Simulation, Integration, and Economic Analysis of Gas-to-Liquid Processes. (December 2008) Buping Bao, B.S., Zhejiang University Chair of Advisory Committee: Dr. Mahmoud M. El-Halwagi Gas-to-liquid (GTL) process involves the chemical conversion of natural gas (or other gas sources) into synthetic crude that can be upgraded and separated into different useful hydrocarbon fractions including liquid transportation fuels. A leading GTL technology is the Fischer Tropsch process. The objective of this work is to provide a techno-economic analysis of the GTL process and to identify optimization and integration opportunities for cost saving and reduction of energy usage and environmental impact. First, a basecase flowsheet is synthesized to include the key processing steps of the plant. Then, computer-aided process simulation is carried out to determine the key mass and energy flows, performance criteria, and equipment specifications. Next, energy and mass integration studies are performed to address the following items: (a) heating and cooling utilities, (b) combined heat and power (process cogeneration), (c) management of process water, (c) optimization of tail-gas allocation, and (d) recovery of catalystsupporting hydrocarbon solvents. Finally, an economic analysis is undertaken to determine the plant capacity needed to achieve the break-even point and to estimate the return on investment for the base-case study. After integration, 884 million $/yr is saved from heat integration, 246 million $/yr from heat cogeneration, and 22 million $/yr from water management. Based on 128,000 barrels per day (BPD) of products, at least 68,000 BPD capacity is needed to keep the process profitable, with the return on investment (ROI) of 5.1%. Compared to 8 $/1000 SCF natural gas, 5 $/1000 SCF price can increase the ROI to 16.2%.
Operating Fischer−Tropsch synthesis (FTS) in supercritical fluid (SCF) media offers advantages over conventional gas-phase FTS operation including in situ extraction of heavy hydrocarbons from the catalyst pores coupled with enhanced incorporation of α-olefins in the chain growth process. In this study, hydrocarbon product distributions in near-critical and supercritical hexane phase FTS (SCH-FTS) was studied over a 15% Co/Al2O3 in a high-pressure fixed-bed reactor system. The critical point of the hexane−syngas−products reaction mixture as collected from the reactor outlet was measured using a variable-volume view cell apparatus. All reactions were carried out in near-critical and supercritical regimes by tuning either the reaction temperature (230−260 °C) or the reaction pressure (30−80 bar). Deviations from the standard Anderson−Schultz−Flory (ASF) chain growth model were observed in most cases; however, the degree of deviation depends on the reaction conditions within the near-critical and supercritical regions and varies from gaslike density to liquidlike density within the supercritical region. As an attempt to understand this phenomenon, a modification to the chain growth model in SCH-FTS, is presented. The model attributes the deviation from the standard ASF model to enhanced α-olefin incorporation within the middle distillate hydrocarbon products due to enhanced adsorption/desorption dynamic in the SCF medium. The proposed enhanced olefin incorporation model was found to be in a good agreement with our experimental results.
Reforming of natural or shale gas is the primary industrial method for the production of syngas. There are four major approaches to reforming: steam reforming, partial oxidation, dry reforming, and combinations of these reforming reactions. The selection of the reforming approach is not a straightforward task. It is highly dependent on process objectives and availability of material and energy resources. Furthermore, the reformer selection has a significant impact on: process yield, energy requirement, CO 2 emissions, and wastewater generation. It also has operational implications including: catalyst life, process safety, and control. The purpose of this work is to develop a systematic tool to aid in the modeling and selection of appropriate reformers to achieve an objective. This can include particular process or economic objectives and constraints such as H 2 :CO ratio. An equilibrium model is developed and solved using optimization software (LINGO). Economic, energy, and environmental constraints are included in the optimization formulation. Top-level economic scenarios were investigated including the inclusion of carbon tax, natural gas, and energy price fluctuations. The work is also extended to shale gas reforming and shows that the composition of the shale gas has a significant impact on potential yields. Given the potential variability in shale gas composition, the optimization model is also used in selecting a gas reservoir (feedstock) for specific objectives and constraints of the reforming process. The inclusion of strict energy and environmental constraints can favor some reforming options over others.
in Wiley InterScience (www.interscience.wiley.com).The Fischer Tropsch Synthesis (FTS) reaction has been studied and for nearly a century for the production of fuels and chemicals from nonpetroleum sources. Research and utilization have occurred in both gas phase (fixed bed) and liquid phase (slurry bed) operation. The use of supercritical fluids as the reaction media for FTS (SCF-FTS) now has a 20-year history. Although a great deal of progress in SCF-FTS has been made on the lab scale, this process has yet to be expanded to pilot or industrial scale. This article reviews the research activities involving supercritical Keywords: Fischer-Tropsch synthesis, supercritical fluids, reaction engineering, cobalt catalyst, iron catalyst, ruthenium catalyst Introduction to supercritical fluids for catalysisThe concept of using supercritical fluids (SCFs) as solvents has been in circulation since their discovery in the nineteenth century.1 Industrial utilization of SCFs has received considerable attention since the early 1980s, starting with the development of technologies for extraction of commodity chemicals and fuels.2 By the mid 1980s, research on new applications of SCFs shifted toward more complex and valuable substances that undergo a much broader range of physical and chemical transformations.2 A great deal of research activities have taken place in studies of reactions, separations, and materials processing of polymers, foods, surfactants, pharmaceuticals, and hazardous wastes.3 SCFs are recognized as a unique medium for chemical reactions, offering single phase operation, a density that is sufficient to afford substantial dissolution power, a higher diffusivity, and lower viscosity than in liquids. These properties can result in significant enhancement of mass transfer and/or heat transfer. Additionally, conducting chemical reactions at near-critical conditions affords excellent opportunities to tune the reaction environment (solvent properties) through modest changes in temperature and pressure. These properties can help to eliminate transport limitations on reaction rates, integrate reaction and product separation processes, 4 and enhance in situ extraction of low volatility products (e.g., heavy hydrocarbons) from porous catalysts. 5The main areas of heterogeneously catalyzed hydrogenation reactions were classified by Hyde et al. 6 according to the following categories: (1) the hydrogenation of food compounds such as fatty acids or oils to produce higher value derivatives; (2) the formation of precursor building blocks for pharmaceuticals and fine chemicals, and (3) asymmetric hydrogenation. In their book, McHugh and Krukonis, 2 address the uniqueness of applying a supercritical medium to many of the above classes of heterogeneous reactions, as well as to homogenous reactions such as selective oxidations, hydrogenations, hydroformylations, alkylations, and
Water-gas-shift (WGS) reaction plays a significant role in industrial application of Fischer-Tropsch synthesis (FTS) for coal-to-liquid (CTL) processes with iron-based catalysts. This reaction provides necessary hydrogen for synthesis gas with low H2/CO molar ratio, and has influence on concentrations of reactants, water and carbon dioxide, which in turn has effect on product distribution, rate of FTS and catalyst deactivation. We provide information on the effect of process conditions (H2/CO feed ratio, reaction temperature and pressure), syngas conversion, and catalyst composition and activation procedure on the WGS activity. H2/CO consumption (or usage) ratio and the exit H2/CO ratio vary with conversion and the extent of WGS reaction. The extent of variation is much greater for H2/CO feed ratios greater than 1.7, than it is for the CO rich syngas (H2/CO = 0.5-1). This in turn places limits on maximum practical single pass conversion which can be achieved with different feed compositions and results in different types of operation (low single pass conversion with tail gas recycle, and high once through single pass conversion).
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