Steam ejector pumps

Water vapor is frequently removed by pumps that operate with water or steam as a pump fluid, for example, water ring pumps or steam ejector pumps. This depends considerably on circumstances, however, because the economy of steam ejector pumps at tow pressures is generally far inferior to that of rotary pumps. For pumping a vapor - gas mixture in which the vapor portion is large but the air portion is small, the vapor can be pumped by condensers and the permanent gases, by relatively small gas ballast pumps (see Section 2.1.5). 

Comparatively, then, a pump set consisting of a Roots pump, condenser, and backing pump, which can transport 100 kg/h of vapor and 18 kg/h of air at an inlet pressure of 50 mbar, has a power requirement of 4 -10 kW (depending on the quantity of air involved). A steam ejector pump of the same performance requires about 60 kW without altering the quantity of air involved. 

The steam ejectors pump away the remaining vapor pressure of water, hydrocarbons and inerts. Ejector systems typically have two stages or three by 50% ejectors. Because of the criticality for tower operation most systems are overdesigned and it may be possible for the tower to operate with one 50% ejector in each stage. Intercondensers (1st stage) and after condensers (2nd stage) condense the steam from the ejectors, tower steam and condensable hydrocarbons. Motive steam flow must be maintained for best operation. 

Because of the low efficiency of steam-ejector vacuum systems, there is a range of vacuum above 13 kPa (100 mm Hg) where mechanical vacuum pumps are usually more economical. The capital cost of the vacuum pump goes up roughly as (suction volume) or (l/P). This means that as pressure falls, the capital cost of the vacuum pump rises more swiftly than the energy cost of the steam ejector, which iacreases as (1 /P). Usually below 1.3 kPa (10 mm Hg), the steam ejector is more cost-effective. 

Other factors that favor the choice of the steam ejector are the presence of process materials that can form soflds or require high alloy materials of constmction. Factors that favor the vacuum pump are credits for pollution abatement and high cost steam. The mechanical systems require more maintenance and some form of backup vacuum system, but these can be designed with adequate reflabiUty. 

A represents mechanical pump or steam ejector B, booster pump D, cryo, turbomolecular, sorption, ion, or trapped diffusion pumps. 

Vacuum Distillation - Heavier fractions from the atmospheric distillation unit that cannot be distilled without cracking under its pressure and temperature conditions are vacuum distilled. Vacuum distillation is simply the distillation of petroleum fractions at a very low pressure (0.2 to 0.7 psia) to increase volatilization and separation. In most systems, the vacuum inside the fractionator is maintained with steam ejectors and vacuum pumps, barometric condensers, or surface condensers. 

Condensers and vacuum pumps will be needed for evaporators operated under vacuum. For aqueous solutions, steam ejectors and jet condensers are normally used. Jet condensers are direct-contact condensers, where the vapour is condensed by contact with jets of cooling water. Indirect, surface condensers, are used where it is necessary to keep the condensed vapour and cooling water effluent separate. 

In-plant management practices may often control the volume and quality of the treatment system influent. Volume reduction can be attained by process wastewater segregation from noncontact water, by recycling or reuse of noncontact water, and by the modification of plant processes. Control of spills, leakage, washdown, and storm runoff can also reduce the treatment system load. Modifications may include the use of vacuum pumps instead of steam ejectors, recycling caustic soda solution rather than discharging it to the treatment system, and incorporation of a more efficient solvent recovery system. 

Even in spite of their low investment costs vrater jet pumps and steam ejectors are being replaced in the laboratories more and more by diaphragm pumps because of the environmental problems of using vrater as the pump fluid. Solvent entering the vrater can only be removed again through complex cleaning methods (distillation). 

Instead of rotary pumps, large water jet, steam ejector, or water ring pumps can be used. For batch evacuation, and the production of hydrocarbon-free fore vacuum for sputter-ion pumps, adsorption pumps (see Section 2.1.8.1) are suitable. If the use of oil-sealed rotary vane pumps cannot be avoided, basically two-stage rotary vane pumps should be used. The small amount of oil vapor that backstreams out of the Inlet ports of these pumps can be almost completely removed by a sorption trap (see Section 2.1.4) Inserted In the pumping line. 

The vacuum system can be equipped with steam ejectors or with liquid ring pumps (vacuum pumps), or a combination. 

The vacuum system is provided independently for each column and usually consists of a mechanical vacuum pump and a steam ejector to achieve the highest vacuum. A centrahzed vacuum system is not recommended, as a process upset in one stage can readily affect the other stages. The heat source can be a shell-and-tube heat exchanger using thermal oil. Figure 5 shows a Lurgi fractionation system that has two columns to produce three separate fractions. 

Rotary, combustion gas heated 16. Steam ejectors and vacuum pumps... 

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