Solids handling, apparatus design

Several problems in solids handling were encountered in operating the laboratory apparatus. Originally solids were transferred from the pyrolysis bed to the combustion bed by means of a weir and dip leg. Because gas tended to flow up through the dip leg, this system was abandoned in favor of a simple solids downcomer with a specially designed solids flow-control valve. 

Scale-up is also difficult [Bermingham 2000]. Because solids are handled, the minimum size of test apparatus is more strongly limited than in the case of, for example, distillation. The minimum size for evaporative and cooling crystallizers is about 5 L. Process steps in which solids are produced must be very carefully designed as they directly influence downstream steps such as filtration, drying, storage of solids, and compaction. If possible, tests in a miniplant in which the total process is mirrored should be supplemented by individual tests in a larger semitechnical plant. 

The range of applications of size reduction is very wide. It includes the preliminary breakup of large masses into pieces that can be handled in a process and the grinding of smaller particles into fine powders. The size and design of equipment reflect this situation. Broadly, size reduction apparatus can be divided into those types which depend on mechanical crushing and those which use impact to fracture the solids. Table 3-13 shows some of the common types of equipment with their principal characteristics. Major hazards are those associated with the machinery and with the material being processed. 

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