Vapor recompression system

Heat Pumps. A heat pump is a refrigeration system that raises heat to a useful level. The most common appHcation is the vapor recompression system for evaporation (qv) (Fig. 14). Its appHcation hinges primarily on low cost power relative to the alternative heating media. If electricity price per unit energy is less than 1.5 times the cost of the heating medium, it merits a close look. This tends to occur when electricity is generated from a cheaper fuel (coal) or when hydroelectric power is available. 

The vapor recompression system uses recovered heat (left). The pressure of such a distillation process can be controlled by modulating the speed of the compressor or the bypass around it (right). 

More insight is yet available from the data in Table II. In the refrigerated process, the two condensers and the throttle valve involve more than 50% of the lost work remaining. One way to eliminate the inefficiencies of the condensers is to recycle the latent heat of the overhead vapor in a heat pump (vapor recompression) system, as shown in Figure 3. The distillation tower pressure, and hence its overhead temperature are kept the same, but the overhead, instead of being condensed, is compressed to a pressure at which it will condense at 77°F (about 180 psig). 

Falling-film evaporators are well suited for vapor recompression systems (Wl) because they operate at low-temperature-difference values and have very Tittle entrained liquid which can cause problems in the compressor. Vapor recompression has been used in distillation towers where the overhead vapor is recompressed and used in the reboiler as the heating medium (M2). 

Water is evaporated from purified brine using multiple-effect or vapor recompression evaporators (Figs. 3 and 4). Multiple-effect systems typically contain three or four forced-circulation evaporating vessels (Fig. 4) connected together in series. Steam from boilers suppHes the heat and is fed from one evaporator to the next to increase energy efficiency in the multiple-effect system. 

One physical method that has attracted some commercial interest is evaporation several evaporative plants were installed in Japan in the early 1970s, nearly all followed by incinerators for the sludge produced (60). They are, however, expensive in both capital and operating costs. The most recent evaporation systems use a process known as vapor recompression, which has the claimed advantage of much lower operating costs than the eadier evaporative processes used in the wool industry. Capital costs of these processes are still high. 

Evaporators require a source of heat to operate. This heat may be supplied from a boiler, gas turbine, vapor compressor, other evaporator, or a combination of sources. Multiple effect evaporators are very popular when cheap, high pressure steam is available to heat the system. A Mechanical Vapor Recompression evaporator would use electricity or a gas turbine to drive a compressor that recycles the heat in the evaporator. 

The mechanical vapor recompression (MVR) evaporator uses a turbofan compressor to evaporate water that separates the water from dissolved solids. The MVR system discussed in this... 

THERMAL RECOMPRESSION. In a thermal recompression system the vapor is compressed by acting on it with high-pressure steam in a jet ejector. This results in more steam than is needed for boiling the solution, so that excess steam must be vented or condensed. The ratio of motive steam to the vapor from the solution depends on the evaporation pressure for many low-temperature operations, with steam at 8 to 10 atm pressure, the ratio of steam required to the mass of water evaporated is about 0.5. 

A C2 splitter column uses vapor recompression. Because of the low temperature required to stay below the critical temperatures of ethylene and ethane, the auxiliary condenser must be cooled by a propane refrigeration system. 

Several plants based on the vapor recompression cycle have been constructed and tested in Europe. One system uses a horizontal conveyor band in a tunnel drying setup (Bertin, France). Another manufacturer has developed a fluidized bed dryer using high pressure steam (NIRO, Denmark). The results of both systems have been encouraging. 

Extension of heat pump technology to large-scale continuous operations and temperatures above 120°C points to water vapor as a working fluid, provided steam compression can be done efficiently. In such a system, water evaporated from material being dried is compressed and used for heating incoming material. The principle of operation is similar to the well-known mechanical vapor recompression (MVR) systems. 

FIGURE 54.9 Mechanical vapor recompression + 1 (MVR + 1) Carver-Greenfield process configuration (1) fluidizing tank (2) MVR stage (3) separator (4) vacuum system (5) final evaporator stage (6) oil separation system (7) deoiler and (8) centrifuge. 

By means of evaporation, dissolved pollution is concentrated with the aim of obtaining distilled purified water from wastewater. In mechanical vapor recompression (MVR), the influent is inserted in the system, where it is distributed across heat elements and as a consequence is partly evaporated. This vapor is compressed by a compressor and is then transported to the inner surface of the heat element where it condenses and is collected. The concentrated wastewater is deposited onto the bottom of the device and is subsequently transported by the concentrate pump, after which the cycle starts all over. The technique is effective (ca. 99%), which is dependent on the influent and the type of pollution. 

Zhou, Y., Shi, C., Dong, G., 2014. Analysis of a mechanical vapor recompression wastewater distillation system. Desalination 353, 91-97. 

Vapor recompression evaporators Evaporation rate < design fouled heat transfer surface/uneven movement of liquid over heat transfer surface/feed property changes/excessive noncondensibles from leaks or present in feed/flooded condensate, trap malfunction. Section 5.1/feed temperature < design/water leakage into the system/lower compressor suction pressure, see also Section 2.1. 

Steam economy in a mechanical recompression system can be very high, equivalent to that obtained in a 10- or 20-efifect system. It may be limited by the fact that less vapor... 

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