Energy Use Profile in Concentrated and Powdered Milk Manufacture

Energy Use Profile in Concentrated and Powdered Milk Manufacture V. K. GOEL, 1 W. K. J O R D A N , and M. A. R A O a Institute of Food Science Department of Food Science

Cornell University Ithaca, NY 14853 ABSTRACT

Direct energy requirements in a plant (Plant A) producing evaporated and condensed milk were 508 kcal/kg processed while those in a plant (Plant B) producing condensed and dehydrated milk were 544 kcal. Thermal energy was the major component accounting for about 82% of the direct energy consumed. Consumption of electricity in the plants was less than in fluid milk plants studied earlier. This was due to the absence of refrigerated storage areas. In Plant A, packaging energy was nearly equal to the thermal energy input; steel cans for evaporated milk contributed 87% of the packaging energy. In Plant B, packaging energy was 11 kcal/kg of milk processed. About 14% of the thermal energy consumed was lost into discarded waste streams and hot equipment surfaces. Heat recovery and insulation techniques can reduce the present losses by about 60%. INTRODUCTION

Food system accounts for about 17% of a]l the energy consumed by the US (2). Unger (19) showed that food and kindred products industries as a whole are sixth among the leading energy using industries. Food processing plants were the object of a number of studies aimed at better management of energy use (3, 12, 13, 14). Goel et al. (8) showed that fluid milk requires significant quantities of energy for processing, packaging, and transportation. However, the energy consumed was less than that for the production of milk on the farm.

Received August 21, 1978. 1World Bank, Washington, De. 2New York State Agricultural Experiment Station, Geneva 14456. 1979 J Dairy Sci 62:876-881

Surplus milk can be utilized for making evaporated milk, condensed milk, or powdered milk, with butter being made from the excess fat. Schwartzberg (15) estimated that the manufacture of evaporated and condensed milk in the US annually requires the removal of 3.8 billion kg of water; the volume of oil consumed for this purpose has been estimated at about 108,000 m 3. Dehydration is also an energy intensive operation; in particular, a single stage spray drier requires six times the energy consumed by a conventional triple effect evaporator (17). Concentration and dehydration reduce energy use in subsequent operations such as packaging and transportation. It is desirable to include energy used for packaging and transportation in the analysis o f energy used for concentrated and powdered milk. Further, if the energy consumed for the production of milk also is included, the guidelines of the International Federation of Institutes for Advanced Study suggest that one can account for most o f the energy consumed for a product (1). One objective of our investigation was to determine the energy requirements for processing, packaging, and transportation in two plants (Plants A and B) utilizing surplus milk. The other objective was to determine energy losses and the savings that can be achieved. Plant A produced evaporated milk, condensed milk, and butter, while Plant B produced condensed milk, milk powder, and butter. Plant A was located in western New York while Plant B was located in central New York. METHODS

Energy consumed on the plant premises was designated as direct energy (1, 8) while that for transportation and packaging were part of the indirect energy requirements. Direct Energy Sources

Data on the sources of direct energy were 876

ENERGY USE obtained from the records maintained in the plants. For Plant A, data were available for each month during 1975 and 1976; for Plant B, such data were available for only 1976. The sources of direct energy consumed in the plants were natural gas, liquified propane, and electricity in 1976; a small amount of No. 6 fuel oil also was used in Plant A during 1976. Energy consumed in the plants was expressed in kilocalories of fossil fuels. For natural gas, fuel oil number 6, and propane, the kilocalories were calculated from their respective calorific values (6). Electricity is a secondary energy source and is derived, for the most part, from fossil fuels. For this reason, the energy losses during generation and transmission were taken into account for an efficiency of 29.4% (18); i.e., 1 kwh = 2,923 kcal. The practice of considering the efficiency of generation of electricity was adopted in (2, 5, 8). Energy for Packaging

