E
nginnyring
Special Heat Transfer Problems of the Food Industry
pocess development I
1. P. BOLANOWSKI AND D. D. LINEBERRY THE GIRDLER CORP., VOTATOR DIVISION, LOUISVILLE, KY.
T
HIS paper discusses a heat exchanger of special design and
76" to 196' F. In cooling the thickened mass back to 80" F an over-all heat transfer coefficient of 316 was attained. Data obtained in commercial production are shown on Table 11. As space does not permit a detailed review of all the applications of this heat exchanger in the food industry, this discussion will be limited to its use in plasticizing of lard, shortening, and margarine, and short time-high temperature sterilization (d).
its use in the food industry. J t is employed in the continuous cooking, cooling, pasteurization, sterilization, crystallization, emulsification, aeration, mixing, caramelization, and extrusion of various food products. Some .examples of its applications are given in Table I.
IN FQOD INDUSTRY TABLE I. APPLICATION OF HEATEXCHANGER
Crystallization Shortening Lard Margarine Cocoa butter Orange concentrate Sweetened condensed milk Mixing Cake batter Cake icings Sterilization Baby food purees
Emulsification Salad dressing
Aeration Marshmallow Egg whites
Extrusion Gelatin Paraffin wax Starch cooking and oooling Salad dressing
Pasteurization Eggs Custard Caramelization
Custards
CHILLING AND PLASTICIZING LARD, SHORTENING, AND MARGARINE
PROCESS AND METHODS. The solidification or plasticizing of lard and shortening may be reviewed under one heading as the process and apparatus involved are the same. There is one distinguishing difference in procedure that deserves reference. Lard and some of the compounds are subjected to less mechanical working in the plasticizing stages because of their inherent behavior and reaction, It has been established that lard, particularly, may be overworked a t this stage with resultant impairment of plasticity. Shortenings, specifically the all-hydrogenated variety, respond favorably to additional work and both consistency and creaming qualities are benefited by the additional work imparted. In the instance of lard, overworking the product produces a rubbery consistency ( I ) .
Sweetened oondensed milk Toppings
DESCRIPTION
Figure 1 shows a cross section of the heat exchanger. The product being processed enters and leaves through openings in the head, 1. The direction of flow is optional. The product passes from the inlet through the annular space, 8, to the outlet. The heat transfer medium enters and leaves through the connections, 3, and changes temperature as it moves through the annular space, 4. The heat transfer tube, 6, separates the product and the heat transfer medium. The mutator shaft, 6, which rotates and carries the scraper blades, 7, is the essential device for effecting rapid heat transfer, maintaining a homogeneous mixture or solution of the product end preventing localized overheating. The heat transfer medium can be water, steam, Dowtherm, ammonia, or brine. Conventional insulation, 8, is applied to the exterior of the machine. Construction materials of the tube and shaft may be iron, nickel, or stainless steel. Choice of the materials of construction and the design of the scraper blades are governed by the physical properties of the product. The rotational speed of the mutator shaft is determined by the flow characteristics of the product. The diameter of the shaft is approximately three fourths of the internal diameter of the cylinder, thus providing a relatively small annular space. Figure 2 shows a three-cylinder assembly used in an industrial prodess requiring heating with steam, cooling with water, and final cooling with ammonia. The over-all heat transfer coefficients are substantially greater than those obtained with any device operating on principles different from that of the equipment discussed. For example, in making starch pastes for salad dressing continuously at the rate of 1800 pounds per hour, an over-all heat transfer coefficient [B.t.u. per (hour)(square foot) (' F.)] of 583 has been obtained while heating the mixture from
The desirable attributes of uniform consistency, texture, and color in lard and shortening led to the development of improved methods of crystallizing and plasticizing. Following the now obsolete method of converting liquid oils to solid fats in a waterjacketed tank equipped with agitators the internally refrigerated chill roll was utilized for plasticizing. The hot refined oil or rendered liquid fat was fed onto the rotating surface of a cylindrical, refrigerated drum. There the liquid fat was transformed to a semisolid state by contact with t h e refrigerated surface. The crystallized mass was removed by a cutting blade and fell in sheets into a trough. The trough or picker box was equipped with paddles and the partially solidified fat was agitated and mixed. Then the