Ejector problems

Goff JA, Coogan CH. (1942) Some two-dimensional aspects of the ejector problem. J. Appl. Mech., 9(4) A-151-154. 

Vacuum tower wash oil, 165 Vacuum lowers, 281—300 bottoms-pump suction pressure loss, 281— 285 high flash-zone pressure, 285-288 ejector problems, 288-292 black gas oil, 292-293 trim gas oil production, 293 pumparound draw temperatures, 293 light resid, 294 steam-to-healer passes, 295-297 gas-oil recovery improvement, 297-298 transfer-line failures, 298-299 troubleshooting problems, 299-300... 

The prevacuum technique, as its name implies, eliminates air by creating a vacuum. This procedure faciUtates steam penetration and permits more rapid steam penetration. Consequendy this results in shorter cycle times. Prevacuum cycles employ either a vacuum pump/steam (or air) ejector combination to reduce air residuals in the chamber or rely on the pulse-vacuum technique of alternating steam injection and evacuation until the air residuals have been removed. Pulse-vacuum techniques are generally more economical vacuum pumps or vacuum-pump—condenser combinations may be employed. The vacuum pumps used in these systems are water-seal or water-ring types, because of the problems created by mixing oil and steam. Prevacuum cycles are used for fabric loads and wrapped or unwrapped instmments (see Vacuum technology). 

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). 

The most commonly used steam is 100 psig with 10-15° superheat, the latter characteristic in order to avoid the erosive effect of liquids on the throats of the ejectors. In Figure 7.31 the steam consumptions are given as lb of motive steam per lb of equivalent air to the first stage. Corrections are shown for steam pressures other than 100 psig. When some portion of the initial suction gas is condensable, downward corrections to these rates are to be made for those ejector assemblies that have intercondensers. Such corrections and also the distribution of motive steam to the individual stages are problems best passed on to ejector manufacturers who have experience and a body of test data. 

Motionless mixers and ejectors are useful for applications requiring short residence times (on the order of seconds or less). If long residence times are required, e.g., if the reaction is relatively slow, the use of a motionless mixer alone would lead to a very long mixer, which may not be practical. One way to overcome this problem is to use a loop reactor, which combines a high-intensity mixer, such as a motionless mixer or ejector, with a separation tank. 

Description Ammonia and carbon dioxide react at 155 bar to synthesize urea and carbamate. The reactor conversion rate is very high under the N/C ratio of 3.7 with a temperature of 182-185°C. Unconverted materials in synthesis solution are efficiently separated by C02 stripping. The milder operating condition and using two-phase stainless steel prevent corrosion problems. Gas from the stripper is condensed in vertical submerged carbamate condenser. Using an HP Ejector for internal synthesis recycle, major synthesis equipment is located on the ground level. 

As in the convention techniques, urea is produced at about 180 to 7XXPC. On the other hand, the residual carbamate is decomposed at the synthesis pressure, by reducing its partial pressure by means of gas stripping. The recombination of the reactants thus liberated occurs alter their condensation, by passage in an absorber or a scrubber, which also serves to recondense the fractions vaporized during the reaction, and to achieve recycling entirely in liquid form. To minimize corrosion problems, the different effluents are normally caused to flow by gravity, or by vaporization, or even by means of ejectors. 

The most commonly used vacuum pumps are steam-jet ejectors and several positive-displacement pumps, which are shown in Figures 5.1 and 5.2. Some of the characteristics of vacuum pumps are given in Table 5.1. A prime consideration when selecting a vacuum pump is the compatihility of a gas with a seal fluid. To avoid these problems, there is a trend toward using dry pumps where a seal fluid or lubricant is not used [60]. 

Low steam pressure. This applies to steam ejectors only. The cause is low line pressure, wet steam or blockage in the steam line. This reduces the driving force ofthe ejector and reduces its air handling capacity. By removing the cause of the low steam pressure, the problem of insufficient vacuum is corrected. 

Reactants that are not transformed into urea are recycled to the reactor by means of an ejector. The plant is free from pollution problems. All vents are efficiently washed so that they are discharged to atmosphere practically free of ammonia and urea. Liquid discharge may have to be physically treated to meet local regulations or client requests. Also, discharge water can be reused as boiler feed water (BFW). 

Jet ejectors require very little attention and maintenance and are especially valuable with corrosive gases that would damage mechanical vacuum pumps. For difficult problems the nozzles and diffusers can be made of corrosion-resistant metal, graphite, or other inert material. Ejectors, particularly when multistage, use large quantities of steam and water. They arc rarely used to produce absolute pressures below 1 mm Hg, Steam jets are no longer as popular as they once were, because of the dramatic increase in the cost of steam. In many instances where corrosion is not a serious consideration, they have been replaced by mechanical vacuum pumps, which use much less energy for the same service. 

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