The choice of catalyst influences reactor temperature, product distribution, and plugging potential. Section 7.4.2 discussed the catalysts used in SCW gasification. They are selected on the basis of the desired product. Catalyst deactivation is an issue assigned to most catalyzed reactions because the deac­tivated catalysts must be regenerated. If they are deactivated because of carbon deposits, as happens in a fluid catalytic cracker (FCC), they can be combusted by adding oxygen, preferably in a separate chamber. The combustion reaction reactivates the catalysts and can additionally provide enough heat for preheat­ing the feed.

7.7.1 Reactor Size

Consider a simple reactor receiving Wf of feed while producing Wp of product per unit of time. The product comprises a number of hydrocarbon components represented by species i. The total carbon in the product gas is its total in the individual gaseous hydrocarbons:

Total carbon production in the product gas = X WpCa kmol/s (7.9)

where aiT is the number of carbon atoms in component i in the gas product; Ci is mole fraction of i in the gas product; and Wp is the product gas flow rate (kmol/s). The amount of carbon in the feed is known from the feed rate, Wf (kg/s), and its carbon fraction, Fc. The carbon gasification yield, Y, is defined as the ratio of gasified carbon to the carbon in the feed:

where 12 is the carbon’s molecular weight (kg/kmol).

From Eq. (7.8) the reaction rate is given in terms of conversion as

ln (1 — Xc)

T

where t is the residence time in a reactor of volume V. For a continuous stirred-tank reactor,

Thus, for a known reaction rate, kg, and a desired conversion, Xc, we can esti­mate the reactor volume required for gasification.