GB2120528A - High temperature treatment of liquids - Google Patents

Abstract

Ultra high-temperature treatment of liquids, such as milk, with a minimum of thermal damage is carried out using a high-temperature heat-exchanger split into two or more parts (15,16) and (17,18), and a line (30), in the service circuit, which may be used to by-pass the first high- temperature heat exchanger by opening valve (29). This reduces the effective residence time of the process-liquid within the high temperature section when plant throughput is reduced. Alternatively the by-pass line may be in the process circuit in which the by-passed part may be cleaned by an in situ cleaning plant. <IMAGE>

Description

SPECIFICATION High-temperature treatment of liquids This invention is concerned with the hightemperature treatment of liquids, such as milk, with a minimum of thermal damage.

The microbiological degradation of foodstuffs may be prevented, delayed or reduced by two processes: chilling and freezing or thermal sterilization. Extended chilling and freezing are generally uneconomical and may be unsuccessful with certain psychrophilic micro-organisms, for example, the milk-colonizing Pseudomonas fragi.

Thermal sterilization, to an aseptic product, is therefore desirable. Unfortunately thermal sterilization can damage heat-sensitive components and the constraints which this places on plant design limit the throughput range.

The aim of the present invention is to suggest an improvement to an "ultra-high-temperature" treatment (hereafter known as a UHT treatment) for the sterilization of process-liquids with a heatsensitive component, such that the increases in thermal damage caused by operating the plant at reduced capacities are minimised. The method and apparatus are applicable to a range of process-liquids, for example, milk, fruit juices, yeast and meat extracts, vegetable oils and chemical preparations.

Two types of UHT treatment have been practised. Firstly there is a direct treatment, in which steam is injected directly into the processliquid. Secondly there is an indirect treatment in which some form of indirect heat-exchanger is used.

During the treatment two effects may be considered as occuring simultaneously: a sterilizing effect, and a thermochemical effect which results in thermal damage. Both effects are increased, by an increase in the treatment time.

The problem of economic UHT-treatment plant design, is to produce a plant which will maintain a variable throughput of process-liquid at the sterilizing temperature for a sufficient time for the -process-liquid to be sterilized but will minimise the thermal-damage due to the elevated temperature, by not holding the process liquid at this temperature for longer than is necessary.

In accordance with a first aspect of the invention there is provided a method of indirect ultra-high-temperature treatment for a processliquid, in which the effective residence time of the process-liquid within a heat exchanger is maintained as the flow rate of the process-liquid varies, by a selected variation in the effective area of the heat transfer surface of the heat-exchanger.

During an indirect UHT treatment, the sterilizing effect increases with the residence time of the process-liquid within the heat-exchanger, as does the thermal damage. An optimum residence time exists for a given temperature and flow rate. As the flow rate of the process-liquid within the heatexchanger is, for example, decreased, the effective residence time may be maintained by a reduction in the area of the surfaces of the heat-exchanger across which heat is being transferred.

Preferably, the selected variation in the effective area of the heat-transfer surface is obtained by diverting all or part of the service-fluid through selected service-circuit flow spaces of the heat-exchanger. Alternatively, the selected variation in the effective area of the heat-transfer surface is obtained by diverting all or part of the process-liquid through selected process-circuit flow spaces of the heat-exchanger.

By diverting either the service-fluid or the process-liquid through only selected flow spaces of their respective circuits, the effective residence time may be maintained. As the flow rate of the process-liquid, is, for example, reduced, then the area of the heat transfer surface of the heatexchanger is also reduced.

By diverting the service-fluid, the process-liquid flow path is unaltered. This may be an essential requirement in some cases, where a parallel flow path in the hygenic, process-liquid circuit is unacceptable.

By diverting the process-fluid, it may be possible in some cases to both reduce the capacity of the plant and allow cleaning in place to occur in those flow spaces through which the process-fluid is no longer passing.

Conveniently, the heat-exchanger is a plate heat-exchanger.

By the expression plate heat-exchanger is meant a heat exchanger comprising a separable pack of gasketed, normally corrugated plates, arranged in a spaced face-to-face relationship to define flow-spaces between adjacent plates. The plates are formed with aligned holes forming ports or manifolds for the supply and discharge of media to and from the flow spaces, the gasketin3 includes gaskets which control the flow between the manifolds or ports and flow spaces normally so that one medium flows through alternate flow spaces and another medium through the intervening flow spaces.

In an embodiment the process-liquid is milk and the portion of flow-spaces through which the service fluid is diverted is a continuous, terminal, distal portion of the flow-spaces.