Packaging materials consumed in the plants were steel cans for evaporated milk and paper board boxes for cartoning the cans in Plant A

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and plastic and paper bags for milk powder in Plant B. Both plants shipped condensed milk in bulk tankers, and the milk to be processed also was received in tankers. Because they are reused many times, the energy required to manufacture the tanks is small per kilogram of milk or condensed milk handled. In contrast, cans, paper boxes, plastic bags, and paper bags were not recycled. For these reasons, the energy expended for the manufacture of these packaging materials was estimated from the energy intensive data of Berry and Makino (5). The quantities of packaging materials consumed were determined from data made available by the plant management. Additional discussion on packaging energy intensities can be found in studies by Rao (10) and Vergara and Rao (12). Energy for Transportation

Energy consumed for transportation of milk to the processing plants and for the products to consuming and distribution centers was estimated from information supplied by the plant manager. The information consisted of estimates of transport distances, modes of trans-

TABLE 1. Production and characteristics of energy use of the plants. Item

Plant A

Plant B

Milk processed, kg × 106 Skim condensed milk processed, kg × 10 6

98.34 ....

88.93 3.31

Products, kg X 10 6 Evaporated milk Condensed milk Powdered milk Butter Cream

14.61 14.46 .... 4.18 ....

"5164 3.62 2.02 1.54

Thermal energy consumed, kcal × 109 Per kg of milk, kcal Per kg of solids, kcal

4O.68 414 3,310

Electricity consumed, kcal × 109 Per kg of milk, kcal Per kg of solids, kcal

9.23 94 751

Energy for packaging, kcal × 109 Per kg of milk, kcal Per kg of solids, kcal

4O.O6 407 3,260

Energy for transportation, kcal × Per kg of milk, kcal Per kg of solids, kcal

10 9

8.01 82 652

40.99

461 3,747 7.42 83 678 0.96

11 87 5.14 58 470

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GOEL ET AL.

portation, and energy intensiveness of each transportation mode. Energy Losses and Conservation

Sources o f energy loss were identified during visits to the plants. Magnitudes of heat losses into discarded streams were estimated b y energy balances while those from hot equipment and steam pipes were estimated from well known heat transfer correlations (11). Heat losses from buildings were estimated b y the degree day method (4). The same methods were used to estimate energy savings possible by heat exchange and insulation. The data and equations are described in the references and will not be discussed. RESULTS AND DISCUSSION Location and Size of Plants

Both plants were located in small towns, and neither had waste water treatment facilities. In 1976, Plant A received milk and produced evaporated and condensed milk and butter. Plant B received milk and condensed skim milk; its products were milk powders, whole condensed milk, butter, and cream. The input and output of materials as well as the characteristics of energy use are in Table 1. The quantities of milk processed during the year were nearly equal for the two plants; however, there were differences in the monthly milk consumption (Figure 1).

intensive sterilization operation necessary for evaporated milk in Plant A and can be attributed to the higher energy intensity of the drying operation. In contrast to these plants, the thermal energy requirements for fluid milk processing in two plants were 67 and 103 kcal/kg (8). Goel et al. (8) reported that consumption of electricity in two fluid milk processing plants was 149 and 253 kcal/kg of milk. The lower consumption of electricity in the plants of our study was due to the relatively lower use of refrigeration as the products did not require refrigerated storage. Profiles of consumption of the sources of direct energy were similar to the production profiles in Figure 1. Energy use for peripheral operations such as space heating was not a major component. Total consumption of direct energy in Plant A during 1975 was about 8% less than that in 1976; however, the consumption per unit weight of output was about the same for both years. Energy for Packaging

Plant A shipped condensed milk in bulk by

Journal of Dairy Science Vol. 62, No. 6, 1979

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Natural gas and fuel oil number 6 were used in boilers; hereafter these will be referred to as sources o f thermal energy. The steam generated was used for concentration or dehydration, space heating during winter, and hot water for cleaning; in Plant A it also was used for sterilizing evaporated milk in continuous atmospheric retorts. Electricity was used for lighting and driving electric motors. Propane was used in fork-lift trucks for movement of goods in the plants. As shown in Table 1, consumption of thermal energy in the plants was at least four orders of magnitude higher than for electrical energy. Also, consumption of thermal energy in Plant B, per unit o f milk input, was 10% higher than in Plant A. This was in spite of the energy

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Figure 1. Milk consumption profiles in Plants A and B during 1976.

