED-RE} with {(R → LC&PRC): RD-EDRE}, the latter can be achieved when propylene conversion is 100%, which has been validated through simulation. Therefore, {(R → PRC): PC-RD-LC-ED-RE} can also be eliminated. On the other hand, the combination of {(R → PRC): PC-RD-LCED-RE} and {(R → ED&RE): PC-RD} could not generate any new integrated solution, and so the final level two integrated retrofit design is {(R → LC&PRC): RD-ED-RE}. Next, level three integration, which combines level two integration with a separate root retrofit, should be considered. According to the methodology, any level two retrofit solution will not be integrated with the root retrofit it was derived from. As such, the only candidate is {(R → LC&PRC): RD-ED-RE} integrated with {(R → ED&RE): PC-RD}. However, recall that the former requires excess water in the RD while the latter needs excess propylene, and so it is inappropriate to combine them. Hence, there is no level three integration possible. Up to now, all the possible retrofit designs are explored. After screening by heuristics, process knowledge and simulation results, only two remain for final cost estimation and optimization. These two retrofit designs are {(R → ED&RE): PC-RD} (root integration) and {(R → LC&PRC): RD-ED-RE} (level two integration). Table 13 summarizes the number of theoretical

are used to assess the feasibility of this retrofit solution. As can be seen, at 1 atm the proposed product (IPA−water mixture) has a boiling point between 355.65 and 373.13 K, while the main remaining component (propane) has a boiling point of 231.15 K. With such significant difference, even when the pressure increases to 1770 kPa, the remaining component will not affect the proposed product. Simulation results also confirm this. The second integrated retrofit design, {(R → ED&RE): PC-RD} requires full consumption of water in the RD, which means propylene should be fed in excess. Since there is no water left unreacted, the ED and regenerator columns are no longer needed. Normal boiling points of both the proposed product (IPA) and the remaining components (propylene and propane) indicate this RD-based retrofit solution is feasible. In fact, this type of design has already been studied.31,34 Note that, according to the proposed methodology, roots R → ED and R → RE should be considered separately. However, in this case study, the regenerator is dependent on the ED, which means that regenerator is not needed if ED is not required. Therefore, the root is marked as R → ED&RE rather than R → ED. On the other hand, R → RE also leads to the same root, namely, R → ED&RE and the same integrated retrofit design. All the root integrations involving the reactor have been considered. The next step is to investigate root integrations between the remaining contributing units. The lights column is operating at a much higher pressure (902 kPa) than other contributing distillation columns (120−150 kPa), and is therefore unlikely to be integrated with them. The remaining contributing units are preconcentrator, ED, and regenerator columns. Liang et al.35 have combined the preconcentrator and regenerator into a dividing wall column (DWC), which is then followed by an ED column. However, DIPE is not considered in their system; if DIPE is involved and not removed before azeotropic distillation, it can form an azeotropic mixture with DMSO and might affect the purity of the IPA product collected from the top of ED column. Therefore, root PRC → RE is not considered. Thus, all possible root integrations have been analyzed, and the feasible options are {(R → PRC): PC-RD-LC-ED-RE}, {(R → LC): RD-PRC-ED-RE} and {(R → ED&RE): PC-RD}. Thus, heuristics, guidelines for using integrated units and process understanding, provide fast screening, removing infeasible roots in the early stage. The next step is to explore a combination of two root integrations among the feasible ones. Level two integration is then considered. {(R → LC): RD-PRC-ED-RE} and {(R → ED&RE): PC-RD} are unlikely to be combined, because the former requires excess water feed into the RD while the latter calls for excess propylene as the water will be fully consumed in the RD. {(R → PRC): PC-RDLC-ED-RE} and {(R → LC): RD-PRC-ED-RE} are possible to be combined, resulting in a level two integrated retrofit solution as below:

Table 13. Theoretical Retrofit Designs and Screening

{(R → LC&PRC): RD‐ED‐RE} K

item

number

theoretical number of root retrofit designs number eliminated based on heuristics number eliminated based on process knowledge number eliminated based on process simulation duplicated designs number of root designs remaining theoretical number of level two retrofit designs number eliminated based on heuristics number eliminated based on process knowledge number eliminated based on process simulation duplicated design number of level two retrofit designs remaining theoretical number of level three retrofit designs number eliminated based on heuristics number of level three retrofit designs remaining number of total retrofit designs remaining root retrofit designs eliminated due to level two integration level two retrofit designs eliminated due to level three integration number of retrofit designs remaining for optimization and cost estimation

