FOOD PROCESSING PRACTICAL REPORT

EXERCISE VISCOLAB

Students : NGO LAM TUAN ANH 01008142

EKTA GOSAIN 01007813

XU HUI 01007183

ARUNKUMAR SURIYAMOORTHI 01008144

2010 – 2011

1. INTRODUCTION

In this practical work, a process of heat treatment of raw milk was carried out by

using tubular heat exchanger and scraped surface heat exchanger. The sequence

of process containing a number of steps and devices is introduced. Some

important parameters should be established on the basis of calculating the flow

rate of milk which is matched with the holding tube’s configuration, the inlet and

outlet temperature of water in tubular heat exchanger (THE). After setting up the

parameters, we can choose the best-suited configuration of process for the

viscolab.

2. GENERAL DESCRIPTION OF THE PROCESS

Raw milk firstly enters the feeding tank. From where, it is pumped to tubular

heat exchanger (THE) at a certain flow rate (preferably between 100-300 l/h)

depending on the holding time of heat treatment process applied, pasteurization

(70oC, 5 min) or sterilization (120oC, 12 min). THE is an equipment used for heat

treatment of liquids. Hot water or condensing steam constitutes the heating

medium. The differences in temperature between the heating agent and the

incoming liquid are greater. Tubular heat exchangers can readily be applied to

obtain very high temperatures. It can be built of regeneration, heating, holding

and cooling sections. The milk is in counterflow with the water throughout the

apparatus. The water is kept circulating and is heated by means of indirect steam

heating, immediately before it should heat the milk to the maximum desired

temperature. The primary objective in using a heat exchanger is to transfer

thermal energy from one fluid to another. In this practical work, THE is a

monotube and counter-current type in nature. At this step milk is preheated to

the desired temperature (60oC) before passing through homogenizer.

In the next step milk from heat exchanger goes to homogenizer. The

purpose of homogenization is to reduce fat globule size in the milk and thus it

will stabilize the emulsion of milk. Moreover, an advantage in homogenizing after

heating is that it prevents or reverses protein—protein and fat globule—protein

aggregation. It also retards the formation of sediment of heat precipitated

proteins.

Homogenized milk is then transferred to Scraped Surface Heat

Exchanger (SSHE) to increase temperature further. SSHE consists of a cylinder

in which the milk is there and an outer jacket through which the heating or

cooling medium flows. In the centre of the cylinder is a driven shaft supporting

scraper blades. The milk is pumped through the space between the cylinder and

shaft, contacts the heated cylinder surface and is continuously scraped off and

mixed into the bulk of the product by the scraper blades.

Figure 1. Schematic of milk heat treatment. (A): pasteurizing milk at 70oC, 5 min;

(B) Sterilizing milk at 120oC, 12 min.

For the effective pasteurization, milk is then kept in holding cell which

can be set up for keeping a concrete resident time of pasteurization and

sterilization. By changing the number of loops and the flow rate of feeding milk,

we can modify the necessary time for effective heat treatment. After heating and

holding, the treated milk have to be cooled as quickly as possible to the suitable

temperature by THE or SSHE.

The process of heat treatment milk is introduced in figure 1.

3. DATA TREATMENT METHODS

3.1. Input data

All parameters are determined and calculated by following the input data given in

Table 1.

Table 1. Data given for calculation

Parameters Value Unit

Milk flow rate Between 100 and 300 l/h Starting temperature raw milk 10 °C Homogenisation temperature milk 60 °C Volume tubular heat exchangers (two possibilities)

5 and 7.5 l

Length tubular heat exchangers (two possibilities)

10 and 15 m

Density milk 1.020 kg/l Density water 1 kg/l Heat capacity milk 4 kJ/kg/°C Heat capacity water 4.18 kJ/kg/°C Overall heat transfer coefficient of tubular heat exchanger

600 J/s/m²/°C

Flow rate water in tubular heat exchanger

650 l/h

Volume holding tube 5 loops available with a volume of 4 litres

3.2. Calculation procedure

3.2.1. Calculation the flow rate of milk

The holding tube only has 5 fixed levels of volume capacity: 4, 8, 12, 16, 20 litters.

Hence, to ensure the sufficiency of time for heat treatment, 4 min for

pasteurization and 12 min for sterilization, we have to choose the suitable

number of loops and modify the flow rate of the product stream. Table 2 shows

the flow rate of milk in holding tube corresponding with the number of loops

used in the holding tube and holding time. As can be seen in the table, we can

pick out:

- Two flow rates of milk, 192 and 240 l/h, for heat treatment at

70oC/5min. The required number of loops is 4 and5 respectively.

- One flow rate of milk, 100 l/h, for heat treatment at 120oC/12min. The

required number of loops is 5 (there is none of loops < 5 that is seen to be

considerably lower than 100 l/h and thus we selected flow rate for 5

loops which was found out to be 100 l/h).

The flow rate can be adjusted by feeding pump.

