Synthesis gas processing

Jn a potentially far reaching application for melt catalysis by the transition metals, we at Texaco have demonstrated the synthesis of a range of commodity chemicals and fuels directly from CO/H2 via the use of ruthenium-containing molten salt catalysis. Products include ethylene glycol, Ci-C4 alcohols, acetic acid, acetate esters, C2+ olefins and vicinal glycol esters. In its simplest form, this new class of melt catalyst comprises one or more ruthenium sources, e.g. ruthenium carbonyls, oxides, complexes, etc. dispersed in a low-melting (m.p. 150 C) quaternary phosphonium or ammonium salt (e.g. tetrabutylphos-phonium bromide). The key components are selected such that  

The melt catalyst enjoys, thereby, certain of the intrinsic advantages of both homogeneous and heterogeneous systems the inherent high selectivity and reproducibility of liquid homogeneous catalysts under normal CO hydrogenation conditions, and the ease of product-catalyst separation, typical of heterogeneous catalyst systems, once reaction is complete. 

Optionally, a second or third transition metal may be added to the ruthenium to further modify its performance and to steer the product distribution toward specific aliphatic oxygenates. These modifiers may be derivatives of rhodium, cobalt, manganese, rhenium, zirconium and titanium, in either halogen-free or halogen-containing forms. 

Some specific applications of this technology include (eq, 1-7)  

1) Ethylene glycol synthesis directly from CO/H2 using Ru-Rh catalysis or Ru alone (5,6). 

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The picture becomes considerably more complicated when the additional components of synthesis gas processes are added. The various types of components involved are portrayed in Figure 1 where each type of component is indicated by a circle. The components of existing oil and gas processes, shown on the right-hand portion of the figure, primarily involve the light hydrocarbons C-, Cg, Cg, C , and Cg the oil fractions Cg+ and the acid gas components composed primarily of and CO2. 

Cresol-Type Components. Large amounts of cresol-type components are produced in synthesis gas processes. 

This discussion can probably be best summarized by saying that the magnitude of work necessary for the development of thermodynamic data on synthesis gas processes is very large because six different types of compounds are involved rather than three types involved in oil and gas processes. These additional types introduce twelve new interactions between the various types of compounds compared with three for oil and gas systems. Thus the amount of work could be as much as five times as much as work already done on oil and gas systems. 

The problems associated with new synthesis gas processes are far greater than problems associated with gas processing plants or refineries because of water, salt, sludge, ammonia, and cresols present in the process streams. This paper attempts to identify the magnitude of the problems and methods for solving these problems. The problem of predicting the thermodynamic properties of nonpolar-polar mixtures by means of equations of state is also identified as an area needing study. 

Why synthesis gas And where does the nitrogen come from Synthesis gas, of course, provides the hydrogen air provides the nitrogen. And if the. synthesis gas process is partial oxidation, then there was probably an air separation plant associated with it. That separates the oxygen from the nitrogen for making the synthesis gas, and leaves the nitrogen for feed to the ammonia plant. 

If sulfur is present as H2S or COS or if C02 is present, any of these compounds will be a poison for many catalysts and will pardy or completely inhibit catalyst activity. As shown in Figures 5.4, 5.5, 5.13 and 5.17, the point at which sulfur removal is employed depends on the synthesis gas process that is used. Table 5.31 lists many of the processes that are available46. 

JORGE A. CAMPS has BS and MS degrees in Chemical Engineering from Louisiana State University. He worked for five years with Exxon Corporation at various U.S. and overseas locations. He joined Davy Powergas International in 1974 and is now a Principal Process Engineer of Synthesis Gas Processes. His most recent experience was as the Lead Process Engineer for a 2300-STPD methanol plant for SCT in Saudi Arabia. Mr. Camps is also an adjunct Professor of Chemical Engineering at the University of South Florida in Tampa. 

The point at which sulfur removal is employed depends on the synthesis gas process that is used. Table 22.11 lists many of the processes that are available.46... 

Make a material balance and a qualitative flow sheet for the synthesis gas process described in Prob. 13. Assume an operating factor of 95 percent and a feed stock with an analysis of 84.6 percent C, 11.3 percent H2, 3.5 percent S, 0.13 percent 02, 0.4 percent N2, and 0.07 percent ash (all on a weight basis). The oxidant in this process will be oxygen having a purity of 95 percent. Production is to be 8.2 m3/s. 

