Dairy Engineering

Module 6. Sterilizing & packing equipment

Lesson 22
CONTINUOUS STERILIZATION PLANT, UHT STERILIZATION-I

22.1 Introduction

Sterilization at higher temperatures is usually referred to as UHT sterilization. The heating to this high temperature for only few seconds, demands that the heating and subsequent cooling has to be as quick as possible to reduce heat damage. The other factors like cost, the technical expertise in handling and maintaining sterility are also important.

Fig. 22.1 Classification of UHT plants and heating energy source

22.2 Indirect Heating

This type of heat exchanger can consist of an assembly of plates, or tubular or a combination of both. If the product is very viscous, use of Scraped-surface heat exchanger is necessary. As the product in the plant must not boil at the highest pressure, a back pressure must exist which is equal to the vapour pressure of the product at this maximum temperature. At a temperature of 135°C, a back pressure of about 2 bar is needed. Further, to avoid bubbling of any dissolved air, at least 1 bar over and above the pressure determined by the sterilization temperature is needed. Another factor is the hydrodynamic pressure drop resulting from the product being pumped through the heat exchanger. Hence, the highest internal pressure in the heat exchanger may reach 6 - 8 bars.

22.2.1 Plate heat exchanger

Generally, plate heat exchangers are limited to lower temperatures, but, here the special design considerations are required as the operative pressures have to be higher.

To withstand the severe conditions of temperature and pressure, the gasket materials used with plates for UHT plant must be more sophisticated. Nitrile rubber is suitable for temperatures up to about 138°C. For higher temperatures, up to 160°C, resin-cured butyl rubber or EPDM (Ethylene Propylene Diene Methylene) or silicon gaskets are suitable. Gasket performance is also limited by the adhesion of the gasket to the stainless steel plate. A safer option will be with plate and gasket designs where gasket is not bonded to the plate, but is held in place mechanically and so can be replaced easily when necessary. High internal pressures, and varying high temperatures within the heat exchanger, may cause flexing and distortion of the stainless steel plates. Hence the designs are such that multiple contact points between adjacent plates are provided, to give mutual mechanical support and increase the rigidity of the whole plate pack.

In the sequence of operation of the UHT plant, the heat exchangers can be of regeneration between milk to be heated and the milk already heated, as in normal pasteurizer. A more efficient one is where an entire water circuit takes care of the heat exchanges progressively. (Refer fig. 22.2)

1. Balance tank

2. Positive displacement pumps

3. First plate heat exchanger

4. Holding tube (110 to 115°C)

5. Second heating section

6. Holding tube (14°C)

7. First cooling section

8. Homogenizer

9. Second cooling section

10. Cold water cooling stage

11. Restrictor

12. Steam heat exchanger

13. Sterilization cooler

14. Back pressure wall

Fig. 22.2 Developed plate-type indirect UHT plant

The untreated product is pumped from balance tank by centrifugal pump to the first plate heat exchange section. The end of this heating section, at a temperature of 110-115°C, there may be a holding tube to provide a few seconds holding to reduce fouling of heating surfaces later in the plant. The next heating section brings the product to the final sterilizing temperature of about 140°C, where it is held for the required holding time in the holding tube. It is then followed by a cooling section which cools the product to homogenization temperature of about 55°C to 75°C.

The homogenizer being positioned after the sterilization process will have to be of the type of Aseptic type of homogenizer. It is generally considered that if milk has to be processed, the homogenization is before the sterilization process, while if it is products like cream, then the homogenization is preferred after sterilization.

After aseptic homogenization, the product passes through a further cooling heat exchanger section, and finally through cold water cooling stage to bring the product to the required outlet temperature. A restrictor is placed after this to provide back pressure required in the plant.

The heat is transferred between product and water circulating in the counter-current flow in a closed circuit. The water in this circuit is heated to a little above the required sterilization temperature by steam under pressure in a heat exchanger section. The low differential of not more than 3°C, reduces the amount of fouling on the heat exchange surfaces, and allow longer operating times before the heat exchanger is shut down to be cleaned. The level of regeneration in such a plant can be above 90%, and the operating energy costs are correspondingly low, as compared to plant where heat exchange in regeneration is between milks.

22.2.2 Tubular heat exchanger

Tubular heat exchangers used as components of UHT plants are mainly of two types, concentric tubes, or some form of shell-and-tube heat exchanger.

The concentric-tube systems are formed by assembling two or three stainless steel tube lengths one inside another with a spacer, often a spiral wire, in each inter-tube space to maintain them concentric. The multiple tubes are then wound into a coil, for assembly into an outer cylindrical housing for hygienic and mechanical protection. The two-tube system is used for simple heating and cooling, where the product flows in the centre tube and the heating or cooling medium flows in the annulus, and it is also fused for regenerative heat exchange where product flows in both spaces.

The triple-tube system is used for the final stage of heating to the sterilization temperature, where the product flows in the annulus between the inner two tubes and the heating medium flows in the centre tube and in the outer annulus. In this way, the available heat transfer area is doubled and the rate of heating is increased. Triple tubes may also be used in final cooling sections, especially where cooling rates are restricted by high product viscosity so that increased transfer area is used to compensate for reduced transfer coefficients.

A Typical flow diagram for an indirect UHT plant using concentric tube heat exchanger sections is show in fig 22.3. The product is pumped from the balance tank (1) by a centrifugal pump, (2), through the steam-heated sterilizing heater which, during products sterilization, is inactive. The product is first heated in the regenerative heater (4) by the outgoing product. Connections are made to the homogenizer (5) at a suitable temperature point within the regenerator.

A valuable characteristic of all tubular heat exchangers is that the tubes carrying the product are strong enough to withstand the full pressure required for effective homogenization. It is therefore possible to install the high-pressure reciprocating pump of the homogenizer before the sterile section of the plant and quite independently of the position of the homogenizer valve. The hygienic problems of installing a homogenizer after sterilization lie with the pump, not with the homogenizer valve, and the extra complication and cost of a sterile homogenizer arises from modifications that have to be made to the pump to avoid bacterial contamination. If the homogenizer pump can be included before the sterile section, it need not be of aseptic design. Furthermore, if the heat exchanger can withstand the full homogenizing pressure, the homogenizing valve can be put at any point downstream from the pump, even after the sterile section. In fact, in a plant of the type shown, two homogenizing valves may be fitted, one at (5) in association with the homogenizer pump and one after the sterile section where the product is at a suitable temperature during cooling, e.g. at (5a). The product can therefore be homogenized before sterilization, after sterilization, or both.

Fig. 22.3 Typical concentric-tube indirect UHT plant

After the first homogenizing point at (5), heating continues in the regenerator (6) and then in the stem-heated section (7) to the final sterilizing temperature. After the holding tube, (8), the product returns to the regenerator for cooling. Final cooling to the outlet temperature is in a water-cooled section (9). Back pressure is maintained by an orifice plate or back pressure valve at (10).

The required sterilization temperature in the holding tube is maintained by a controller (C) which senses the temperature in the holding tube and varies the steam supply to the final steam heating section (7) as necessa