Optimal Design of Synthesis Gas Production Process with Recycled

Dec 12, 2007 - The strategy is based on the integrated superstructure optimization that assists the formulation of the optimal process design problem ...
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Ind. Eng. Chem. Res. 2008, 47, 323-331

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PROCESS DESIGN AND CONTROL Optimal Design of Synthesis Gas Production Process with Recycled Carbon Dioxide Utilization Seungjune Choi, Jehun Park, Chonghun Han,* and En Sup Yoon Department of Chemical and Biological Engineering, Seoul National UniVersity, 151-744, Seoul, Korea

Many countries are trying to reduce their primary energy demand and greenhouse gas (such as carbon dioxide (CO2)) emissions. There has been much research on CO2 utilization to reduce CO2 emission and increase economic efficiency in industries. In this paper, an integrated optimization strategy of an industrial synthesis gas production plant with additional CO2 recycling processes using carbon dioxide as a reactant is presented. The CO2 recycling process allows three different kinds of alternative synthesis gas reaction processes in parallel: steam reforming, dry methane reforming, and reverse water-gas shift reaction. The strategy is based on the integrated superstructure optimization that assists the formulation of the optimal process design problem such that mixed integer programming can be derived. The mathematical programming problem which has flexibility in selecting different synthesis gas reaction processes is used to find the optimal configuration of the process. The industrial synthesis gas plant case studies have been applied to present the optimization strategy. With the optimum configuration, the annualized profit increased by 14% and CO2 emission decreased by 31% from the base case to the optimal structure. Three other extensions to the optimal design were evaluated for the case studies. 1. Introduction Scientists have indicated that the global climate change is due to the greenhouse gas effect in which man-made greenhouse gases alter the amount of thermal energy stored in the Earth’s atmosphere, thereby increasing atmospheric temperature. The greenhouse gas generated in the most significant quantities is carbon dioxide (CO2). The primary source of man-made CO2 is combustion of fossil fuels. Stabilizing the concentration of atmospheric CO2 requires a variety of actions including a reduction of CO2 emissions. Utilization of carbon dioxide has become not only one of the global issues for controlling atmospheric CO2 concentrations but also an important source of carbon products. Furthermore, CO2 utilization contributes to enhancing sustainability since various chemicals, materials, and fuels can be produced using CO2 which should be a sustainable way in the long term when renewable sources of energy are used as energy input.1 There are many articles on the range of carbon dioxide reactions in connection to its use as a raw material for commercially important production. Song et al. proposed strategies that include increasing the commercial applications of products from CO2.2 New reaction pathways could replace hazardous substances with carbon dioxide, e.g., carbon dioxide used in the production of dimethyl carbonate rather than the conventional route with phosgene as an intermediate. It can be used as the whole molecule in the reactions, and it can be used as a carbon source or as an oxygen source, e.g., in the dehydrogenation of ethylbenzene to styrene.3 For example, commercially important products can be obtained from hydrogenation and hydrolysis of carbon dioxide, and these include methanol, ethanol, methane, ethylene, formic acid, acetic acid, * To whom correspondence should be addressed. Tel.: +82-2-8741581. Fax: +82-2-872-1581. E-mail: [email protected].

adipic acid, and graphite. Also, carbon dioxide can be used in the production of methyl amines and as a building block for isocynates supplanting phosgene. One of the ways to produce useful chemicals using CO2 as a reactant or feedstock is the synthesis gas production. Synthesis gas is a mixture of CO and H2 and one of the most important feedstocks in the chemical industry. From synthesis gas, mixtures having a wide variety of products or intermediates such as organic acid, phosgene, polycarbonates, and agricultural chemicals can be manufactured.4-6 In most cases the manufacturing cost of these chemicals is directly determined by the production cost of synthesis gas.7 Industrially, the most common method of producing synthesis gas is steam reforming of natural gas. Although this process is the most widely used process to produce synthesis gas (mostly for methanol production), it is very expensive because it requires a large amount of heat. So alternative routes to producing synthesis gas have been studied and include dry methane reforming (DMR), autothermal reforming (ATR), partial oxidation of methane or reverse water-gas shift reaction (RWGS), etc.8-12 Combinations of these methods such as DMR and partial oxidation of methane or steam reforming have been reported. Among them, the alternative processes using carbon dioxide as raw material may have economic advantages through low material and operating costs and sustainable cost with reduction of greenhouse gas emissions. Although there have been numerous papers on the production of synthesis gas using carbon dioxide, little research has been done on the optimization of the synthesis gas processes. Asadullah et al. have proposed an experimental approach to process parameter optimization. They presented the optimum equivalence ratio and biomass feeding rate, to maximize product formation rate and carbon-based conversion to gas and minimize the formation of carbon which deposit on the catalyst surface, for producing synthesis gas from biomass.13 Marnasidou et al.

