Mixed-feed evaporator

A cocurrent evaporator train with its controls is illustrated in Fig. 8-54. The control system applies equally well to countercurrent or mixed-feed evaporators, the princip difference being the tuning of the dynamic compensator/(t), which must be done in the field to minimize the short-term effects of changes in feed flow on product quality. Solid concentration in the product is usually measured as density feedback trim is applied by the AC adjusting slope m of the density function, which is the only term related to x. This recahbrates the system whenever x must move to a new set point. 

Many other mixed-feed arrangements are possible which combine the individual advantages of each type of arrangement. Figure 3.13 shows a three-stage evaporator in temperature-enthalpy terms, assuming that inlet and outlet solutions are at saturated conditions... 

Heat Requirement of the Process. Heat is required for vaporization in the extractive distillation column, and for the reconcentration of magnesium nitrate solution. Overall thermal effects caused by the magnesium nitrate cancel out, and the heat demand for the complete process depends on the amount of water being removed, the reflux ratio employed, and the terminal (condenser) conditions in distillation and evaporation. The composition and temperature of the mixed feed to the still influence the relative heat demands of the evaporation and distillation sections. For the concentration of 60 wt% HNO3 to 99.5 wt% HNO3 using a still reflux ratio of 3 1, a still pressure of 760 mm Hg, and an evaporator pressure of 100 mm Hg, the theoretical overall heat requirement is 1,034 kcal/kg HNO3. 

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 In crystallizing evaporators, where a slurry of crystals and mother liquor is withdrawn, feed may be admitted directly to each effect to give what is called parallel feed, as shown in Fig. 16.10rf.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

Calculate the degrees of freedom of the mixed-feed triple-effect evaporator shown below. Assume the steam and all drain streams are at saturated conditions and the feed is an aqueous solution of dissolved organic solids (two-component streams). Also, assume that all overhead streams are pure water vapor with no entrained solids (one-component streams). 

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. 

A new addition to Clariant s PA catalyst portfolio is the NAPHTHALIMAX catalyst series, whichhas been especially developed for mixed o-xylene/naphthalene as well as 100% naphthalene feed streams. For the development of this catalyst type, a bench-scale unit as well as a pilot reactor had to be built which allowed the evaporation of naphthalene and its variable mixture with o-xylene streams at all mixing ratios. These two reactor units are not equipped with twin-switch condensers for safety reasons, but with a direct combustion unit for the product stream in order to reduce the potential hazard of exposure to the highly toxic naphto-quinone, a by-product of the mixed-feed process. This new family member shows very promising performance results at the pilot scale, with reaction gas yields up to 110 wt%. 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

If the crystallizer is not of the evaporative type but rehes only on adiabatic evaporative cooling to achieve the yield, the heating element is omitted. The feed is admitted into the circulating hue after withdrawal of the slurry, at a point sufficiently below the free-hquid surface to prevent flashing during the mixing process. 

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