Steam vacuum jets

Figure 16.3 A typical vacuum steam jet performance curve.

In the following table are summarized the quantities of the various gases and the total quantity of gases that the vacuum steam jet must remove. It is necessary to remove the water vapor because it is carried along with the other gases. 

The bottom stream from the first fractionating column goes into a second column operated under vacuum. Steam jet ejectors are used to create the vacuum so that the absolute pressure can be as low as 30 to 40mmHg (about 0.7 psia). The vacuum permits hydrocarbons to be vaporized at temperatures much below their normal boiling points. Thus, fractions with normal boiling points above 650" F can be separated by vacuum distillation without causing thermal cracking. 

Steam-Jet Systems. Low pressure water vapor can be compressed by high pressure steam in a steam jet. In this way, a vacuum can be created over water with resultant evaporation and cooling water, therefore, serves as a refrigerant. This method frequently is used where moderate cooling (down to 2°C) is needed. The process is inefficient and usually is economically justified only when waste steam is available for the motive fluid in the steam jet. 

Uses of Ejectors For the operating range of steam-jet ejectors in vacuum applications, see the subsection Vacuum Systems. ... 

Applications The common Heliflow apphcations are tank-vent condensers, sample coolers, pump-seal coolers, and steam-jet vacuum condensers. Instant water heaters, glycoLwater seivdces, and cryogenic vaporizers use the spiral tube s ability to reduce thermally induced stresses caused in these apphcations. 

The vacuum pump is usually of the steam-jet type if high-pressure steam is available. If nigh-pressure steam is not available, more expensive mechanical pumps may be used. These may be either a water-ring (Hytor) type or a reciprocating pump. 

Vacuum is applied to the chamber and vapor is removed through a large pipe which is connected to the chamber in a manner such that, if the vacuum is broken suddenly, the inrushing air will not greatly disturb the bed of material being dried. This line leads to a condenser where moisture or solvent that has been vaporized is condensed. The noncondensable exhaust gas goes to the vacuum source, which may be a wet or diy vacuum pump or a steam-jet ejector. 

A vacuum condenser has vacuum equipment (such as steam jets) pulling the noncondensibles out of the cold end of the unit. A system handling flammable substances has a control valve between the condenser and Jets (an air bleed is used to control nonflammable systems). The control method involves derating part of the tube surface by blajiketing it with noncondensibles that exhibit poor... 

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. 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which carmot 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. 

Replacement of the steam-jet ejector with a vacuum pump. The distillation operation will not be affected. The operating cost of the ejector and the vacuum pump are comparable. However, a capital investment of 75,000 is needed to purchase the pump. For a five-year linear depreciation with negligible salvage value, the annualized fixed cost of the pump is I5,000/year. 

Figure 2-44. Friction factor for streamlined flow of air at absolute pressures from 50 microns Hg. to 1mm Hg. By permission, Standards for Steam Jet Ejectors, 3rd. Ed., Heat Exchange Institute, 1956 [54] and Standards for Steam Jet Vacuum Systems, 4th Ed., 1988. Note f on same basis as Figure 2-3 [58].

Standards for Steam Jet Ejectors, 3rd ed.. Heat Exchange Institute, 1956, and Standards Jor Steam Jet Vacuum Systems, 4th ed., 1988, Cleveland, Ohio. 

Steam Jet syphons Steam Jet exhausters Single-stage vacuum pumps... 

Steam jet syphons Steam jet slurry heater Single-stage vacuum pumps Using air as the motive fluid Air jet exhausters Using liquid as the motive fluid Water jet eductors... 

The process designer or mechanical engineer in a process plant is not expected to, nor should he, actually design a mechanical vacuum pump or steam jet, biit rather he should be knowledgeable enough to establish the process requirements for capacity, pressure drops, etc., and understand the operation and details of equipment available. 

Mechanical vacuum pumps are eight to ten times more efficient users of energy than steam jets although, steam Jets are reliable and cost less [23], See Table 6-17. 

Ketterer, S. G. and C. G. Blatchley, Steam Jet Vacuum Pumps, Schutte and Koerting Co. 

Croll-Reynolds Co., Design and Application of Steam Jet Vacuum Ejectms, Croll-Re)Tiolds Co., Westfield, N.J. 

Decker, L. O., Consider the Cold Facts About Steam-Jet Vacuum Cooling, Chem. Eng. Prog., American Institute of Chemical Engineers, V. 89, No. 1, p. 74 (1993). All rights reserved. 

When the steam leaves a condensing turbine, it passes to a surface-type condenser for recovery of the condensate. Vacuum equipment (jets or pumps) are necessary to achieve high vacuums on the condenser. 

---