Utilization of Solar Heated Water in Shell and Tube Heat Exchanger for Pasteurization of Milk

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Utilization of Solar Heated Water in Shell and Tube Heat Exchanger for Pasteurization of Milk

(Milk Pasteurization by using Solar Energy)

Pankaj Bisen

P.G Scholar, Deptt. of Dairy Engg. College of Dairy Science and Food Technology,

(CGKV) Raipur 492012, India.

Dr. A.K. Agrawal

Prof. & Head, Deptt. of Dairy Engineering College of Dairy Science & Food Technology, CGKV,

Raipur-492012 (India)

Geetesh Sinha

SRF, (Dairy Engineering)

College of Dairy Science and Food Technology, (CGKV) Raipur 492012, India.

Abstract- The present investigation is aimed to evaluate the performance of solar water heated shell and tube heat exchanger, to develop milk pasteurization system using solar heated water and to assess the chemical and microbial qualities of pasteurized milk prepared using solar heated water. The shell and tube heat exchanger performance was evaluated considering solar water flow rates of 3, 5, and 7 l/min at 85oC and cold water flow rates at 0.5, 1.0 1.5 l/min at 35oC. The parameters like effectiveness of heat exchanger (), tube side (hi) and shell side (ho) convective heat transfer coefficient and tube wall temperature (Tw) were calculated. The highest values for and Tw were observed to be

    1. and 75.460C at flow rate of hot water at 7 l/min and flow rate of cold water at 0.5 l/min. Therefore this combination of flow rates of hot water and milk was subsequently utilized for pasteurization. The pasteurized milk was evaluated for its chemical and microbial qualities and was compared with raw milk. The pasteurized milk had 0.15% acidity, 3.49% fat, 8.44% solid not fat and 11.93% total solids. In this way pasteurized milk was found more or less similar with raw milk. In microbiological quality evaluation, methylene blue reduction time was found 1.4 h for raw milk and 5.4 h for pasteurized milk. The total bacterial count were found 4.27 log 10 cfu/ml and coliform was found to be absent in 0.01 dilution for pasteurized milk. All the microbiological test findings found within the BIS standards specified for pasteurized milk. The cost of processing in conventional and solar water heated pasteurization was estimated to be Rs. 2.75 and Rs. 1.90 respectively. The finding of this investigation have shown that solar water heating system assisted shell and tube heat exchanger system can be used to produce pasteurized milk with acceptable quality.

      Key words: Solar water heater, solar energy, 1-2 heat exchanger, non-renewable energy

      1. INTRODUCTION

        Solar thermal system can greatly contribute to energy saving during the production process in the dairy sector, which demand water temperature of <800C. The hot water produced by solar collectors can also be used for pre-hating the water entering the steam boiler. In this case, the energy contribution of the solar system is relatively small both in comparison with the total energy demand. The Indian dairy sector has acquired substantial growth momentum from the 9th plan onwards. As a result of this India is first amongst the world milk producing nations achieving an annual output of about 140 million tonnes during 2013-14 [1].

        In dairy processing plant milk and milk products are subjected to various unit operations like filtration, clarification, homogenization, pasteurization, sterilization (UHT treatment), cooling, packaging,

        refrigeration etc. Among various heat treatments the main operation is pasteurization in which steam required is 0.3kg/l of milk. Traditionally, fossil fuels (coal, gas, diesel etc.) are utilized to generate heat to produce steam/hot water and are subsequently used as a heating medium in PHE. Burning of these fossil fuels contribute to pollution as well as to green house gases (GHGs) resulting into global warming. Further, these energy sources are conventional. If solar heated water is used for fully or partially, it would reduce the cost of fuel charges (Katre and Prasad, 1989). Several renewable energy sources like wind, geothermal, solar etc are available. Among various types of renewable energy sources, solar energy (from sun) is the most readily available and important source of energy because it is non-polluting, renewable, clean and inexhaustible. Solar energy also helps in reducing pollution and maintenance of eco balance [2].

