Entrainment vapor

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.)... 

As the spent catalyst falls into the stripper, hydrocarbons are adsorbed on the catalyst surface, hydrocarbon vapors fill the catalyst pores, and the vapors entrained with the catalyst also fall into the stripper. Stripping steam, at a rate of 2 to 5 lbs per 1,000 lbs (2 kg to 5 kg per 1,000 kg,) is primarily used to remove the entrained hydrocarbons between catalyst particles. Stripping steam does not address hydrocarbon desorption and hydrocarbons filling the catalyst pores. However, reactions continue to occur in the stripper. These reactions are... 

Spent catalyst from the reactor/cyclones discharges into the stripper. Stripping steam displaces hydrocarbon vapors entrained with the catalyst and removes volatile hydrocarbons from the catalyst. 

Excessive vapor entrainment down the dipleg can increase erosion and possibly catalyst attrition. On the reactor side, excessive entrainment will send more cracked product vapors to the stripper. 

In order to minimize vapor entrainment down the cyclone dipleg, ensuring the primary cyclone diplegs are sufficiently submerged in the dense bed and maintaining the dipleg valves on the secondary cyclone diplegs are essential. 

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)]. 

The sulfate process has, in some cases, been supplanted by the chloride process because of by-product character and disposal. However, a continuous process that uses relatively dilute sulfuric acid (25 to 60%) to temper the violent, original reaction and to reduce the amount of water-vapor-entrained particulates is available. As the process uses more dilute acid than the older batch process, more of the spent acid can be recycled. 

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. 

Equation (6.59) was derived from the orifice equation with an orifice coefficient of 0.6 (3), and assuming pure liquid is passing under the downcomer. Tests by Lockett and Gharani (43) showed that Eq. (6.59 gives conservative predictions, even under conditions of significant vapor entrainment in the downcomer underflow (Sec. 6.2.3). 

Liquid and vapor entrainment. Both represent a recycling of lower-purity materia] which contaminates the tray liquid or vapor, both counteract the mass transfer process and lower efficiency. Liquid and vapor entrainment are discussed in Secs. 6.2.11 and 6.4.5, respectively,... 

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. 

One of the major weaknesses in the method is that it does not account for varying vapor entrainment into the downcomer with changes in vapor rate. 

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. 

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. 

Absorption and Desorption, Stripping, Vapor-Entrainment Distillation... 

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) ... 

---