Evaporative cooling evaporators

As with distillation, the dominant heating and cooling duties associated with an evaporator are the vaporization and condensation duties. As with distillation, there will he other duties associated with the evaporator for heating or cooling of feed, product, and condensate streams. These sensible heat duties will usually be small in comparison with the latent heat changes. 

Like distillation, evaporation can be represented as a box. This again assumes that any heating or cooling required by the feed and concentrate will be included with the other process streams in the grand composite curve. 

The solution obtained is evaporated somewhat, cooled in a vacuum desiccator and the crystals of the tetraoxo-acid filtered off too drastic evaporation causes formation of the heptaoxodiphosphoric acid by loss of water. 

The hot filtered solution is then without delay poured into a lipped beaker or a conical flask not into an evaporating-basin, since it is crystallisation and not evaporation which is now required), the beaker covered with a watch-glass, and then cooled in ice-water. As cooling proceeds, the solution should be stirred from time to time to facilitate crystallisation, and when crystallisation appears complete, the cooling should be continued for at least another 15 minutes. 

Yield, about 6g. To obtain a second (but necessarily less pure) crop of the sulphate, evaporate the filtrate further on the water-bath, and cool as before. 

Hydrolysis of Potassium Ethyl Sulphate. Dissolve about i g. of the crystals in about 4 ml. of cold distilled water, and divide the solution into two portions, a) To one portion, add barium chloride solution. If pure potassium ethyl sulphate were used, no precipitate should now form, as barium ethyl sulphate is soluble in water. Actually however, almost all samples of potassium ethyl sulphate contain traces of potassium hydrogen sulphate formed by slight hydrolysis of the ethyl compound during the evaporation of its solution, and barium chloride almost invariably gives a faint precipitate of barium sulphate. b) To the second portion, add 2-3 drops of concentrated hydrochloric acid, and boil the mixture gently for about one minute. Cool, add distilled water if necessary until the solution has its former volume, and then add barium chloride as before. A markedly heavier precipitate of barium sulphate separates. The hydrolysis of the potassium ethyl sulphate is hastened considerably by the presence of the free acid Caustic alkalis have a similar, but not quite so rapid an effect. 

Required Sulphuric acid, 27-5 ml. aniline, 24 ml. sodium nitrite, 20 g. dry potassium carbonate, 3-4 g. (To ensure that the potassium carbonate is dry, it should be gently heated in an evaporating-basin over a small Bunsen flame for 4-5 minutes with stirring, and then allowed to cool in a desiccator.)... 

Mix 6 2 ml. (6 4 g.) of pure ethyl acetoacetate and 5 ml. of pure phenylhydrazine in an evaporating-basin of about 75 ml. capacity, add 0 5 ml. of acetic acid and then heat the mixture on a briskly boiling water-bath (preferably in a fume-cupboard) for I hour, occasionally stirring the mixture with a short glass rod. Then allow the heavy yellow syrup to cool somewhat, add 30-40 ml. of ether, and stir the mixture vigorously the syrup may now dissolve and the solution shortly afterwards deposit the crystalline pyrazolone, or at lower temperatures the syrup may solidify directly. Note. If the laboratory has been inoculated by previous preparations, the syrup may solidify whilst still on the water-bath in this case the solid product when cold must be chipped out of the basin, and ground in a mortar with the ether.) Now filter the product at the pump, and wash the solid material thoroughly with ether. Recrystallise the product from a small quantity of a mixture of equal volumes of water and ethanol. The methyl-phenyl-pyrazolone is obtained... 

Example. Dissolve 0 3 g. of />-chlorobenzoic ncid in a small quantity of warm ethanol (about 10 ml.), and ctlrefully add 5 o aqueous sodium hydroxide drop- wise until the solution is just pink to phenolphthalein. Evaporate to dryness on a water-bath. Dissolve the sodium -chlorobenzoate in a minimum of water, add a solution of 0-5 g. of phenacyl bromide in ethanol (about 5 ml.), and boil the mixture under reflux for i hour, and then cool. The phenacyl ester usually ciy stallises on cooling if it does not, add water dropnise with stirring to the chilled solution until separation of the ester just begins. Filter the ester, wash on the filter with water, drain and recrystallise from ethanol m.p. 90 . The /)-bromophenacyl ester is similarly prepared, and after recrystallisation from aqueous ethanol has m.p. 128 . (M.ps., pp. 543-545.)... 

C) Phenacyl and p-Bromophenacyl esters. Ammonium salts in aqueous-ethanolic solution do not however usually condense satisfactorily with phenacyl and />-bromophenacyl bromide. The aqueous solution of the ammonium salt should therefore be boiled with a slight excess of sodium hydroxide to remove ammonia, and the solution then cooled, treated with hydrochloric acid until just alkaline to phenol-phthalein, and then evaporated to dryness. The sodium salt is then treated as described (p. 349) to give the ester. Filter the ester, and wash with water to remove senium halide before recrystallisation. 

B) Methiodi s. Members of Classes (i), (ii) and (iv) combine wdth methyl iodide (some very vigorously) to form quaternary methiodides. It is best to add the amine to an excess of methyl iodide dissolved in about twice its volume of methanol, allow any spontaneous reaction to subside, and then boil under reflux for 30 minutes (extend to 1 hour for Class (iv) except pyridine and quinoline). The methiodide may crystallise when the reaction-mixture cools if not, evaporate the latter to small bulk or to dryness, and recrystallise, (M.ps., pp. 553-554 )... 

Place about 1 g. of the nitro-hydrocarbon in a boiling-tube and add 5 ml. of cone. HCl and several pieces of granulated tin. Warm the mixture and shake continuously to break up the oily drops of the nitro-compound. When all the oil has disappeared (about 3 minutes heating) pour off the liquid from any undissolved tin into a 100 ml. conical flask. Cool and add cautiously 30% aqueous NaOH solution until the precipitate formed redissolves to give a dark-coloured solution. Cool the latter thoroughly and shake well with about 15 ml. of ether. Separate the ethereal layer in a separating-funnel, wash with water and evaporate the ether in a basin on a previously heated water-bath in a fume-cupboard atoay from all flames. The residue is either... 

A simple form of apparatus is that in which a cooled condensing surface is supported a few cm. above a shallow, heated pool of liquid, and the whole is enclosed in a highly evacuated chamber (compare Fig. II, 26, 1) this offers the least hindrance to the flow of vapour from the evaporating to the condensing surface. The rate of distillation is then determined by the rate at which the liquid surface is able to produce vapour. When the evaporating... 

The simplest form of apparatus consists of a small porcelain evaporating dish covered with a filter paper which has been perforated with a number of small holes a watch glass of the same size, convex side uppermost, is placed on the filter paper. The substance is placed inside the dish, and the latter heated with a minute flame on a wire gauze or sand bath. The sublimate collects in the Fig. II, 45, 1. watch glass, and the filter paper below prevents the sublimate from falling into the residue. The watch glass may be kept cool by covering it with several pieces of damp filter... 

Cuprous bromide. The solid salt may be prepared by dissolving 150 g. of copper sulphate crystals and 87 5 g. of sodium bromide dihydrate in 500 ml. of warm water, and then adding 38 g. of powdered sodium sulphite over a period of 5-10 minutes to the stirred solution. If the blue colour is not completely discharged, a little more sodium sulphite should be added. The mixture is then cooled, the precipitate is collected in a Buchner funnel, washed twice with water containing a little dissolved sulphurous acid, pressed with a glass stopper to remove most of the liquid, and then dried in an evaporating dish or in an air oven at 100 120°. The yield is about 80 g. 

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