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T H E J O L - R S A L OF I S D C S T R I A L A X D EiVGISEERISG CHEMISTRI-
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EDITORIALS MECHANICAL REFRIGERATION IN MANUFACTURES Mechanical refrigeration. first used only for ice making by men still active, a n d next applied to brewery cellars a n d beer wort, soon was extended t o all kinds of cold storage purposes and became the foundation of che modern fresh food distribution system. These uses of mechanical refrigeration are today so common a n d xell known as t o obscure a more recent a n d valuable extension of mechanical cooling as a p a r t of manufacturing processes. JT7hilethese are constantly increasing in number a n d variety there are even more instances m-here this t y p e of machinery could be economically used and where, for some reason, its application has not been undertaken. TtTherever t h e general process includes such steps as gas, liquid or solid cooling, condensation, crystallization, fermentation, mechanical refrigeration must be carefully considered as a possible means of improving t h e product, increasing rate of production or permitting recovery of new products. Solidification of various hot molten materials is most commonly carried o u t with water from a n y natural source of available temperature, b u t is better controlled b y mechanically cooled water or brine, as, for example, in t h e manufacture of candles, wherever t h e speed of cooling a n d t h e constancy of t h e r a t e contributes t o cost reduction or t o improvement of quality of product. T h e o u t p u t of moulds or moulding machines used for these purposes if cooled with city or surface water, which is subject t o temperature changes during t h e several seasons of t h e year, decreases as t h e temperat u r e of t h e water increases, making i t necessary t o carry more moulds, t o instal more machines t h a n t h e average sales of t h e product warrant, to lose profitable business during t h e summer months, or t o keep goods stored for a longer time, all of which cost items may be balanced against t h e mechanical refrigeration costs. Promotion of separation b y cooling is another typical case. Water cooled t o between 36' t o 40' F. is used t o separate t h e lime-treated beet sugar solution from i t s molasses. By this means t h e solution is maintained a t from 4 j 0 t o j o o F., t h e proper temperature for best results. Separation of lard, paraffine a n d like products, from the liquor, whether held in suspension or in solution, is promoted by cooling or partial freezing. This process is standard with some products b u t is not used a t all with others t o which i t might well be applied. A few 'examples include: lard recovery from rendered fats by solidifying on refrigerated drums rotating in t h e liquid; paraffine from t h e mineral oils, by cooling t o 18' or z o o F , a n d filtering in filter presses; sulfuric acid concentration b y crystallization after cooling t o low temperatures; acid sulfate of soda separation into sulfate'of soda a n d sulfuric acid; similarly, gas purification or separation, b y cooling i t t o temperatures a t which one or more of t h e mixed vapors will liquefy with or without t h e assistance of pressure control. Substitution of mechanical for free water
cooling m a y be equally advantageous in all processes of making solutions having exothermic heats of reaction or absorption, or in cooling baths which m a y become heated otherwise, either from t h e air or from hot dips. Such cooling of bleaching baths t o temperatures lower t h a n can be maintained by t h e use of t h e existing water supplies has been found advantageous, a n d t h e same is t r u e of some of t h e baths used in making mercerized silk. Mechanical refrigeration might be used t o a much greater extent t h a n is a t present customary for concentrating aqueous solutions by blasting air or freezing t h e solutions. It is in direct competition with heated vacuum systems for concentrating liquors and is in some cases less expensive. Air cooling a n d drying, whetherthe object be temperat u r e or humidity control, has passed through t h e stages of free water contact which in most i m p o r t a n t cases has been abandoned in favor of mechanical refrigeration. Numerous systems are in use where this cooling a n d drying has been performed b y surface, city or well water, a n d because t h e results have not been uniformly satisfactory either in obtaining t h e desired temperature or in reducing t h e humidity in t h e air t o t h e proper degree for all seasons a n d weather conditions, t h e y have been replaced by refrigerating equipment. All uncertainties a n d variations are removed b y t h e substitution of mechanical refrigeration and yet its use is far from being as universal as i t should a n d could be. I n some classes of factory operation i t has been found t h a t t h e amount of labor performed by t h e workmen, especially during t h e summer months, was increased so much b y mechanically cooled breathing air t h a t t h e expense of this cooling is more t h a n offset by t h e greater o u t p u t of t h e operators. For humidity control, by far t h e largest example of aircooling b y mechanical refrigeration is the Gayley process equipment for steel furnaces, t h e success of which is beyond question, a n d yet t h e application of similar equipment t o other industries where humidity control is necessary or desirable is painfully slow. S u m e r o u s as are t h e special instances where moist u r e contkct of air must be minimized, there is an equally large number n-here low temperature of air or other gas is requirccl directly, such, for example. as concentrating solutions for crystallization of citric acid by dry air contact a t low temperatures, or drying products like gelatine, t o which high temperatures or moist air are unsuitable. A similar dry, cool air requirement is imposed on storage rooms, carrying hygroscopic or high vapor pressure products or organic m a t t e r such as camphor, calcium chloride a n d hides a n d these are increasing in number. I n t h e organic field t h e most interesting development is in t h e control of germination whether in storage or in process of seeds, silkworms, tobacco curing a n d fermentation, in addition t o t h e retardation of decay growth of all foodstuffs. Originally designed t o freeze water just below 32' F.,
me ch ani ca.1 r t:f rig e r a t ion. p r o c e s ses a n d e c! 11 i p men t are nom- available a t all temperatures, ranging from a fevi degrees allox-e absolute zero t o a few degrees belo117 atmospheric temperature, in great \-ariety of plants a n d ranges of first a n d operating costs. T h e tendency of t h e d a y is t o produce greater a n d greater variety of appliances. each better a d a p t e d t o a special application a n d to correspondingly increase t h e n u m ber of applications. T h e initiative must. h o v e v e r , come from t h e user, a n d t h e user of t h e f u t u r e is t h e chemical manufacturer w h o must meet t h e refrigerating machine producer half way, so t h a t b y joint effort t h e industries m a y receive maximum benefit.