10.1021/ie070961n CCC: $40.75 © 2008 American Chemical Society Published on Web 12/12/2007

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have given qualitative reasoning for maximizing the conversion of methane to synthesis gas by partial oxidation in a spoutedbed reactor by optimizing the parameters such as the freeboard design, bed depth, and particle size.14 Larentis et al. have developed empirical models based on experiment and then optimized parameters of the synthesis gas process combined CO2 reforming and partial oxidation of natural gas.15 Mohanty used the objective functions given by Larentis et al. to optimize the process parameters using a real parameter nondominated sorting genetic algorithm.16 The objective of this paper is to optimize the existing synthesis gas production plant with additional synthesis gas processes using recycled carbon dioxide utilization. The existing process studied in this paper is a widely used synthesis gas production process using steam reforming of methane or butane. However, more detailed information about the existing process cannot be elucidated due to proprietary problems. The optimization problem is formulated using an integrated superstructure of the global synthesis gas production process which includes alternative synthesis gas reactions using carbon dioxide as a raw material. The integrated superstructure is the basis for the mathematical programming that can provide the best alternative process and operating conditions for maximizing the annual profit and at the same time satisfying process constraints. Application of the proposed methodology to the industrial synthesis gas process problem is introduced as a case study. 2. Design Problem and Optimization Approach In the industrial synthesis gas production process, transportation of pressurized CO of synthesis gas is limited because of its toxicity. This fact and the economical aspects have result in synthesis gas being produced on-site. The primary method of synthesis gas production has been steam reforming. It is very useful to use low-cost materials to produce synthesis gas, and most existing synthesis gas plants use a steam and natural gas or LPG mixture via the steam-reforming reaction at high temperature. But oil prices have soared and political pressure is growing to cut greenhouse gases, of which CO2 is by far the most prominent. An economically and environmentally efficient process design to produce synthesis gas can be determined with the appropriate network of existing steam-reforming process and alternative synthesis gas processes using the recycled carbon dioxide. Recycled CO2 can be separated from the outlet stream by a membrane or absorption system and reused as a reactant in the process. The synthesis gas process design problem statement can be described as follows. Given a set of raw materials, the reaction paths and kinetics, process specifications, and performance measure determine the optimal configuration of the synthesis gas production process using recycled carbon dioxide along with a set of design options with the integrated optimization approach. This article addresses the integrated superstructure optimization approaches that exploit network and combinations between existing process and alternative synthesis gas processes such that annual profit is maximized. We propose an optimization strategy as illustrated in Figure 1. This is adapted from the approaches of economic potential introduced by Douglas et al.17 Optimization strategy starts with a problem statement which considers objectives, constraints, and evaluation criteria. Once the problem statement has been properly defined, the next step is the generating of alternatives. In this step, a large number of alternative synthesis gas production processes using carbon dioxide are generated and evaluated. Engineering analysis (usually starting with mass and energy balances) is applied to each alternative to make predictions about the

Figure 1. Optimization strategy.