        Tubular heat exchangers are so important and so widely used in the process industry that their design has been highly developed. When the required heat transfer surface is large, the recommended type of exchanger is the shelland-tube [3]. In the present investigation, 1-2 shell and tube heat exchanger is used for pasteurization of milk. Shell and Tube heat exchangers are applied where high temperature and pressure demands are significant and can be employed for a process requiring large quantities of fluid to be heated or cooled. Due to their design, these exchangers offer a large heat transfer area and provide high heat transfer efficiency in comparison with others.The 1-2 exchanger is normally arranged so that the cold fluid and the hot fluid enter at the same end of the exchanger, giving parallel flow in the first tube pass and counter flow in the second. This permits a close temperature approach, at least at the exit end of the exchanger. With a view to overcome various disadvantages of using conventional fuels, the investigation was carried out to evaluate the performance of solar water heating system assisted shell and tube heat exchanger for milk pasteurization.

      2. MATERIALS AND METHODS

        Solar water heating system works on the principle of thermosyphon. Thermosyphon systems are similar in configuration to active systems; with the exception that circulation through the collector loop is achieved through the fluid buoyancy forces, resulting from temperature gradients in the storage tank and collector. This buoyant force is utilized by placing the collector outlet at a lower level than the tank outlet that supplies fluid to the collector, producing a difference in fluid density that drives the circulation.

        A. Design and length of holding tube (Holder)

        The holder ensures the product is held for a specified time, not less than 15 s, when heated to 720C or more. The time of 15 s must be elapsed by most rapidly moving particle passing through the holding

        tube. For a particular holder, timing is based on the milk pump flow rate. The milk temperature is determined at the discharge of the holder.

        Cc = Heat capacity rate ratio of lower temperature stream (dimensionless)

        Ch = Heat capacity rate ratio of higher temperature stream (dimensionless)

        1. Convective heat transfer coefficient (h)

          1. Tube side coefficient (hi)

            i

            i

            h = NNuk

            d

            (8)

            NNu= 0.023NRe0.8 NPr0.3 (9)

            µ

            µ

            NRe = vd

            (10)

            NPr

            Where,

            = µCpc

            k

            (11)

            Fig. 1: Schematic diagram of solar water heating system

            () =

            Milk flow rate is 0.5 l/min, than

            () = 2L

            0.5 l/m

            () = 2L

            0.0083 l/s

            (1)

            (2)

            (3)

            NNu = Nusselt number (Dimensionless) NRe = Reynolds number (Dimensionless) NPr = Prandtl number (Dimensionless)

            d = Inner diameter of tube (m)

            k = Thermal conductivity of cold water (W/m0C) = Density of cold water (kg/m3)

            v = Velocity of cold water (m/s)

            µ = Viscosity of cold water (N m/s)

            Cpc = Specific heat of cold water (kJ/kg0C)

          2. Shell side coefficient (h0) By Donohue equation [4]/p>

        () = 2L

        (4)

        h = 0.2 k/D

        (DoGe)0.6 (Cphµ)0.33 ( µ )0.14 (12)

        0.0000083 3/s

        In present case, a stainless steel pipe of length 0.064 m having inner

        o

        Where,

        o µ k µw

        radius ri is 0.025 m and outer radius ro is 0.026 was taken for holder. In eqn. (2) by putting the value L=0.064 m, ri = 0.025 m, the required residence time of 15 s is achieved.

        µ = Viscosity of the shell fluid (hot water) (N.s/m2)

        µw = Viscosity of the shell fluid at tube wall temperature (N.s/m2)

        Do = Outside diameter of tube (m) here,

        c

        c

        Ge = GbGc (13)

        b

        b

        G = m Sb

        Where,

        and G = m Sc

        (14)

        Fig. 2: Holding tube attached at hot milk outlet of shell and tube heat exchanger

        1. Experimental Set up

          Experiments were conducted on a 1-2 Shell and tube heat exchanger.

