Parallel feed

Parallel-feed operation is illustrated in Fig. 3.12c. Fresh feed is added to each stage, and product is withdrawn from each stage. The vapor from each stage is still used to heat the next stage. This arrangement is used mainly when the feed is almost saturated, particularly when solid crystals are the product. 

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 Evaporators of the types shown in Fig. 11-122Z or e are used, and the produc t is withdrawn as a shiny.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. 

To maintain continuity and achieve an unintemipted process line a second parallel feed route is provided through a second pay-off reel no. 2. 

Spiral-wound modules consist of several flat membranes separated by turbulence-promoting mesh separators and formed into a Swiss roll (Figure 16.18). The edges of the membranes are sealed to each other and to a central perforated tube. This produces a cylindrical module which can be installed within a pressure tube. The process feed enters at one end of the pressure tube and encounters a number of narrow, parallel feed channels formed between adjacent sheets of membrane. Permeate spirals roward the perforated central tube for collection. A standard size spiral-wound module has a diameter of about 0.1m, a length of about 0.9 m and contains about 5 m2 of membrane area. Up to six such modules may be installed in series in a single pressure tube. These modules make better use of space than tubular or flat sheet types, but they are rather prone to fouling and difficult to clean. 

One of the most frequently used micro reactor types relies on the use of micro-structured platelets with multiple parallel channels, typically manufactured by methods other than routinely used for chip processing, encased in a housing [3,4, 12, 13, 18, 28-39]. If more than one platelet is used, which is usually done to increase throughput, a stack-like arrangement is preferred for parallel feed. Such stacks are either welded directly from the outside [29, 30], are encompassed by a cover [3,18, 31, 32, 37-39], have end caps with fluidic connectors [12,13, 33] or are inserted into a recess of a housing, which is typically composed of two parts [4, 28, 34-36, 40 1]. 

In the unit considered in Example 14.2A, the weak liquor is fed to effect 0 and flows on to 0 and then to . The steam is also fed to 0, and the process is known as forward-feed since the feed is to the same unit as the steam and travels down the unit in the same direction as the steam or vapour. It is possible, however, to introduce the weak liquor to effect and cause it to travel from to to , whilst the steam and vapour still travel in the direction of 0 to to . This system, shown in Figure 14.6, is known as backward-feed. A further arrangement for the feed is known as parallel-feed, which is shown in Figure 14.7. In this case, the liquor is fed to each of the three effects in parallel although the steam is fed only to the first effect. This arrangement is commonly used in the concentration of salt solutions, where the deposition of crystals makes it difficult to use... 

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

A wax filtration plant has six filters that operate in parallel, feeding from one common feed tank. Each filter can handle 1000 gpm when running, but the filters must be taken ofi-hne every six hours for a cleaning procedure that takes ten rninutes. The operating schedule calls for one filter to be cleaned every hour. 

The fairly uniform color formation in the mixing channel demonstrates that mixing is close to completion [20], A total flow of 8 1 IT1 at a pressure drop of 2.5 bar can be achieved owing to the parallel feed of many inlet channels. 

The design of this mixer is based on a series of fork-shaped elements (see Figure 1.124) [43, 144], Initially, two parallel feed channels are placed so that one... 

Plow system and reactor The experimental flow system employs a parallel feed arrangement, with one stream containing a pure benzene/hydrogen mixture and another containing thiophene in addition to benzene/hydrogen feed. A four-way valve was used to allow one feed to go through the reactor while another to be diverted to a bypass line. 

The designer has a number of options to achieve the greatest energy economy with a given number of effects. These are usually associated with the location of the feed in respect to the introduction of the steam. Figure 17 illustrates several methods of operation which are forward feed, backward feed, mixed feed, and parallel 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. 

Hydroxy-2-methylbutyric acid was not a precursor for the tigloyl moieties of (56) and (57), which indicates that introduction of the double bond is by dehydrogenation rather than dehydration. (A parallel feeding experiment was carried out with 2-methylbutyric acid in an attempt to ensure the reliability of this negative result. )... 

In the parallel-feed arrangement Figure 8.43c) one feed pump is required, and predetermined flows Fi, F2 and F3 are passed into the corresponding effects. A thick liquor product is taken from each effect for this purpose pumps are generally necessary. Parallel feeding is often encountered in crystallization practice, e.g. in the salt industry, and it is useful if a concentrated feedstock is being processed. 

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

D. C. Jeun and V. Flokos, In-Band Scattering from Arrays with Parallel Feed Network, IEEE Trans. Antennas Propag., Vol. 44, No. 2, February 1996, p. 172. 

The separation efficiency depends on the equilibrium location and the ratio of the phase fluxes, but is often low in an individual separation stage. To separate a mixture and obtain pure products, several separation stages are connected in series. This is done most effectively with countercurrent phase flow. Phase cross flow and parallel feed of one phase to individual separation stages are sometimes used. However, cocurrent flow is of non importance. 

Some multiple units operate in cascade, with the brine following any of several paths through the unit. When nearly saturated brine is the feed solution, there is less to be gained by staging the brine, and parallel feed is common. 

---