Description

Overview This report surveys the technologies used to generate mixtures of carbon monoxide and hydrogen, commonly referred to as synthesis gas (syngas), that are important intermediate feedstocks for the production of large volume chemicals such as methanol, ammonia, synthetic hydrocarbon liquids, and hydrogen. Synthesis gas can be prepared from a variety of hydrocarbon materials, ranging from natural gas (methane) to petroleum-based gases and liquids (refinery gases, LPG, naphtha, heavy residues) and even solids such as coal and petroleum coke. Conversion processes used include several variations of reforming (steam reforming, combined reforming, autothermal reforming, others) and both catalytic and non-catalytic partial oxidation (gasification) process. The choice of technology for syngas generation is strongly influenced by the available feedstock as well as the gas composition best suited to the downstream consuming process. Table 1 illustrates the typical capacities, preferred hydrogen to carbon monoxide ratios, and representative process selections for downstream methanol, ammonia, GTL, and hydrogen applications. It should be noted that SMR, combined reforming, and ATR are not the only possible technologies for producing syngas for methanol synthesis. Nor are the representative technologies presented for other end use processes necessarily the optimal technologies for every case. Syngas technology selection depends heavily on a number of factors, including the quality of feedstock available and availability of other raw materials such as oxygen, and the decision of which technology is most appropriate for a particular plant must be assessed on a case-by-case basis. Table 1: Syngas Characteristics by End-Use Application Detailed technology descriptions are provided for steam-methane reforming, combined reforming, autothermal reforming, partial oxidation/gasification, and variations/combinations of these as offered by the leading vendors supplying syngas technology as tailored for the target end use applications. Most of these descriptions are based on natural gas as feedstock, but description of coal gasification as a route to syngas is also included. For completeness of understanding, end use processes utilizing syngas are presented in moderate detail so as to familiarize the reader with the technologies offered by the top providers and highlight interactions between syngas and end use processes. Another objective is to contrast the features of the major licensed end use processes, particularly tradeoffs in capital costs, operating costs, and operational flexibility and reliability. Syngas for Methanol Production Natural gas based synthesis gas can be produced via partial oxidation (Reaction (1) below) and/or steam reforming (Reaction (2) below) of methane as follows: Currently, the synthesis gas generation technologies are basically grouped in accordance with Reaction (1) and (2) above as follows: Non-catalytic partial oxidation Catalytic partial oxidation Steam reforming Combined reforming The steam reforming reaction is a highly endothermic reaction. It takes place inside the catalyst filled tubes of a reformer furnace. The endothermic heat is supplied externally by firing additional amounts of natural gas. Simultaneous to the steam reforming reaction, the water/gas shift reaction also takes place: The steam reformer requires a high steam to carbon ratio to prevent carbon from being deposited on the catalyst, thereby reducing its activity: High steam-to-carbon ratios imply high consumption of energy in the process of vaporizing the required steam and also increased hydrogen production due to the water/gas shift reaction, i.e. Reaction (3). Alternatively, synthesis gas can be produced via catalytic or non-catalytic partial oxidation of methane, Reaction (1). Reaction (1) is an exothermic reaction, thus it does not require additional heat. However, there is an implicit energy input in the form of power in the generation of pure oxygen from atmospheric air in an air separation unit (ASU). The conversion of synthesis gas to methanol is a strongly exothermic process. The methanol synthesis reactions can be represented as follows: The ideal synthesis gas composition for producing methanol would have molar ratio of just over two when calculated according to the following formula: Addition of CO2 is one option for adjusting this ratio. Other methods relate to the process design configuration, particularly around the reformer. Reforming The main options with respect to reformer configuration are shown in much simplified block diagram form in Figure 1. The terminology can vary between licensors. Discussion in the report covers each of the illustrated variants. Partial Oxidation Commercial POX processes can be operated at elevated pressures using 98 volume percent or greater oxygen as the oxidant. They are also capable of employing a wide variety of feedstocks such as natural gas, refinery off-gas, LPG, naphtha, gas oil, vacuum residual fuel oil, shale oil, asphalt residual fuel oil, or even whole crude oil. Figure 1: Selected Methanol Reformer Options POX involves the combustion of a hydrocarbon in a flame with less than stoichiometric quantities of oxygen to form carbon dioxide and steam, which, in turn, react with the unreacted hydrocarbon to produce carbon monoxide and hydrogen. Usually, only about 35 percent of the total oxygen required for complete oxidation is fed to the system. The highly exothermic oxidation reaction consumes essentially all the available oxygen. The liberated energy is used for the endothermic reforming reaction; the overall POX process remains exothermic. Gasification Higher molecular weight carbonaceous materials such as petroleum fractions and solids such as coal and coke can also be used as syngas feedstock via gasification. Gasification consists of a series of controlled chemical reactions taking place at up to 1,000 psig or more and nominally 2,600oF, resulting in corrosive slag and hydrogen sulfide