10 1 5 1 0 3 3 1 0 0 1 1 1 1 0 3+1+0=4 2 0 4−2=2

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DIPE. The lights mixture (mostly propylene and propane) is rectified into the top stream. After depressurizing to 905 kPa, it is split into two streams (split ratio of 1.49:1), which are respectively sent to propane column-1 and propane column-2 for recovering and recycling unreacted propylene. Two propane columns are required in the retrofit scheme {(R → ED&RE): PC-RD} for the following reasons. First, the original propane column is proposed to be reused (to minimize capital investment for retrofitting). Second, propylene is in excess in the retrofitted process, resulting in a higher flow rate of the propylene/propane mixture to the propane column (175.5 kmol/h compare to 112.2 kmol/h in the original process). Third, the original propane column will not be able to handle this increased flow rate. Therefore, a new propane column (propane column-2) is added while the original propane column is labeled as propane column-1 in the retrofitted process (Figure 7). The retrofitted process in Figure 7 is optimized to minimize the manufacturing cost per unit product ($/kg); and the objective function should include the amortized capital investment ($/yr) as in eq 7. Decision variables for optimization are selected after analyzing the degrees of freedom for the relevant unit operations as well as their effect on the objective function. The chosen decision variables are feed flow rate of water (f water), reflux ratio of RD (RRD), distillate rate of propane column-1

solutions and candidates remaining after each step of screening. Note that the “root retrofit designs eliminated due to level two integration” refers to removal of {(R → PRC): PC-RD-LC-ED-RE} and {(R → LC): RD-PRC-ED-RE} from the solution pool due to the level two integrated retrofit solution {(R → LC&PRC): RD-ED-RE}. 3.4. Optimization and Cost Estimation of Promising Integrated Solutions. Two promising integrated retrofit designs, namely, {(R → ED&RE): PC-RD} and {(R → LC&PRC): RD-ED-RE} are remaining. In the last step, optimization and cost estimation are conducted for choosing one of these designs. The complete process flow diagram of {(R → ED&RE): PC-RD} is shown in Figure 7. Similar to the original process, fresh propylene (400 kmol/h) and recycled propylene are mixed and pressurized to 1755 kPa, and then fed to stage 5 of the RD column; water (380.3 kmol/h) is also pressurized to 1750 kPa, and fed to stage 3 of this column. The water to propylene feed ratio in the RD column is 1:1.39. Reactions occur on stages 3 to 5 (reaction zone), where the temperature is between 126 and 133 °C to minimize generation of DIPE. IPA with a mole fraction of 0.9999 is collected from the bottom of the RD column. Note that the IPA production rate increases from 349.9 kmol/h to 379.5 kmol/h, mainly because RD-based retrofit design minimizes generation of

Figure 7. RD-based integrated retrofit design of IPA process using propylene in excess; values shown correspond to the optimized case. L

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Figure 8. RD-based integrated retrofit design of IPA process using water in excess; values shown correspond to the optimized case.

optimizer in Aspen Plus gave the following optimal values of decision variables: RD reflux ratio = 29.9, distillate rate of propane column-1 = 62.1 kmol/h, distillate rate of propane column-2 = 92.5 kmol/h, and split ratio =1.49:1, and the value of the objective function is 0.096 $/kg. The complete process flow diagram of {(R → LC&PRC): RD-ED-RE} is shown in Figure 8. Fresh propylene (400 kmol/h) is pressurized to 1815 kPa, and after producing chilled water in E1, the stream temperature rises to 12.8 °C; then it is heated to 48.9 °C in heat exchanger E2, and fed to stage 4 of the RD column. Water (819.4 kmol/h) is also pressurized to 1804 kPa, and fed to stage 2 of this column. The water to propylene feed ratio in the RD column is 2.05:1. Reactions occur on stages 2 to 4 (reaction zone). The lights mixture with a flow rate of 30.0 kmol/h is vented from the top; it has about 30 mol % propylene (i.e., 9 kmol/h) and the rest is mostly propane. Note that the propylene vented from the RD column is only 2% of fresh propylene to the process, and therefore there is no need to recover and recycle it, thus avoiding the expensive propylene− propane splitter. The bottom stream of the RD is depressurized to 135 kPa and then split into two streams (split ratio = 2.57:1) for respectively sending to ED column-1 and ED column-2 for removing water from the azeotropic mixture. Similar to the reasoning for two propane columns in {(R → ED&RE): PC-RD}, two ED columns are required because water is in excess in the retrofitted process, resulting in a higher flow rate of the azeotropic mixture. Therefore, a new ED column (ED column-2) is added while the original ED column is labeled as ED column-1 in the retrofitted process (Figure 8). IPA with a mole fraction of 0.9999 (and total production rate of 370.3 kmol/h) is collected from the top of both ED column-1 and ED column-2, while rich solvent from the bottom of these columns is sent to their respective regenerators (regenerator-1 and regenerator-2), where the lean solvent (DMSO), after cooling, is returned to

