Entrainment column diameter

The column diameter is normally determined by selecting a superficial velocity for one (or both) of the phases. This velocity is intended to ensure proper mixing while avoiding hydrodynamic problems such as flooding, weeping, or entrainment. Once a superficial velocity is determined, the cross-sectional area of the column is obtained by dividing the volumetric flowrate by the velocity. 

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 principal factor that determines the column diameter is the vapour flow-rate. The vapour velocity must be below that which would cause excessive liquid entrainment or a high-pressure drop. The equation given below, which is based on the well-known Souders and Brown equation, Lowenstein (1961), can be used to estimate the maximum allowable superficial vapour velocity, and hence the column area and diameter,... 

Entrainment. If entrainment is excessive, column diameter or tray spacing are usually increased. It has been recommended (2,67) that entrainment from the tray should not exceed about 0.10 lb liquid entrained per pound of liquid flow. At higher values, significant efficiency reduction occurs (34). Depending on the service, a lower or higher value can be set (4). For instance, if the column overhead stream is compressed and no knock-out drum is present, the entrainment that can be tolerated is smaller. Also, for trays operating at a high liquid-to-vapor ratio, 0.1 lb of liquid entrained per pound of liquid is an excessive quantify of entrained liquid, and a lower limit is set. 

Many of the correlations presented here are of general applicability although some may be particularly suited for certain tray types as indicated. The main objectives are to determine the tray or column diameter for a given application, to calculate the tray liquid holdup and the pressure drop between the trays, and to check for flooding (entrainment and downcomer) and weeping. 

Most columns also require clarifying zones at each end to provide for coalescence and to minimize entrainment. These zones also are dependent upon column diameter. The combined height required (Z) for the clarifying zones can be approximated by ... 

Entrainment flooding. At incipient flooding, die minimum column diameter is fixed. 

A gas absorption column to handle 3630 kg/hr of a gas is being designed. Based on pressure drop, entrainment, and foaming consideration, the maximum vapor velocity must not exceed 0.61 m/sec. If the density of the vapor is 0.801 kg/m , what is the column diameter ... 

Fig. 7.9 summarizes the hardware design of the RD column. Despite the large column diameter required in the reactive section (5.7 m), it is interesting to note that only 25 % of the column volume is available for holding up the liquid and carrying out the reaction. The rest of the column volume is taken up by freeboard above the tray that is necessary to minimize liquid entrainment to the tray above that, causes flooding. Distillation tray columns are not efficient devices for carrying out slow liquid-phase reactions. 

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. 

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. 

Entrtiinment values of 0.05 lbs liquid/lb vapor are usually acceptable, %vith 0.001 and 0.5 Ib/lb being the extremes. The specific design dictates the tolerance on entrainment From the calculated vapor velocity, Vc, the diameter of the column can be calculated using ... 

You have a sample of crushed coal containing a range of particle sizes from 1 to 1000 gm in diameter. You wish to separate the particles according to size by entrainment, by dropping them into a vertical column of water that is flowing upward. If the water velocity in the column is 3 cm/s, which particles will be swept out of the top of the column, and which will settle to the bottom (SG of the solid is 2.5.)... 

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