Evaporators Thermal compression

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. 

Thermal compression is the term used to describe the application of a steam jet ejector or thermocompressor to an evaporator in order to increase the steam economy. A typical system is illustrated in Figure 18-1. Steam jet thermocompressors can be used with either single or multiple-effect evaporators. As a rule-of-thumb, the addition of a thermocompressor will provide an improved... 

Perhaps the highest thermal vapor compression technology in any field can be found in the water desalination industry. Technology in that industry has advanced to designs with more than eight successive effects with steam economies over 12. Thermal compression evaporators beyond seven effects In process industries does not appear to be economical because mechanical vapor compression systems offer a better alternative. 

Thermal Compression—Ot the two vapor-compression methods, thermal compression requires less capital but yields lower heat recovery than does mechanical compression. A steam-jet booster is used to compress a fraction of the vapor leaving the evaporetor so that the pressure and temperature are raised. Thermal compression is normally applied to the first effect on existing evaporators or where the conditions are right for the application in single-effect units. 

The thermal energy savings depend on the available motive-steam pressure end the compression ratio. From steam-jet performance curves, the ratio of suction vapor to motive steam flow rates is obtained and matched with the total steam requirements for the evaporator. The difference in the low rates between the existing evaporator steam without thermal compression and motive steam is the steam savings. The motive steam rate is the new steam consumption. The... 

Elbs et al. introduced the concept of the vitrification concentration, the concentration of polymer in a solvent-polymer blend above which a polymer is glassy and mobility is no longer possible. " Kim and Libera showed that PS-b-PB-b-PS (PS cylinders form by thermal annealing) forms parallel hexagonal cylinders when cast from solution and allowed to evaporate slowly. " Compression in the direction of film thinkness indicates that the film further shrinks after the polymer film reaches the vitrification concentration. 

To calculate the flow fields outside the evaporating meniscus we use the onedimensional model, developed by Peles et al. (1998, 2000, 2001). Assuming that the compressibility and the energy dissipation are negligible (a flow with moderate velocities), the thermal conductivity and viscosity are independent of the pressure and temperature, we arrive at the following system of equations ... 

The HRUBOUT process is a mobile in situ or ex situ thermal desorption process designed to remediate soils contaminated with volatile organic compounds (VOCs) and semivolatile organic compounds (SVOCs). For the ex situ process, excavated soil is treated in a soil pile or in a specially designed container. Heated compressed air is injected into the soil, evaporating soil moisture and removing volatile and semivolatUe contaminants. Heavier hydrocarbons are oxidized as the soil temperature is increased to higher levels over an extended period of time. The vapor is collected and transferred to a thermal oxidizer (incinerator) for destruction. 

The sample to be analyzed, say C60 fullerene, is mixed with an appropriate amount of KBr in an agate mortar and then transferred into a press and compressed at 4,000 Kg into a pellet with a diameter of 1.2 cm and a thickness of 0.2 cm. The pellet was mounted into the sample holder of the Specac variable temperature cell and inserted into the cell. The cell was then evacuated with the aid of a pump to a vacuum of 0.1 torr and then heated gradually at 120°C in order to permit the humidity absorbed on the internal surfaces of the cell and in the KBr pellet to evaporate. The sample was then cooled to the desired temperature to record the infrared spectrum. In order to go below room temperature, use was made of liquid nitrogen, added cautiously and in small amount in the cavity present inside the cell. Such cavity is connected with the sample holder and permits to cool the sample to the desired temperature. The temperature of the sample was monitored with adequate thermocouples. The lowest temperature reached with this apparatus was -180°C (93K) while the highest temperature was +250°C. Heating is provided by the Joule effect and an external thermal control unit. 

In order to clarify these ideas, we need to compare the irreversible entropy productions (or the exergy destruction) in cycles that utilize regenerative heating of compressed air, thermal recuperation in the form of evaporation and superheating of the methanol fuel, and chemical recuperation through either reforming or cracking reaction with methanol. The next section presents such a comparison in a simplified form to illustrate the utility of thermodynamic analyses. 

Basic Refrigeration Methods Three basic methods of refrigeration (mentioned above) use similar processes for obtaining refrigeration effect evaporation in the evaporator, condensation in the condenser where heat is rejected to the environment, and expansion in a flow restrictor. The main difference is in the way compression is being done (Fig. 11-71) using mechanical work (in compressor), thermal energy (for absorption and desorption), or pressure difference (in ejector). 

The present volume is concerned with the general physical chemistry of the liquid state.. It deals with the theory of liquids, density, thermal expansion, compressibility, viscosity, thermal conductivity, surface tension, specific heat, vapour pressure, boiling-point, latent heat of evaporation, and the critical state. 

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