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Ind. Eng. Chem. Res. 2009, 48, 794–800
PROCESS DESIGN AND CONTROL Optimal Gas-To-Liquid Product Selection from Natural Gas under Uncertain Price Scenarios Chul-Jin Lee, Youngsub Lim, Ho Soo Kim, and Chonghun Han* School of Chemical and Biological Engineering, Seoul National UniVersity, San 56-1, Shillim-dong, Kwanak-gu, Seoul, 151-742, Korea
Gas-to-liquid (GTL) products have come into the spotlight for alternative energy carriers as an environmentally benign and highly profitable alternative to petroleum resources. There have been various studies conducted to explore the economic and environmental benefits of individual GTL products, but few researchers have performed a comparative economic assessment, determining which product would make the greatest profit among various GTL products. Furthermore, considering the inherent characteristics of process industries, profits from one product are deeply dependent on the price of its raw material. Thus, it is important to choose an optimal GTL product covering those price variations. In this study, we performed economic analyses for three GTL products used for transportation fuel through process modeling and investigated the profitability according to variations in feedstock cost and product price. 1. Introduction Abrupt increases in the price of oil in recent years have aroused people’s concerns about obtaining replaceable resources for petroleum. By virtue of its cleanness and economic benefits, natural gas has been used for various sources, including fuel for vehicles or heating. Because of its several advantages, we could utilize natural gas over certain scales of gas fields, by using exploitation economics.1 However, GTL (gas-to-liquid) technology has made smallsized gas reservoirs (1∼3 Tcf) available so that stranded gas can be utilized as a raw material. Since GTL products are physically identical to petroleum-based products like diesel, amid soaring oil prices, GTL products can become prominent alternatives for petroleum-based products. In addition, natural gas-based products are environmentally benign compared to petroleum byproducts. Thus, GTL products could be the energy choice when stricter environmental regulations are enacted.2,3 Considering the environmental and economic benefits of GTL products, it is meaningful to evaluate the in-depth economics of GTL products (Fischer-Tropsch (FT) diesel, DME, MeOH) under various scenarios. There have been several economic evaluation studies for GTL processes consuming natural gas as feedstock. Han et al. assessed an economic analysis for methanol synthesis using the CAMERA process, and Zhou et al. analyzed the economic and environmental effects for DME production.4,5 In general, however, most researchers focused on individual analyses for a single GTL product by adopting the Fischer-Tropsch process, so there have been only a few studies that have performed a comparative economic analysis to select the optimal product to maximize economic profit. Along these lines, Gradassi et al. compared the economic benefit of several GTL products,6 and Morita focused on three profitable GTL products (DME, FT diesel, methanol) utilized for transportation fuels and reflecting real world situations.7 * To whom correspondence should be addressed. E-mail: chhan@ snu.ac.kr. Tel.: 82-2-880-1887.
However, these studies did not consider all the characteristics of the process industry, and each product is largely influenced by raw material cost and product selling price. In this paper, we modeled three GTL processes producing DME, FT diesel, and methanol, using ASPEN PLUS, and performed an economic assessment of each of them. Furthermore, considering the variations in the price of natural gas and the expected product price, we analyzed the profitability of each through sensitivity analysis. As the results show, the priority of profitability was reversed according to the variations in raw material cost and product price. 2. Theoretical Background 2.1. Gas-To-Liquid Process. The gas-to-liquid process includes (1) producing synthesis gas (or “syngas”) through a reforming process consuming natural gas, and (2) using this syngas to produce various chemical products in the liquid phase by applying it to targeted processes. 2.2. Reforming Process. Production methods from natural gas to syngas are largely divided into three types: steam reforming, oxyreforming, and CO2 reforming. For each reaction mechanism, the enthalpy change and stoichiometric H2/CO ratio are described in Table 1.8 2.2.1. Steam Reforming. The steam reforming process is mainly used for hydrogen and syngas production. Syngas produced from steam reforming has the high synthetic ratio (H2/ CO) of 3 and is thus usually consumed by hydrogen production or ammonia synthesis processes. For methanol production, with a primary synthetic ratio of 2, we adjusted the synthetic ratio by lowering the hydrogen concentration through a water-gas Table 1. Reactions and Applications of Syngas reaction CH4 + CO2 f 2CO + 2H2 CH4 + 1/2O2 f CO + 2H2 CH4 + H2O f CO + 3H2 CH4 + H2O f CO + 3H2 CO + H2O f CO2 + H2
H2/CO ∆Ho (298K) 1 2 3 >3
10.1021/ie800879y CCC: $40.75 2009 American Chemical Society Published on Web 12/19/2008
206kJ/mol -36kJ/mol 247kJ/mol 247kJ/mol -47kJ/mol
application oxoalcohols FT synthesis methanol synthesis H2 production & ammonia synthesis
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 795
Figure 1. Process flow diagram of dimethylether (DME).