          The Fig. 3 shows the schematic diagram of the heat exchanger. The Fig. 1 shows the schematic diagram and of solar water heating system. The solar water heating system was connected to S&THE using an insulated pipe. The hot water was circulated in the shell and flow through it, while the medium to be heated was allowed to flow

          m = Mass flow rate (kg/s)

          Ge = Average mass velocity of the fluid flowing parallel and across the tube (kg/m2s)

          Gb = Mass velocity of fluid flowing parallel with the tube (kg/m2s)

          Gc = Mass velocity of fluid flowing across the tube (kg/m2s) Sb = Area for flow in baffle window (m2)

          Sc = Area for transverse flow between the tube (m2) Here,

          through the tubes.

          S = f

          Ds2 N

          Do2

          (15)

        2. Experimental observations

          Preliminary trials were carried out between water to water for

          performance evaluation. The performance evaluation of solar water heating system assisted shell and tube heat exchanger was done with two parameters viz. flow rate of hot water at 3, 5 and 7 l/min and flow rate of cold water at 0.5, 1.0 and 1.5 l/min. The following heat transfer parameters were considered for evaluating the solar water heating system assisted S&THE.

        3. Heat exchanger Effectiveness ()

        Heat capacity rate ratio, C* is the ratio of the smaller to the larger heat capacity rate to hot and cold fluids.

        mccpc = Cc & mhcph = Ch (5)

        b b 4 b 4

        Where,

        fb = Fraction of cross sectional area of shell occupied by baffle window (0.1955), [5]

        Ds = Inside diameter of shell (m)

        Nb = Number of tube in baffle window

        Nb = fbx total no. of tubes Nb = 0.1955 x 32

        Nb = 6.25 (say, 6)

        and

        p

        p

        Sc = Ds (1 Do) (16)

        Where,

        Then, = TcbTca

        ThaTca

        When Cc < Ch

        (6)

        p = Center to centre distance between tubes (m) P = Baffle pitch (m)

        Where,

        = ThaThb

        ThaTca

        When Cc > Ch (7)

        F. Tube wall temperature (Tw)

        For heating

        = Heat exchanger effectiveness

        For cooling

        Tw = T + Ti (17)

        Here,

        Tw = T Ti (18)

        1

        T = hi T (19)

        C. Effect of flow rates of hot and cold water on tube side convective heat transfer coefficient (hi)

        Where,

        i 1 + Di hi Doho

        T = Average fluid temperature (0C)

        T = Overall temperature drop (0C)

        T i = Temperature drop through inside tube (0C)

        G. Pasteurization of milk

        The pasteurized milk was prepared using time temperature combination (71.50C for 15 s) [6]. Samples were analyzed for their chemical [7], microbial [8] qualities. The pasteurized milk was also checked for alkaline phosphatase activity [9]. Statistical analysis of chemical parameters was carried out using t-test.

      3. RESULTS AND DISCUSSION

        1. Performance evaluation of solar water heating system assisted S&THE

          The results on performance evaluation of solar water heating system assisted shell and tube heat exchanger for various heat transfer parameters is presented in Table 1.

          TABLE I AVERAGE HEAT TRANSFER PARAMETERS

          Fh

          Fc

          hi

          ho

          Tw

          l/min

          l/min

          kW/m2oC

          kW/m2oC

          oC

          3

          0.5

          0.73

          0.38

          0.58

          71.93

          1.0

          0.68

          0.67

          0.57

          67.40

          1.5

          0.63

          0.91

          0.56

          64.20

          5

          0.5

          0.77

          0.39

          0.76

          74.00

          1.0

          0.73

          0.68

          0.75

          69.30

          1.5

          0.68

          0.95

          0.74

          66.37

          7

          0.5

          0.79

          0.40

          0.95

          75.46

          1.0

          0.75

          0.70

          0.94

          71.32

          1.5

          0.71

          0.97

          0.93

          69.16

        2. Effect of flow rates of hot and cold water on effectiveness of heat exchanger

        Figure. 3: Effect of flow rates of hot and cold water on effectiveness of heat exchanger ()

        From Fig.3, the values of effectiveness of heat exchanger ranged from 0.63 at 3 l/min to 0.71 at 7 l/min. The ANOVA had shown that both flow rate of hot and cold water had significant (p0.05).

