Synthesis reactors

The Fischer-Tropsch reaction is essentially that of Eq. XVIII-54 and is of great importance partly by itself and also as part of a coupled set of processes whereby steam or oxygen plus coal or coke is transformed into methane, olefins, alcohols, and gasolines. The first step is to produce a mixture of CO and H2 (called water-gas or synthesis gas ) by the high-temperature treatment of coal or coke with steam. The water-gas shift reaction CO + H2O = CO2 + H2 is then used to adjust the CO/H2 ratio for the feed to the Fischer-Tropsch or synthesis reactor. This last process was disclosed in 1913 and was extensively developed around 1925 by Fischer and Tropsch [268]. 

Methanol. Methanol is produced by stoichiometric reaction of CO and H2. The syngas produced by coal gasification contains insufficient hydrogen for complete conversion to methanol, and partial CO shifting is required to obtain the desired concentrations of H2, CO, and CO2. These concentrations are expressed in terms of a stoichiometric number, ((H2 — CO)/(H2 + CO2), which has a desired value of 2. In some cases CO2 removal is required to achieve the stoichiometric number target. CO and H2 are then reacted to form methanol in a catalytic methanol synthesis reactor. 

The compressed synthesis gas is dried, mixed with a recycle stream, and introduced into the synthesis reactor after the recycle compressor. The gas mixture is chilled and liquid ammonia is removed from the secondary separator. The vapor is heated and passed into the ammonia converter. The feed is preheated inside the converter prior to entering the catalyst bed. The reaction occurs at 450-600°C over an iron oxide catalyst. The ammonia synthesis reaction between nitrogen, N2, and hydrogen, Hj, is... 

Table 8.6. Typical composition of the feed gas when it enters the ammonia synthesis reactor.

Ammonia synthesis reactor. The shaded area contains the catalyst. [Adapted, with permission, from Murase et al., Optimal Thermal Design of an Auto-thermal Ammonia Synthesis Reactor, Ind Eng Chem Process Des Dev 9 504 (1970). Copyright, American Chemical Society.]... 

In order to illustrate this approach, we next consider the optimization of an ammonia synthesis reactor. Formulation of the reactor optimization problem includes the discretized modeling equations for a packed bed reactor, along with the set of knot placement constraints. The following case study illustrates how a differential-algebraic problem can be optimized efficiently using (27). In addition, suitable accuracy of the ODE model can be obtained at the optimum by directly enforcing error restrictions and adaptively adding elements. Finally, bounds on the continuous state profiles can be enforced directly in the optimization problem. 

Fig. 4. Schematic of ammonia synthesis reactor. Reprinted with permission from Comp. Chem. Eng., 14, No. 10, 1083-1100, S. Vasantharajan and L. T. Biegler, Simultaneous Optimization of Differential/Algebraic Systems with Error Criterion Adjustment, Copyright 1990, Pergamon Press PLC.

Murase, Akira, Roberts, Howard L., and Converse, Alvin O., Optimal thermal design of an autothermal ammonia synthesis reactor, l EC Proc. Des. Dev. 9(4), 503 (1970). 

As to efforts to carry out ammonia synthesis in a technical direction, studies along that line had been started in the B. A. S. F. after Wilhelm Ostwald had suggested such work in 1900. In laboratory experiments considerable yields of synthetic ammonia had been obtained by W. Ostwald (27). However, all attempts to reproduce these yields on a larger scale were futile, and finally Ostwald had to admit that in his original experiments, ammonia had probably been erroneously introduced into the synthesis reactor with a foreign source, presumably in form of an iron nitride, which had been formed by a previous treatment of the iron catalyst with ammonia. 

Modem ammonia synthesis reactors operate at -200 atm at -350°C and produce nearly the equihhrirrm conversion ( 70%) in each pass. The NH3 is separated from unreacted H2 and N2, which are recycled back to the reactor, such that the overaU process of a tubular reactor plus separation and recycle produces essentially 100% NH3 conversion The NH3 synthesis reactor is fairly small, and the largest components (and the most expensive)... 

Figure 3-17 Plot of eqdlibrimn convereion Xg vasus terr5)er-ature for ammonia synthesis starting with stoichiomeh ic feed. While the equilibrium is favorable at anbient tar5)erature (where bactaia fix N2), the convasion r dly falls off at elevated temperature, and commercial ammonia synthesis reactors operate with a Fe catalyst at pressiues as high as 300 atm to att 2 high equilibrium conversion.

Synthesis gas is compressed to pressures of about 250 psig before entering the methanol synthesis reactor and conversion to methanol. Also, after methanol synthesis, any unreacted synthesis gas is again compressed and recycled back through the reactor. 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

It has to easily recover the original oxidation state by contact with air. Usually the reoxidation of a reduced catalyst is the rate-determining step, and therefore residence times in the regenerator are much higher than in the synthesis reactor. 

Before entering the synthesis vessel, the gas is mixed with the recycled gas which is then heated by exchange with synthesis reaction heat the gas enters the first reactor at 240°C and leaves the second at 270°C. In this exchanger the exit gas exchanges heat with the gas that enters the synthesis reactor and afterwards it is cooled to 35°C, which condenses the methanol vapors. The liquid methanol is separated and the non condensed gas is recycled. 

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