expected performance of the process. Economically feasible alternative processes are selected by this analysis and lead to the determination of the superstructure. For the definition of superstructure, the available process design information for synthesis gas production is incorporated into the integrated superstructure. Superstructure including existing plant process and selected alternatives can be determined by all feasible connections satisfying constraints. But taking into account all feasible connections leads to a mathematical problem of high complexity. To derive a sensible solution, it is necessary to find a way to eliminate all those connections that should not be realized as they are information which is not part of the mathematical model. Furthermore, it is possible to reduce the solution space estimating which connection will most probably not be part of an optimal solution. Subsequent to the steps above, the determined superstructure is formulated via mathematical optimization to search for the alternatives that advance objectives. As objective function, annual profits are considered. Therefore, fixed and variable, revenue, investment, and operating cost coefficients are determined for every process unit and the piping. The optimization problem is solved using mixed interprogramming with binary variables. It determines an optimal structure and process parameters satisfying constraints. The optimization strategy allows for the iteration to incorporate the insight gained by analyzing the solution during the optimization process. The identification of additional problem

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constraints, the revamping process units with positive impact on the system performance, or inclusion of rectified model is done during the iteration process. 3. Synthesis Gas from CO2 For the development of new processes, one of the preferred approaches is to search an alternative process for using CO2 from industrial effluent gases as a raw material for making chemicals that have a relatively large market or demand. The production of synthesis gas is a potential area for large-scale CO2 conversion and utilization. Many synthesis gas production processes using carbon dioxide have been studied and reported extensively in the literature. In this section, three kinds of industrial synthesis gas production processes using CO2 have been introduced. 3.1. Steam Reforming with Recycled CO2. Synthesis gas can be produced from a range of feedstocks (naphtha, coal, biomass, etc.). For heavy hydrocarbons as feedstock, partial oxidation with steam and oxygen is used for producing synthesis gas. When synthesis gas is produced from natural gas or light hydrocarbons, steam reforming is the most widely used process if a high H2/CO molar ratio is desired. CH4 + H2O f CO + 3H2

(steam reforming, +206 kJ/mol) (1)

CO + H2O f CO2 + H2 (water-gas shift, WGS, -41 kJ/mol) (2) CH4 + CO2 f 2CO + 2H2

(CO2 reforming, +247 kJ/mol) (3)

The process is typically operated at 15-30 bar and the outlet temperature of the reactor is typically 1123-1223 K with a H2O/ CH4 ratio higher than 0.4. The steam reforming is highly endothermic and it is carried out at high temperature and at pressures ranging between 15 and 30 bar.18 The composition of the gas at the reactor outlet reflects the equilibrium of reactions (1) and (2) above. In some cases, it is advantageous to add CO2 at the inlet of the steam reformer. In this case steam reforming can be blended with CO2 reforming to adjust H2/CO ratio. This can be done to save hydrocarbon feedstock such as methane or butane (LPG) and decrease the H2/CO ratio in the product gas. The CO2 coming out from the reformer is then recycled, and in some cases CO2 is also imported. CO2 then is supposed to react to form CO via the reverse water-gas shift reaction, and this is favored at low H2/CO ratios. However, low H2/CO ratios lead to high methane concentrations in the outlet. To compensate for this, a higher temperature can be used.19 3.2. Dry Methane Reforming. The major source for synthesis gas is from the steam-reforming reaction. This process has several limitations such as high-energy requirement, a high H2/CO product ratio, and poor selectivity for carbon monoxide. In addition, catalyst deactivation caused by carbon deposition is another problem. A proposed alternative to the steam-reforming process is dry methane reforming (DMR, i.e., in the absence of water). This reaction was first proposed by Fischer and Tropsch in 1928. This reaction has some advantages over steam reforming. It produces a synthesis gas with a lower H2/CO ratio and has higher energy efficiency in conversion to hydrocarbons. It is an effective way to utilize two greenhouse gases (CH4 and CO2).20