        Figure. 4: Effect of flow rates of hot and cold water on tube side convective heat transfer coefficient (hi)

        The average values of tube side convective heat transfer coefficient ranged from 0.65 kW/m2oC at 3 l/min to 0.69 kW/m2oC at 7 l/min. The highest hi value found was 0.69 kW/m2oC at 7 l/min and the lowest value of 0.65 kW/m2oC at 3 l/min and these values are statistically significant (p0.05) at each flow rate of hot water.

        1. Effect of flow rates of hot and cold water on shell side convective heat transfer coefficient (ho)

          The average values of shell side convective heat transfer coefficient ranged from 0.57 kW/m2oC at 3 l/min to 0.94 kW/m2oC at 7 l/min. The highest ho was found 0.94 kW/m2oC at 7 l/min and the lowest

          0.57 kW/m2oC at 3 l/min and they are statistically significant (p0.05) at each flow rate of hot water.

          Figure. 5: Effect of flow rates of hot and cold water on shell side convective heat transfer coefficient

        2. Effect of flow rates of hot and cold water on tube wall temperature (Tw)

          The average values of tube wall temperature increased from 67.84oC at 3 l/min to 71.98oC at 7 l/min. The highest value of tube wall temperature found was 71.98oC at 7 l/min and lowest 67.84oC at 3 l/min and each flow rate had shown significant (p0.05) effect on tube wall temperature.

          Figure. 6: Effect of flow rates of hot and cold water on tube wall temperature (Tw)

          Similarly, while considering flow rate of cold water, tube wall temperature values ranged from 66.57oC at 1.5 l/min to 73.79oC at

          0.5 l/min had also shown significant (p0.05) at each flow rate of cold water. The highest tube wall temperature found was 73.79oC at

          0.5 l/min and lowest was recorded as 66.57oC at 1.5 l/min.

          All heat transfer parameters show increase in their values with increase in flow rates. The values of heat exchanger effectiveness (0.79) and tube wall temperature Tw (75.460C) were found to be maximum in combination of flow rate of hot water at 7 l/min and flow rate of cold water at 0.5 l/min.

          Therefore, combination of flow rate of solar heated water (85oC) at 7 l/min and flow rate of cold water at 0.5 l/min was adopted for pateurization. For pasteurization, cow milk was fed at 35oC for heating and it attained maximum temperature of 73oC in a tube side (stainless tubes) in the heat exchanger, while the heating medium i.e. solar heated water was admitted at 85oC in the shell side and its was allowed to discharge from the outlet side where its temperature was recorded to be 69oC. The pasteurized milk was analyzed for its chemical and microbial quality and compared with raw milk samples.

        3. Chemical quality of raw and pasteurized milk

          The samples of raw and pasteurized milk were analyzed for chemical parameters like acidity, fat, solid not fat and total solids and the data are presented in the Table 2

          Parameter s

          Raw milk (Mean± SD)

          Pasteurize d

          Milk (Mean± SD)

          t-stat

          t-test

          Acidity

          (0.150±0.0085)

          (0.151±0.00

          93)

          -1.483

          NS

          Fat

          (3.491±0.066)

          (3.466±0.06

          5)

          1.914

          NS

          Solid not fat

          (8.446±0.032)

          (8.438±0.05

          3)

          0.560

          NS

          Total solids

          (11.938±0.079)

