Vapors, compression and

So far only the energy requirement for a process in the form of work has been considered. Freezing, vapor compression, and reverse osmosis processes are examples of processes that require a work input. There are, however, other important processes, such as multiple-effect evaporation and flash evaporation, for which the energy input is in the form of heat. How does one relate the energy requirement of these processes to the minimum work of separation One method is to convert the heat requirement to a work equivalent by means of the Carnot cycle. If T is the absolute temperature of the heat source and T0 the heat-sink temperature, then one can use the familiar relation... 

Some Process Comparisons. Let us make a rough comparison between vapor compression and multiple-effect boiling evaporation under the following assumed conditions ... 

Table V. Comparison of Vapor-Compression and Multiple-Effect Boiling Evaporators...

Fig. 2.8 Schematic of refrigeration system a vapor compression and b vapor absorption...

Nevertheless, there is a long way to go before these systems can reach a commercial application level. There are two main issues. Power density (specific to membrane area unit) attainable by the current membranes (approximately 1 W/m ) is too low to make the technology cost-effective. However, the development of membranes for a specific purpose has just been started and significant improvements are expected in the next future in terms of performance, durability and cost. The second main issue is fouling caused by particles entrained by the streams contacted to membranes, which has to be controlled by expensive and possibly polluting water pretreatment processes. The latter problem is definitely avoided by the other two alternatives proposed, reverse vapor compression and hydrocratic generator, which on the other hand have not yet proved their technical feasibility. 

Necessary temperature rise < 50 K with direct vapor compression and <40 K... 

Fig. 7-15. Nomogram to find the specific energy costs for each ton of water evaporated from an aqueous solution for the case of mechanical vapor compression and multistage evaporation. Representation according to Mannesmann-Engineering AG, Messo-Chemietechnik [7.29].

Evaporator systems are major pieces of process equipment and are often purchased on a total responsibility basis. This is especially true of vapor compression and highly heat integrated systems. Specific design information and fabrication are often proprietary to vendors. Evaporator manufacturers generally are rather specialized. Few offer a complete range of evaporator types some specialize in one type only. 

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. 

Constmction of new power plants in the coal region of the western United States presents serious problems in states whose laws dictate zero effluent. In these plants, cooling-tower water withdrawn from rivers cannot be returned to them. In these situations, cooling-tower effluent is purified by distillation (vapor-compression plants have predominated) and by a combination of distillation and membrane technology. The converted water then is used as boiler feedwater the plant blowdown (effluent) is evaporated from open-air lined pools, and pool sediment is periodically buried back in the coal mine with the flue ashes. 

Eig. 7. Schematic flow diagram of a basic horizontal-tube vapor compression (VC) desalination plant, shown (a) with a mechanical, motor-driven compressor and (b) with a thermocompressor, using an ejector, where (------) represents vapor (—), brine and (-), product. 

The compressor can be driven by electric motors, gas or steam turbiaes, or internal combustion (usually diesel) engines. The compressor can also be a steam-driven ejector (Fig. 7b), which improves plant reUabiUty because of its simplicity and absence of moving parts, but also reduces its efficiency because an ejector is less efficient than a mechanical compressor. In all of the therm ally driven devices, turbiaes, engines, and the ejector mentioned hereia, the exhaust heat can be used for process efficiency improvement, or for desalination by an additional distillation plant. Figure 8 shows a flow diagram of the vertical-tube vapor compression process. 

Fig. 8. Pictorial view (a) and flow diagram (b) for a vertical-tube vapor-compression process. Courtesy of Resources Conservation Co.

Vapor-Compression Evaporation and Waste Heat Evaporation. Both of these processes remove water from contaminants rather than contaminants from water. They are better suited for industrial installations where excess energy is available. The water thus produced is of high quaUty and can be used directly. An important advantage is the concentration of waste-residue volume with attendant economies of handling and transportation... 

The saturation temperature of a vapor rises when it is mechanically compressed and its latent heat is available at a higher temperature. AppHcation of this heat to an aqueous stream evaporates part of the water, producing a distillate of pure water. AppHcation of vapor compression has grown significantly since 1960. 

Seawater Distillation. The principal thermal processes used to recover drinking water from seawater include multistage flash distillation, multi-effect distillation, and vapor compression distillation. In these processes, seawater is heated, and the relatively pure distillate is collected. Scale deposits, usually calcium carbonate, magnesium hydroxide, or calcium sulfate, lessen efficiency of these units. Dispersants such as poly(maleic acid) (39,40) inhibit scale formation, or at least modify it to form an easily removed powder, thus maintaining cleaner, more efficient heat-transfer surfaces. 

The liquefied gas must be maintained at or below its boiling point. Refrigeration can be used, but the usual practice is to cool by evaporation. The quantity of liquid evaporated is minimized by insulation. The vapor may be vented to the atmosphere (wasteful), it may be compressed and reliquefied, or it may be used. 

Vapor-Compression Cycles The most widely used refrigeration principle is vapor compression. Isothermal processes are realized through isobaric evaporation and condensation in the tubes. Standard vapor compression refrigeration cycle (counterclockwise Ranldne cycle) is marked in Fig. ll-72<7) by I, 2, 3, 4. 

Although the T-s diagram is veiy useful for thermodynamic analysis, the pressure enthalpy diagram is used much more in refrigeration practice due to the fact that both evaporation and condensation are isobaric processes so that heat exchanged is equal to enthalpy difference A( = Ah. For the ideal, isentropic compression, the work could be also presented as enthalpy difference AW = Ah. The vapor compression cycle (Ranldne) is presented in Fig. H-73 in p-h coordinates. 

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