Evaporation Using Vapor Recompression

In the single-effect evaporator the vapor from the unit is generally condensed and discarded. In the multiple-effect evaporator, the pressure in each succeeding effect is lowered so that the boiling point of the liquid is lowered in each effect. Hence, there is a temperature difference created for the vapor from one effect to condense in the next effect and boil the liquid to form vapor. 

In a single-effect vapor recompression (sometimes called vapor compression) evaporator the vapor is compressed so that its condensing or saturation temperature is increased. This compressed vapor is returned back to the heater of steam chest and condenses so that vapor is formed in the evaporator (B5, Wl, Zl). In this manner the latent heat of the vapor is used and not discarded. The two types of vapor recompression units are the mechanical and the thermal type. 

Sometimes it is necessary to add a small amount of makeup steam to the vapor line before the compressor (B5, K2). Also,-a small amount of condensate may be added to the compressed vapor to remove any superheat, if present. 

Vapor recompression units generally operate at low optimum temperature differences of 5 to 10°C. Hence, large heat transfer areas are needed. These units usually have higher capital costs than multiple-effect units because of the larger area and the 

Some typical applications of mechanical vapor recompression units are evaporation of sea water to give distilled water, evaporation of kraft black liquor in the paper industry (L2), evaporation of heat-sensitive materials such as fruit juices, and crystallizing of salts having inverse solubility curves where the solubility decreases with increasing temperature (K2, M3). 

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Evaporation is energy-intensive. The latent heat associated with separation of a ton of dry NaCl from saturated solution is about 6.5 GJ. With KCl, the equivalent figure depends more on the temperature assumed for the brine as produced but is in the range of 3.5-4 GJ. Energy economization is therefore a major consideration in evaporation process design. Two different approaches are in widespread use, multiple-effect evaporation and vapor recompression. 

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. 

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... 

A variation on standard evaporation technology that is much less energy intensive is the mechanical vapor recompression vaporization process. This uses the same evaporation... 

Crystallization that occurs during evaporation can potentially be intensified by use of vapor recompression and spinning discs. In this scenario, the evaporated vapor is compressed and then condensed on the bottom of the discs to heat the crystallizing fluid (58). This approach may permit operation at higher temperatures, lower surface area, and less time. 

MJ/kWh (10,400 Btu/kWh), mechanical vapor recompression can vaporize 1 kg of water for less than 0.46 MJ (1.0 lb for less than 200 Btu). The Carver-Greenfield process is based on combining mechanical vapor recompression with multiple-effect evaporation to dry high-water-content biomass and other solid suspensions. Many full-scale units have been placed in operation since the first facility was installed in 1961. One unit was used at the Hyperion wastewater treatment plant in Los Angeles from 1987 to early 1995 to dry 40 t/day of biosolids wetcake to 99+% total solids content (Haug, Moore, and Harrison, 1995). The process has since been replaced by rotary steam dryers because it was not possible to reach the design capacity of the unit. 

A vapor-recompression evaporator is to concentrate a very dilute aqueous solution. The feed rate is to be 30,000 Ib/h the evaporation rate will be 20,000 Ib/h. The evaporator will operate at atmospheric pressure, with the vapor mechanically compressed as shown in Fig. 16.12 except that a natural-circulation calandria will be used. If steam costs 8 per 1000 lb, electricity costs 3 cents per kilowatthour, and heat-transfer surface in the heater costs 70 per square foot, calculate the optimum pressure to which the vapor should be compressed. The overall compressor efficiency is 72 percent. Assume all other costs are independent of the pressure of the compressed vapor. To how many effects will this evaporator be equivalent ... 

Vapor recompression technology is widely used in continuous evaporators. Billet has discussed this subject in... 

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. 

Vapor recompression vapor is compressed and used to heat the feed in the single effect forced circulation or MSMPR use for materials having relatively low boiling point elevation and where a lot of heat is required for the evaporation. E.g. sodium sulfate, NaCl, sodium carbonate monohydrate. See also Section 4.1. 

B. Vapor-Recompression Evaporation. The existence of the BPR in a solution means that the condensing temperature of the vapor raised in an evsqiorator will be lower than the boiling point of the solution from which it came. In other words, the vapor as it forms is superheated. When the vapor is used in another effect, the superheat provides very little thermal energy, and the vapor temperature quickly drops to the saturation temperature of pure water at the operating pressure. 

The value of that vapor would be increased if it were boosted to higher pressure and higher condensing temperature. It could then be used in place of steam to produce more evaporation. This is the fundamental idea of vapor-recompression evaporation. If the vapor produced by evaporation has its pressure increased by the black box in Fig. 7.10, it can be returned to the heating zone of the same effect. 

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