Distillation columns loading-flooding

Example 9 Loading/Flooding of a Distillation Plate An available sieve plate column of 2.5-m diameter is being considered for an etbylben-zene/styrene separation. An evaluation of loading at the top plate will be made. Key dimensions of the single-crossflow plate are ... 

This matrix will contain information regarding loading characteristics such as flooding hmits, exchanger areas, pump curves, reactor volumes, and the like. While this matrix may be adjusted during the course of model development, it is a boundary on any possible interpretation of the measurements. For example, distillation-column performance markedly deteriorates as flood is approached. Flooding represents a boundary. These boundaries and nonlinearities in equipment performance must be accounted for. 

Vacuum distillation columns are mostly operated at very low liquid loads ul, in particular in the rectifying section. For this reason, it is important to ascertain the lower loading line ul,U) Fig. 2-20, as well as the flooding point. Kirschbaum [19] and Schmidt [7, 32] have looked at methods to determine the lower loading line. 

For total-reflux distillations carried out in packed columns, regions of loading and flooding are identified by their effects on mass-transfer efficiency, as shown in Fig. 14-47. Gas and liquid rate increase... 

In this strategy the feed mixture is charged in the reboiler (at the beginning of the process) to its maximum capacity. For a given condenser vapour load Vc, if the reflux ratio R (which governs the distillate rate, LD, kmol/hr) and the solvent feed rate F (kmol/hr) are not carefully controlled the column will be flooded. To avoid column flooding Tran and Mujtaba (1997) developed the following necessary and sufficient condition ... 

In the laboratory it is usual to employ columns having diameters of 10—30 mm for distillations at normal pressures and of 20—50 mm for distillations at reduced pressures. For very large throughputs, even at normal pressure, a diameter greater than 50 rnm is necessary. The recent view that a column should be loaded to just below the point of flooding makes it necessary to settle on its diameter beforehand, so that the desired throughput may be realized. 

For vacuum distillation, a preliminary test for leakage is necessary. The apparatus is evacuated when the required pressure has been reached the vacuum pump is stopped and the rate of increase of pressure is observed (section 5.4.1). A further test for tightness is performed when the column is in operation and the thermal load has been established. The still pot heater is switched on only when the desired pressure is attained, and the column jacket heater as soon as vapour evolution starts. The cooling water should be tiumed on before the heaters are put into operation during the initial period its rate of flow should be checked at intervals since it usually tends to vary somewhat at the start with the expansion of tap washers. When the vapour has reached the colunm head the load is adjusted to the required value (section 4.10.7) unless the column is to be flooded (section 4.10.8) to wet the packing. The apparatus is then allowed to run for at least half an hour at total reflux, so as to allow the initial boiling point to be accurately determined, and only then is it adjusted to the required reflux ratio (section 4.10.4). 

---