Reaction conversion and selectivity can deteriorate as a catalyst ages. Optimize continuous catalyst makeup and replacement rate to improve profitability.
Part 1 of this series (Jan. 2019, pp. 44–50) (1) addressed the economics of replacing aged catalyst in fixed-bed systems, which involves balancing the significant upfront cost of replacement with the long-term benefits of improved process performance. It is best to employ a cash-flow analysis to optimize the replacement strategy, because catalyst lifespans are typically on the order of years.
This article considers fluidized beds and slurry reactors with a continuous catalyst makeup and purge. From the perspective of an economic evaluation, the catalyst makeup is treated as an ongoing variable cost, rather than an initial investment that pays back over time. The principles outlined here can also apply to homogeneous catalyst systems with a large system inventory and continuous makeup and purge.
Fluidized-bed and continuous slurry reactors are characterized by catalyst suspended and well-mixed in the reacting medium, rather than fixed in place in a packed tube or vessel. Commercial applications of these designs in the refining and chemical process industries (CPI) include reactors for (2):
- refinery fluid catalytic crackers
- Fischer-Tropsch synthesis
- oxychlorination of ethylene to vinyl chloride
- oxidation of n-butane to maleic anhydride
- production of acrylonitrile from propylene and ammonia
- hydrogenation of nitrobenzene to aniline.
Reactor performance in terms of conversion of raw materials or selectivity of desired products can deteriorate as catalysts age. Rather than periodic changes of the entire catalyst charge, continuous or frequent removals and additions are common. This makeup rate affects the age distribution of catalyst particles in the fluidized bed and thus the reactor performance.
This article describes how to optimize the catalyst makeup rate for maximum process profitability in a fluidized-bed reactor.
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