In the DMR, steam is completely substituted by CO2 as shown in eq 3, resulting in a H2/CO ratio of 0.42:1. Because dry methane reforming of synthesis gas has a lower H2/CO ratio compared to steam reforming, this reaction can be used to adjust the H2/CO ratio in the product stream of steam reforming. The reduction of by-products and the high concentration of CO reduce the specific requirement of utilities and offer substantial advantages with respect to selecting purification technologies.21 If the CO is used to produce phosgene as an intermediate for the polycarbonate production, high-purity carbon monoxide is required. CH4 and H2 will form CCl4 and HCl as impurities of phosgene, which would result in an impaired quality of the polycarbonate or in an additional phosgene purification process. Therefore, a process design with small amounts of CH4 impurities in the CO is advantageous. The dry methane reforming process meets this requirement without requiring any additional CH4 removal and moreover it can be industrially used to produce pure CO.22 3.3. Reverse Water-Gas Shift (RWGS) Process. The water-gas shift reaction has been intensively studied for the last several decades for H2 production from synthesis gas. In contrast, a reverse water-gas shift reaction has attracted little attention due to its poor demand. The water-gas shift (WGS) (eq 2), the reaction of carbon monoxide and water to produce carbon dioxide and hydrogen, is frequently used in industries in conjunction with the production of pure hydrogen via the steam reforming of hydrocarbons (eq 1). On the other hand, the reverse water-gas shift (RWGS) (eq 4), the reaction of carbon dioxide and hydrogen to produce carbon monoxide and water, is also a reaction of considerable industrial importance: CO2 + H2 f CO + H2O

(4)

The hydrogenation of CO2 usually reaches also the production of CH4 on catalysts at atmospheric pressure. Methanation of CO2 means conversion of CO2, to CO through the RWGS reaction, followed by the methanation of CO, viz. CO2 f CO f CH4

(5)

In eq 5, methane is less valuable than CO because methane is a stable molecule and its reactivity is very low, while CO is an important raw material for many chemicals such as formaldehyde, methanol, and acetic acid. Therefore, the RWGS reaction can open a route for the production of valuable CO from CO2, using hydrogen produced as a byproduct. For such purpose, RWGS processes for producing CO in the CO2 hydrogenation have been developed.23 The CO2 conversion to CO of the process is mainly affected by temperature and recycle ratio. As the temperature and recycle ratio increase, the CO2 conversion to CO increases due to the endothermic nature of reverse water-shift gas reaction.24 4. Generation of Superstructure To define an optimal configuration problem, it is necessary to devise an adequate superstructure of the integrated process with new synthesis gas production processes that is rich enough to account for all potential configurations and connectivity. A number of synthesis units and streams network are required to capture all the different design options that exist for a process. However, the representation potential of a superstructure strongly increases with the number of synthesis units that are included in the formulation and a superstructure which is too small will result in missed chances. On the other hand, the

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Figure 2. Superstructure of the process.

addition of too many units unnecessarily further increases the complexity of the mathematical formulation. It is difficult to determine the minimum number of process units to be included in a superstructure for representation of all feasible design options in the process synthesis problem. Therefore, to find a technical sensible solution, it is necessary to prohibit all those connections that must not be realized due to information that is not part of the mathematical model. It is possible to reduce the feasible solution set estimating which connection will most probably not be part of an optimal solution. This estimation is based on rules that compare the stream properties. The parameters that are used therefore are fixed very carefully. Of course, there is a risk to cut off a good solution if the parameters are too stringent. To avoid this, the values used in the estimation are related to problem-specific data as far as possible. The superstructure can identify its interconnections which have their corresponding feed compositions and flow rates that maximize the total annualized profit of the synthesis gas production process. A superstructure for the synthesis gas production process, containing feasible alternative design, is presented in a simplified form in Figure 2. In this superstructure, there are three types of commercially available synthesis gas production units considered: RWGS, DMR, and steam reforming with recycled carbon dioxide. To complete the superstructure, auxiliary units have been included. So it consists of a steam reformer reactor with recycled CO2, two DMR units, two RWGS reactors, a MDEA process for CO2 separation, a membrane, a PSA (pressure swing adsorption) unit, etc. The carbon dioxide from the each process is recycled to the feed stream. The refined synthesis gas is compressed and split into the carbon monoxide and hydrogen from the membrane and the hydrogen is refined in the PSA unit and partially recycled to the feed stream. 5. Mathematical Formulation Although the superstructure generated all feasible alternatives of the synthesis gas process, it does not address process-specific description. This section presents the formulation of a mathematical model of the synthesis process superstructure for the optimal process design. To develop a mathematical model,

continuous and binary variables are associated with the superstructure. The continuous variables represent the stream flow rates and capacities of the process units. Furthermore, the binary variables are assigned to the existence or the nonexistence of the corresponding process units at a given operating condition. Mathematical expressions for constraints and cost functions are generated by the operating condition and environment of the process along with production capacities, product demand, and raw material availability. The annualized maximum profit of the process is also obtained from the objective function. Hence, the proposed optimization model is formulated as shown below. 5.1. Objective Function. The maximization of annualized process profit is max profit ) PR - OC - RMC - CC - SC