plasticized product was picked up by a steam pump and transferred t o a n agitation tank for holding or further mixing, or in same instances it was delivered t o the filling machine. This method of chilling and plasticizing had many apparent deficiencies. The product was exposed to the atmosphere during processing and was subjected t o the acquisition of impurities and moisture from condensate. In the picker box, undetermined and varying amounts of air were whipped into the product so that neither plasticity nor color factors were constant. Also the pro-oxidant effect of the incorporated air wwi undesirable and adversely affected the keeping qualities. As early as the middle nineteen thirties, the closed, continuous internal chiller and plasticizer came into wide use and has substantially replaced the chill roll. This advance in processing methods and chilling and plasticizing procedures is described in the following section. INTERNAL PLASTICIZER. The rendered fat or refined oil is pumped from storage tanks to a small float-controlled supply tank adjacent to the chilling unit. It is pumped by a gear pump, an integral part of the unit, through a water-jacketed coil-type 6S7
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INDUSTRIAL AND ENGINEERING CHEMISTRY
precooler, There the tempeiatule is dropped from the storage temperature of 140" to 160" F. to about 120' F. Inert gas is drawn into the hot oil line in predetermined quantities through a valve a t the pump inlet. The gas is, in effect, dissolved in the
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second pump picks up the product a t the discharge end of the tempering section and delivers it a t high pressuie to the fillei. A texturizing valve is located in the line between the tempering section and the filler. The function of this texturizing tieatment is to remove any residual lumps and to increase the sheen and finish. Claimed advantages of this closed system method of plasticizing are: 1. The product is protected fioni atmospheric impurities and undesnable moisture additions from condensate or humidity. 2. Radiation losses are reduced with consequent savings in refrigei ation. 3. The control factors that govein Figure 1. Cross Section of Process Cylinder of the Special Heat Exchanger air or gas incorporation, rate of heat 1. Product connections 5. Heat transfer tube 2. Product chamber 6. Mutator shaft transfer, temperature of product and 3. Heat transfer medium connections 7. Scraper blades refrigerant, and operating 4. Heat transfer medium chamber 8. Insulation _ _pressures yield a product superior in color, texture, and consistency. 4. The homogeneity of the finished product is buch that oil, thereby ensuring a uniform dispersion or distribution of the gas separation is largely inhibited, with a beneficial effect on keepparticles. ing qualities. Plasticizing or solidification takes place in the chilling section, which is either a single cylinder or a multiple of cylinders exter&IARGARINE nally refrigerated. As the process material is pumped, it is forced through the narrow annular space between the shaft and the heat There are two manufacturing processes in wide use. The first transfer tube wall. The mechanically driven shaft rotates a t is the continuous, closed system process most widely used in the 500 to 700 r.p.m. and the continuous scraping action of its blades LTnitedStates. The second is the int,ermittent process that \vi11 against the heat transfer surface removes the thin film of crysbe identified in this discussion as the European process. tallized product. As a result of the high ratio of heat transfer Both processes are continuous in the crystallizing or plasticizing surface to the volume of material treated and the rapid film rephase. From this st,age hoJl-ever their points of similarity moval, the time of crystallization is short and the rate of heat diverge broadly. transfer abnormally high. The over-all coefficient is usually The eont,inuous in the range of 300 B.t.u. per square foot per F. difference. process e m p l o y s Residence time of the product in the chilling zone is from 6 to 20 the internal chilling seconds, depending on the flow rate and the designed capacity and p I a s t,i c i z i n g of the particular model. unit described in The product flow rate may be varied to synchronize with the the section on lard capacity of the filling equipment. The product temperature is and s h o r t e n i n g . controlled by regulating the temperature of the cooling medium. The apparatus itUniform crystallization takes place and all of the fatty material self is modified in is chilled to the same temperature. design and t,he temUniform pressures are maintained throughout the system by pering section, a n means of a pressure-regulating valve located between the feed a u x i l i a r y of the pump and the chilling section. chilling