ENERGY USE tankers while evaporated milk was shipped in steel cans boxed in paper cartons. Energy expended for the steel cans and the paper board totaled 40.1 billion kcal with the former accounting for 87% of the total. In Plant B, paper accounted for 82% of the packaging energy and plastics for the rest. For illustrative purposes the energy for packaging milk was about 270 kcal/kg of milk in two fluid milk plants (8). The energy intensity of the throwaway steel can is seen readily from the estimates of packaging energy. Savings in energy by evaporation instead of drying are lost with the use of energy intensive packaging for evaporated milk. These data also point out the need to consider energy inputs at all stages in the food cycle of a product or its market form. Accounting for inputs in only one stage, such as processing, does not provide the true picture of energy use. Studies on energy inputs at various stages in the food cycle have been reported for a few commodities (9, 10). Energy for Transportation

Energy consumed for the transportation of milk to the plants and the products to their destinations was estimated from the information provided by the plant management and tabulated in Table 1. In Plant A, energy consumed for transportation was 44.5 kcal and 37.2 kcal/kg of milk for procurement and distribution. The corresponding figures for Plant B were 33.3 kcal and 24.5 kcal. The higher energy use in Plant A for procurement was due to relatively long distances traversed while for distribution it was due to the relatively larger quantities of products. Energy for Water

Both plants utilized water from wells. Because of the nature of the operation, Plant A used more water and required more energy for pumping than Plant B. Based on data for the Cornell University water supply system, the energy for pumping water was estimated to be 25 kcal/kg of solids in Plant A and 4 kcal/kg of solids in Plant B. Thermal Energy Losses and Conservation

Because thermal energy was a substantial portion of the direct energy consumed in the

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plants, the magnitude of its loss and the potential for conservation were determined. In food processing plants, losses o f thermal energy occur from discarded hot streams and poorly insulated surfaces (11). In Plant A, the discarded hot streams were condensate from the first effect of the evaporator system, water used to cool the sterilized cans of evaporated milk, overflow from the tank supplying hot water to pasteurizers, and vapors from the evaporators that were condensed by direct contact with cold water. Surfaces of energy loss were those of the evaporators, continuous retorts, uninsulated steam pipes, as well as poorly insulated building walls and ceilings. The magnitudes of these losses, summarized in Table 2, amounted to 15.2% of the thermal energy consumed in Plant A. In contrast to Plant A, Plant B did not have continuous retorts; however, it had a spray drier for dehydration. Thermal energy losses in Plant B are also in Table 2. The magnitude of the heat loss in the discarded hot air from the spray drier was estimated from the results of Varshney et al. (20), 278 kcal/kg of milk powder produced. This procedure was used because data on the flow rate of air to the spray drier and its inlet and outlet temperatures were not available. Losses in Plant B amounted to 13.7% of the input of thermal energy. Magnitude of thermal energy that can be recovered was estimated with assumptions: 1) condensate can be returned to the boilers with 90% heat recovery; 2) 50% of the energy is recovered from the hot water used to cool cans, overflow from hot water tanks, and cooled product vapors; 3) temperature o f the storage areas heated during winter is reduced from 21.2 C to 15.6 C and the ceilings are provided with 10 cm insulation; 4) steam pipes and hot equipment surfaces are provided with 2.5 cm insulation; and 5) 50% of the heat in the discarded air from the spray drier is recovered by spraying milk. Table 3 is a summary of the thermal energy that can be conserved; it also shows the percentage of each loss that can be avoided. It is emphasized that the techniques o f heat recovery from hot water and insulation of hot surfaces are well known and have been either implemented or suggested previously in food processing plants (3, 12). Recovery of heat from the spray driers' exhaust air by heat Journal of Dairy Science Vol. 62, No. 6, 1979

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GOEL ET AL.