Table 2. Configuring holding cell and milk flow rate required

Configuring Holding Cell

Number of loops, N 1 2 3 4 5 Volume capacity, litter 4 8 12 16 20

For heat treatment at 70oC, 5 minutes

Holding time, min 5 5 5 5 5 Flow rate of milk, l/h 48 96 144 192 240

For heat treatment at 120oC, 12minutes

Holding time, min 12 12 12 12 12 Flow rate of milk, l/h 20 40 60 80 100

3.2.2. Configuring the tubular heat exchanger

Objective of this part is to determine the temperature of heating medium coming

into the THE and the possibility of using one of 2 THEs for viscolab.

Calculations:

Temperature of water can be determine via the heat balance of the process in

the THE. There are 3 heat loads:

(1) The energy for heating milk from initial temperature (Tmi = 10oC) to the temperature before coming into homogeniser (Tmo = 60oC), called Qm (kW)

Qm = Fm * Cpm * (Tmi – Tmo) Where: Fm : mass flow of milk, kg/s

Cpm : specific heat capacity of milk, kJ/kg.oC

Tmi : Inlet milk temperature, oC Tmo : Outlet milk temperature, oC

(2) The energy releasing from hot water to milk, Qm (kW): Qw = Fw * Cpw * (Twi – Two)

Where: Fw : mass flow of water, kg/s

Objectives

Cpw : specific heat capacity of water, kJ/kg.oC Twi : Inlet water temperature, oC Two : Outlet water temperature, oC

(3) The overall energy transfer in the THE, Q (kW):

Q = K * A * ∆Tm

Where: A : The heat exchanged area of tubular, m2

K : Overall heat transfer coefficient, W/m2.oC

∆Tm : The mean temperature difference, oC

The heat exchanged area of tubular (A) can be calculated by the formula:

A = π.L.φ = π.L. L

V

*

*4

π

With: L : Length of THE (m)

V : Volume of THE (m2)

φ : The diameter of the tubular tube (m)

As supposed that there are no heat losses in the system. There is a balance

of heat exchange:

Q = Qw = Qm

Thus, by solving the 2 simultaneous equations of heat balance, we can find out the value of Twi and Two, which are unknown.

Outcome data:

The outcome of mathematical calculations is represented in the table 3 and 4:

As can be seen in table 3, where the calculation of heat treatment of milk at

70oC/3min is performed, we have some remarks:

- The flow rate of 240 l/h is considerably the highest that can be performed

in practical work because of its high productivity.

- Two of THE can be used in our works. However, the larger one, which has

7.5 litter and 15m in volume and length respectively, is mostly preferable

to use in viscolab because it contributes the larger heat transfer area.

Thus, that leads to the reduction of energy to heat water before coming

through THE. In fact, it is clear from the table 3 that the temperature of

inlet water in case of using THE 2 (7.5x15) in lower than the other.

Nevertheless, it may take more places to the lab area because of its length.

- The inlet temperature of water using as heating medium for this process

is around 67.35oC, the out let temperature is 57.51oC.

According to the data from table 4, where the calculation of heat

treatment of milk at 120oC/12min is performed, we can see that:

- The temperature of water entering to both heat exchangers is not

considerably difference, which are approximately 1oC indifference. Thus,

within the same flow rate of milk and energy consumed, we suggest using

the THE 1 (5l x 10m) which is smaller and does not take more places in

the lab. It is therefore more suitable than the large one (THE 2)

- The inlet and outlet temperature required for this preheating process are

61.24 and 53.73oC respectively.

4. CONCLUSION

In conclusion, 2 heat treatment procedures of milk can be performed with the

parameters as followed:

- For the process of pasteurization at 70oC/5min, the tubular heat

exchanger 2, with dimensions of V x L = 7.5 l x 15 m, is used. Water as a

heating medium enters to this equipment at the initial temperature of

67.35oC. The number of loops of the holding cell is 5, providing the

highest productivity of a process.

- For the process of sterilization at 120oC/12min, the smaller heat

exchanger, with dimensions of V x L = 5 l x 10 m, can be used. This

preheating step requires hot water entering the device at 61.24oC. 5 loops

are set up in the holding cell to ensure the sufficient time for treating milk.

Table 3: Calculation for heat treatment of milk in 2 tubular heat exchangers at 70oC / 5 min

Heat treatment of milk at 70oC, 5 min

Parameters Notation Equation / Formula explanation Value Unit

Milk

Flow rate

Fm

Choose b/w 100-300 l/h 144.000 196.000 240.000 l/h

Converted to kg/s 0.041 0.056 0.068 kg/s

Temperature milk inlet Tm,i Given 10.000 10.000 10.000 oC

Temperature milk outlet Tm,o Given 60.000 60.000 60.000 oC

Density of milk ρm At room temperature (given) 1.020 1.020 1.020 kg.l

Heat capacity of milk cpm 4.000 4.000 4.000 kJ/kg.oC

Tubular heat exchanger 1 - L x V = 10 m x 5 liter

Flow rate of water

Fw

Given in l/h 650.000 650.000 650.000 l/h

Converted to kg/s 0.181 0.181 0.181 kg/s

Temperature water inlet Tw,i Calculating from heat balance 64.448 69.998 75.532 oC

