Evaporation tube bundle

Following extensive laboratory trials, four experimental sleeves were first fitted to operational evaporators towards the end of 1980. A further 11 evaporator weld leaks occurred in 1981 and a fUrther 41 sleeves were fitted to by-pass defective and suspect welds. In parallel, work was in progress to examine whether the sleeving technique could be applied on a routine basis. A trial installation of 200 sleeves was conducted on the spare evaporator tube bundle in the latter part of 1982. A decision was then made to sleeve all the 3000 tube-to-tubeplate junctions in the three evaporators. Work on two of the units was completed in 1983 and the third unit was sleeved by March 1984. It was installed in the summer and PFR operated for the first time with three fiilly-sleeved evaporators in August 1984. 

Kettle-type reboilers, evaporators, etc., are often U-tube exchangers with enlarged shell sec tions for vapor-liquid separation. The U-tube bundle replaces the floating-heat bundle of Fig. 11-36. ... 

Just inside the shell of the tube bundle is a cylindrical baffle F that extends nearly to the top of the heating element. The steam rises between this baffle and the wall of the healing element and then flows downward around the tubes. This displaces non-condensed gases to the bottom, where they are removed at G. Condensate is removed from the bottom of the heating element at H. This evaporator is especially suited for foamy liquids, for viscous liquids, and for those liquids which tend to deposit scale or crystals on the heating surfaces. Vessel J is a salt separator. 

The vapor composition at the top of the condenser (Y,i) is different from that at the bottom (Y, ). The condenser may be compared to a fractional distillation problem in reverse. Butane, having a higher boiling point, will condense out faster than the propane, although both are condensing at the same time. Thus, the vapor and liquid mol fractions from the top to the bottom of the condenser tube bundle are always changing. Proceed as follows The vapor at the top has the same composition as the gas leaving the evaporator. Therefore, Y,. = Y,. 

On being pumped to the next stage, the liquid flashes through the nozzle and the mixture travels down the tube bundle as it did in the first stage. Vapor from the first-effect separator provides the heat for evaporation in the second stage. 

The heat given up by the steam condensing in the tubes causes water to evaporate from the brine solution at the pressure of 0.60 bar maintained in the effect. The exiting brine contains 5.5 wt% salt. The steam generated in the first effect is fed to a tube bundle in the second effect. The condensate from the bundle and the steam generated in the second effect at a pressure of 0.20 bar constitute the fresh water produced in the process. 

Seawater containing 3.5 wt% dissolved salts is to be desalinated in an adiabatic six-effect evaporator. (See Problem 8.58.) Backward feed is to be used the seawater is fed to the last evaporator, and successively concentrated brine solutions flow countercurrent to the direction of flow of steam from one effect to the next. Saturated steam at P = 2 bar is fed to the tube bundle in the first effect. The operating pressures in bars of the six effects are, respectively, 0.9,0.7,0.5,0.3,0.2, and 0.1. The brine leaving the first effect contains 30 wi% salt. The flowchart shows Effects 1,5. and 6. 

The heating element of an evaporator is sometimes referred to as a calandria. Usually this term is applied to a heating system in which the liquor rises through a vertical tube bundle surrounded by the heating medium and then descends through a central well. 

Fig. 2. A vertical short-tube evaporator. (A) Bundle of tubes (B) shell (C) exit of concentrated liquor (D) vapor exit.

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