Specifying evaporators

Evaporator systems are major pieces of process equipment and are often purchased on a total responsibility basis. This is especially true of vapor compression and highly heat integrated systems. Specific design information and fabrication are often proprietary to vendors. Evaporator manufacturers generally are rather specialized. Few offer a complete range of evaporator types some specialize in one type only. 

The purchaser s task often is to define the process and mechanical limitations accurately to enable vendors to engineer economical systems and offer fair process guarantees. Accurate costs for various energy forms must be established. Overall plant energy balances must be determined and any limitations described to the vendor. Performance guarantees come in a variety of forms and may not be advantageous to the purchaser. 

Pilot tests may be required to evaluate new processes or products. Most evaporator manufacturers maintain test facilities and the purchser is encouraged to avail himself of this opportunity, especially if little experience is available for his specific product. Often a vendor s broad experience with all types of evaporators may contribute as much to the solution of new problems as intensive experience in the application of a specific design. Pilot tests may need to be treated confidentially and should not be used indiscriminately. 

Many intangible factors may influence the selection of evaporator fabricator vendor experience with the process or product, his reputation and responsibility, value of the offered guarantee, prior experience with a vendor, confidence in the vendor. These factors are more difficult to evaluate. 

The process engineer should carefully evaluate the total system proposed by any vendor. Heat transfer rates, steam economy and frequency of cleaning are the obvious factors to be evaluated. But other, smaller details are often almost as important. Minor equipment must be given proper engineering. Mechanical equipment, instrumentation, condenser type, and piping must all be given careful evaluation. 

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Parker, N. H. (1963b) Chem. Eng., NY (July 22nd) 135. How to specify evaporators. 

Evaporation the analytical procedure may specify evaporation to a smaller volume or evaporation to dryness. Both operations can be conveniently carried out in a porcelain evaporating dish or casserole the capacity of the vessel should be as small as possible for the amount of liquid being reduced in volume. The most rapid evaporation is achieved by heating the dish directly on a wire gauze. For many purposes a water bath (a beaker half-filled with water maintained at the boiling point is quite suitable) will... 

Any control system which results in the controlled operation of the evaporation process must maintain both an energy and material balance across the evaporator boundaries, Consequently, a control system designed to maintain a specified evaporation must simultaneously manipulate product withdrawal and energy supplied to the evaporator, if efficient operation is to be achieved. The control system must be able to treat changes in feed flowrate or composition. The control system should function to reduce heat input with a reduction in feed rate. It should also function to change the overhead vapor rate (amount evaporated) as changes in the feed composition occur. 

Thermospray interface. Provides liquid chromatographic effluent continuously through a heated capillary vaporizer tube to the mass spectrometer. Solvent molecules evaporate away from the partially vaporized liquid, and analyte ions are transmitted to the mass spectrometer s ion optics. The ionization technique must be specified, e.g., preexisting ions, salt buffer, filament, or electrical discharge. 

Odor. Odor is excluded in military appHcations and in the four grades specified for human breathing. The gas is tested by smelling a flowing stream or by smelling a beaker from which a Hquid sample has just evaporated. Oxygen that is produced in modem ASUs employing turbine compressors or nonlubricated piston machines is odorless. 

Emulsified asphalt used as a protective coating is specified by ASTM D1227 (10). These emulsions ate appHed above freezing by bmsh, mop, or spray, and bond to either damp or dry surfaces. Such appHcation is not recommended for inclines <4° to avoid the accumulation of water. However, curing by water evaporation can be slow, and these emulsions may remain water-susceptible. 

Polymerization-grade chloroprene is typically at least 99.5% pure, excluding inert solvents that may be present. It must be substantially free of peroxides, polymer [9010-98-4], and inhibitors. A low, controlled concentration of inhibitor is sometimes specified. It must also be free of impurities that are acidic or that will generate additional acidity during emulsion polymerization. Typical impurities are 1-chlorobutadiene [627-22-5] and traces of chlorobutenes (from dehydrochlorination of dichlorobutanes produced from butenes in butadiene [106-99-0]), 3,4-dichlorobutene [760-23-6], and dimers of both chloroprene and butadiene. Gas chromatography is used for analysis of volatile impurities. Dissolved polymer can be detected by turbidity after precipitation with alcohol or determined gravimetrically. Inhibitors and dimers can interfere with quantitative determination of polymer either by precipitation or evaporation if significant amounts are present. 

