Vapor recompression

Although this technique is limited to those materials having relatively low boiling point elevation and to those cases wherein a 

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Various heat pumping schemes have been proposed as methods for saving energy in distillation. Of these schemes, use of the column overhead vapor as the heat pumping fluid is usually the most economically attractive. This is the vapor recompression scheme shown in outline in Fig. 14.6. 

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. 

In Texas, subterranean sulfate brines are pumped to the surface where the brines are first saturated with NaCl before they are cooled by mechanical refrigeration to form Glauber s salt (7,8). This salt is then separated from its mother Hquor, melted, and dehydrated with mechanical vapor recompression evaporators (9). 

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. 

Ethanol removed by the vapor stream can be recovered by condensation, vapor recompression, or scmbbiag. Ia the first two methods, the coaceatratioa of the recovered ethanol depends on the relative humidity of the sweep stream and the ratio of sweep and permeation rates. In scmbbiag, the rate of water deflvery to the Hquid-gas coatactor affects the ethanol concentration ia the recovered stream. 

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. 

Reflux overhead vapor recompression, staged crude pre-heat, mechanical vacuum pumps Fluid coking to gasification, turbine power recovery train at the FCC, hydraulic turbine power recovery, membrane hydrogen purification, unit to hydrocracker recycle loop Improved catalysts (reforming), and hydraulic turbine power recovery Process management and integration... 

For heat pumping to be economic on a stand-alone basis, it must operate across a small temperature difference, which for distillation means close boiling mixtures. In addition, the use of the scheme is only going to make sense if the column is constrained to operate either on a stand-alone basis or at a pressure that would mean it would be across the pinch. Otherwise, heat integration with the process might be a much better option. Vapor recompression schemes for distillation therefore only make sense for the distillation of close boiling mixtures in constrained situations3. 

Figure 21.6 Heat pumping in distillation. A vapor recompression scheme. (From Smith R. and Linnhoff B, 1988, Trans IChemE ChERD, 66 195, reproduced by permission of the Institution of Chemical Engineers.)...

Mechanical textile finishing, 24 622 Mechanical tire shredding, 21 470-472 Mechanical vapor recompression (MVR) crystallizers, sodium carbonate recovery via, 22 789 Mechanics, classical and quantum, 16 734-740... 

A distillation column operates with vapor recompression. 

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. 

Vapor recompression eliminates the need for a conventional heat source, such as steam, to drive the reboiler. There is, however, an electrical energy requirement to drive the compressor which is not present in conventional distillation. The key advantage of vapor recompression is that the cost of running the compressor is often lower than the cost of driving a conventional reboiler. Under ideal conditions, the operating cost of a vapor recompression... 

Vapor recompression is another means of improving energy efficiency. As shown on the left in Figure 2.88, the overhead vapor from the distillation column is compressed to a pressure at which the condensation temperature is greater than the boiling point of the process liquid at the tower bottoms. This way, the heat of condensation of the column overhead is reused as heat for reboiling the bottoms. This scheme is known as vapor recompression. 

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). 

To improve the LoHeat process, a fifth stage of solution flashing was added along with mechanical vapor recompression (MVR) to boost the fifth-stage flashed steam back to the pressure of the regenerator column. The combination of ejectors plus MVR for multi-stage heat recovery is referred to as the Benfield Hybrid LoHeat Process. If this improvement is combined with ACT-1 activator the energy consumption can be reduced to 600 kcal/Nm3 of C02 removed. 

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). 

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