Pressure vapor entrainment

At high liquid loads (> 15 gpm/ft of bed cross section) and high pressures, vapor entrainment in the liquid may restrict structured packings capacity well before flooding is approached. This phenomenon causes efficiency to rapidly diminish as throughout is raised. Random packings also experience this type of limitation (386), but to a lesser extent, because of the unrestricted lateral movement of vapor and liquid. 

The pressure at any point in the suction line must never be reduced to the vapor pressure of the liquid (see Equation 3-6). Both the suction head and the vapor pressure must be expressed in feet of the liquid, and must both be expressed as gauge pressure or absolute pressure. Centrifugal pumps cannot pump any quantity of vapor, except possibly some vapor entrained or absorbed in the liquid, but do not count cm it. The liquid or its gases must not vaporize in the eye/entrance of the impeller. (This is the lowest pressure location in the impeller.)... 

Variations in temperature, pressure, vapor composition, and possible liquid entrainment into the relieving system during the depressuring period... 

At pressures exceeding 10 to 20 bar (150 to 300 psia), and especially at high liquid rates, vapor entrainment into the downcomer liquid becomes important, and tray efficiency decreases with further increases in pressure [Zuiderweg, Int. Chem. Eng. 26(1), 1 (1986)]. 

Operating conditions for the absorption and stripping towers are important design parameters for the process. Due to vapor pressure and entrainment, propane will be present in the effluent gas streams from both the absorber and stripper. Usually this quantity of propane is not recovered and is considered an economic loss. The amount of propane in the gas phase is mainly dependent on the operating temperature and pressure of the towers. 

Downcomer vapor underflow ("vapor entrainment" or "gas recycle1 ) is analogous to liquid entrainment. It reduces both tray capacity and efficiency (17,44,45). In low- and medium-pressure distillation systems, where gas density is significantly lower than liquid density, it takes only a small quantity of gas to generate volumes comparable to the liquid volumetric flow rate. The quantity of gas recycle is therefore small, and it has little effect on tray performance. At high pressures, the quantity of gas recycled is significant. An analysis of some FRI data (44) for iC4-nC4 distillation showed vapor entrainment increases from about 7 percent at 165 psia to about 50 to 60 percent at 400 psia on a molar basis. 

In the emulsion regime [high pressure (> 150 psia) and/or high liquid rates], vapor entrainment through the downcomer (Sec. 6.4.5) is not large enough to affect efficiency. 

An alternative explanation, preferred by the author, is in terms of mechanisms postulated by Kurtz et al. (31a). As liquid rate increases, more vapor is entrained down the bed. This drops efficiency. Because structured packings permit far less lateral movement of fluids than random packings, far more vapor will be carried downward. The vapor entrainment will be most detrimental to efficiency when fluid lateral movement is restricted most. This can be expected with narrow flow channels (e.g., wire-mesh structured packings), at high liquid rates and high pressure. 

A liquid level is maintained with an overflow weir while the vapor comes up through the perforated floor at sufficient velocity to keep most of the liquid from weeping through. Hole sizes may range from 1/8 to 1 in., but are mostly l/4-l/2in. Hole area as a percentage of the active cross section is 5-15%, commonly 10%. The precise choice of these measurements is based on considerations of pressure drop, entrainment, weeping, and mass transfer efficiency. The range of conditions over which tray operation is... 

In summery, the downcomer can limit column capacity when liquid flow rains are high, as in absorbers and pressure fractionators. Two viewpoints are used (and these ate not necessarily independent of each other) height of froth baildup in the downcomer, obtained from a pressure balance, and residence time in the downcomer, obtained from an entrainment velocity limitation. When the downcomer backs up liquid, the vapor entrains more liquid, and a flooding condition can be approached. 

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 ... 

A special case of desorption is vapor-entrainment distillation, which is used for the thermally mild separation of high-boiling substances that are immiscible with water. Steam is generally used as entraining vapor (steam distillation) since it can subsequently simply be condensed, but nitrogen is also used. The maximum temperature is limited to the boiling point of water at the operating pressure (Equation 2.3.3-9) ... 

The entrainer should alter the separation factor in a way that the separation factor becomes different from unity. Since the entrainer has no influence on the pure component vapor pressures, the entrainer has to shift the ratio of the activity coefficients selectively. Although in practice a concentration of 50-80% entrainer is used in the columns, for the selection of a selective entrainer in the first step the selectivity at infinite dilution is used. 

We assign the efficiencies i, and t] to these regimes. The unit enthalpy of the mass Ml of motive steam at supply conditions is Hi. After isentropic expansion to the pressure in the nozzle, this enthalpy becomes H2. The steam entrains the mass M2 of low-pressure vapor, and the unit enthalpy of the mixture at low pressure is H3. The mixmre, of mass Ml + M2, is compressed in the diffuser and acquires the unit enthalpy H4 at the outlet of the ejector. 

Under sufficient pressure to permit a Hquid phase at 55—56°C, the acetaldehyde monoperoxyacetate decomposes nearly quantitatively into anhydride and water in the presence of copper. Anhydride hydrolysis is unavoidable, however, because of the presence of water. When the product is removed as a vapor, an equiUbrium concentration of anhydride higher than that of acetic acid remains in the reactor. Water is normally quite low. Air entrains the acetic anhydride and water as soon as they form. 

The vapor pressure may be dependent on the amount of the dissolved, not the entrained, air in the Hquid. This issue is important to appHcations of cooling-water double-suction pumps (58,59). Because of the unknowns, a safety margin is always recommended for use to minimise the effects of cavitation. 

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