(DISpro‑1), distillate rate of propane column-2 (DISpro‑2), and split ratio (SP). Note that the reflux ratio of each propane column is used to achieve the purity of the top stream: 0.95 propylene and 0.05 propane, and hence it is not selected as a decision variable. Bounds on decision variables and process constraints are RRD > 0

(14)

DISPro ‐ 1 ≤ 1.1 × DIS 0Pro

(15)

0 ≤ DISPro ‐ 2 ≤ 175.5

(16)

0 < SP < 1

(17)

fPro ‐ 1 ≤ 1.1 × f 0Pro

(18)

In the above equations, f Pro‑1 and are the inlet flow rate of propane column-1 and of the propane column in the original process, respectively. Equation 14 ensures a positive value for the reflux ratio of RD, eq 15 provides the upper bound of the distillate rate for propane column-1, which should not exceed 10% of its original value (DIS0Pro).36 Equation 16 specifies the upper bound on distillate rate for propane column-2; as it is a new unit operation, this bound should be reasonably large. Here, it is based on the inlet flow rate before splitting. Equation 17 refers to the split ratio, which should be between 0 and 1. Equation 18 is an extra constraint for the inlet follow rate of propane column-1. Equations 15 and 18 are related to the reused unit operation (distillation column) from the original process. When the original distillation column needs to be reused, it should not exceed the design margin of the original unit (usually 10%).36 Finally, similar to the optimization for solution without capital investment, equality constraints, which include mass and energy balances for each unit operation in the process, are taken care by Aspen Plus simulation and its convergence. The built-in f 0Pro

M

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explanation for higher capital investment is that, when propylene is fed in excess, unreacted propylene has to be evaporated and recycled from the top, resulting in a large diameter and high reboiler duty for the RD column. Moreover, another propylene/propane column, which is large and expensive, is required for {(R → ED&RE): PC-RD} using propylene in excess. On the other hand, {(R → LC&PRC): RD-ED-RE} using water in excess requires a smaller RD column with lower reboiler duty. Therefore, although two extra columns are required for azeotropic separation of IPA and the water mixture, the manufacturing cost of {(R → LC&PRC): RD-ED-RE} is lower than that of {(R → ED&RE): PC-RD}. Hence, the former integrated solution involving an RD column with excess water is recommended for retrofitting the original IPA process. Table 15 summarizes the status of unit operations for both {(R → ED&RE): PC-RD} and {(R → LC&PRC):

the respective ED column. Note that regenerator-1 is the original regenerator, while regenerator-2 is the newly added unit. Water from the distillate streams of both regenerators is not recycled because it contains a trace amount of DMSO that could affect the catalyst in the RD column. Similarly, the retrofitted process {(R → LC&PRC): RD-EDRE} is optimized to minimize the manufacturing cost per unit product ($/kg, eq 7). As before, decision variables for optimization are selected after analyzing the degrees of freedom for the relevant unit operations as well as their effect on the objective function. The chosen decision variables are reflux ratio of RD (RRD′), split ratio (SP′), distillate rate of regenerator-1 (DISreg‑1), and distillate rate of regenerator-2 (DISreg‑2). Constraints are similar to those for the process {(R → ED&RE): PC-RD}. The built-in optimizer in Aspen Plus gave the following optimal values for the decision variables: RD reflux ratio = 30.1, split ratio = 2.57:1, distillate rate of regenerator-1 = 322.7 kmol/h, and distillate rate of regenerator-2 = 125.5 kmol/h, and the value of the objective function is 0.082 $/kg. Note that in {(R → ED&RE): PC-RD}, less fresh water is required as propylene is fed in excess (6845.4 kg/h compared to 11775.6 kg/h in the original process). Taking the cost of process water as 0.064 $/1000 kg,26 the total manufacturing cost decreases by 2524 $/yr due to less fresh water being required. Similarly, in {(R → LC&PRC): RD-ED-RE}, extra fresh water is required (14749.2 kg/h compared to 11775.6 kg/h in the original process), which increases the total manufacturing cost by 1594 $/yr. Both these values are included in the cost estimation (Table 14).

