Since the mid-1960s the world has seen remarkable increases in production rates for commercial polymers. Single-train line rates have increased by factors of 20 to 40 for polymers such as polyethylene, polypropylene, and polystyrene. A rate of 1 t h ร1 that would have been economical in 1964 has become 20-40 t h ร1 . Part of this improvement is attributable to improved catalysis and innovative processes, but the greatest increase in productivity is due to classical scaleup. Larger plants produce more polymer using substantially the same manpower and with reduced capital investment per ton of annual capacity. The selling prices of the major commodity polymers have increased by factors of 3 to 4 since the 1960s, but the US consumer price index has increased by a factor of 6. The result has been an ongoing decrease in the real cost of polymers that leads to their greater use and a continuing incentive to build still larger plants.
The success of the petrochemical industry in the 20th century is attributable in large part to the scalability of typical chemical processes. This chapter discusses the technical basis for achieving capacity increases without major changes in technology. These techniques are broadly applicable to the development of new polymers and new polymerization processes. They continue to be applied to existing processes, although no process is infinitely scalable. Eventually, a limit must be reached.
10. 2
The Limits of Scale Consider the scaleup of a stirred tank reactor. It is common practice to scale using geometric similarity so that the larger reactor will have the same shape as the small reactor. Suppose the volume scaleup factor is given by Eq. (1).