Evaporators economy

A control scheme is to be developed for the evaporator shown in Fig. E18.20. The feed and product streams are mixtures of a solute and a solvent, while the vapor stream is pure solvent. The liquid level is tightly controlled by manipulating the feed flow rate, wp. The product composition, JCp, and the feed flow rate, wp, are to be controlled by manipulating the product flow, Wp, and the steam flow rate, The evaporator economy is approximately constant, because E kg of solvent are evaporated for each kg of steam. The flow rates have units of kg/min, while the compositions are expressed in weight fraction of solute. 

Vapor-Compression Evaporation and Waste Heat Evaporation. Both of these processes remove water from contaminants rather than contaminants from water. They are better suited for industrial installations where excess energy is available. The water thus produced is of high quaUty and can be used directly. An important advantage is the concentration of waste-residue volume with attendant economies of handling and transportation... 

Because each effect of an evaporator produces almost as much vapor as the amount it condenses, the total evaporation accompHshed per unit of prime steam, or steam economy, iacreases ia almost direct proportioa to the number of effects used. The total heat load is also spHt up betweea the effects so that each effect has a much lower heat duty than a single effect for the same total evaporation load. However, the total available AT is also spHt up similarly so that each effect of a multiple effect requites about as much heating surface as a single effect operating over the same total temperature difference. Thus ia selecting the number of effects to use ia any iastallatioa, steam cost savings and capital cost of effects have to be balanced. Even before... 

For a steam-heated evaporator, each unit of steam Wq applied produces a known amount of evaporation based on the number of efrec ts and their fractional economy E ... 

Evaporator performance is rated on the basis of steam economy— kilograms of solvent evaporated per kilogram of steam used. Heat is... 

Seawater Evaporators The production of potable water from saline waters represents a large and growing field of application for evaporators. Extensive work done in this field to 1972 was summarized in the annual Saline Water Conversion Repoi ts of the Office of Sahne Water, U.S. Department of the Interior. Steam economies on the order of 10 kg evaporation/kg steam are usually justified because (1) unit production capacities are high, (2) fixed charges are low on capital used for pubhc works (i.e., they use long amortization periods and have low interest rates, with no other return on investment considered), (3) heat-transfer performance is comparable with that of pure water, and (4) properly treated seawater causes httle deterioration due to scahng or fouhng. 

Flash Evaporators The calculation of a heat and material balance on a flash evaporator is relatively easy once it is understood that the temperature rise in each heater and temperature drop in each flasher must all be substantially equal. The steam economy E, kg evap-oration/kg of I055-kJ steam (Ib/lb of lOOO-Btu steam) may be approximated from... 

To nelp consei ve steam economy, venting is usually done from the steam chest of one effecl to the steam chest of the next. In this way, excess vapor in one vent does useful evaporation at a steam economy only about one less than the overall steam economy. Only when there are large amounts of noncondensable gases present, as in beet-sugar evaporation, is it desirable to pass the vents directly to the condenser to avoid serious losses in heat-transfer rates. In such cases, it can be worthwhile to recover heat from the vents in separate heat exchangers, which preheat the entering feed. 

By far the best application of computers to evaporators is for working up operators data into the basic performance parameters such as heat-transfer coefficients, steam economy, and dilution. 

Modules Eveiy module design used in other membrane operations has been tried in peivaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is veiy low (O.I-I Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for neat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construc tion was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capiUaiy bundles. The capillaiy device with the feed on... 

In the above example, 1 lb of initial steam should evaporate approximately 1 lb of water in each of the effects A, B and C. In practice however, the evaporation per pound of initial steam, even for a fixed number of effects operated in series, varies widely with conditions, and is best predicted by means of a heat balance.This brings us to the term heat economy. The heat economy of such a system must not be confused with the evaporative capacity of one of the effects. If operated with steam at 220 "F in the heating space and 26 in. vacuum in its vapor space, effect A will evaporate as much water (nearly) as all three effects costing nearly three times its much but it will require approximately three times as much steam and cooling water. The capacity of one or more effects in series is directly proportional to the difference between the condensing temperature of the steam supplied, and the temperature of the boiling solution in the last effect, but also to the overall coefficient of heat transfer from steam to solution. If these factors remain constant, the capacity of one effect is the same as a combination of three effects. 

For economy of cost, and to reduce the viscosity (and so improve heat transfer), solutions weaker than eutectic are normally used, provided there is no risk of freezing at the evaporator. 

The steam economy of an A/-stage battery is approximately 0.8A/ lb evaporation/lb of outside steam. 

The drums are usually heated with steam, and steam economies of 1.3 kg steam per kg of water evaporated are typically achieved. 

An evaporator operating on the thermo-recompression principle employs a steam ejector to maintain atmospheric pressure over the boiling liquid. The ejector uses 0.14 kg/s of steam at 650 kN/m2, and is superheated by 100 K and the pressure in the steam chest is 205 kN/m2. A condenser removes surplus vapour from the atmospheric pressure line. What is the capacity and economy of the system and how could the economy be improved ... 

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 ... 

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