apparatus, The rapid rate of chilling, accompanied by the rapid removal of is of radically difthe film of crystallized material from the heat transfer surface, ferent design. In forms an exceedingly minute fat crystal. The relationship this instance the between crystal size and consistency in this field is critical and, tempering section is therefore, this shock chilling seems most desirable from the Figure 2. Three-Cylinder Assemmore truly what' the standpoint of the finished product's behavior and performance. bly of Special Heat Exchanger name implies, beThe supercooled product is pumped from the chilling zone at a Cause almost COmUsed in process entailing heating with temperature of 60" to 65" F. to the tempering zone where the steam, cooling with water, and final cool1e t e 1, e e I' i ing with ammonia solidification and final plasticizing is substantially completed as takes place in the the heat of crystallization raises the temperature of the produst system. to about 75" to 80" F. Mild agitation intermixes the crystallized Since lactic acid, mill%,and salt ale present in the emulsion, and uncrystallized components and prevents the solidification the apparatus is constructed of corrosion resistant metals process from going too far. The agitation in this section is The European process utilizes a chill roll for the crystallizing provided by the rotation of a shaft with spirally located pins that stage and for the same reason its materials of construction ale intermesh with stationary pins on the cylinder wall. The temcorrosion-resistant metals, in most instances pering chamber is not refrigerated. COWTINUOUS PROCESS.As the prior processing involved i n The product is now a finished shortening and ready for direct margarine manufacture is not directly pertinent, this discussion filling. Only a slight rise in temperature or change in conRill proceed directly uith the initial manufacturing step, which IS sistency will take place. The fat components have approached the crystallizing operation. equilibrium although the final set takes place in the container The emulsion is pumped from supply tanks to the inteinal and the product is usually further tempered after the packaging chiller. The temperature of the emulsion for table margarines is operation. approximately 100" F. at the time of introduction to the chilling One pump on the internal plasticizer has forced the product zone. The emulsion is chilled t o a temperature of approximately through the chilling section and the tempering section. A 45' to 50' F. in t h k section. llthough it has passed through the
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659
TABLE 11. COMMERCIAL APPLICATIONS OF SPECIALHEATEXCHANGER Product Data Operation Chilling and plasticizing Cooling Heating Cooling Cooling Sterilization Cooling
Material Lard and shortening Margarine Starch Starch
Slush freezing (7 to 8 % ice)
Pounds per hour 10,000 4,800 2,500 2,500 3,800 9,000 6,000
Temp., In 120 100 80 195 46 180 80 42
3,500
F.
Q*
Out 60 50 195 120 33 230 224
B.t.u./hr. 350,000 240,000 245,000 120,000 40,000 404,000 252,000
U H.T!C. 250 300 300 360 210 411 300
16
146,500
300
Heat Exchanger Heat Transfer Medium Heat transfer Type Temp., O F . area,sq. ft. NHs NH3 Steam Water
NHa
Steam NHs
SHa
10 0 240 70 14 335
.o
0
Const. matl
18 11 6.0 6.0 7.6 7.6 18
Carbon steel Nickel Stainless steel Stainless steel Nickel Stainless steel Nickel
18
Nickel
PLANT STERILIZATION STCDIES TABLE 111. PILOT Operation Heating Heating Heating Heating Heating Heating Cooling Heating Cooling
Product Data pounds Material per hour 270 Liver soup 217 Spinach 319 Peas Applesauce 740 730 Pears 214 Milk, homogenized 214 Milk 18% cream 250 250 18% cream
Temp., In 175 161 162 180 170 50 284 58 279
F. Out 285 281 285 235 232 290 110 279 120
Q*
B.t.u./hr. 26,700 23,400 35,300 36,000 40,700 45,900 34,300 47,000 31,000
U, H.T.C. 600 383 600 41 1 470 590 460 572 418
Heat Exchanger Heat Transfer Medium Heat transfer Type Temp., OF. area,sq. ft. Steam 310 0.676 Steam 323 0.676 Steam 324 0.676 Steam 335 0.076 Steam 334 0.676 Steam 323 0.676 Water 63 0.676 Steam 322 0.676 Water 63 0.676
Const. matl. Nickel Nickel Nickel Nickel Nickel iiickel Nickel Nickel Nickel
normal congeal or solidification point it is in a supercooled, fluid state and does not set up until it is delivered into the semiquiescent tempering zone. The tempering section is a cylindrical tube of approximately 7 inches internal diameter and varies in length from 3 t o 10 feet or longer, depending on the characteristics and setting properties of the fat components and the hourly throughput. The solidified mass of crystallized margarine is s l o ~ l yforced through the tempering chamber by the pressure of the emulsion or feed pump. The residence time in this zone varies from 60 seconds to 2 minutes and a t the time of discharge, the mass has a consistency and state of solidification suitable for direct delivery t o automatic print forming machines. The heat of crystallization has been almost entirely dissipated in the tempering zone. Very little change in temperature occurs in the package. -4typical margarine processing line is shown in Figure 3. SHORT TIME-HIGH TEMPERATURE STERILIZATION
