Vapor recompression heat pumping

These heat pump systems are the vapor recompression heat pump (VRHP) at which the top vapor of the column is compressed and then boils up the bottom liquid in a heat exchanger, the bottom flashing heat pump (BFHP) where the bottom liquid has been flashed through a valve and then condenses the top vapor by exchanging the heat, and the absorption heat pump (AHP) with a cycle of water/ ammonia working pair. 

Vapor recompression heat pump system (VRHP)... 

In the past vapor recompression ( heat pumps ) has often been considered for distillation of materials boiling at low temperatures. The incentive in many instances was to be able to use water-cooled condensers, thus avoiding the expense of refrigeration. Another factor favoring vapor recompression is a small temperature diference between the top and bottom of the column. 

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. 

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.)...

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). 

Figure 4.13. Prefractionation arrangements (a) removing light keys with absorber (b) removing heavy keys with stripper (c) heat pumping (d) vapor recompression (e) reboiler flashing. B bottom product, D distillate, V valve.

In some respects this patent is like the deFillippi and Vivian patent, U.S. 4,349,415 although the deFillippi patent claims all near-critici liquids and supercritical fluids for extracting all organics while this one claims propylene as the extractant for ethanol. Both patents describe that the solvent and the extracted organic are separated by distillation. The major difference is in the supply of energy to the reboiler in the solvent distillation column. U.S. 4,349,415 utilized vapor recompression while this one uses a heat pump. 

It would have b n infoimadve if the patentee had compared the heat pump and vapor recompression processes. 

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. 

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. 

Understand several approaches to designing energy-efficient distillation trains, including the adjustment of tower pressure, multiple-effect distillation, and heat pumping, vapor recompression, and reboiler flashing. 

Heat Pumping, Vapor Recompression, and Reboiler Flashing... 

Figure 10.45 Distillation configurations involving compression (a) heat pumping (b) vapor recompression (c) reboiler flashing.

Another popular heat pump concept, if not widely adopted, is the direct use of the overhead vapor as the heat pump working fluid, an open cycle heat pump termed mechanical vapor recompression (MVR). In MVR the compressor raises the pressure of the overhead vapor to the point where it will condense at a temperature sufficient to drive the reboiler. This has the advantage that the reboiler heat exchanger serves as both the reboiler and the condenser. One disadvantage is the process vapor may not be the most efficient working fluid for the temperatures involved. Nevertheless, primary energy savings over 50% are anticipated [21]. 

An NGL plant was selected to analyze several distillation assisted heat pump processes when compared to conventional distillation. The depropanizer column which is the third column of the NGL plant was suitable for retrofitting by heat pump systems. This conventional process, along with top vapour recompression, bottom flashing and absorption heat pumps, were simulated using the Aspen Plus software, in order to determine economically the best alternative. Distillation with both top vapor recompression and bottom flashing heat pumps allows reduction of operation (energy) costs by 83.3% and 84%, respectively. This improves the economic potential (incorporating capital costs) by 53% and 54%, respectively. 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

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