C. E. LUCKE ~
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THE THEORY OF THE PERFECT SHEET ASPHALT SURFACE T h e theory of 11 perfect sheet asphalt surface is based upon a s t u d y of t h e behavior of surfaces a n d films. T h e most modern conception of t h e chemistry of colloids is likewise based on t h e relation of surfaces a n d films. Colloid chemistry is, therefore, of great value in interpreting t h e behavior of such surfaces and in guiding t o their rational construction. As long ago as 1 9 0 j t,he n-riter called attention in “ T h e Modern -1sphalt P a v e m e n t , ” published in t h a t year, t o t h e fact t h a t t h e more or less satisfactory or unsatisfactory n a t u r e of a sheet asphalt pavement depends on t h c surface area of t h e particles composing t h e mineral aggregate of s a n d a n d filler entering i n t o i t s construction, t h e a m o u n t of bitumen which m a y be used i n such a mixture, without being present in excess, being dependent on t h e extent of t h e subdivision of t h e aggregate a n d its available surface area t o which bitumen m a y adhere. At t h a t time he went no further into t h e consideration of a n y effect t h a t this surface area might have upon t h e mixture. With t h e development of t h e modern conceptions of colloidal or dispersoid chemistry a n d of adsorption which have been formulated since t h e n , i t is a p p a r e n t t h a t t h e extent of surface area presented in a n y mineral aggregate. especially i n connection with t h e presence of colloid material, is of much greater importance t h a n h a d previously been appreciated. I n order t o unders t a n d this, one m u s t have some comprehension of t h e principles of physical chemistry a n d . especially, of t h e modern conceptions of colloid material. Colloidal chemistry originated i n t h e investigations of G r a h a m in t h e sixties of t h e last century. It t h e n lay d o r m a n t f o r f o r t y years a n d has only recently been developed t o a n extent commensurate with t h e importance which i t is now recognized as having, T h e original idea of G r a h a m was t h a t substances could be classified as crystalline a n d colloidal, with a sharp line of division between t h e two. according t o whether or not t h e y m-ould, in solution, diffuse through a n animal or semi-permeable membrane. I t is now recognized t h a t colloids are merely a s t a t e of m a t t e r , one in a highly dispersed or subdivided condition. By appropriate means, in a suitable medium or phase wiTh which t h e substance does r,ot form a molecular so!urion. cryst:illine substances can be
obtained in such a degree of subdiL5sion or dispersion t h a t t h e y exist in a colloid s t a t e . i. e . , t h e y will remain suspended in a medium indefinitely. T h e chemistry of colloid m a t t e r t h e n differs from t h e chemistrl- of m a t t e r in i t s ordinary form, merely liy t h e degree of i t s subdix-ision or dispersion. I n order t o be dispersed in colloid form a substance must exist in a system of a t least t w o phases: a n interior or disperse one \\-hich m a y be solid or liquid, a n d an exterior or continuous phase i n which i t is dispersed. T h e main characteristic of t h e disperse phase is its s t a t e of subdi7-ision or degree of dispersion. Bancroft has characterized this a s fo1lows:l “If we drop a stone into water, i t sinks very rapidly; if we grind t h e stone i n t o coarse particles these sink less rapidly; if we grind t h e stone i n t o fine particles, these sink slowly; if we grind t h e stone into very fine particles we should expect t h e m t o sink very slov-ly, t h e r a t e being a function of t h e diameter of t h e particles. This is not t h e case, howe\-er. Very fine particles do not follow Stokes’ equation a n d do not settle a t all, because of t h e Brownian movements which are negligible for coarse particles. We, therefore. conclude t h a t a n y substance can be brought into a s t a t e of colloidal solution provided we make the particles of t h a t phase so small t h a t t h e Brownian movements will keep t h e particles suspended, a n d provided we prevent agglomeration of t h e particles b y a suitable surface film.” T h e Brownian motion t o m-hich Bancroft refers is one which can be discerned under a n instrument known as t h e ultra-microscope which makes visible particles which are invisible in t h e ordinary instrument. I t serves as a means of detecting m a t t e r in a colloid s t a t e which has been unavailable except within t h e last decade. T h e size of particles, when in a colloid s t a t e , is ordinarily a s small or smaller t h a n 0.0001 m m . in diameter, a n d in t h e case of some solids no larger t h a n 0.000006 m m . A realization of t h e enormous surface area possessed by disperse solid colloids of this size m a y he arri7-ed a t f r o m t h e f a c t t h a t if a n a m o u n t of material represented b y a cube, one side of which has a dimension of one centimeter, is reduced b y decimal subdivision only t o t h e coarsest colloidal size, a I O thousandth of a millimeter in dimension, t h e number of cubes produced would be I O t o t h e I j t h power. while t h e surface area would be increased t c ) 60 square meters: or I O O , O O O times t h a t of t h e original cube. T h e great increase in surface area of t h e finer material over t h a t exhibited b y t h e surface area of a single cube is a t once made evident, a n d t h e importance of having such a material present in a sheet asphalt surface mixture is apparent. if a large surface area is 2necessary feature of t h e mineral aggregate of a perfect surface, which: in t h e light of actual cxxperience, appears t o be t h e case. I t is also of great importance because of t h e surface energy developed thereby. in addition t o t h e fact t h a t t h e large surface area permits t h e us? of ;% greater 1
J . Phys. C h e m . , 18, 549.