Figure 2. Process flow diagram of FT diesel.
shift reaction, or by a secondary reforming reaction, such as autothermal reforming.9 2.2.2. Oxyreforming. The oxyreforming process (e.g., autothermal reforming) uses oxygen as a reactant to produce syngas and is generally used to adjust a synthetic ratio. Oxygen, or air with a high concentration of oxygen, is fed into the autothermal reforming reaction (or “ATR”) by oxidant, and to the case, steam is added with the feedstock. This reaction is the most preferable when taking into account the thermodynamics (∆H < 0) among several reforming processes and can produce syngas with a synthetic ratio of 2, which is suitable for subsequent use in Fischer-Tropsch synthesis. This H2/CO ratio can be obtained through recirculation of CO2 or a COrich off gas, as well as by reducing the amount of steam in the feed.10 In this research, the autothermal reforming process was used to produce syngas with a synthetic ratio of 2, which is being used by the DME pilot plant (50 kg/day) of the Korean gas company.12 2.2.3. Carbon Dioxide Reforming. For carbon dioxide reforming (CO2 reforming), the synthetic ratio is normally
fixed to 1, and the maximum yield for syngas can be obtained when the feed gas is supplied with a CO2/CH4 ratio of 1. Conversion of methane and carbon dioxide increases as the reaction temperature rises along with a reforming steam. The production economics of CO2 reforming is known to be equivalent to the steam reforming process.8 3. Modeling and Simulation Because all the targeted products are produced at a high temperature (∼1000 °C) and high pressure (∼300 kPa), Peng-Robinson EOS can be applied as a thermodynamic package model for GTL processes. The three candidate processes have the same reaction at the beginning of syngas production. Because the reforming processes of ATR are simultaneously conducted in one reactor, it has the advantage of relative compactness on size attribute. Also the synthetic ratio of ATR is the nearest to 2, which can satisfy the requirement of syngas needed without additional treatment. Therefore it is most cost-effective, and preferred, to use an
796 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009
autothermal reforming to obtain syngas. The syngas then enters each chemical reaction targeted at candidate productssDME, FT diesel, MeOH. The primary reactions and simulated specifications are described below. The three GTL processes were modeled using ASPEN PLUS. 3.1. Dimethylether. DME (dimethylether) is a chemical product made from natural gas and has been usually used to produce intermediate chemicals such as dimethyl sulfate or oxygenated compounds. Because of its environmentally benign characteristics, DME has been mainly utilized as the propellant aerosol, replacing chlorofluorocarbons. The use of DME has been relatively small, however. As DME research expands and DME is used in diesel engines as an alternative fuel, there may be exponential growth in the demand for DME. Combustion and emission tests have proven that after minor modifications of a diesel engine, DME is suitable for use with diesel fuels, lowering smoke, and NOX emissions, with the same thermal efficiency.11 Furthermore, DME can be easily liquefied at ambient temperature, and its liquefied product has similar physical properties to LPG, with expectations that it will used as a substitution for LPG. There are two kinds of reaction pathways to produce DME: direct synthetic reaction and indirect synthetic reaction. In this modeling, indirect synthetic reaction was used because it has been much more commercialized than direct synthetic reaction (Figure 1). In the indirect reaction, natural gas is reformed into syngas, syngas then reacts to produce MeOH. In the next phase, 2 mol of MeOH is consumed to produce 1 mol of DME. A detailed reaction procedure is described below. Syngas production part, 1000 °C, 3000 kPa: CH4 + 1/2O2 f CO + 2H2 CH4 + H2O f CO + 3H2
∆H ) -36 kJ ⁄ mol (1) ∆H ) + 247 kJ ⁄ mol (2)
DME reaction part, 230 °C, 3000 kPa: CO + 2H2 f CH3OH 2CH3OH f CH3OCH3 + H2O CO + H2O f CO2 + H2
∆H ) -91 kJ ⁄ mol
(3)
∆H ) + 5.6 kcal ⁄ mol (4) ∆H ) -47 kJ ⁄ mol
(5)