By the diversion of the service fluid into a continuous series of flow-spaces, the valve means required are minimised. Diversion into only the distal flow spaces is preferred as the processliquid will remain at whatever temperature it leaves the effective portion of the heat exchanger at. Should the diversion be, for example, entirely proximal the effective residence time would not be reduced, at a constant throughput, by diverting the service-fluid. Hence the distal terminus of the heat exchanger provides the best location for one end of the continuous series of flow spaces for both thermodynamic and mechanical reasons.

The portion of the flow-spaces through whicìl the service-fluid is diverted may be one quarter of the flow-spaces.

The ratio of the flow rates of the service fluid to the process liquid may be maintained substantially at unity.

In accordance with a second aspect of the invention there is provided an apparatus for the ultra-high-temperature treatment of a processliquid comprising a heat-exchanger, means for circulating a service-fluid through one set of flowspaces of the heat exchanger and means for circulating a process-liquid selectively through all or part of the second set of flow-spaces.

In accordance with a third aspect of the invention the invention there is provided an apparatus for the ultra high temperature treatment of a process liquid comprising a heat exchanger, means for circulating a process-fluid through one set of flow spaces of the heat exchanger and means for circulating a service-fluid selectively through all or part of the second set of flow spaces.

The invention will be further described with reference to the accompanying drawings of which Figure 1 is a diagrammatic representation of a UHT treatment milk sterilization plant incorporating one embodiment of the invention: and Figure 2 is a diagrammatic representation of a part of a UHT treatment milk sterilization plant incorporating a further embodiment of the invention.

Turning now to Figure 1, the diagram illustrates an apparatus for the indirect UHT treatment of milk with a diversion of the service-fluid.

Milk enters along line 1, at a temperature of 50C, from milk reception or bulk milk storage and enters one set 2 of flow spaces of a first heatexchanger. The temperature of the raw milk is raised to 740C by heat exchanged with the processed milk in the other set of flow spaces 3.

This direct regeneration serves to conserve energy.

The pre-heated raw milk leaves the heatexchanger 2, 3 via outlet line 4 to a homogenizer 5. The homogenizer provides the pressure required to drive the milk through the plant to the aseptic filling plant which follows, and may also act as a flow controller.

From the homogenizer 5, the milk passes via line 6, to one set of flow spaces 7 of a regenerative heat-exchanger 7, 8. The regenerative heat-exchanger 7, 8 raises the temperature of the milk to 1 220C. The heat required for this rise in temperature is supplied by the regenerator-fluid circulating through the other set of flow spaces 8.

The regenerator circuit comprises the flow spaces 8, a return line 9, a cooler 10, a line 11, flow spaces 1 2 and a line 13. The purpose of the regenerator circuit is indirect transfer the heat removed from the sterilized milk as it cools, to the pre-heated raw milk. The regenerator fluid at 780 leaves cooler 10 along line 11 and enters the flow-spaces 12 of the regenerative heatexchanger 12, 25 where it removes a quantity of heat sufficient to raise its temperature to 1200 C.

The regenerator fluid then passes via line 13 to flow spaces 8 where the majority of this quantity of heat is transferred to the pre-heated raw milk in flow spaces 7. The temperature of the regenerator fluid falls to 820C and it is returned along return line 9 to the cooler 10, which prevents overheating. The incorporation of such a regenerator circuit, gives the UHT treatment a high thermal economy.

Line 14 carries the heated milk to one set of flow spaces 1 5, of the first heat-exchanger in the high-temperature section 1 5, 16. The hightemperature heat-exchangers 1 5, 1 6 and 1 7, 18 heat the milk to 1 380C. At this temperature sterilization occurs rapidly, usualiy in two seconds.

The line 19, links the two high-temperature heatexchangers and carries milk from the set of flow spaces 15 to the set of flow spaces 17. In this example, heat-exchanger 1 5, 1 6 is three times the length of heat-exchanger 1 7, 1 8.

In a conventional UHT treatment plant there would be only one high-temperature heatexchanger. The present invention provides two or more heat-exchangers at this stage. These may be in the same frame.

The service-fluid circuit comprises the flow spaces 16, the line 20, the heater 21, the line 22, the flow spaces 18 and the line 23. The servicefluid at 1400C leaves the heater 21 along line 22 and passes through flow spaces 18, via line 23 and flow spaces 16 to emerge into line 20 at 1240 C. It is returned to the heater 21 by line 20.

Valve 28 is normally open and valve 29 is normally closed. Line 30 by-passes flow spaces 16 when valve 29 is opened and valve 28 closed.

The ratio of the flow rates of the service fluid through flow spaces 16 and 18 to that of the process liquid in flow spaces 1 5 and 17, is maintained substantially at unity by a flow controller which may, for example, be incorporated into the heater 21. With a flow ratio of unity, the temperature difference across the heat transfer surfaces will be constant throughout the high-temperature section, and the temperature profile will be linear. A non-linear temperature profile results in increased thermal damage. Furthermore, the rate of deposition of fouling materials is a function of the temperature difference; a non-linear profile is known to enhance deposition.

