Steam recompression

The compressors to be covered in this book are those using mechanical motion to effect the compression. These types of compressors are commonly used in the process and gas transport/distribution industries. A partial list of these industries includes chemical, petrochemical, refinery, pulp and paper, and utilities. A few typical applications are air separation, vapor extraction, refrigeration, steam recompression, process and plant air. 

Figure 7.7 Simplified process flow diagram of superheated steam dryer configuration to recover energy from dryer exhaust a) excess steam with recuperation by high-pressure steam b) excess steam recompressed to superheat c) excess steam with recuperating by electricity.

Note Low-heat design incorporates steam recompression and heat exchange techniques to increase energy efficiency. 

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. 

A comparable statement can be made with regard to the power apphed to a mechanical recompression evaporator.) In summary, the steam flow required to increase the sohd content of the feed from Xq to X. is... 

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. 

It is therefore necessary to provide an additional source of heat. Although low-pressure steam is not available, presumably a high-pressure supply (say 1135 kN/m2) exists, and vapour recompression using a steam-jet ejector could be considered. 

An evaporator operating on the thermo-recompression principle employs a steam ejector to maintain atmospheric pressure over the boiling liquid. The ejector uses 0.14 kg/s of steam at 650 kN/m2, and is superheated by 100 K and the pressure in the steam chest is 205 kN/m2. A condenser removes surplus vapour from the atmospheric pressure line. What is the capacity and economy of the system and how could the economy be improved ... 

The utilities required for the refrigeration system other than power are therefore very much less than for recompression with steam, although the capital cost and the cost of power will be much higher. 

The wax cake is washed on the filter with cold propane to displace the oil-solvent solution trapped in the cake. The cake is discharged continuously from the filter drum and then is transferred from the filter by means of a screw conveyor. The propane is recovered from both the wax cake and oil solution by distillation and is recompressed for re-use. The last traces of propane are removed at atmospheric pressure and with steam. 

Steam jets are also employed to recompress low-pressure steam to a higher-pressure steam. Jets are sometimes used to compress low-pressure hydrocarbon vapors with higher-pressure hydrocarbon gas (instead of steam). They are really wonderful and versatile machines. 

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... 

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. 

Sulphur is detrimental to the sjmthesis and trace amounts of sulphur are removed using zine oxide prior to synthesis. After the production of synthesis gas, the methanol sjmthesis requires compression to about lOObar. The methanol synthesis loop con rises a reactor, a separator and recompression of the reeyele gas. A purge gas can be used to produce power supplemented by steam raised in the methanol reactor and the coal gasifier. The crude methanol produced can be upgraded to chemical grade product by distillation. The intermediate methanol is passed into storage. The reaction stoichiometry is ... 

Thermodynamically, the reforming reaction. Equation 3.5.1, shows that the reformer should be operated at die lowest pressure and highest temperature possible. The reforming reaction occurs on a nickel-oxide catalyst at 880 C (1620 "F) and 20 bar, which results in a 25 "C approach to the equihbrium temperature [25,29]. Methane conversion increases by reducing the pressure, but natural gas is available at a high pressure. It would be costly to reduce the reformer pressure and then recompress the synthesis gas later to 100 bar (98.7 atm) for the converter. The steam to carbon monoxide ratio is normally in the range of 2.5 to 3.0 [30]. The ratio favors both the conversion of methane to carbon monoxide and the carbon monoxide to carbon dioxide as indicated by Equations 3.5.1 and 3.5.3. If the ratio is decreased, the methane concentration increases in the reformed gas, but if the ratio is set at three, the unreacted methane is small. The methane is a diluent in the synthesis reaction given by Equation 3.5.2. 

Benfield LoHeat System (UOP) [677], The high thermal efficiency of this two-stage adsorption process using lean and semi-lean solvent is achieved by recompression of the flash steam with an injector or a mechanical vapor compressor. UOP has also developed a number of other process configurations [404], [669], [678] -[680], The diethanolamine promoter enhances the mass transfer by carbamate... 

Psia, (b) co-current CO2 rinse at that pressure with recycle of effluent gas as feed, (c) counter-current evacuation to - 2.5 Psia with simultaneous steam pui e, and (d) counter-current pressurization with a part of CO2 depleted product gas from step (a). The effluent gas from step (c) is partly used as the purge gas in step (b) after recompression, and the balance is withdrawn as the recovered CO2 product gas. The inert gas and CO2 recoveries from the feed gas are, respectively, - 100 and 78% [21]. This application may be attractive for green house gas emission control (CO2 sequestration from a hot flue gas or CO2 removal from combustion gases for power generation). 

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. 

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