Table 15. Status of the Columns for the RD-Based IPA Processes retrofitted process: {(R→ED&RE): PC-RD} RD column propane column-1 propane column-2

{(R→ED&RE): PC-RD}

capital investment for added unit 14,959,577 operations ($) cost of utilities ($/yr) 8,652,000 cost of operations ($/yr) 1,362,987 cost of maintenance ($/yr) 1,501,166 cost of operating overhead ($/yr) 472,336 property taxes and insurance ($/yr) 326,340 cost of depreciation ($/yr) 1,305,362 general expense ($/yr) 1,306,716 cost of water reduced/increased ($/yr) −2,524 Summary total manufacturing cost ($/yr) 14,924,384 amortized capital investment ($/yr) 2,558,088 annual IPA production (kg/yr) 182,438,598 manufacturing cost per unit product 0.096 ($/kg)

new existing unit new

retrofitted process: {(R→LC&PRC): RD-ED-RE} RD column ED column-1 and regenerator-1 ED column-2 and regenerator-2

column status new existing units new

RD-ED-RE} processes. As can be seen, the number of columns in {(R → ED&RE): PC-RD} is only three compared to seven in the original process in Figure 8 (excluding pumps and heat exchangers); on the other hand, the number of columns in {(R → LC&PRC): RD-ED-RE} is five. In both retrofitted processes, existing LC, PC, and ether column are not used. Hence, plot space should not be a limitation in retrofitting the existing IPA process; however, this may require shutdown of the existing process for several months, and so requires advanced planning and execution. 3.6. Control of RD-Based IPA Processes. The RD column is involved in both the base case and retrofit designs. Compared to the conventional unit operations, it is more complicated and difficult to control. Even then, it has found a number of industrial applications. Further, there have been many studies on design and control of RD. See the book by Luyben and Yu33 for details on these. In particular, Wang and Wong34 investigated dynamics and control of a RD column for IPA production, through rigorous simulations. They discussed multiplicities and nonlinearities involved, and then used variable transformation to overcome the strong nonlinearity in the temperature−composition loops of the RD column for IPA production. However, Wang and Wong34 considered only the RD column with pure propylene feed, thus not requiring the propylene−propane splitter. Hence, their study needs to be extended to investigate dynamics and control of realistic and complete RD-based IPA processes. In general, not all steady-state designs lead to controllable processes and so control studies should be performed on one or more promising retrofit solution(s) before implementation. Usually, these are performed after steady-state design and analysis. Accordingly, the present work is focused on steady state design. Further studies are required to include control strategy into the proposed retrofitting methodology.

Table 14. Cost Estimation of the RD-Based IPA Processes item

column status

{(R→LC&PRC): RD-ED-RE} 7,303,771 7,529,200 2,243,867 1,008,595 602,182 219,260 877,039 1,275,213 1,594 13,758,544 1,248,945 178,040,240 0.084

3.5. Solution Comparison. Cost estimation of the two integrated retrofit solutions, after optimization, is summarized in Table 14, where {(R → ED&RE): PC-RD} uses propylene in excess and {(R → LC&PRC): RD-ED-RE} requires water in excess. Both are better than the improved solution without capital investment (with a manufacturing cost of 0.103 $/kg, which is 5.5% lower than that of the original process in Table 10). The integrated solution, {(R → ED&RE): PC-RD} achieves 11.9% savings in manufacturing cost of the original process, whereas the integrated process {(R → LC&PRC): RD-ED-RE} achieves 22.9% savings. The capital investment for added unit operations and the cost of utilities makes the former more costly than the latter (Table 14). One possible N

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4. CONCLUSIONS In this study, a heuristic-based, systematic method is developed for process retrofitting via process intensification/integration. It consists of four main steps: (1) base case analysis, retrofit target, and contributing unit operations specification; (2) generation of improved solution without capital investment; (3) generation of integrated solutions; and (4) optimization and cost estimation of integrated solutions. This methodology is focused to achieve integrated retrofit solutions. It is illustrated for retrofitting a conventional IPA process. It leads to two integrated retrofit solutions, namely, {(R → ED&RE): PC-RD} and {(R → LC&PRC): RD-ED-RE} involving a RD column with propylene in excess and a RD column with water in excess, respectively. The former reduces the number of columns from 7 in the original process to 3 and also the manufacturing cost from 0.109 $/kg to 0.096 $/kg, whereas the latter has 5 columns and a manufacturing cost of 0.084 $/kg. Hence, the integrated retrofit solution {(R → LC&PRC): RD-ED-RE} is the recommended design.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (65) 6516 2187. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.W.N. thanks the National University of Singapore for supporting his doctoral research. Both authors acknowledge Yonghui Lin for the initial study on reactive distillation options for the IPA process.

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REFERENCES

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DOI: 10.1021/acs.iecr.5b02707 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX