Evaporation parallel feed

Parallel-feed multiple-effect evaporators. Parallel feed in multiple-effect evaporators involves the adding of fresh feed and the withdrawal of concentrated product from each effect. The vapor from each effect is still used to heat the next effect. This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt. 

Parallel feed involves the introduc tion of raw feed and the withdrawal of produc t at each effect of the evaporator. It is used primarily when the feed is substantiallv saturated and the product is a sohd. An example is the evaporation of brine to make common salt. Evaporators of the types shown in Fig. 11-122Z or e are used, and the produc t is withdrawn as a shiny. In this case, parallel feed is desirable because the feed washes impurities from the salt leaving the body. 

Figure 14.7. Parallel-feed arrangement for a triple-effect evaporator...

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 In crystallizing evaporators, where a slurry of crystals and mother liquor is withdrawn, feed may be admitted directly to each effect to give what is called parallel feed, as shown in Fig. 16.10rf.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. 

Fig. 7.2-1 Three effect evaporation unit with parallel-feed operation... 

True parallel feed is rare in caustic soda and potash evaporation. It is more common with brine. 

Multiple parallel SCWO reactors are used to process the accumulated hydrolysate held in the SCWO feed tank. Liquid effluent from the SCWO system containing inorganic salts is processed in an evaporator/crystallizer, where salts are concentrated into salt cakes for disposal and clean water is recycled. The gaseous effluent from the SCWO, containing primarily carbon dioxide and oxygen, is scrubbed, monitored, and filtered through activated carbon before being released to the atmosphere. 

Sap flows from the back pan into the front pan, sometimes using another float or valve to regulate the entry of sap. The bottom of the front pan is flat, and the front "pan" may actually be made up of one or more pans, with each separate pan itself divided into several partitions. Front pans are made in one of two configurations those in which the sap runs parallel to the main axis of the evaporator, termed reverse-flow evaporators, and pans in which the sap flows from side-to-side, termed cross-flow evaporators. In both cases, syrup flows out the last partition of the front pan via a manual or automatic draw-off valve into a pail or directly into a pipeline feeding a filtration system. 

The principle of operation of the diffusion still is first described as a single-stage device. This is illustrated in Figure 1, in which the system is shown comprised of two parallel heat transfer surfaces—one supplied with heated feed water, the other cooled. The surfaces are closely spaced. The heated water is supplied to surface a for evaporation. 

The parallel product flow configuration for a two-effect evaporative forced-circulation crystallizer is illustrated in Figure 9.16. The feed rate is maintained at a constant rate by feedback flow control. The level is controlled by varying the mother liquor return from downstream solid-liquid separation to the suction of a centrifugal pump. 

---