Evaporation backward feed

Another factor that can be important in the design of evaporators is the condition of the feed. If the feed is cold, then the backward-feed arrangement has the advantage that a smaller amount of liquid must be heated to the higher temperatures of the second and first stages. 

The feed to a multiple-effecl evaporator is usually transferred from one effect to another in series so that the ultimate product concentration is reached only in one effect of the evaporator. In backward-feed... 

From assumed feed temperature (forward feed) or feed flow (backward feed) to the first effect and assumed steam flow, calculate evaporation in the first effect. Repeat for each succeeding effect, checking intermediate assumptions as the calculation proceeds. Heat input from condensate flash can be incorporated easily since the condensate flow from the preceding effects will have already been determined. 

A triple-effect backward-feed evaporator concentrates 5 kg/s of liquor from 10 per cent to 50 per cent solids. Steam is available at 375 kN/m2 and the condenser operates at 13.5 kN/m2. What is the area required in each effect, assumed identical, and the economy of the unit ... 

Figure 14.6. Backward-feed arrangement for a triple-effect evaporator...

It will also be possible by relatively minor piping changes to convert the forward-feed evaporator to backward feed, which might be more favorable if the calcium sulfate scale problem can be solved. Except for tubes, pump shaft sleeves, impellers, etc., the plant will be built exclusively of steel and cast iron. Tube materials will be evaluated by tubing different evaporator effects and heat exchangers with steel, admiralty metal, aluminum brass, and 90/10 cupronickel. The copper alloy tubes will be used exclusively in the final condenser and in the few heat exchangers that are in contact with nondeaerated sea water. 

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. 

Figure 17. Patterns ofliquid flow in multiple-effect evaporators Forward feed, Backward feed, fcj Mixed Feed.

Other patterns of feed are sometimes used. In mixed feed the dilute liquid enters an intermediate effect, flows in forward feed to the end of the series, and Is then pumped back to the first effects for final concentration, as shown in Fig. I6.10c. This eliminates some of the pumps needed in backward feed and yet permits the final evaporation to be done at the highest temperature. In crystallizing evaporators, where a slurry of crystals and mother liquor is withdrawn, feed may be admitted directly to each effect to give what is called parallel feed, as shown in Fig. 16.10rf. In parallel feed there is no transfer of liquid from one effect to another. 

Patterns of liquor flow in multiple..effect evaporators (rz) forward feed b) backward feed (c) mixed feed d) parallel feed, (omn) Liquor streams. (-) Steam and vapor condensate streams. 

A triple-effect evaporator is to be used to produce a 50 percent NaOH solution from a feed containing 25 percent NaOH. Steam is available at 320°F, and the vapor from the last stage is condensed at 120°F. Backward feed is used. ( If equal amounts of water are removed in each effect, what would be the concentrations in the intermediate effects, the boiling-point elevation in each effect, and the net temperature differences available for heat transfer (6) With the same terminal temperatures and more than three effects, what would be the maximum number of effects that could be used ... 

Fig. 7.2-3 Three effect evaporation unit with backward-feed operation...

Example. Here we consider a double-effect evaporator concentrating 35% NaOH to 50%. Figure 9.68 shows a backward-feed system using medium-pressure steam. The material balance table is based on one ton of NaOH. 

This section describes the controls for a triple-effect membrane-cell caustic evaporator. Section 9.3.3.1 explains some of the reasoning behind the selection of the number of effects and the progression of flow of caustic through the evaporators. To illustrate the control systems here, we assume backward feed of the caustic. 

Figure 8.2-4. Simplified diagram of backward-feed triple-effect evaporator.

In mixed feed operation the feed enters as intermediate effect, flows in forward feed through the later effects, and is then pumped back to the earlier effects for further concentration. Operation in the earlier effects can be either backward feed or forward feed. This eliminates some of the pumps needed in backward feed and permits final evaporation at the highest temperature. Mixed feed operation is used only for special applications. Sometimes liquid at an intermediate concentration and certain temperature is required for additional processing. The feed temperature may be close to that of an intermediate stage and mixed feed may result in greater energy economy. 

Caustic Soda Evaporation. A flow diagram for a typical triple-effect caustic soda evaporator is shown in Figure 44. The evaporator is of the backward-feed design and concentrates 10 -11.3 wt % NaOH cell liquor to 50 wt % NaOH. Liquor flows from... 

---