Recycle operation optimum conversion

Optimum Recycle Operations. When material is to be processed to some fixed final conversion Xp f in a recycle reactor, reflection suggests that there must be a particular recycle ratio which is optimum in that it minimizes the reactor volume or space-time. Let us determine this value of R. 

These figures indicate that no matter what reactor system is selected, when the fractional conversion of A is low the fractional yield of R is high. Thus, if it is possible to separate R cheaply from a product stream, the optimum setup for producing R is to have small conversions per pass coupled with a separation of R and recycle of unused A. The actual mode of operation will, as usual, depend on the economics of the system under study. 

Significant amounts of CH4 and C2H2 are also formed but will be ignored for the purposes of this example. The ethane is diluted with steam and passed through a tubular furnace. Steam is used for reasons very similar to those in the case of ethylbenzene pyrolysis (Section 1.3.2., Example 1.1) in particular it reduces the amounts of undesired byproducts. The economic optimum proportion of steam is, however, rather less than in the case of ethylbenzene. We will suppose that the reaction is to be carried out in an isothermal tubular reactor which will be maintained at 900°C. Ethane will be supplied to the reactor at a rate of 20 tonne/h it will be diluted with steam in the ratio 0.3 mole steam 1 mole ethane. The required fractional conversion of ethane is 0.6 (the conversion per pass is relatively low to reduce byproduct formation unconverted ethane is separated and recycled). The operating pressure is 1.4 bar total, and will be assumed constant, i.e. the pressure drop through the reactor will be neglected. 

An optimum permeabil- 20 ity is detected above which reactant loss in the permeate and product back-permeation reduce the attainable conversion. Reactant recycle or intermediate feed helps in overcoming the above limitations. The choice between cocurrcnt and countercurrent patterns depends on operating conditions. 

Egly and Smith have studied the effect of operating variables on methylamine production from methanol and ammonia over activated alumina. Nine reactions are postulated as probable under commercial operating conditions. These include reaction of methanol with di- and trimethylamines as well as decomposition of the amines. Optimum space velocities were found for a given temperature and pressure at which maximum conversion was obtained (e.g., a 97 per cent conversion of alcohol to a product consisting of 54 per cent mono-, 26 per cent di-, and 20 per cent trimethylamine was attained at 50 C with a space velocity of 720 per hr). The existence of optimum space-velocity conditions was interpreted qualitatively on the basis of the rate of the various possible reactions. Trimethylamine in the product can be reduced by adding water to the feed or eliminated by recycling. 

V decreases with increasing conversion of A. Thus, if it is possible to remove small amounts of V cheaply from large volumes of the reaction mixture, the optimum reactor configuration and mode of operation would involve the use of a plug flow reactor with low conversions of A per pass coupled with a separator to remove the product V and to recycle unconverted reactants. The exact conversion level to be employed will depend on an economic analysis of the combined reactor-separator system. 

Since substrate costs generally make up about 50 % of production costs substrate conversion is a key parameter for the economy of bioreactions. The lower the intensity of longitudinal medium dispersion, the higher the substrate conversion in continuous bioreactors under corresponding operational conditions. However, at a low dispersion intensity, cell washout occurs. To avoid washout and to achieve high substrate conversion,tower reactors with optimum longitudinal dispersion or tower loop reactors with an optimum recycling rate can be used (1). ... 

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