Overhead Vapor Compression

Figure 16-2 Distillation column with heat pumping by overhead vapor compression.

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which cannot be condensed at the low pressures employed. This gas must be vented by compressing it to atmospheric pressure. This is accomplished by means of steam jet ejectors. Ideally, it would be possible to operate a vacuum pipe still without ejectors, with the overhead vapors composed only of steam. In practice, however, leakage of air into the system and the minor cracking which occurs make it necessary to provide a means of removing non-condensibles from the system. In addition to the distillation of atmospheric residuum, the lube vacuum pipe still is also used for rerunning of off specification lube distillates. 

In the aforementioned process, the heat for the reboiler is usually available as waste heat from the steam cracker, for example, and is essentially cost-free. If this heat is not available, a heat pump can be used. The heat pump can upgrade the heat, at an exergetic cost, to the desired temperature level. If the separation is viewed in isolation, this means that the heat rejected by the condenser at relatively low temperature, can be upgraded to be the higher temperature heat input for the reboiler. A schematic of the heat pump process is given in Figure 10.2. The overhead vapors are heated slightly in the reflux subcooler, which enables these vapors to be compressed and cooled in the condenser-reboiler. 

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. 

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). 

For example, the reboiler and condenser from the same column may be heat-integrated by using a heat pump, as discussed by Null [Chem. Eng. Prog., 72(7), 58 (1976)]. A heat pump may use an external fluid which is vaporized in the condenser, then compressed and condensed in the column reboiler (Fig. 13-75 ). Another heat pump (Fig. 13-75b) uses column overhead vapor, which is compressed and condensed in the reboiler and then returned to the top of the column as reflux. A third possibility is to use the column bottoms, which is let down in pressure and vaporized in the condenser, then compressed and fed back to the bottom of the column as boil-up (Fig. 13-75c). 

The cracked gas in this scheme shown on the next figure is compressed before being caustic-washed and dried. The dried cracked gas is sent to the depropanizer who e the overhead is compressed. It then goes to a fi ont-end acetylene reactor where the acetylene is hydrogenated to ethylene and ethane. After hydrogoiation, the gases are partially condensed to xovide reflux for the depropanizer. The net overhead liquid and vapor, which contain the C3 and lighter... 

Rather than setting the pressure of a distillation column at a level sufficiently high to permit the use of cooling water in the overhead condenser, one may specify a lower pressure and use a refrigerant in the condenser. For example, the separation of propylene-propane, as specified in Fig. 17.2, can be conducted by low-temperature distillation at a 100-psia column overhead pressure, as shown in Fig. 17.8, if a feed system such as shown in Fig. 17.9 is provided. There, the feed mixture is compressed in two stages with an intercooler. A refrigerant-cooled condenser, which follows a water-cooled aftercooler, prepares a saturated liquid feed for the distillation operation. In Fig. 17.8, refrigerant must be supplied to the partial condenser to condense the overhead vapor to obtain reflux at 43 F. At... 

Conventional, (Fig. 17.8) Heat Pump with External Refrigerant, (Hg. 17.11) Heat Pump with Compression of Overhead Vapor, (Fig. 17.12) Heat Pump with Reboiler Liquid Rashing, (Fig. 17.13)... 

Figure 17.10. Use of heat pumps in distillation, (a) Heat pump with external refrigerant, (h) Heat pump with compression of overhead vapor, (c) Heat pump with reboiler liquid flashing. [H. R. Null, Chem. Eng, Progr., 72 (7), 58-64 (1976).]...

Figure 17.12. Low-temperature distillation using heat pump with compression of overhead vapor for separation of propylene-propane system...

Figure 2-39 gives a detailed diagram of a rectification unit with direct vapor compression of the overhead product vapor. 

Compression evaporation could be defined as an evaporation process in which part, or all, of the evaporated vapor is compressed by means of a suitable compressor to a higher pressure level and then condensed the compressed vapor provides part of all of the heat required for evaporation. Compression evaporation is frequently called recompression evaporation. All compression methods use the vapors from the evaporator and recycle them to the heating side of the evaporator. Compression can be achieved with mechanical compressors or with thermal compressors. Thermal compression uses a steam jet to compress a fraction of the overhead vapors with high pressure steam. Mechanical compression uses a compressor driven by a mechanical drive (electric motor or steam turbine) to compress all the overhead vapors. 

To reduce the energy required for a distillation process with almost pure water as overhead vapor, a heat-pump arrangement is proposed. The column is operating at atmospheric pressure. The reboiler needs a vapor condensing at 150 °C. The overhead water vapor should be compressed by a reversible adiabatic compression process such that it can be tbe condensing vapor in tbe reboiler. Determine the heat delivered by the condensing vapor per pound of the vapor calculate the ratio of the heat delivered in the reboiler to the work required in the reversible compressor. Use tbe steam tables from Smith and Van Ness (1975) or any other suitable source. (Ans. 1020 Btu/fb (567 cal/g) 7.254.)... 

Water washing of the overhead systems of debutanizers and depropanizers is indicated only if serious fouling problems occur. Normally, these streams are quite dry and should be kept that way to minimize corrosion and hydrogen blistering problems. With proper water washing of the compressed wet-gas stream, water washing of the overhead vapor streams of the debutanizer and depropanizer towers becomes unnecessary. 

The rich oil from the absorber is expanded through a hydrauHc turbiae for power recovery. The fluid from the turbiae is flashed ia the rich-oil flash tank to 2.1 MPa (300 psi) and —32°C. The flash vapor is compressed until it equals the inlet pressure before it is recycled to the inlet. The oil phase from the flash passes through another heat exchanger and to the rich-oil deethanizer. The ethane-rich overhead gas produced from the deethanizer is compressed and used for produciag petrochemicals or is added to the residue-gas stream. 

Chevron s WWT (wastewater treatment) process treats refinery sour water for reuse, producing ammonia and hydrogen sulfide [7783-06-04] as by-products (100). Degassed sour water is fed to the first of two strippers. Here hydrogen sulfide is stripped overhead while water and ammonia flow out the column bottoms. The bottoms from the first stripper is fed to the second stripper which produces ammonia as the overhead product. The gaseous ammonia is next treated for hydrogen sulfide and water removal, compressed, and further purified. Ammonia recovery options include anhydrous Hquid ammonia, aqueous Hquid ammonia, and ammonia vapor for incineration. There are more than 20 reported units in operation, the aimual production of ammonia from this process is about 200,000 t. 

Butyl slurry at 25—35 wt % mbber continuously overflows from the reactor through a transferline to an agitated flash dmm operating at 140—160 kPa (1.4—1.6 atm) and 55—70°C. Steam and hot water are mixed with the slurry in a nozzle as it enters the dmm to vaporize methyl chloride and unreacted monomers that pass overhead to a recovery system. The vapor stream is compressed, dried over alumina, and fractionated to yield a recycle stream of methyl chloride and isobutylene. Pure methyl chloride is recovered for the coinitiator (AlCl ) preparation. In the flash dmm, the polymer agglomerates as a coarse cmmb in water. Metal stearate, eg, aluminum, calcium, or zinc stearate, is added to control the cmmb size. Other additives, such as antioxidants, can also be introduced at this point. The polymer cmmb at 8—12 wt % in water flows from the flash dmm to a stripping vessel operated under high vacuum to... 

Administration Building. To reduce the risk from the potential blast that could occur from either gas compression/reaction or feed purification/vaporization, safety film was installed over the windows, a catch system was provided to capture flying window fragments, and overhead fixtures were secured. This reduced the risk ranking for the administration building to IV. 

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