Temperature water outlet Tw,o Calculating from heat balance 53.636 55.282 57.512 oC

Heat capacity of water cpw 4.180 4.180 4.180 kJ/kg.oC

Density of water ρw 1.000 1.000 1.000 kg/l

Volume tubular Vt Choose between 5.0-7.5 => 5 0.005 0.005 0.005 m3

Length tubular Lt Choose between 10-15 => 10 10.000 10.000 10.000 m

Diameter of tubular Φ Φ = SQRT(4.V/(Π.L) 0.025 0.025 0.025 m

Total area Α A = Π.D.L 0.792 0.792 0.792 m2

Overall heat transfer coefficient K Given 0.600 0.600 0.600 kW/m2K

Energy for heating milk

Qm Qm = Fm.Cpm(Tmo-Tmi) 8.16 11.11 13.60 kW

Q=Qm=Qw Heat balance 8.16 11.11 13.60 kW

Qw Qw = Fw.Cpw(Twi-Two) 8.16 11.11 13.60 kW

Q Q = A.K.∆Tm 8.16 11.11 13.60 kW

Tubular heat exchanger 1 - L x V = 15 m x 7.5 liter

Flow rate of water

Fw

Given

650.000 650.000 650.000 l/h

0.181 0.181 0.181 kg/s

Temperature inlet Tw,i 61.320 64.085 67.352 oC

Temperature outlet Tw,o 50.507 49.368 49.332 oC

Heat capacity of water cpw Given 4.180 4.180 4.180 kJ/kg.oC

Density of water ρw Given 1.000 1.000 1.000 kg/l

Volume tubular Vt Choose between 5.0-7.5 => 7.5 0.008 0.008 0.008 m3

Length tubular Lt Choose between 10-15 => 15 15.000 15.000 15.000 m

Diameter of tubular Φ Φ = SQRT(4.V/(Π.L) 0.025 0.025 0.025 m

Total area Α A = Π.D.L 1.189 1.189 1.189 m2

Overall heat transfer coefficient K Given 0.6 0.6 0.6 kW/m2K

Energy for heating milk

Qm Qm = Fm.Cpm(Tmo-Tmi) 8.16 11.11 13.60 kW

Q=Qm=Qw Heat balance 8.16 11.11 13.60 kW

Qw Qw = Fw.Cpw(Twi-Two) 8.16 11.11 13.60 kW

Q Q = A.K.∆Tm 8.16 11.11 13.60 kW

Table 4: Calculation for heat treatment of milk in 2 tubular heat exchangers at 120oC / 12 min

Heat treatment of milk at 120oC, 12 min

Parameters Notation Equation / Formula explanation Value Unit

Milk

Flow rate

Fm Choose b/w 100-300 100.000 l/h

0.028 kg/s

Temperature milk inlet Tm,i 10.000 oC

Temperature milk outlet Tm,o 60.000 oC

Density of milk ρm 1.020 kg.l

Heat capacity of milk cpm 4.000 kJ/kg.oC

Equipment - Tubular heat exchanger 1 - L x V = 10 m x 5 liter

Flow rate of water

Fw 650.000 l/h

0.181 kg/s

Temperature water inlet Tw,i 61.2369 oC

Temperature water outlet Tw,o 53.7286 oC

Heat capacity of water cpw 4.180 kJ/kg.oC

Density of water ρw 1.000 kg/l

Volume tubular Vt Choose between 5.0-7.5 => 5 0.005 m3

Length tubular Lt Choose between 10-15 => 10 10.000 m

Diameter of tubular Φ Φ = SQRT(4.V/(Π.L) 0.025 m

Total area Α A = Π.D.L 0.792 m2

Overall heat transfer coefficient K Given 0.600 kW/m2K

Energy for heating milk

Qm Qm = Fm.Cpm(Tmo-Tmi) 5.667 kW

Q=Qm=Qw Heat balance 5.667 kW

Qw Qw = Fw.Cpw(Twi-Two) 5.667 kW

Q Q = A.K.∆Tm 5.667 kW

Equipment - Tubular heat exchanger 1 - L x V = 15 m x 7.5 liter

Flow rate of water

Fw Given 650.000 l/h

0.181 kg/s

Temperature inlet Tw,i 60.203 oC

Temperature outlet Tw,o 52.695 oC

Heat capacity of water cpw Given 4.180 kJ/kg.oC

Density of water ρw Given 1.000 kg/l

Volume tubular Vt Choose between 5.0-7.5 => 7.5 0.008 m3

Length tubular Lt Choose between 10-15 => 15 15.000 m

Diameter of tubular Φ Φ = SQRT(4.V/(Π.L) 0.025 m

Total area Α A = Π.D.L 1.189 m2

Overall heat transfer coefficient K Given 0.6 kW/m2K

Energy for heating milk

Qm Qm = Fm.Cpm(Tmo-Tmi) 5.667 kW

Q=Qm=Qw Heat balance 5.667 kW

Qw Qw = Fw.Cpw(Twi-Two) 5.667 kW

Q Q = A.K.∆Tm 5.667 kW