3.2. Fischer-Tropsch Diesel. Fischer-Tropsch (FT) diesel can be made from natural gas using Fischer-Tropsch processes, as presented in equations 6-8 (Figure 2). When the price of crude oil was lower, synthesizing diesel was less profitable than obtaining diesel by distillation. At the current oil price (Dubai oil, $85/bbl, 2007.12.), however, there are reports of the economic competitiveness between FT diesel and traditional diesel. For this reason, several major petrochemical companies have already moved into countries with natural gas to construct FT diesel plants.1 FT diesel has been spotlighted because it has similar physical properties to diesel made by distillation. Because the liquid phase product through the FT process originates from natural gas, it does not contain sulfur, NOx or other aromatics. Currently, many countries impose restrictions on the sulfur content of diesel to protect against air pollution, and these restrictions appear to be for the long-term. Because FT diesel is consistent with the severe regulation against air pollution, the demand for FT diesel will continue to increase over time.2,3,10 The low-temperature Fischer-Tropsch (LTFT) fixed-bed reactor was used to produce FT diesel. The operating conditions were set to produce a high wax selectivity, which could be selectively catalytically hydrocracked to yield mainly diesel fuel. With the LTFT reaction, 77 wt % of the final product is diesel
and 10 wt % naphtha. The quantity and quality of the diesel fuel made by using the fixed-bed Fischer-Tropsch route is known to be superior to that produced by the fluidized-bed route.15,16 Syngas production part, same as DME. FT diesel reaction part, 250 °C, 3000 kPa: CO + 2H2 f -CH2 - + H2O CO + H2O f CO2 + H2 2CO f C + CO2
∆H ) -165 kJ ⁄ mol (6) ∆H ) -47 kJ ⁄ mol ∆H ) -134 kJ ⁄ mol
(7) (8)
3.3. Methanol. Methanol (MeOH) is one of the primary raw materials used to produce various chemical products, such as formaldehyde (HCHO) and other organic chemicals. It is mainly used to synthesize MTBE (methyl tert-butyl ether), the gasoline additive which increases the octane number for better engine efficiency. Since many countries have used gasoline with the additive MTBE for fuel, the demand for MeOH has continuously increased over time. The main feedstock of DMFC (direct methanol fuel cell) is also projected to be used as a substitution for gasoline and diesel.8 Where MeOH synthesis is based in carbon oxides (CO and CO2) and hydrogen, equilibrium reactions are involved that are exothermic in the direction of methanol formation as shown in eq9 and 10. As the exothermic reaction proceeds with volume contraction, one obtains maximum MeOH production at low temperatures and high pressure (Figure 3). CO conversion of MeOH synthesis is known to be around 0.95 at 230 °C, 3000 kPa, and CO2 conversion is 0.18 under the same conditions.17 Syngas production part, same as DME. MeOH reaction part, 230 °C, 3000 kPa: CO + 2H2 f CH3OH CO2 + 3H2 f CH3OH + H2O
∆H ) -91 kJ ⁄ mol
(9)
∆H ) -50 kJ ⁄ mol (10)
3.4. Simulation Results. In the case of consuming natural gas as a main feedstock at the same amount of 200MMSCFD for three candidate processes, the simulated production quantity of each is described in Table 2. 4. Economic Analysis 4.1. Basic Assumptions. Several assumptions are adopted to conduct the economic analysis and to determine the comparative profitability of the three candidate products. (1) FT diesel, MeOH, and DME plants will be constructed separately in the Middle East, with all products being sold in S. Korea. (2) The transportation distance is 5000 km between the production site and the point of sale. (3) The feedstock (mainly natural gas) is uniformly consumed at the amount of 200 MMSCFD (see Table 3). (4) DME is sold at the LPG price to substitute for LPG demand. (5) Labor and land costs are the same for three cases. (6) The selling price is13 as follows: (i) FT diesel, $780/ton (diesel price before tax at 2007.4 in Korea); (ii) naphtha, $607/ ton (Southeastern C&F price at 2007.4); (iii) MeOH, $337/ton (export price of Methanex to Asia at 2007.4); (iv) DME, $561/ ton (export price of Aramco to Asia at 2007.4). (7) The exchange rate is 933 won/$. 4.2. Payout Time. The payout time is defined as the period in years it takes to recover the funds invested. It was used in
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 797
Figure 3. Process flow diagram of methanol (MeOH). Table 2. Production Quantity of the GTL Process
FT diesel naphtha DME MeOH
Table 5. Total Manufacturing Cost of Each Process for a Year
unit
production quantity
ton/day ton/day ton/day ton/day
3074 396 3742 6868
Table 3. Natural Gas Cost and Required Quantity
natural gas price total gas volume required required scale of gas field
unit
FT diesel
DME
MeOH
US$/MMBtu MMSCFD TCF
7.92 200 1.3
7.92 200 1.3
7.92 200 1.3