gas as co-products. In comparison with conventional combustion using a stoichiometric excess of oxidant, gasification typically uses one-fifth to one-third (alt data: 30-50%) of the theoretical oxidant so as to only partially oxidize the combustible constituents of the feedstock. The major combustible products of gasification are carbon monoxide and hydrogen, with only a minor portion of the carbon completely oxidized to carbon dioxide or converted to methane. The heat produced by the partial oxidation provides most of the energy required to break chemical bonds in the feedstock, increase gasifier products to reaction temperature, and drive endothermic (heat-absorbing) gasification reactions. Although there are various types of coal gasification reactors, with different design and operating characteristics, all fall into one of three generic categories: Moving-bed reactors (also referred to as fixed-bed reactors) Fluidized-bed reactors Entrained-flow reactors The report discusses numerous examples in each of these categories. End-Use Process Types Methanol Process descriptions of key 5,000+ MTPD methanol technologies from the following licensors are included in the report: JM Catalysts Low Pressure Methanol Process JM Catalysts Leading Concept Methanol Process Davy Process Technology Improved Low Pressure Methanol Process Uhde Methanol Process Lurgi Mega Methanol Process Ammonia Most of the world's ammonia output is synthetic material manufactured by combining hydrogen and nitrogen over a catalyst according to the following equation: Synthesis gas (containing hydrogen and nitrogen in a 3:1 mole ratio), is fed to a converter where it is catalytically transformed into ammonia. Air is the ultimate source of nitrogen, and methane or heavier hydrocarbons are usually the main source of hydrogen. Of the hydrogen feedstock sources - natural gas, coal, and petroleum fractions - natural gas is the most often employed in commercial ammonia plants, representing about three quarters of world production. Natural gas is favored for several reasons: Its availability and ease of delivery, its high hydrogen content, and the simplicity and low capital cost of plants designed for natural gas. The report includes detailed descriptions of a generic ammonia process, as well as specifics of the Kellogg Brown & Root KRES-KAPP ammonia process and several variations of Uhde's ammonia process. Synthetic Hydrocarbon Liquids Synthetic hydrocarbon liquids can be prepared from syngas, with an optimal hydrogen to carbon monoxide molar ratio being 2.0. The products can range in molecular weight and properties - depending on the feedstock, reactor type, operating conditions, and catalyst used - from those of naphtha to those of kerosene and gas oil. The Sasol Slurry Phase Distillate process and the Shell Middle Distillates Synthesis process are discussed in detail, including product distributions and properties. Hydrogen Hydrogen production is an integral part of the process for making ammonia. Steam reforming of natural gas is the most prevalent means of hydrogen production. If the reforming involves an autothermal step, pure oxygen must be used to avoid introducing nitrogen, which is undesirable for syngas not destined for ammonia production. High purity hydrogen is typically obtained by the use of pressure swing adsorption to remove carbon oxides. Steam reforming of natural gas is discussed in detail, with brief description of naphtha as a reforming feedstock. The partial oxidation of refinery gases (HyTEX process) is detailed. Vacuum residue and other low-value fuel oil blend stocks can also be used in producing hydrogen needed by refineries coping with heavy, high-sulfur crudes and lower sulfur specifications on products. Economics Methanol is taken as an illustrative end use for which three alternative syngas generation routes are compared: Steam Methane Reforming (SMR) Combined Reforming Autothermal Reforming (ATR) The second segmentation presents the cost of syngas production across the various downstream products for which it is an important feedstock: Methanol Ammonia Gas to Liquids (GTL) Hydrogen Each of these processes requires a different syngas capacity and composition, and so a typical technology offering is presented with a corresponding typical capacity for each process. In the case of ammonia and methanol, large scale facilities have been presented in accordance with the current industry trend toward these increased capacities. Commercial Analysis Global syngas demand by end-use is estimated for 2003 and for 2015. Demand is segmented by region for these same years. Currently the biggest end use market for syngas is ammonia production. Ammonia is principally consumed in fertilizer applications, in some cases applied directly to the soil in its pure form, although more commonly used in the production of other solid or liquid fertilizers such as urea, ammonium nitrate, and diammonium phosphate. Methanol was the second largest syngas consumer in 2003. Methanol is mainly used in the production of formaldehyde, MTBE (methyl tertiary butyl ether), and acetic acid. Other users include methyl methacrylate, for acrylic sheet, surface coatings, and molding resins, and methyl halides such as methyl chloride, an intermediate in silicone production. In 2003, hydrogen production was the third largest demand for syngas. Ninety percent of this demand is for refineries, where hydrogen is used in hydrocracking to improve gasoline yields and hydrotreating to lower the sulfur content of fuels and help meet increasingly stringent clean air regulations. A second important industrial application for hydrogen is in metals processing, where the gas is used in annealing and high temperature reduction of iron. Gas-to-liquids (GTL) syngas applications represented a small fraction of total syngas demand in 2003. The sector is expected to experience explosive growth to 2015, however, by which time it will occupy second place behind ammonia.

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