(6)

As part of the objective function, the product revenue is defined as PR )

∑P

k k ProdFProd

(7)

k

k where PProd is the material cost of component k in the product k is the flow rate of component k in the product stream and FProd stream. Operation costs are defined as

OC )

∑

ut (OCfix i Wi + OCi Gi)

(8)

i∈utilities ut where OCfix i is the fixed operation cost at utility i, OCi is the cost associated with the use of utility i, Wi is the binary variable equal to 1 if equipment i is selected in the final structure, and Gi is the flow rate capacity of utility i. Raw material costs are defined as

RMC )

∑P

k k FeedFFeed

(9)

k

where PkFeed is the material cost of component k in the feed stream and FkFeed is the flow rate of component k in the feed stream.

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component at unit i, and νki is the stoichiometric coefficient component k at i.

Capital costs are defined as

∑

CC )

WiCCi

(10)

i∈equipment

(5) flow rate capacity of units

where CCi is the capital cost for equipment i. Sustainable costs are defined as the summation of induced carbon taxes such as SC )

∑C

-

2 (FCO gen

tax

2 FCO con )

(11)

Ctax

is the carbon tax per unit mole of CO2 flow and where CO2 2 (or F FCO gen con ) is the generated (or consumed) flow rate of CO2. 5.2. Constraints. (1) material balance: n

∑

n

Fkij -

j

∑F

k ji

∀ i ∈ N, ∀ k ∈ K

)0

(12)

j

where Fkij is the flow rate of component k at stream (i,j). (2) energy balance: NC

Qsi )

∑ ∆H

∀i∈N

k i

(13)

n

Gi )

∑∑F k

k ji

j

UP GLO i ‚Ei e Gi e Gi ‚Ei

(19)

where GLO (or GUP i i ) is the minimum or maximum capacity of unit i. The problem of optimizing a synthesis gas process given by the objective function and set of process constraints corresponds to a mathematical model for which structural parameter optimization can be performed in the process superstructure. To obtain the value of the parameters in the mathematical model, the following information is required: (a) Cost correlations for all units and utilities; (b) conversion factor of reactant materials; (c) enthalpy data for each stream; (d) data on the utility demands. This mathematical programming problem can be solved with standard mixed integer programming codes so as to provide the optimal configuration from a superstructure which has included feasible alternatives.

k

6. Industrial Application n

∑ F ‚(H

∆Hki )

k ij

n

k ij

- Hkref) -

j

∑ F ‚(H k ji

k ji

- Hkref)

(14)

j

This last equation can be simplified if we consider constant specific heats, n

∆Hki

)

∑ j

Fkij‚

(

)

Cpkij + Cpkref 2

‚(Tij - Tref) -

n

∑

(

Fkji‚

j

)

Cpkji + Cpkref 2

‚(Tji - Tref) (15)

where Qsi is the energy supply to unit i, Hkij is the enthalpy per unit mole of flow in stream (i,j), Cpkij is the specific heat component k at stream (i,j), and Tij is the temperature of stream (i,j). (3) stream splitter: n

k ) (i)‚ Fij(i)

∑F

k ji

(16)

j)1

where (i) is the splitting factor of unit i and j(i) is the key outlet stream form unit i. (4) synthesis gas production equipment model: conversion of key component n

∑F

Cik(i) ) xi‚

k ji

∀i∈N

(17)

j)1

Dki )

νki νk(i) i

‚Ck(i) i

(18)

where k(i) is the reactant component at unit i, Cki is the reacted component k moles at unit i, xi is the conversion of the key

