Liquid entrainment

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. 

For distillations, it is often of more interest to ascertain the effect of entrainment on efficiency than to predic t the quantitative amount of liquid entrained. For this purpose, the correlation shown in Fig. 14-26 is useful. The parametric curves in the figure represent approach to the entrainment flood point as measured or as predicted by Fig. 14-25 or some other flood correlation. The abscissa values are those of the flow parameter discussed earher. The ordinate values y are fractions of gross hquid downflow, defined as follows ... 

Entrainment Due to Gas Bubbling/Jetting through a Liquid Entrainment generally hmits the capacity of distiUation trays and is commonly a concern in vaporizers and evaporators. Fortunately, it is readily controllable bv simple inertial entrainment capture devices such as wire mesh pads in gravity separators. 

The major variable in setting entrainment (E, weight of liquid entrained per weight of vapor) is vapor velocity. As velocity is increased, the dependence of E on velocity steepens. In the lowest velocity regime, E is proportional to velocity. At values of E of about 0.001 (around 10 percent of flood), there is a shift to a region where the dependence is with (velocity) ". Near flood, the dependence rises to approximately (velocity). In this regime, the kinetic energy of the vapor dominates, and the bulk of the dispersion on the plate is often in the form of a coarse spray. 

Gases and liquids may be intentionally contacted as in absorption and distillation, or a mixture of phases may occur unintentionally as in vapor condensation from inadvertent cooling or liquid entrainment from a film. Regardless of the origin, it is usually desirable or necessary ultimately to separate gas-liquid dispersions. While separation will usually occur naturally, the rate is often economically intolerable and separation processes are employed to accelerate the step. 

The amount of material released to the atmosphere will be further reduced because liquid entrainment from the two-phase flashing jet resulting from a leak will be reduced or eliminated. 

Figure 1 is based upon 5% of the liquid entrained in the vapor. This is adequate for normal design. A mist eliminator can get entrainment down to 1%. 

Hats on chimney tray excessively restricted vapor resulting in liquid entrainment. Design error. 

In addition to the large increases in pressure drop, high liquid entrainment into the exiting gas stream also occurs. 

Vapor space velocities normally should not exceed 100% of critical experience demonstrates that this keeps liquid entrainment into the flare line within acceptable limits. However, a velocity of 175% of critical is permitted when one is applyingthe 1.5 times Design Pressure Rule to remote contingencies,... 

Liquid entrainment separator Any device that removes and collects moisture present in an airstream. 

In the drying cycle, the wet inlet gas first passes through an inlet separator where free liquids, entrained mist, and solid particles are removed. This is a very important part of the system because free liquids can damage or destroy the desiccant bed and solids may plug it. If the adsorption... 

The unit shown in Figure 4-49 has been used in many process applications with a variety of modifications [18,19,20]. It is effective in liquid entrainment separation, but is not recommended for solid particles due to the arrangement of the bottom and outlet. The flat bottom plate serves as a protection to the developing liquid surface below. This prevents re-entrainment. In place of the plate a vortex breaker type using vertical cross plates of 4-inch to 12-inch depth also is used, (Also see Reference [58].) The inlet gas connection is placed above the outlet dip pipe by maintaining dimension of only a few inches at point 4. In this type unit some liquid will creep up the walls as the inlet velocity increases. 

The work of Simkin, Strand, and Olney [64] correlates most of the work of other investigators, and can be used for estimation of probable entrainment from bubble cap trays as shown in Figure 8-116. It is recommended that the liquid entrainment for design be limited to 0.10 mols/mol dry vapor. 

Calculate liquid entrainment, W e, as pounds of liquid per square foot of net tray area, equals (Step... 

We = liquid entrainment mass velocity, lbs entrainment per minute per/ft, based on net tray area. 

The tower diameter may be calculated for first approximation by the Souders-Brown method however, this has been found to be conservative, since it is based on no liquid entrainment between trays. Actually, some entrainment can be tolerated at negligible loss in efficiency or capacity. 

Jet Jlooding occurs due to liquid entrainment induced by vaporjets passing through the liquid flowing on the tray. The entrained droplet may carry into the tray area above and reduce tray efficiency and capacity. 

Weight of liquid entrained per unit weight of vapor flowing, Ib/lb... 

Liquid entrainment mass velocity, lb entrainment/ (min) (ft ), based on net tray area of tower minus twice downcomer area Assumed allowable liquid entrainment mass velocity derived from assumed allow able loss mols liq-uid/mol vapor, Ib/hr (ft ), based on net tray areas same as for We... 

Liquid entrainment mass velocity corrected for liquid properties and plate spacing, lb entrainment/(hr) (ft ), based on net tray area as for Wg... 

Recommended [131] rule of thumb to avoid liquid entrainment from the entering liquid by the vapor out of the tower Limit the vapor capacity factor Cj to about 0.4 ft/sec,... 

With increasing vapor rate, the contact between liquid and vapor increases to increase the rate of mass transfer and the HETP value will improve in efficiency of contact and drop from point C to E to point D. With increasing vapor rate, liquid entrainment will occur into the vapor phase and lower the efficiency (and raise the HETP) to... 

E = superficial liquid entrainment rate, Ib/hr/ff Fj, Fj = correlation factors defined by equations... 

E = superficial liquid entrainment, Ib/hr/fL or, heat transfer effectiveness. 

Because reciprocating compressors should be supplied with clean gas, inlet filters are recommended in all applications. They cannot satisfactorily handle liquids entrained in the gas, although vapors are no problem if condensation within the cylinders does not take place. Liquids will destroy the lubrication and cause excessive wear. 

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,... 

For preliminary design, liquid entrainment is usually used as a reference. To prevent entrainment, the vapor velocity for tray columns is usually in the range 1.5 to 3.5 ms-1. However, the entrainment of liquid droplets can be predicted using Equation 8.3 to calculate the settling velocity. To apply Equation 8.3 requires the parameter KT to be specified. For distillation using tray columns, KT is correlated in terms of a liquid-vapor flow parameter FLV, defined by ... 

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