Control of supersaturation is an important factor in obtaining crystal size distributions of desired characteristics, and it would be useful to have a model relating rate of cooling or evaporation or addition of diluent required to maintain a specified supersaturation in the crystallizer. Contrast this to the uncontrolled situation of natural cooling in which the heat transfer rate is given by... 

Figure 9.2-2 shows a data input screen in which general characteristics are input by radio buttons and numerical data is typed. The program calculates distances to specified in.sic concentrations and other requested consequence levels automatically. Results are available in a variety of formats including cloud footprints, sideview, cross section, pool evaporation rate, concentration vs distance and heat flux contours. Figure 9.2-3 shows the calculated results as a toxic plume. superimposed on the map with and without oligomerization. 

Therefore, if the solubility data for a substance are known, it is a simple matter to calculate the potential yield of pure crystals that could be obtained from batch crystallization (equations 7.4 and 7.6). Conversely, the degree of evaporation to produce a specified yield may be estimated (equation 7.8). 

Ton of refrigeration The heat equivalent to melting 2,000 lb (one ton) of ice in 24 hours. One ton equals 12,000 Btu/hr or 200 Btu/min. To be comparative, refrigeration equipment must have the refrigerant level (or evaporation temperature) specified. 

Basically the system design consists of the selection of component parts to combine and operate in the most economical manner for the specified conditions. Unfortunately, the specific conditions are not only for the evaporator where the refrigerant is actually used hut include all or part of the following. These conditions are identified whether the system is a separate component selection or a package furnished assembled by a manufacturer. 

To compare the performance between single component refrigerants and blends it will be necessary to specify the evaporating temperature of the blend to point A on the diagram and the condensing temperature to point B. 

The catalogue-rated cooling capacity of a room air-conditioner, if not qualified, will be based on ASHRAE Standard 16-1983. This specifies test conditions of air onto the evaporator at 80°F dry bulb, 50% relative humidity (26.7°C, 49.1% saturation), and air onto the condenser at 95°F dry bulb, 75°F wet bulb (35°C and 23.9°C). The original basis for this specification was the ambient condition prevailing in the mass-market area of the USA. 

A solution of a l-(2-acyloxyphenyl)-2-azidoethanone 3 (10 mmol) in benzene (30 mL) was slowly treated with (EtO)jP (1.66 g, 10 mmol) at 5-10 C under N2. The mixture was stirred for 30 min and then heated under reflux fora specified time (see Table). The resulting solution was evaporated under reduced pressure and the residue was chromatographed (silica gel, EtOAc/hexane 1 1) to give the colorless product, which was recrystallized from Et20 (4a-c) or EtOAc/EUO (4d). 

A solution of a benzene-1,2-diaminc (15mmol) and an cnol ether 4 (15mmol) in o-xylene (lOmL) was heated in a domestic microwave oven for a specified time. The reaction mixture was cooled and evaporated under reduced pressure, and the residue was purified by chromatography (silica gel. hexane/ CHCl3 1 4). 

Method A MsOH (0.96 g, 10 mmol) was added to a suspension of salicylohydrazide (1 1.52 g, 10 mmol) in toluene (50 mL) and the mixture was stirred for 10 min. An acid chloride or anhydride (10 mmol) was added and the mixture was heated under reflux for a specified time. The mixture was cooled, neutralized with sat. aq NaHCO, and the layers were separated. The aqueous layer was extracted with CH2C12 (2 x 20 mL) and the combined organic phases were dried (MgS04) and evaporated in vacuo. Chromatography (silica gel, CH2C12) gave the product, which separated from EtOH as colorless crystals. 

A 7-chloro-5-phenyl-l//-l,3,4-benzotriazepine-2(3//)-thione24 (2.5 mmol) in dioxane (l 5 mL) was treated with powdered Pb(OAc)2 (1.63 g. 5 mmol) and an amine was passed for 30 min into the suspension. The mixture was left for a specified time. H,S was passed through the mixture and the precipitated lead sulfide was filtered off. The filtrate was evaporated in a stream of air to leave the product, which was recrystallized (cyclohexane). 

Now suppose that, at a particular composition of the liquid, the total pressure increases as the liquid becomes richer in a specified component, say I. Expansion (i.e., distillation) can then, by the above rule, only diminish the concentration of I. in the liquid. The concentration of I. in the vapour is therefore, at that point, not less than its concentration in the liquid, for if it could be less, any possible evaporation would necessarily increase the concentration of I. in the liquid. 

The dimensions of the column used by the checkers were the same as those specified in Note 12, and the product was obtained from fractions 10-20. The submitters evaporated the rmclions with a rotary evaporator operated at water aspirator... 

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