Short time-high temperature sterilization, or flash sterilization as it is sometimes called, was carried out in both commercial and pilot scale equipment, in conjunction with aseptic canning and closing devices to sterilize whole milk, 18 to 30% cream, evaporated milk, concentrated milk, acid and nonacid baby food purees, cream style corn, pumpkin, chocolate milk, etc. These pilot plant tests were conducted with two types of aseptic canning units, one, the H C F unit developed by the American Can Co., Maywood, Ill., and the other, the Martin aseptic canning unit developed by the James Dole Engineering Co., Redwood City, Calif. Both of these companies collaborated on the test projects, with useful results. Table I11 shows some of the data obtained in this work. The merit of the equipment in the field of sterilization is its ability to operate a t high temperatures with heat sensitive material, very viscous material , and material that undergoes a change of state, such as thickening, without scorching or overcooking. In a general comparison of the conventional canning process and the aseptic canning process some points of contrast become readily evident. In conventional canning the prepared food is filled into containers. The closed containers are then placed in a stationary or agitating retort and heated for a prescribed time at a prescribed temperature to effect sterilization. The processing time at a prescribed temperature must be sufficient to allow the very center of the can or glass contained product to become heated, and must be held long enough to ensure absolute sterilization. This means that the product next to the container walls and ends comes to temperature very much sooner than the product in the center of the container, but must be held a t this temperature
Figure 3.
Modern Margarine Processing Line
Special h e a t exchanger is principal c o m p o n e n t
until the whole mass has been penetrated. Therefore, with very visoous or heat sensitive products as, for example, purees and dairy products, respectively, overheating or caramelization results. In addition, as the container size increases, process time increases and thus the quality of the product is not the same in a small container as it is in a large container. The temperature to which a closed container can be safely heated is dependent on its strength. The processing of some heavy viscosity products entails 70 minutes a t 250' F. in a No. 2 can. The continuous aseptic canning process entails the separate but synchronized sterilization of the product, the container, and the cover. The food material is rapidly and continuously heated to high temperatures (280" to 290' F.), held, then cooled to 80" to 90' F. as it is pumped successively through a heater, a holding section, and a cooler of a closed heat exchange system. Then it is delivered cold to the presterilized containers and covers. The filling and closing operations are executed in a sterile chamber. The time involved in heating, holding, and cooling is just a matter of seconds. Tests carried out with food products and equipment inoculated with heat-resistant organisms prove that this method of sterilization is dependable and practical. The sterilization temperatures referred to when speaking of short time-high temperature sterilization are in the neighborhood of 280" to 290" F. The holding time a t these temperatures will vary from approxi-
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mately 10 seconds upn-ard, depending on the nature and state of the product. Commercially, the aseptic process has several advantages over the conventional method. The most important are as follows: 1. A better flavor, texture, and color are produced Kith the flash sterilization method. 2. The flash sterilization method, with aseptic filling and closing, provides a very, wide range between scorching of the product and sufficiently safe sterilization, thus permitting a large factor of safety without sacrificing quality. 3. Regardless of container size the quality of the finished pioduct is the same. ' From this very general comparison of conventional canning and the aseptic canning process it must be realized that the heat exchanger system required to supply a sterile product must have several requisites. The heat exchanger under discussion provides these various requisites and affords many unique advantages. The high ratio of heat transfer surface to the volume of material treated fulfills the requirement for a very high heating rate. The constant and rapid cleaning of the heat transfer surface1000 to 1400 times per minute-prevents scorching and localized overcooking. Turbulence is created by the revolving shaft and blades rather than by forcing the product a t a high velocity through small diameter tube exchangers, thus eliminating the necessity of building the heat exchanger to withstand high pressures. As a matter of fact, the pressure drop across this heat exchanger has to be imposed by a back pressure valve thus making it possible to operate a t a process pressure high enough only to prevent flashing a t the sterilization temperature. Because of the spinning shaft and blades and the flexibility