A synthesis gas process is described in Probs. 13 through 18 of Chap. 2. Prepare a plant layout for a production of 25 MM scf/day which can use either air or 95 percent purity oxygen as the oxidant in this process. 

The operating conditions for the synthesis gas process using the given feed stock, a heavy fuel oil, are presented in an article by S.C. Singer and L.W. ter Haar in Chem. Eng. Progr, 57(7), 68(1961). The material balance may be made either with software programs by ASPEN PLUS or... 

The full pseudo-homogeneous 2D axi-symmetric model, consisting of (11.19) to (11.23), was used to simulate the synthesis gas process. The model was simulated with a grid 17x257 for 3 seconds until the steady state solution was obtained. The time increment in the simulations was At = 10 s. To ensure mass conservation the convergence criteria was set to an error limit of 10 of the residual error. 

In the synthesis gas process CO is produced from CH4 and H2O. The reversed water gas shift reaction consumes CO in the z-interval from 0-1.5 m. The mole fraction of CO2 is increasing from the reactor entrance and reaches... 

The upper part of Fig. 1 shows the synthesis gas plant which is fed from the right-hand side with methane (stream l) and air (stream 2) for a combustion process to match the heat requirements of the synthesis gas process. The combustion process delivers the exhaust gas (stream 3). The synthesis gas is produced by methane, water vapor and air (streams U, 5 6) in a primary and secondary reformer and a converter (units REF1, REF2 and CON). The raw gas (stream 28) passes the gas conditioning (SEPl) which has been detailed in Fig. 3 and the synthesis gas (stream 29) enters the ammonia plant shown in the lower part of Fig. 1. The ammonia... 

A dominant problem for all synthesis gas processes is the metal-dusting corrosion (MDC) phenomenon. Further improvement of the reforming technology requires a minimization of MDC. Aging of flie alloys and defects in the metal oxide layer as well as the gas atmosphere might explain this corrosion effect. A possibility for MDC prevention is the displacement of CO by a purge gas [28]. 

Sasol produces Cj-Cg a-olefins from coal-derived synthesis gas processing followed by olefin extraction by fractionation. Sasol has announced that they will extend the process to also recover higher carbon number a-olefins. The extraction process has the ability to further recover Cg a-oleflns and there is the potential to also produce Cjj-Cjg a-olefins by the same process. 

Catalyst manufacturers have addressed these issues and have produced various shapes of catalysts to achieve maximum activity and maximum heat transfer while minimizing the pressure drop. These catalysts typically exhibit a lifetime over more than 5 years. There are a number of catalyst vendors for synthesis gas processes, including Johnson Matthey Catalysts (formerly Synetix), Slid Chemie, Umicore, and Haldor Tqpsoe. 

Preparation of heat and mass balances for synthesis gas processes thus requires methods to calculate mass and heat balances and chemical equilibrium. 

For a systematic structure, it is practical to distinguish between the actual synthesis gas generation and the synthesis gas processing. In processes requiring more or less pure oxygen (partial oxidation, autothermal reforming), the air separation plant that may have to be installed has to be considered with regard to the selection of the process and the economic evaluation [5.23]. 

In the synthesis gas process CO is produced from CH4 and H2O. The reversed water gas shift reaction consumes CO in the z-interval from 0-1.5 m. The mole fraction of CO2 is increasing from the reactor entrance and reaches a maximum at a distance of 1.5 m from the inlet. At z = 1.5 m the reaction is reversed, and a maximum occurs in the CO2 profile. Then there is a slight decrease to the point where equilibrium is achieved. A radial temperature gradient with a maximum at the wall is observed at the reactor entrance. Further away from the reactor entrance, the radial profile is flat. The mole fraction profiles also confain marked radial gradients within the first 1.5 m of the reactor. The radial gradients observed in the species concentration profiles are caused by fhe limited heat flux added to the reactor through the wall. The reactions are endothermic and the heat transferred through the wall and/or from the wall into the bed is not sufficient to smooth out the temperature profile, thus the chemical conversion becomes non-uniform. 

TABLE 9.2. Ammonia Synthesis Gas Process Developments since 1920. 

Smart, S., Ding, L. P. and Diniz Da Costa, J.C. (2011) Inorganic membranes for synthesis gas processing. In Advanced membrane science and technology for sustainable energy and environmental applications. BasUe, A. and Nunes, S. P. (Eds.). Cambridge, Woodhead Publishing Ltd, 215-254. 

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