Plate columns entrainment

The nonuniformity of drop dispersions can often be important in extraction. This nonuniformity can lead to axial variation of holdup in a column even though the flow rates and other conditions are held constant. For example, there is a tendency for the smallest drops to remain in a column longer than the larger ones, and thereby to accumulate and lead to a locali2ed increase in holdup. This phenomenon has been studied in reciprocating-plate columns (74). In the process of drop breakup, extremely small secondary drops are often formed (64). These drops, which may be only a few micrometers in diameter, can become entrained in the continuous phase when leaving the contactor. Entrainment can occur weU below the flooding point. 

The term in equation 42 is called a Souders-Brown capacity parameter and is based on the tendency of the upflowing vapor to entrain Hquid with it to the plate above. The term E in equation 43 is called an E-factor. and E to be meaningful the cross-sectional area to which they apply must be specified. The capacity parameter is usually based on the total column cross section minus the area blocked for vapor flow by the downcomer(s). Eor the E-factor, typical operating ranges for sieve plate columns are... 

Plate-Column Capacity The maximum allowable capacity of a plate for handling gas and liquid flow is of primaiy importance because it fixes the minimum possible diameter of the column. For a constant hquid rate, increasing the gas rate results eventually in excessive entrainment and flooding. At the flood point it is difficult to obtain net downward flow of hquid, and any liquid fed to the column is carried out with the overheaa gas. Furthermore, the column inven-toiy of hquid increases, pressure drop across the column becomes quite large, and control becomes difficult. Rational design caUs for operation at a safe margin below this maximum aUowable condition. 

These two types of flooding are usuaUy considered separately when a plate column is being rated for capacity. For identification purposes they are caUed entrainment flooding (or priming ) and downflow flooding. When counterflow action is destroyed by either type, transfer efficiency is lost and reasonable design hmits have been exceeded. 

Entrainment Entrainment in a plate column is that liquid which is carried with the vapor from a plate to the plate above. It is detrimental in that the effective plate efficiency is lowered because hquid from a plate of lower volatility is carried to a plate of higher volatility, thereby diluting distillation or absorption effects. Entrainment is also detrimental when nonvolatile impurities are carried upward to contaminate the overhead product from the column. 

Column diameter for a particular service is a function of the physical properties of the vapor and liquid at the tray conditions, efficiency and capacity characteristics of the contacting mechanism (bubble trays, sieve trays, etc.) as represented by velocity effects including entrainment, and the pressure of the operation. Unfortunately the interrelationship of these is not clearly understood. Therefore, diameters are determined by relations correlated by empirical factors. The factors influencing bubble cap and similar devices, sieve tray and perforated plate columns are somewhat different. 

The intermediate-level waste concentrator handles the low-level waste concentrate, contaminated aqueous solutions from solvent washing, and many other streams with appreciable solids content. With more exhaustive entrainment removal, as by partial reflux of condensate through a bubble-plate or sieve-plate column, water sufficiently pure for return to process can be produced. If concentrator bottoms are concentrated to the point of incipient crystallization, they are routed to waste storage. If still unsaturated, they are routed to the high-level waste concentrator. 

The term lower limiting velocity is used to denote the vapour velocit - (referred to unit cross section of the empty column) below which the effectiveness of the column begins to fall off, the term upper limiting velocity for the vapour velocitj at which flooding is so intense that, in plate columns, the layer of liquid on a j)late is entrained upwards, and in packed columns a quantity of spraying liquid rises from the foot of the column to the top. Since the fundamental publications by Mach [219],... 

A basic requirement for all plate columns is that the distance between the plates should be sufficient to prevent mechanical entrainment of liquid. Liquid carried upward by the vapour flow would markedly decrease the efficiency. Wagner et al. [48] made experiments with radioactive tracers in a vacuum distillation apparatus to determine non-volatde impurities in the distillate. In connection with mechanical entrainment Newitt et al. [49] made theoretical and experimental studies of the mechanism of droplet formation and of droplet size distribution. 

At low operating pressures, due to the large gas volumes, plate columns exhibit pronounced droplet entrainment, which lowers the separation performance due to backmixing of the liquid. The throughput can be increased by introducing into the spray layer a 5-10 high packed layer that acts as a mist collector. 

Plate columns are only used in processes that do not require a solid catalyst and for which relatively long contact times are needed. Since the liquid flow is evenly distributed over the complete height of the column, large diameters can be used. The interfacial area per unit volume of gas liquid mixture is larger than in packed columns, but on the other hand plate columns only have gas-liquid mixtures on the plates themselves. Whether there is more interfacial area per unit total volume of column in the plate column depends on the plate spacing which is determined by the presence or absence of downcomers, foaming, entrainment, and so on. 

Several investigators have published quantitative data on the amount of entrainment in bubble-plate columns. Most of their investigations have been on systems involving air and water. 

Holbrook and Baker (Ref. 16) studied entrainment in an 8-in. bubble-plate column using steam and water. Curves C are based on a portion of their data. They conclude that the plate spacing and vapor velocity were the main factors in determining the amount of entrainment and that the amount of liquid flow and slot-vapor velocity were of less importance. 

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

For cross-flow plates, net area is the column cross section less that area blocked by the downcomer or downcomers (Fig. 14-22). The vapor velocity in the net area represents an approach velocity and thus controls the level of liquid entrainment. For counterflow plates, net area is the same as the column cross section, since no downcomers are involved. 

The gas risers must have a sufficient flow area to avoid a high gas-phase pressure drop. In addition, these gas risers must be uniformly positioned to maintain proper gas distribution. The gas risers should be equipped w ith covers to deflect the liquid raining onto this collector plate and prevent it from entering the gas risers where the high gas velocity could cause entrainment. These gas riser covers must be kept a sufficient distance below the next packed bed to allow the gas phase to come to a uniform flow rate per square foot of column cross-sectional area before entering the next bed. 

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