Dairy Engineering

Lesson 23. CONTINUOUS STERILIZATION PLANT, UHT STERILIZATION-II

Module 6. Sterilizing & packing equipment

Lesson 23
CONTINUOUS STERILIZATION PLANT, UHT STERILIZATION-II

23.1 Direct Heating System

There are certain disadvantages in the indirect heating system, especially the fouling of heat exchange surface. To overcome this, a system where the heating medium viz. Steam, is directly brought in contact with product to be heated. This eliminates the following disadvantages of Indirect heating system.

1. Thickness of heat exchange wall slows down the rate of coming up time, cooling rate.
2. Fouling of heat transfer surface, and thereby reducing the operating time.

The product is first raised to 80-85°C by Indirect heating system, and then the product is raised to sterilizing temperature of 140-150°C by direct injection of steam. The temperature rises rapidly, but also causes dilution of product due to steam condensing in to the milk, almost to an extent of 10% or more. Hence the holding section coming later will have to be designed for greater capacity to this extent. After the designed holding time at the sterilization temperature, expansion cooling follows. The vacuum in the expansion vessel will normally be controlled at a level corresponding to a boiling temperature little above that of the product before mixing with steam. This will result in removal of exactly the amount of water added as condensed steam.

The general sequence of operations in Direct Heating Systems is shown is figures below.

Fig. 23.1 Diagrammatic representation of a direct-heating system

Fig. 23.2 Time -temperature relationships during direct heating

Fig. 23.3 Diagrammatic representation of injection and infusion system

However, depending on the method used for mixing the product with steam to achieve the final sterilizing temperature, the heating systems are classified as 1) Injection system 2) Infusion system.

23.1.1 Injection system

In this system, the steam at a pressure higher than that of the product is injected into the product stream through a suitable nozzle, and is condensed to give the required temperature. There are many different injector designs, for meeting the basic requirement of keeping the steam and product thermally separated as far as possible until they reach the mixing zone. This will prevent surface fouling of the injector.

The important requirement in these systems is the rapid condensation of steam, to prevent the passage of bubbles of uncondensed steam into the holding tube, which would reduce the effective holding time at the sterilization temperature. The rapid condensation is achieved by introducing the steam into the liquid in the form of small bubbles or in the form of a thin sheet. It is also essential that sufficient back pressure in the liquid is maintained, above that needed to prevent boiling, at the steam injector.

Fig. 23.4 Typical steam injector designs

Fig. 23.5 Typical injection type UHT plant

1. Balance tank 2. Pre Heating section 3. Heat exchanger(75 to 85°C)
4. Steam injector 5. Expansion vessel 6. Condenser 7. Homogenizer

The product is pumped from a Float Controlled Balance Tank (FCBT) through a heating section where it is preheated by outgoing product. It is then further preheated to a constant temperature in the approximate range 75-85°C in a heat exchanger by condensation of steam under vacuum or by hot water circulation. The temperature of the product at the outlet of this pre-heater is controlled by controlling the heating vapour supply or the temperature of the hot water. A high-pressure pump then supplies the product to the steam injector and the holding tube, where the pressure must be high enough to prevent boiling of the product. An adequate back pressure to prevent the product from boiling is required at this stage.

The sterilized product is then lead to Expansion cooling vessel, which is kept at a suitable vacuum by the condensation of the released water vapour in a condenser. The vacuum corresponds to a boiling temperature of about 1-2°C higher than that of the product at the outlet of the final pre-heater. This temperature difference will ensure that condensed steam, in the form of water vapour is removed. A restrictor valve at the end of the holding tube retains the high pressure in the injector and holding tube, and as the product enters the expansion vessel, it boils and is rapidly cooled. The product is then lead from the bottom of expansion vessel, to an aseptic homogenizer through a pump. The homogenization after sterilization stage prevents formation of casein aggregates and associated defects. The matching of flows in the different sections is important to meet the defined holding time for the process. It is essential to maintain balance between water addition as steam and water extraction as vapors from the expansion vessel, by maintaining adequate difference between the temperature of the product before mixing with steam and after expansion cooling.

A closed circuit is used for plant cleaning and sterilization, in which the cleaning solutions return to the balance tank through a sterilizing water cooler and restrictor valve. By not applying vacuum to the expansion vessel during this operation, a full sterilizing temperature is achieved.

23.1.2 Infusion system

This system is similar to injection system in all aspects except for the method of mixing of product with steam. The infusion is done by dropping heated product into a steam pressure vessel with a conical base. The size and proportions of the vessels differ, as do the methods adopted for distribution of the product into the steam. Two designs of infusers are shown below:

Fig. 23.6 Infusion –type UHT sterilizer

1. Pump 2. Infusion vessel 3. Positive displacement pumps 4. Restrictor
5.Expansion vessel 6. Condenser 7. Aseptic pump 8. Aseptic Homogenizer

In the first one the product is supplied to a hemispherical bowl with a loose circular disc closing the top. The product flows from the space between the bowl and the loose disc to form a thin umbrella with access to the heating steam from both sides. If the product has particles, the loose disc can lift to allow particles to pass out of the bowl with liquid. The other distributor has product flow in a series of parallel and horizontal distribution tubes. These have thin slits along the bottom, from which the product falls in thin, laminar free-falling films.

Shape of the infuser contributes to the volume of pool of product in its base, and this increases the holding time as determined by the dimensions of the holding tube and the flow rate through it. Further, for operational stability the steam pressure at the infuser should be at least 0.5 bar above that equivalent to the sterilization temperature.

Table 23.1 Comparison of direct heating and indirect heating