Multi-bed adiabatic reactor

Oxidation of SO2 into SO3 is a classic example of exothermic reversible reaction. Optimal temperature regime for such a reaction requires starting at as high temperatures as a catalyst can handle, then the temperature decrease along with progressing conversion of a reactant. It is traditionally performed in multi-bed adiabatic reactors with intermediate cooling. Temperature profile in an RFR (Fig. 4) has lower temperatures at both ends of the catalyst bed, suggesting that an RFR would perform close to the theoretical optimum. 

There are two types of heat exchange for a multi-beds adiabatic reactor Indirect and direct heat exchange. If the amount and composition of entering gas, ultimate content of ammonia and number of beds have been fixed, the remaining problem is how to determine the temperature and gas composition (or conversion) at the inlet and outlet of various beds, i.e., optimum distributions between beds. As an example, the optimum distributions for multi-bed reactors with indirect heat exchange are discussed as follows ... 

In a multi-bed adiabatic reactor, the first bed of catalyst bears about 70% production task of the whole converter, thus the protection of the catalyst in the first bed is very important. The inlet gases of the reactor must not contain water vapor, liquid ammonia, and trace amounts of CO - - CO2 or other poisons. Above all, the temperature in catalyst bed must not exceed the heat-resistant temperature of the catalyst. This is because high temperature can quicken up catalyst-aging, decrease catalytic activity and the net yield of ammonia due to decrease of equilibrium ammonia concentration. 

Ethanol is first pressurised to reactor operating pressure and then evaporated. AftOT that, steam is injected. Steam injection has a positive impact on conversion and selectivity in tanperature regions above 375 C (Kochar etal., 1981). Furthermore, the amount of catalyst needed is lower and its lifetime is inCTeased (Barrocas et al., 1980 Morschbacker, 2009). A multi-bed adiabatic reactor systan consisting of four reactors in series with a SynDol catalyst bed in each reactor is assumed. The reactor tubes are located inside a furnace in order to heat and subsequently reheat the reactants to reaction temperature. A mixture of combustible by-product gases from the ethylene dehydration process and natural gas (additional fuel) is used to heat the furnace. 

Because the adiabatic reaction path is linear a graphical solution, also applicable to multi-bed reactors, is particularly apposite. (See Example 3.7 as an illustration.) If the design data are available in the form of rate data for various temperatures and conversions they may be displayed as contours of equal reaction rate in the (T, Y) plane. Figure 3.14 shows such contours upon which is superimposed an adiabatic reaction path of slope cp/(- AH) and intercept T0 on the abscissa. The reactor size may be evaluated by computing ... 

The overall reactor model comprises, as the heart of it, the single catalyst pellet model which is formulated in an overall framework that includes the changes in the bulk fluid phase. The equations for the catalyst pellet coupled with the equations for the bulk fluid phase represent what we may call in certain cases, the overall reactor model or in a more restricted sense, the catalyst bed module. This catalyst bed module may represent the overall reactor model in certain cases such as the single adiabatic catalytic packed bed reactor. In other cases, this module may represent only the essential part of the overall reactor model such as in non-adiabatic and multi-bed reactors. 

The following subsection briefly describes current industrial processes for methanol production with adiabatic multi-bed and isothermal single-bed reactor design. We will refer to the ICI (adiabatic multiple-bed reactor) and to the Lurgi process (isothermal single-bed reactor), which are important representatives of the different ways of producing methanol commercially nowadays. 

Fixed-bed reactors (multi-tubular and staged adiabatic), fluidized-bed reactors (bubbling bed, turbulent bed, fast, and transport or pneumatic), radial flow reactor, gauz reactor... 

If the reactor were a single adiabatically operated fixed bed, the heat release would raise the temperature to 600 °C, which corresponds to an equilibrium conversion of SO2 of only 70% (Figure 6.3.4), but even this far from sufficient conversion would only be reached for an infinite residence time and reactor length. For isothermal operation, a conversion of about 98% would be possible, but this would require an expensive reactor (e.g., a multi-tubular reactor intensively cooled by a molten salt. Figure 4.10.7). 

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