Hist Pasteurizer Plant and its Components

The HTST system is the most common method used by the dairy plants for pasteurization of milk. The main advantage of HTST pasteurization is its capacity to heat treat milk quickly and adequately with built-in safeguards that prevent improper pasteurization due to under heating of milk. The HTST system employs plate heat exchangers for heating, regeneration and cooling. The system consists of feed pump, plate heat exchanger, holding section, flow diversion valve,instrumentation, essential services and piping system. The entire process is automatic and is ideal for handling of 5000 litres per hour (lph) or higher quantity of milk. This is a continuous flow process and also saves energy due to regeneration section(Figure).In order to understand a pasteurizer let us go systematically for:

•  Flow diagram of process;
•  Different compartments/sections;
•  Plate heat exchanger, which is the main part; and
•  Instrumentation

i. Flow diagram of pasteurization process

The schematic flow diagram of HTST pasteurization is given in below Figure.Raw milk enters the constant heat tank (balance tank), passes to the milk pump and then through a flow controller to the plate heat exchanger. The plate heat exchanger consists of regeneration section, heating, holding and cooling sections.

 Flow Diagram of Pasteurization

The raw milk enters the pre-heating (regeneration section), where hot pasteurized milk (72°C) flows counter current to the raw cold milk, within adjacent plates,transferring heat for pre-heating of raw milk and pre-cooling of pasteurizing milk resulting in energy saving. The partially heated raw milk passes through a filter or clarifier and homogenizer. It then enters the heating section where it is heated to at least 72°C. The hot milk then passes through the holding section to ensure that the fastest moving particles of milk are held at 72°C for at least 15 seconds.

The flow diversion valve diverts the milk to constant head tank if it is not properly heated to pasteurization temperature. Properly pasteurized milk passes forward through the flow diversion valve into the regeneration section where it is cooled by incoming cold raw milk passing in the opposite direction on the other side of the plates. Milk enters the cooling section and is cooled at 4°C before storage.

An indicating thermometer situated at the outlet of the holding section measures the temperature of the hot milk and this is recorded on a revolving thermograph. If the temperature of the milk falls below 72°C, the hot milk-recording pen drops past the set pointer on the thermograph and this activates the flow diversion value, the safe-guard pen and an alarm bell. The flow diversion valve diverts the unheated milk into the constant head tank for re-circulation until the milk reaches the correcting temperature.

 

ii. Components of a HTST Pasteurization Plant

The complete pasteurizer plant consist of:

•  Constant head tank
•  Milk feed pump
•  Flow controller
•  Filters
•  Clarifier
•  Homogenizer
•  Plate heat exchanger consisting of bank of plates compartmentalized into regeneration, heating, holding and cooling sections,
•  Flow diversion valve
•  Instruments associated with indicating controlling and/or recorded functions,
•  Systems for providing steam, air, water, heating and cooling arrangements, and
•  Piping system to link various components

 iii. Plate Heat Exchanger (PHE)

The Plate Heat Exchanger consists of a bank of plates inter-connected (sections) held in a rigid frame (figure). The main function of the PHE is the exchange or transfer of heat from a hot liquid (hot water or hot pasteurized milk) to a cooler one (cold water, chilled water brine or raw milk) across a metal plate. Let us see how the heat is transferred through plates.

Plates: The plates are thin stainless steel sheets usually rectangular in shape. The plates are corrugated and cause a turbulent flow, which increases rate of heat exchange. The rate of heat exchange also depends on the surface area of the plate,the thickness and type of metal used in the plates, the rate and direction of flow of the liquids and the difference in temperature between the two liquids involved in the heat exchange process.

An approximate 3-8 mm space is maintained between the plates by a non-absorbent rubber seal, which is bonded around the edges of the plate. The liquids, which are sandwiched among the plates, enter and leave the interspaces through holes in the corners of the plates. Open and blind holes route the liquids from one set of plates to another. The capacity of the pasteurizer is secured by a corresponding number of plates.

 Plate Heat Exchanger

Regeneration sections: The bank of plates is usually divided into four sections separated by connector grids with inlet and outlet bosses. In the regeneration section, the incoming cold milk is heated by the hot pasteurized milk and the pasteurized milk is cooled by transferring heat to the cooling medium. This heat transfers process work most effectively when the two liquids involved flow in
opposite direction, i.e. counter current flow on either side of the plates. Regeneration section raises the raw milk temperature from 4°C to 67°C and cools the pasteurized milk from 72°C to 10°C. Thus, PHE saves about 92% of heating and cooling energy. Theregeneration efficiency is calculated by using the following formula:

% Regeneration = temperature increase due to regeneration/ total temperature increase

For example: The cold milk enters the pasteurizer at 4°C and attains a temperature of 60°C after regeneration. The final pasteurization temperature is 72°C. Calculate the regeneration efficiency.

Increase in Temperature due to regeneration: 600C-40C=560C

Total Temperature Increase: 720C-40C= 680C

% Regeneration efficiency: 560C/680C = 82.36%

Steam-heated hot water or vacuum steam is used in heating section to raise the partly heated raw milk to pasteurization temperature. The holding section is either plate type or tube type. The plate type will have a number of plates. The partly cooled pasteurized milk is further cooled in cooling section to 4°C.

 

iv. Instrumentation

The instruments associated with the pasteurization plant are used for performing three functions in below Table

Instruments associated with pasteurizer and their functions