TABLE 2. Sources and magnitudes of losses a of thermal energy, kcal × 106. Item

Plant A

Plant B

Discarded hot streams Steam condensate Pasteurizer feed tank overflow Water used for can cooling Vapors condensed Spray drier exhaust air

789(1.9) 479(1.2) 906(2.2) 2,383(5.9) NA

769(1.9) 250(.6) NA 2,337(5.7) I01(.3)

Heat loss surfaces Steam pipes Equipment Building ceilings

86(.2) 502(1.2) 1,042(2.6)

36(.1) 1,059(2.6) 1,066(2.6)

Total

6,187(15.2)

5,618(13.7)

aNumbers in parentheses indicate losses as percent of boiler fuels consumed. NA means not applicable.

exchange with cold air is difficult due to small particles o f milk in the former; the milk particles will foul heat exchanger surfaces (20). A c o u n t e r current spray of milk will recover the heat and the milk powder. In Plant A, additional energy savings can be achieved by i m p r o v i n g the efficiency o f the c o n t i n u o u s atmospheric retorts. Singh (16) showed that c o n t i n u o u s atmospheric retorts have an operating efficiency of only about 30%. This was due mainly to the steam injected through spargers n o t mixing well w i t h the h o t w a t e r in the retort and being lost f r o m the vents. It was d e m o n s t r a t e d (6) that the energy

use efficiency c a n be increased to a b o u t 45% by heating the water for the retort by a heat e x c h a n g e r and p u m p i n g the water c o u n t e r to the f o o d cans.

CONCLUSION

Thermal energy is a major c o m p o n e n t o f the direct energy c o n s u m e d in c o n c e n t r a t i o n and d e h y d r a t i o n of milk; it is at least four orders o f magnitude higher than electricity. C o n s u m p t i o n of electricity per kilogram of milk processed in t w o plants was less than that in two fluid milk plants (8); this was mainly due to the absence

TABLE 3. Potential for conservation of thermal energy by heat recovery and insulation, a kcal × 106 . Item

Plant A

Heat recovery Condensate Hot water Pasteurizer regen, efficiency to 90% Air from spray drier

710(90) 1,884(50) 149(12) NA

692(90) 647(50) 39(13) 50(50)

83(96) 486(97) 869(83)

34(96) 1,026(97) 915(86)

4,181(68)

3,404(61)

Insulation b Steam pipes Equipment Buildings (t = 15.6 C also) Total

aNumbers in parentheses refer to savings expressed as percent of corresponding losses. bThermal conductivity of insulation assumed to be .035 W/m.K. Journal of Dairy Science Vol. 62, No. 6, 1979

Plant B

ENERGY USE o f refrigerated storage facilities for t h e concentrated and d e h y d r a t e d milk products. In Plant A, packaging energy was nearly equal to the thermal energy c o n s u m e d per kilogram of milk processed; cans for evaporated milk constituted 87% o f the packaging energy. C o n s u m p t i o n of energy for processing operations, packaging, and t r a n s p o r t a t i o n for the t w o plants was b e t w e e n 613 and 1,000 kcal/kg o f milk processed. Goel et al. (8) estimated that p r o d u c t i o n o f milk on the farm requires at least 1,200 kcal/kg. Thus, the energy requirements for c o n c e n t r a t i o n or d e h y d r a t i o n , packaging, and transportation in the studied plants do n o t exceed that for milk p r o d u c t i o n on the farm. Losses o f thermal energy in the plants were substantial; t h e y were a b o u t 14% of the thermal energy i n p u t in the plants. R e c o v e r y o f heat f r o m hot waste streams and insulation o f h o t e q u i p m e n t surfaces can reduce the present losses b y a b o u t 60% in b o t h the plants. Upgrading the efficiency of the atmospheric retorts in Plant A also will lead to significant energy savings. ACKNOWLEDGMENTS