Einr;g;r
ng
p m e S S
development
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of pressure adjustment across the heat exchanger system, the equipment lends itself to the processing of a wide variety of food products irrespective of product viscosity and consistency. Some of the products sterilized with this equipment range from whole fluid milk to pumpkin and cream style corn. The ability to control the pressure drop across the system makes possible the elimination of high pressure pumps, By the proper design of the shaft and control of the annular space, products containing discrete particles have been processed, for example, cream style corn. In respect to sanitation the heads and shaft are readily accessible. All surfaces coming in contact with the product can be visually inspected. In conclusion, this heat exchanger has been proved to be ideally suited for applications where the material is heat sensitive, very viscous, and undergoes a change of state during heating or cooling. Examples may be found in starch cooking and cooling, gelatin cooling, freezing of concentrates, chilling and plasticizing of fats and oils. In short, it has been experienced that, in most cases, if the product to be processed is pumpable it can be processed in this closed, continuous sanitary heat exchanger, LITERATURE CITED
(1) Slaughter, J. E., Jr., and McMichael, C. E.,, J . Am. Oil Chemists' SOC.,26, No. 10, 623-8 (October 1949). (2) Sutton, Mack, and Bond, A., Jr., Am. Paint J.,33, S o . 31, 56, 58, 61-2, 64, 66, 68, 70 (1949). RECEIVED for review May 4,1951. ACCEPTED September 28, 1951. Presented as part of the Symposium on Chemical Engineering Aspects of Food Technology before the Divisions of Industrial and Engineering Chemistiy, and Agricultural and Food Chemistry, 119th Meeting, A\IERICAN CHEMICAL SOCIETY,Boston, Mass., dpril 1981.
High Purity Spongelike Copper from Waste Pickling Sludge ROBERT L. RUSHER'
AND
G E O R G E W. B L U M
CASE INSTITUTE OF TECHNOLOGY, CLEVELAND, OHIO
A
PORTION of an investigation of possible methods for the utilization of pickling sludge from the brass and copper industries resulted in the development of a method for the preparation of a high purity copper of particularly fine spongelike texture and large surface area. The waste material utilized is that produced in the acid pickling baths of the brass and copper products manufacturers. It is a highly acidic metallic salts sludge, which a t present constitutes a severe water pollution problem, especially in the Connecticut area where one third of the United States copper and brass products are mariufactured. The analysis of the pickle liquor sludge used in this investigation is shown in Table I. With the assumption that all of the nitrogen present exists as nitric acid and that all of the metals are present as sulfates, the analysis of Table I1 was obtained. The effects of such a waste on waters used for public water supplies for industrial, agricultural, and recreational purposes 1 Present address, Grasselli Chemicals Departmenr, E. I. du Pout de Nemours & Co., Inc., Wilmington, Del.
are obvious. It causes corrosion and deterioration of the sewage treatment plants to take place and seriously affects the biological treatment of sewage. The acids and metal salts are toxic to aquatic life, even in concentrations as small as 2 p.p.m. for copper and zinc. Although it was realized that the underlying emphasis of this problem is normally directed toward the study of pickling wastes for the abatement of pollution, this work was a study of the utilization of the sludge, because the possibility arises for the recovery of copper and zinc which are again critical metals in the present world situation. During the course of preliminary laboratory investigation of the problem, four methods of utilization were considered. 1. The method of thermal decomposition, whereby one third of the copper was recovered from the sludge due to a solubility difference after roasting a t 670" C. for 1 hour, was considered economically not feasible because of the high cost of construction, operation, and maintenance of a rotary kiln operating at 670' C. 2. The use of the acid-free sludge plus additives, such as so-
,