Table 4. Summary of Economic Analysis for Each Process unit revenue NG cost operating cost transportation cost capital cost payout time
FT-diesel
MeOH
direct production cost raw materials utilities maintenance operating supplies labor direct supervision laboratory charges royalty fixed charges plant OVHD manufacturing cost
FT diesel
MeOH
DME
718,876 607,068 8,420 66,216 9,932 1,605 321 241 25,072 49,662 40,885 809,424
727,557 607,068 18,075 65,530 9,829 1,469 293 220 25,070 49,147 40,375 817,080
692,762 607,068 17,250 37,800 5,670 1,546 309 232 22,885 28,350 23,793 744,905
DME
$/year 1,038,681,860 844,024,175 767,077,822 $/year 606,684,750 606,684,750 606,684,750 $/year 8,420,763 18,075,488 11,229,528 $/year 21,678,716 49,062,039 17,158,499 $ 2,374,816,438 2,254,284,540 1,289,015,597 year 5.91 13.24 9.76
this study because of its considerable usefulness for easily comparing the profitability among several projects.14 payout time )
($ × 103)
fixed capital investment + start-up cost profit after taxes + depreciation
Start-up costs are commonly considered to be 10% of a fixed capital investment, with depreciation running about the same. The corporate tax of 27 % applied is the same amount used in Korea. 4.3. Economic Analysis. A summary of the economic analysis for each process is displayed in Table 4. To perform the economic evaluation among the three products, cost estimations, covering capital costs and manufacturing costs, should be conducted. In this study, the Turton’s method was used to calculate the capital cost using CBM:18 CBM)C0pFBM The unit size of each facility could be estimated from the simulation flow rate, heat exchange area, and fluid power. Cp0, the purchased cost for base conditions, could be calculated from these size parameters. FBM, the bare module cost factor related
with operating pressure and construction materials, could be estimated from the pressure factor, and material factor of the process unit. Carbon steel was used as the main material of construction. The manufacturing cost was estimated from the flow rate simulation and other related costs, as summarized in Table 5 The feedstock is supplied at the same rate for each process at 200MMSCFD, and the synthesized GTL products will be transported by ship to Korea a distance of 5000 Km. The labor cost includes 3 shift positions at $25,000 for each position. Related costs were calculated based on the Chemical Engineering Plant Index for 2000. When the natural gas is supplied at the price of $7.92/MMBtu, which was the documented LNG price in Korea during 2006, producing FT diesel is the most profitable scenario, followed by DME, then MeOH. However, the profitability is considerably influenced by the cost of natural gas, so extensive investigations applying variations to feedstock price were performed for a robust economic analysis. 5. Scenario Generation In the former section, the natural gas was set at a price of $7.92/MMBtu, but most experts expect that natural gas can be supplied at $1∼3/MMBtu from stranded GTL gas reservoirs.2,3 Thus, when lowering the feedstock cost to $3.00MMBtu, the ratio of profitability is reversed so that the DME plant becomes more profitable than the FT diesel one, as shown in Figures 4 and 5 and Table 6.
798 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009
Figure 4. Payout time variation curve vs the natural gas price around $7.92/ MMBtu. Figure 7. Manufacturing cost diagram of MeOH.
Figure 5. Payout time variation curve vs the natural gas price around $3.00/ MMBtu.
Figure 8. Manufacturing cost diagram of DME.
Figure 6. Manufacturing cost diagram of FT diesel. Table 6. Evaluation Results for the Profitability NG price ($/MMBtu)
profitability
7.92 3.00
FT diesel > DME > MeOH DME > FT diesel > MeOH
It is interesting to note that there is an abrupt increase in the slope of DME in Figure 4. Furthermore, as shown in Figure 5, when the supplying price of natural gas is reduced to around $3.00/MMBtu, the order of profitability is reversed. The reason why this happens is related to the difference of portion of feedstock cost in manufacturing cost. Natural gas cost takes up the largest portion of manufacturing costs for all three GTL products, but to differing degrees (Figures 6, 7, and 8). The natural gas portion for DME is the largest, which means that the economics of DME production is heavily dependent on the supply price of natural gas. Hence, as the purchasing cost of natural gas decreases, the profitability of DME improves more quickly than the others.