6.1. Optimally Designed Synthesis Gas Process with the Existing Process. The industrial synthesis gas production plant with proposed recycled carbon dioxide processes was optimized to apply the method. Optimal solutions were determined on a 2.21 GHz AMD Athlon 64, 1 GB RAM PC using the BARON algorithm of GAMS.25 A carbon dioxide pipeline from the nearby plant can be connected to the conventional synthesis gas process of the OCTANOL(2-ethylhexanol) plant at the rate of 2 t/h, and the cost of excess high-purity carbon dioxide is essentially pumping cost which amounts to about $3/t since it is being vented to the atmosphere now. The OCTANOL plant is divided into the octanol production process and the synthesis gas process. The octanol process produces octanol, N-butanol, iso-butanol(2-ethyl-1-propanol) using propylene and synthesis gas. The synthesis gas process produces synthesis gas with the H2/CO2 ratio close to 1 using steam reforming of butane-LPG. The sales prices and the cost of raw materials are given in Table 1 along with carbon tax. To save the raw material cost and the operation cost, the synthesis gas process was expanded into a superstructure by integrating the supplementary carbon dioxide processes. They were set up to produce synthesis gas from the high-purity carbon dioxide which is generated as a byproduct from the other adjacent plant. The optimal configuration of the plant as shown in Figure 3 was obtained from the superstructure. This included the DMR process and the direct feeding with recycled CO2 for the synthesis gas production. The result is shown in Table 2. It was more profitable to have the corresponding existing process present. The net profit increased from $15.1 to $17.1 million/year or about 14% from the base case to the optimal structure. Although the product sales did not decrease much, the raw material and the operating cost decreased from $24.3 to $21.0 million as it cut down the raw material cost by using cheap carbon dioxide as raw material and the operating cost by reducing the fuel for

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Figure 3. Optimal configuration of the plant flow rates in metric ton per year.

Figure 4. Maximizing CO2 consumption. Flow rates in metric ton per year. Table 1. Raw Material Cost, Product Prices, and Sustainable Cost raw materials

cost ($/t)

butane-LPG natural gas carbon dioxide

54.2 55 3

products

prices ($/t)

carbon monoxide hydrogen

53.6 75

sustainable cost carbon tax for CO2 production

cost ($/t) 30

the steam reformer. The sustainable cost also decreased by about 31% from the base case as it consumed carbon dioxide. However, the optimal structure could not consume all of the carbon dioxide available and about 2.1 t/h, 69% of carbon dioxide brought from the other plant, is vented to the atmosphere and it is imposed as a carbon tax. So the other extension to the optimization problem was evaluated. 6.2. Maximizing Carbon Dioxide Utilization. The optimal structure was determined and it is maximized CO2 utilization. This result has changed the optimal process structure and the structure contained the RWGS process and DMR process for producing the synthesis gas as presented in Figure 4.

The net profit decreased further to $16.5 million/year, consuming 2 t of the CO2 from the other plant. These declines are a result of changes in sales and all of the associated costs as shown in Table 2. In this case, the process could reduce sustainable cost by the decreased emission of carbon dioxide. However, a decline in the production of high-purity hydrogen has led to the decrease in the total product sales and net profit as the RWGS process requires high-purity hydrogen for the synthesis reaction, 6.3. Revamping Methyl Diethanol Amine (MDEA) Unit for Capacity Expansion. Another case study was carried out by revamping the methyl diethanol amine (MDEA) process for CO2 separation so as to increase the process capacity. In the former case, MDEA unit and membrane unit were operated at almost maximum load. In this case, their capacities were expanded to twice from the base case to debottleneck the synthesis gas refining load by revamping of the units. The optimal structure and a summary of the results from the revised superstructure are shown in Figure 5 and Table 3. Two DMR processes and the direct feeding with recycled CO2 were added on to the base case configuration and associated costs of the process were also changed. Despite additional revamping cost, net profit of the process increased to $18.6 million/year or about 23% compared with the base case.

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Figure 5. Optimal process configuration for capacity expansion. Flow rates in metric ton per year.

