Dairy Engineering

Module 5. Pasteurizers

Lesson 17
PASTEURIZATION: BATCH, FLASH AND CONTINUOUS (HTST) PASTEURIZATION

17.1 Introduction

The process of Pasteurization involves heating milk to a certain temperature, holding at that temperature for certain period of time and then immediately cooling to below 4°C. This is done most commonly in two methods i.e. 1) Low Temperature and Longer Time 2) High Temperature Short Time. The former method is used in most of the Batch processes and involves a multipurpose vat. The later method is used with Plate type of heat exchanger.

17.2 Batch Process

The Batch processes are suitable for small capacity. Batch process involves heating to 63°C and holding at that temperature for 30 minutes. For this process, multipurpose vat is most suitable, and its construction is shown in the figure below.

Fig. 17.1 Batch pasteurizer

The Multipurpose vat used for Batch pasteurization is a jacketed and insulated tank. It is a cylindrical vessel with double jacket. The tank is heated by circulating hot water or steam in the inner jacked. Slow speed agitation provides uniform heating through the wall. The milk is slowly heated and after reaching the required temperature, is held at that temperature for 30 minutes. Then it is cooled to below 4°C. The bringing up time and cooling time are not accounted for the pasteurization time.

The time taken for heating and cooling is given as

where, t_h, t_c = Time taken for heating or cooling respectively

m	= mass of milk
c	= Specific heat of milk
A	= Area of heat transfer
U	= Overall heat transfer co-efficient
To	= Initial temperature of milk
Tm	= Final temperature of milk
Ts	= Hot fluid temperature
TCW	= Chilled water temperature

17.3 HTST Pasteurizer

High Temperature Short Time Pasteurizers (HTST), have proved to be the workhorses of processing in Dairy Industry. For milk the time temperature combination used is 71.5°C for 16 seconds, and then immediately cooled to below 4°C. The heat exchanger that can achieve this duty is the Plate Heat Exchanger(PHE).

The Plate heat exchanger for HTST pasteurizer is made up of groups of plates that are used for heating the milk to the high temperature required, holding at that temperature for the required period and cooling it. The schematic diagram of the pasteurizer is shown below

Fig. 17.2 Schematic diagram of pasteurizer
Adopted from JMD Sonic Engg Ltd manual

Fig. 17.3 HTST pasteurizer

(Adapted from manual of GEA Ahlborn Gmbh &co)

It consists of heat exchange plates, frame for hanging the same. End plates along with tightening bolts are provided to keep these plates pressed together, with gaskets in between. The groups of plates are provided in between with connecting plates which make it possible to introduce or exit milk after each of the processes, like Regeneration, heating, holding, and chilling. The plates are made of die-pressed sheet of stainless steel with a corrugated surface, to give turbulence to the flow and thereby achieve maximum heat transmission, even with very low temperature gradient between the heat exchange fluids.

Fig. 17.4 PHE

In the PHE the process fluid or service fluid can be passed in many ways through channels connected in parallel or in series, in the same direction or in opposite directions to achieve maximum heat transfer. The thickness of plates is about 0.5 to 1.25mm. The gap between plates is about 3 to 6 mm wide, depending on the design and products handled. The types of heat transfer plates, and their gasket arrangement is shown in fig 17.2.

The arrangement of gaskets makes the flow in a particular direction for the milk and hot or cold water. The plates are of different sizes and configuration of corrugations, and may cater to the needs of wide variety of fluid processing. The manufacturer supplies the plate connection diagram to meet the specific heat exchange duty, which shows the sequence of the plates, arrangement of gaskets, and location of the connections. The nomenclature for the arrangement is explained as below:

Fig. 17.5 Plate connection diagram

In the connection diagram, the flows of liquid are shown. The flows drawn on the left side are running along A- plates and the flow on the right side over B-plates. The A-plates have gasket on the right hand side and B-plates have it on the left. The two types are arranged alternately in the PHE, so the process fluid and service fluid flow in the alternate channels. The plates are usually numbered, so as to make it easier in assembling, and relate to the connection diagram. Any damaged plate can be replaced by an identical spare plate.

The milk from storage tank enters the HTST pasteurizer through a Float Balance Tank, which has a float to control the inflow of milk and maintains a constant level of milk in it. The milk is then pumped from here through a flow control valve, which may be ball valve type. Milk enters the Regeneration section where the incoming raw milk is partly heated by the returning hot milk from holding section. As the heat exchange is beneficial by reducing the load on steam and chilled water requirement, the number of plates is more in this section.

The Regeneration section may be divided in two segments, so that after the first segment, when the milk is about 40°C, milk is lead to duplex filters, which has two sets of vessels with filters, to remove any extraneous material in the milk. The filters are provided with air vents, to remove air in the beginning of the process. The outgoing connection at this stage may also lead to the Cream separator if necessary, as this is the ideal temperature for the efficient cream separation. The return line then enters the second segment of Regeneration section, and the milk is lead through again a connecting plate to the heating section. The connecting plate here is meant to direct the milk flow to homogenizer if necessary.

In the heating section, the milk is heated with hot water, coming from hot-water generator, aided by a hot water pump. The hot-water generator is provided with steam supply, through a automatic steam flow control valve, which gets the signal from the hot water temperature controller of the pasteurizer controls. The flow rate ratio of milk: hot water is in between 1:2 to 1:3, depending on the manufacturer’s design. The hot water supply line also has a pressure controlled, outlet for the condensate which accumulates because of steam supply to maintain the temperature of hot water to about 85°C.

Once, the required pasteurization temperature is reached, the milk is lead to holding section, which has stainless steel pipes of sufficient length to keep the milk to the required time. The flow rate of milk has to be fixed, to meet this requirement. In some of the pasteurizer designs, this section also is of PHE type, though it is not very common.

At the end of the Holding section, the Flow Diversion Valve (FDV) is positioned, so that, the milk above the set point of pasteurization temperature only is lead to the Regeneration section again, which is called the Forward flow. If it is not, then the milk is lead to the balance tank, which is called the diverted flow. The operation of FDV is explained later, in detail. (Fig. 17.6: Temperature profile of milk in each section of pasteurizer)

The hot milk after FDV in forward flow will enter the Regeneration section, and will give off its heat to the incoming raw milk, and then enters the Cooling section. Here, the milk is cooled to below 4°C by chilled water at 1°C, or at times brine (glycol + water) when viscous products have to be cooled. Here also, the flow rate of chilled water is 2 to 3 times that of milk. The flow rate difference has to take into consideration the pressure drop on the service fluid side, i.e chilled water or hot water. The difference in flow channels and passes are shown in the diagram below.

Fig. 17.7 Heating and regeneration section

After chilling to below 4°C, the milk is piped to Pasteurized milk storage tank. (Fig. 17.8: Pasteurizer Flow of Milk)