Variation of Product Selling Price. The previous result (Table 6) shows that lowering the cost of raw materials inverted the proportion of profitability. Likewise, in the process industry, the profitability is deeply dependent on feedstock cost and product price. For a robust assessment, we should assess the variability of product prices. Also, it is more reasonable to consider the future product price, rather than the present product price, at the time when the real commercial plant would be constructed. Since the product selling price is heavily influenced by the current market, it is crucial to cover uncertainties in price fluctuations at selecting the optimal GTL product. In the next section, we will predict the product selling price based on historical price data and analyze the profitability using the case of $3.00/MMBtu for the feedstock price. Evaluation Procedure. (1) Configure the product price function by a regression method based on the historical product price data (constrained to R2 > 0.85). (2) Using the obtained price function, predict the selling price for 2012. (3) Make a profitability function (payout time vs selling price variation) when the expected product price for 2012 is varied (20%. (4) Compare the economics of GTL products using the profitability function. Figure 9 depicts how the profitability of the three products is influenced by variations in the product price. Setting the selling price at 0% means that at the product price for 2012,
Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 799 Table A1. Total Manufacturing Cost of DME for a Year ($ × 103)
DME
direct production cost raw materials utilities maintenance operating supplies labor direct supervision laboratory charges royalty fixed charges plant OVHD manufacturing cost
$692,762 $607,068 $17,250 $37,800 $5,670 $1,546 $309 $232 $22,885 $28,350 $23,793 $744,905
Table A2. Simulation Result for Energy Balance Figure 9. Profitability comparison curve vs product price variation. Syngas part
we can compare the process economics among the candidate products. As a result, the order of profitability did not change when subjected to product price variations ranging ( 20% for 2012.
CO2 part DME reaction part
6. Conclusions In this study, natural gas was used as the main feedstock for three GTL productssFT diesel, MeOH, and DME. Comparative economic evaluations were conducted with process design using a process simulator, cost estimation, and profitability analysis by payout time, in that order. After analyzing the results of payout time, it was shown that producing FT diesel is the most profitable among the three products when natural gas is supplied at the price of $7.92/MMBtu for the Korean gas company. On the other hand, the cost of natural gas can be cut down to $3.00/ MMBtu because of stranded gas reservoirs. Also, the cost portion of feedstock as a manufacturing cost is different for each product, reversing the order of profitability. That is, when natural gas is supplied at $3.00/MMBtu, constructing a DME plant is the best choice compared to the others. Applying the product selling price to the economic comparison does not bear as much influence on profitability as does feedstock cost. In conclusion, the profitability of GTL processes are heavily dependent on the feedstock cost rather than the product price, so in-depth considerations of feedstock, such as volatility in price, should be analyzed before selecting a GTL process. Acknowledgment The authors gratefully acknowledge the program for the construction of Eco Industrial Park(EIP) which was conducted by the Korea Industrial Complex Corporation(KICOX) and the Ministry of Commerce, Industry and Energy(MOCIE), the Korea Science and Engineering Foundation provided through the Advanced Environmental Biotechnology Research Center (R11-2003-006) at Pohang University of Science and Technology, the Brain Korea 21 project initiated by the Ministry of Education, Korea, Seoul R&BD Program and No.(R01-2004000-10345-0), Energy Resources Technology Development Project provided through the Korea Energy Management Corporation/Ministry of Knowledge Economy. Appendix Sample calculation of Table 5.14,18 1 direct production cost ) raw materials cost + utilities + maintenance + operating supplies + labor + direct supervision + laboratory charges + royalty (Table A1)
unit model
unit
value
HX-PRE-V CO2-HX COOL1 COOL2 cooler HX pump COMP HX
kW kW kW kW kW kW kW kW kW
16,560 47,447 -430,350 -258,366 -431,128 294,163 143 32,969 44,747
2 utilities (Table A2) yearly cost ) Q× Csteam × t ) Q × $9.83/GJ × 24 × 365 × 0.95 yearly cost ) Q × Ccoolingwater × t ) Q × $0.354/GJ × 24 × 365 × 0.95 yearly cost ) electric power × Celectricity × t ) Q × $0.354/GJ × 24 × 365 × 0.95 3 maintenance cost ) 0.04 × fixed capital cost 4 operating supplies ) 0.15 × maintenance cost 5 labor cost: P ) 0, Nnp ) 24 for DME case operating labor ) 4.5 (at least operator number of shifts) × NOL NOL (number of operators per shift) ) (6.29 + 31.7P2 + 0.23Nnp)0.5 P ) number of processing steps involving the handling of particles solids Nnp ) number of number of nonparticulate processing steps handing steps 6 direct supervision ) 0.2 × labor cost 7 laboratory charges ) 0.15 × labor cost 8 royalty ) 0.03 × total production cost 9 fixed charges ) 0.03 × fixed capital cost 10 plant overhead) 0.72 × labor cost + 0.024 × fixed capital cost
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ReceiVed for reView June 4, 2008 ReVised manuscript receiVed September 24, 2008 Accepted October 13, 2008 IE800879Y