          (11.905±0.0

          85)

          1.483

          NS

          Parameter s

          Raw milk (Mean± SD)

          Pasteurize d

          Milk (Mean± SD)

          t-stat

          t-test

          Acidity

          (0.150±0.0085)

          (0.151±0.00

          93)

          -1.483

          NS

          Fat

          (3.491±0.066)

          (3.466±0.06

          5)

          1.914

          NS

          Solid not fat

          (8.446±0.032)

          (8.438±0.05

          3)

          0.560

          NS

          Total solids

          (11.938±0.079)

          (11.905±0.0

          85)

          1.483

          NS

          TABLE II EFFECT OF HEATING ON CHEMICAL PARAMETERS OF MILK

          Pasteurized milk had 0.151% acidity, 3.46% fat, 8.43% SNF and 11.905 total solids and there were no significant changes between chemical parameters of raw and pasteurized milk. The pasteurization process does not change the level of fat, SNF and solid not fat of milk. The data obtained in this study is in accordance with value reported by [10].

        4. Alkaline phosphates test of pasteurized milk

          It is observed that in all the replications the alkaline phasphatase test had shown negative result. It indicates that efficacy of pasteurization was good and acceptable as per the requirement for pasteurization.

        5. Microbial quality of pasteurized milk

        The methylene blue reduction time was recorded to be 1.4 h for raw milk and after heating to pasteurization temperature it was found 5.40 h [11]. The average standard plate count of raw milk was 3.7 x 105cfu/ml and after pasteurization it was reduced to 1.9 x 104cfu/ml [11]. The coliform was found absent in 0.01 dilution in prepared pasteurized milk [11].

      4. CONCLUSION

It can be concluded that, the values of heat exchanger effectiveness, number of transfer units, and tube wall temperature were found to be maximum in combination of flow rate of hot water at 7 l/min and flow rate of cold water at 0.5 l/min. Solar water heating assisted shell and tube heat exchanger system could be used to produce properly pasteurized milk with acceptable quality without using conventional energy source for heating.

REFERENCES

[1]. Anonynomous. (2014). Rising Milk Production- Future propects. Indian Dairyman. 66(7):15

[2]. Rai, G.D. (2006). Solar Energy Utilisation, Mechanical Engineering Department, Samrat Ashok Technological Institute Vidisha (M.P.).

[3]. Ahmad., T. (2009). Dairy Plant Engineering and Management. 6th Edition. Kitab Mahal, Allahabad.

[4]. Donohue, D.A. (1949) Ind. Eng.Chem., 41:2499

[5]. McCabe, W.L.; Smith J. C. and Harriot P., (2001.)Unit Operations of Chemical Engineering Mc-Graw Hill, International edition. 6

[6]. FSSAI (2012) Manual of Methods of Analysis of Foods: Milk and Milk Products, Food Safety and Standard Authority of India, Ministry of Health and Family Welfare, Government of India. New Delhi.

[7]. BIS: 1479, part I (1981) Methods of test for dairy industry: Rapid examination of milk. Bureau of Indian Standards, Manak Bhavan, Bahadur Shah Zafar Marg,New Delhi, India.

[8]. BIS: SP18, part XI (1981) ISI Handbook of Food Analysis, Dairy Products. Bureau of Indian Standards, Manak Bhavan, New Delhi, India.

[9]. BIS: IS-8479, part-II (1977) Method for determination of phosphatase activity in milk and milk products. Indian Standard Institution. Manak Bhavan, New Delhi, India.

[10]. Winarso, D.; Herawati and Foekh, B.; (2011). The Study of temperature effect and length of pasteurization heating on milk quality, Journal of Agriculture and Food Technology, 1(8): 137-144

[11]. BIS: IS-1479, part III, 1977. Bacteriological Analysis of Milk. Bureau of Indian Standards, Manak Bhavan. Bahadur Shah Zafar Marg, New Delhi, India.

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