Vapor-liquid separators Other types

Horizontal Blowdown Drum/Catch Tank This type of drum, shown in Fig. 26-16, combines both the vapor-liquid separation and holdup functions in one vessel. Horizontal drums are commonly used where space is plentiful, such as in petroleum refineries and petrochemical plants. The two-phase mixture usually enters at one end and the vapor exits at the other end. For two-phase streams with very high vapor flow rates, inlets may be provided at each end, with the vapor outlet at the center of the drum, thus minimizing vapor velocities at the inlet and aiding vapor-hquid separation. 

The condenser is the stage where overhead vapors are condensed and liquid is returned to the top of the column as reflux. The condenser is partial if only part of the vapor is condensed and refluxed and the remainder leaves the condenser as vapor distillate. This type of condenser adds one equilibrium stage to the column trays because it holds a vapor phase and a liquid phase at equilibrium with each other. A total condenser is one where the entire overhead vapor is condensed (cooled to the bubble point temperature or subcooled to a lower temperature), part of the condensate is returned as reflux, and the remaining part is taken as liquid distillate. This type of condenser does not count as an equilibrium stage because no vapor-liquid separation takes place in it. The liquid distillate composition is identical to the composition of the vapor leaving the column top tray. 

So far the discussion has been specific to systems at constant temperature equivalently, pressure could be fixed and temperature and liquid phase composition taken as the variables. Although much experimental vapor-liquid equilibrium data are obtained in constant-temperature experiments, distillation columns and other vapor-liquid separations equipment in the chemical process industry are operated more nearly at constant pressure. Therefore, it is important that chemical engineers be familiar with both types of calculations. 

Falling-Jilm-type evaporator. A variation of the long-tube type is the falling-film evaporator, wherein the liquid is fed to the top of the tubes and flows down the walls as a thin film. Vapor-liquid separation usually takes place at the bottom. This type is widely used for concentrating heat-sensitive materials such as orange juice and other fruit juices, because the holdup time is very small (5 to 10 s or more) and the heat-transfer coefficients are high... 

These three approaches have found widespread application to a large variety of systems and equilibria types ranging from vapor-liquid equilibria for binary and multicomponent polymer solutions, blends, and copolymers, liquid-liquid equilibria for polymer solutions and blends, solid-liquid-liquid equilibria, and solubility of gases in polymers, to mention only a few. In some cases, the results are purely predictive in others interaction parameters are required and the models are capable of correlating (describing) the experimental information. In Section 16.7, we attempt to summarize and comparatively discuss the performance of these three approaches. We attempt there, for reasons of completion, to discuss the performance of a few other (mostly) predictive models such as the group-contribution lattice fluid and the group-contribution Flory equations of state, which are not extensively discussed separately. 

Exchange distillation takes advantage of the fact that condensed phase-vapor phase separation factors can be enhanced (as compared to liquid-vapor factors) by association-dissociation equilibria in one or the other phase. A good example is found in B/ B separations. At the normal boiling point of 173 K, the vapor pressure isotope effect for BF3, In a In [T( BF3)/ P("BF3)] = —0.0075, is sufficiently large for useful separation by distillation (Neetley et al., 1958). Compare this effect, however, with association-dissociation constants (f q) for reactions of the type... 

Plate evaporators may be used as a heating element in a forced circulation system in which boiling is deliberately suppressed. Bolling may also occur within the gasketed-plate evaporator with the mixture of liquid and vapor discharged into a cyclone or other type of separator. 

The above definitions and discussion of surface tension are for a liquid in equilibrium with its vapor. If the liquid is in contact with another substance, then, instead of just having one surface, there is now an interface to be considered. The interface is the region of contact between two dissimilar substances. Consider the situation in Figure 8(a), where a single substance is separated into two parts. If the surfaces are of unit area and no other type of dissipation of work or energy occurs, then the work required to cleave the substance should be just the energy necessary to create the two new surfaces, as shown in Eq. (34),... 

The Lewis and Matheson method appears to be the most satisfactory method of handling the general multicomponent design problem. In most cases, it requires relatively little trial and error and will handle cases of normal and abnormal vapor-liquid equilibria. It is especially well suited to the cases in which the reflux ratio and the separation of the key components are specified and the problem is to determine the number of theoretical plates and the component concentrations in the column. In some specific cases, other methods may have advantages, but unless a number of problems of the same type are to be handled, it is more desirable to have one method that will apply to essentially all cases. 

System type (4) Two miscible phases flow countercurrently in two regions of the device separated by a membrane. One of the phases may be generated from one feed phase by the application of pressure energy. Examples include reverse osmosis, ultrafiltration, microflltration, gas permeation, pervaporation. Examples where the other phase is introduced from outside are electrodialysis, dialysis, sweep vapor/ liquid based system. 

Membrane contactors provide a novel approach to the solution of many such problems (especially of the second and third kind) of contacting two different phases, one of which must be a fluid. Essentially, a porous membrane, most often in hollow-fiber form, is the basic element in such a device. Any membrane in flat or spiral-wound or hollow-fiber or any other form has two interfaces since it has two sides. However, conventional separation processes involve usually one interface in a two-phase system, for example, gas-liquid, vapor-liquid, liquid-liquid, hquid-supercritical fluid, gas-solid, liquid-solid, and the like. Membrane contactors allow the creation of one immobilized phase interface between two phases participating in separation via the porous membrane. Three types of immobilized phase interfaces in two-phase configurations are relevant ... 

Conventional Flare System - The majority of pressure relief valve discharges which must be routed to a closed system are manifolded into a conventional blowdown drum and flare system. The blowdown drum serves to separate liquid and vapor so that the vapor portion can be safely flared, and the separated liquid is pumped to appropriate disposal facilities. The blowdown drum may be of the condensible or noncondensible type, according to the characteristics of the streams entering the system. Selection criteria, as well as the design basis for each type of blowdown drum, are detailed later in this volume. The design of flares, including seal drums and other means of flashback protection, is described later. 

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