Figure 6. Resources utilization of different case optimal configuration.

Figure 7. Optimal process configuration for capacity expansion with CO2 zero emission.

However, this optimal structure could not consume any of the carbon dioxide available and about 1.1 t was vented to the atmosphere per hour. The additional optimal structure was

determined requiring all of the CO2 from the other plant to beconsumed. Figure 6 shows the results of the resource utilization of optimal configuration of different cases. Figure 7

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Table 2. Sales and Costs Associated with the Annualized Profit for Each Case

product sales utility cost raw material operating cost sustainable annualized profit

optimal structure (dollars/year)

max. CO2 consumption (dollars/year)

base case (dollars/year)

39258000 560000 14347000 6678700 521100 17151200

38318000 810000 14274000 6455500 251770 16526730

40131000 0 17199000 7095270 756000 15080730

Table 3. Sales and Costs Associated with the Annualized Profit for Each Revamping Case

product sales utility cost raw material cost operating cost sustainable cost annualized profit

revamping process (dollars/year)

revamping and all CO2 consumption (dollars/year)

base case (dollars/year)

38385000 1770000 11496000 6262200 286030 18570770

37786000 2020000 11062000 6174000 0 18530000

40131000 0 17199000 7095270 756000 15080730

shows that the RWGS process and two DMR processes were included for the optimal structure. In this case, the net profit decreased slightly to $18.5 million/year, but the process did not incur any sustainable cost by zero emission of carbon dioxide.

Nomenclature k PProd (or PkFeed) ) material cost of component k in product (or feed) stream k FProd (or FkFeed) ) flow rate of component k in product (or feed) stream OCfix i ) fixed operation cost at utility i OCut i ) cost associated with the use of utility i Wi ) binary variable equal to 1 if equipment i is selected in the final structure Gi ) flow rate capacity of utility i CCi ) capital cost for the equipment i Ctax ) carbon tax per unit mole of CO2 flow CO2 2 FCO gen (or Fcon ) ) generated (or consumed) flow rate of CO2 k Hij ) enthalpy per unit mole of flow in stream (i,j) Qsi ) energy supply to unit i Tij ) temperature of stream (i,j) Cpkij ) specific heat component k at stream (i,j) j(i) ) key outlet stream form unit i (i) ) splitting factor of unit i k(i) ) reactant component at unit i Cki ) reacted component k moles at unit i xi ) conversion of key component at unit i νki ) stoichiometric coefficient component k at i GLO (or GUP i i ) ) minimum or maximum capacity of unit i

Literature Cited 7. Conclusions In this study, we investigated an integrated optimization strategy of an industrial synthesis gas production plant with additional processes using recycled CO2 utilization. To design the optimal synthesis gas production process from various reaction paths using carbon dioxide as a raw material, three kinds of alternative synthesis gas reaction processes were considered. Global superstructure of the synthesis gas production process including all feasible alternatives was developed and formulated to the mathematical programming. The optimal configurations of the synthesis gas processes were obtained by mathematical programming based on the process constraints and annual profit of product revenue, manufacturing, sustainable costs, and so forth. The proposed integrated optimization approach has been successfully applied to industrial case studies. The results of application showed that the optimal configuration increased the annual profit from $15.1 to $17.1 million/ year or more than 14% from the base case to the optimal structure. The plant could not consume all of the carbon dioxide available from the plant. Three other extensions to the optimal configuration were evaluated. In one of them, the optimum was determined when maximizing only the CO2 consumption. The second extension doubled the revamping of the MDEA process capacity to increase the process capacity and increased the annualized profit to $18.6 million/year or 23% compared with the base case, which is the most annualized profit among cases. In the third, the optimum was determined requiring all of the carbon dioxide from the plant to be consumed and that the MDEA process capacity be doubled. These optimization results could provide the optimal synthesis gas production process designs which show better economic performance such as annual profits than that of the base case. In addition to improving economic performance, the environmental impact of the optimal process alternative could also be dramatically reduced.

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ReceiVed for reView July 16, 2007 ReVised manuscript receiVed September 29, 2007 Accepted October 17, 2007 IE070961N