Vapor-liquid separators Foaming

Demister, 292, 409-41 1 vapor-liquid separation, 409-411 Depressurization (foaming), 412 Depropanizer reboiler limits (hydrofluoric acid alkylation), 152 Depropanizer, 149, 152, 278-279 Desalter operation, 25, 27 Detergent cleaners, 100 Dew point, 13, 26, 124, Appendix Diesel oil, 19-21 stripping, 20-21 Diethanolamine (DEA), 90 Differential pressure, 21 Differential pressure indicator (pressure surveys), 513, 520 Digital computer, 17 Diolefin formation (vapor recovery unit), 208... 

Foam-breaking cyclones can be installed in either vertical or horizontal vessels. In some offshore drilling operations, certain vapor-liquid separation vessels are routinely retrofitted with defoaming separators if there is a known or suspected foaming problem. This is usually possible since the individual tubes can pass through existing manways and be assembled inside the main separator vessel. If necessary, the separator assembly can be supported off the inside walls of the main vessel, as well as the vessel s inlet, thereby avoiding hotwork . 

Conditions sometimes exist that may make separations by distillation difficult or impractical or may require special techniques. Natural products such as petroleum or products derived from vegetable or animal matter are mixtures of very many chemically unidentified substances. Thermal instability sometimes is a problem. In other cases, vapor-liquid phase equilibria are unfavorable. It is true that distillations have been practiced successfully in some natural product industries, notably petroleum, long before a scientific basis was established, but the designs based on empirical rules are being improved by modern calculation techniques. Even unfavorable vapor-liquid equilibria sometimes can be ameliorated by changes of operating conditions or by chemical additives. Still, it must be recognized that there may be superior separation techniques in some cases, for instance, crystallization, liquid-liquid extraction, supercritical extraction, foam fractionation, dialysis, reverse osmosis, membrane separation, and others. The special distillations exemplified in this section are petroleum, azeotropic, extractive, and molecular distillations. 

Columns operated at vapor and liquid flow rates greater than those for which they were designed become flooded. Unexpected foaming can also cause flooding. In a flooded column, liquid cannot properly descend against the upflowing vapor. Poor separation performance results, the overhead condensation circuit fills with process liquid, the reboiler is starved of process liquid, and the column quickly becomes inoperable. 

Neighboring bubble surfaces in a foam interact through a variety of forces that depend on the composition and thickness of liquid between them, and on the physical chemistry of their liquid-vapor interfaces. For a foam to be relatively stable, the net interaction must be sufficiently repulsive at short distances to maintain a significant layer of liquid in between neighboring bubbles. Otherwise two bubbles could approach so closely as to expel aU the liquid and fuse into one larger bubble. Repulsive interactions typically become important only for bubble separations smaller than a few hundredths of a micrometer, a length small in comparison with typical bubble sizes. Thus attention can be restricted to the vapor-liquid-vapor film structure formed between neighboring bubbles, and this structure can be considered essentially flat. 

If a trayed column in high pressure service does not have sufficient downcomer residence time to clarify the liquid phase, the vapor/liquid mixture will be carried down to the next lower tray. This recycling of vapor lowers the capacity, as well as the separation efficiency, of the trays. Conditions preventing rapid disengagement of vapor and liquid in a tray downcomer are high liquid viscosity, low surface tension, and a small density difference between liquid and vapor. Also, any tendency for the liquid to foam will increase the downcomer residence time required. 

The chemical equilibrium in a multiphase system and the corresponding species concentrations in chemical separations are altered when chemical reactions take place. We will illustrate this now for gas-liquid equilibrium (as in gas absorption), vapor-liquid equilibrium (as in distillation), liquid-liquid equilibrium (as in solvent extraction), stationary phase-liquid equilibrium (as in ion exchange, chromatography and crystallization), surface adsorption equilibrium (as in foam fractionation) and Donnan equilibrium. 

The LT evaporator shown in Fig. 11-122/ is typical of those commonly used, especially for black hquor. Feed enters at the bottom of the tube and starts to boil partway up the tube, and the mixture of liquid and vapor leaving at the top at nigh velocity impinges against a deflector placed above the tube sheet. This deflector is effective both as a primary separator and as a foam breaker. 

In a packed column, liquid and vapor flow counter-currently and separation between the liquid and vapor phases takes place continuously. In contrast, in a column with trays, separation occurs in stages. In a packed column, vapor does not bubble through the liquid as in the columns with trays. For this reason, and due to the absence of the vapor-flow orifices, packed columns operate at a much lower pressure drop. In addition, because liquid and vapor contact in a packed column is less agitated than in a trayed column, packed columns are less likely to foam. 

Foam formation in a boiler is primarily a surface active phenomena, whereby a discontinuous gaseous phase of steam, carbon dioxide, and other gas bubbles is dispersed in a continuous liquid phase of BW. Because the largest component of the foam is usually gas, the bubbles generally are separated only by a thin, liquid film composed of several layers of molecules that can slide over each other to provide considerable elasticity. Foaming occurs when these bubbles arrive at a steam-water interface at a rate faster than that at which they can collapse or decay into steam vapor. 

Effectiveness of Internals in Controlling foam. Originally, the separators contained limited internals—an inlet disengaging device, surge baffles, an anti-foam baffle, and a demister at the outlet. After severe foaming occurred, however, a bank of parallel plates (Dixon plates) was filled in each separator on one of the Ninian and one of the Brent trains The plates provide a large surface area and are sloped so that the separated oil flows dow n to the surface of the liquid. The installation is illustrated m Ftg S. The parallel plates arc 4 in. apart and extend from near the top of the vessel in the vapor space down to the normal liquid level. 

High vapor velocities, combined with high foam levels, will cause the spray height to hit the underside of the tray above. This causes mixing of the liquid from a lower tray, with the liquid on the upper tray. This backmixing of liquid reduces the separation, or tray efficiency, of a distillation tower. 

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