The G o v e r n m e n t o f India provided scholarship support to V. K. Goel. The College of Agriculture and Life Sciences also s u p p o r t e d this study. Special thanks are due to the managers and personnel of the plants for their c o o p e r a t i o n and patience.

REFERENCES

1 Anonymous. 1974. Energy analysis workshop on methodology and conventions. Int. Fed. of Inst. Adv. Stud., Stockholm, Sweden. 2 Anonymous. 1976. Energy use in the food system. U.S. Government Printing Office, Washington, DC. 3 Anonymous. 1976. A study of conservation potential in the meat packing industry. U.S. Government Printing Office, Washington, DC. 4 ASHRAE. 1976. 1976 Systems handbook: Amer. Soc. of Heating, Refrigerating, and Air-conditioning

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Eng., Inc., New York, NY. 5 Berry, R. S., and H. Makino. 1974. Energy thrift in packaging and marketing. Technol. Rev. 76(4): 32. 6 Chhinnan, M. S., R. P. Singh, L. D. Pedersen, and H. E. Griffith. 1978. Improvements in energy utilization of an atmospheric retort. 38th Ann. Mtg. Inst. Food Technol., Dallas, TX, June 4 to 7, 1978. 7 Gatts, R. R., R. G. Massey, and J. C. Robertson. 1974. Energy conservation guide for industry and commerce. NBS Handbook 115, U.S. Government Printing Office, Washington, DC. 8 Goel, V. K., W. K. Jordan, and M. A. Rao. 1978. Energy profile analysis in fluid milk plants. Paper presented at the Joint Meeting of ADSA and ASAS, East Lansing, MI. 9 Henig, Y. S., and H. M. Schoen. 1976. Energy requirements: Freezing vs. canning. Frozen vs. canned corn. Food Eng. 48(9): 54. 10 Rao, M. A. 1977. Energy consumption for refrigerated, canned, and frozen peas. J. Food Proc. Eng. 1:149. 11 Rao, M. A., and J. Katz. 1976. Computer estimation of heat losses in food processing plants. Food Technol. 30(3): 36. 12 Rao, M. A., J. Katz, J. F. Kenny, and D. L. Downing. 1976. Thermal energy losses in vegetable canning plants. Food Technol. 30(12):44. 13 Rao, M. A., J. Katz, and V. K. Goel. 1978. Economic evaluation of measures to conserve thermal energy in food processing plants. Food Technol. 32(4): 34. 14 Rippen, A. L. 1975. Energy conservation in the food processing industry. J. Milk Food Technol. 38:715. 15 Schwartzberg, H. G. 1977. Energy requirements for liquid food concentration. Food Technol. 31(3):67. 16 Singh, R. P. 1977. Energy consumption and conservation in food sterilization. Food Technol. 31(3):57. 17 Starkie, G. L. 1975. Some aspects of energy conservation in dairy processing plant. J. Soc. Dairy Technol. 28:121. 18 Summers, C. M. 1971. The conversion of energy. Sci. Amer. 225(3):148. 19 Unger, S. G. 1975. Energy utilization in the leading energy consuming food processing industries. Food Technol. 29(12): 33. 20 Varshney, N. N., A. N. Patil, and T. P. Ojha. 1978. Heat losses from milk spray dryers. J. Food Sci. and Technol. 15: 81. 21 Vergara, W., and M. A. Rao. 1978. Indirect energy requirements for vegetable canning. J. Food Proc. Eng. 2:137.

Journal of Dairy Science Vol. 62, No. 6, 1979