The heated milk is carried by holding tube 24 to one set of flow spaces 25 of the regenerative heat-exchanger 1 2, 25. The dual processes of sterilization and thermal damage continue in the holding tube 24 at 1 380C.

From the holding tube 24 the milk passes to one set flow spaces 25 of the regenerative heatexchanger 12, 25 where the temperature is lowered to 880C, a quantity of heat being removed by the regenerator fluid circulating through flow spaces 12.

The line 26 carries the processed milk from the regenerative heat-exchanger 12, 25 to one set of flow spaces 3 of the heat-exchanger 2, 3. The temperature of the processed milk falls to 1 90C during it's passage of flow spaces 3 at heat is exchanged with the raw milk in the other set of flow spaces 2.

The line 27 carries the processed milk to an aseptic packaging plant.

Thermal damage can occur at temperatures above 800 C, with partial dephosphorylation of caseins and denaturation of serum proteins. The dephosphorylation of caseins leads to the disruption of micelles. The denaturation of serum proteins, such as lactoglobulin can release volatile sulphur compounds such as hydrogen sulphide and certain mercaptans, these will taint the flavour of the milk.

However the UHT treatment temperatures for milk must remain in the range 135-1 500 for at least two seconds, if they are to destroy heat resistant bacterial spores, thermoduric bacteria, thermodurid molds and other resistant organisms, such as the leptospira.

If the temperature-time profile for the UHTtreatment process is known, then the sterilising effect and the thermal damage may be calculated, it should be noted that the regenerative sections contribute to this factor. The choice of timetemperature combination is a compromise for a given plant operating at its design capacity.

At the present time one characteristic of a UHT treatment plant is its inability to handle quantities of milk which fall short of its design capacity. The advantages of small size, suitability for CIPcleaning, thermal economy high product quality and automation cannot be realized with the problem of variable through-put unsolved.

Previously, it has been usual to overcome the problems of turndown by running the UHT treatment plant at full rate and using a sterile buffer tank, between the UHT plant and the aseptic filler. However, this is an expensive solution adding complexity to the plant.

A typical turndown ratio may'be 4 to 1. This would reduce the quality of the product considerably, as the low flow-rate would mean a longer effective residence time and the accompanying thermal damage.

Under turnddwn conditions the heat load on the service-fluid circulating in flow spaces (16) and (1 8) is correspondingly reduced, and the required heat transfer area is reduced by closing valve (28) and opening valve (29). This shunts the servicefluid along line (30) which by-passes flow-spaces (16). The heated milk at 1 220C entering flowspaces 1 5 from line 14 is not heated, but is held at the regeneration temperature of 1 220C. Flow space 1 5 acts as an intermediate holding tube under these conditions.

In this embodiment it is necessary to maintain the process flow-path 4, 1 5, 1 9, 1 7, 24 unaltered and modify the service side only, since it would be unacceptable to have a parallel flow path such as 16, 30 on the hygienic, process side.

The regenerative heat-exchangers may also be modified in the same way, although the benefits are not so pronounced since the process-liquid is at a lower temperature. There may be more than the single division of the high-temperature section into the high-temperature heat-exchangers such as 15, 1 6 and 17, 18 this would add to the range of through-puts which the system could handle.

Turning now to figure 2 the diagram illustrates an apparatus for the indirect UHT-treatment of milk with a diversion of process-liquid, The line 31 carries milk from a regenerative heat-exchanger substantially as in figure 1, to one set of flow spaces 32 of a high-temperature heatexchanger 32, 33. The line 34 carries milk to a second set of flow spaces 35 of a hightemperature heat-exchanger 35, 36. The milk then passes into a holding tube 37.

The service circuit comprises heater 38, line 39, flow spaces 36 line 40, flow spaces 33 and return line 41.

In this embodiment the line 31 possesses a valve 45, line 42, communicates with line 31 and possesses valves 46 and 48, line 34 possesses valves 47 and 49.

At design capacity valves 45, 47 and 49 are open, valves 46 and 48 are closed. When operating at a reduced throughput, either hightemperature heat-exchanger 32, 33 or 35, 36 is taken out of the process circuit. To isolate hightemperature heat-exchanger 32, 33, valves 45 and 47 are closed and valve 46 is opened. Flowspaces 32 may now be cleaned-in-place. A cleaning-in-place-plant 50 may be provided.

A closure of valves 46,49 and 51 with valves 45, 47 and 48 open will isolate flow spaces 35.

The cleaning-in-place plant may be that 50, used by flow spaces 32.

Although this example is illustrated by reference to an UHT treatment plant for milk. The invention may also be applied in other cases where thermal sterilization of thermo degradable products is desired.

Various modifications may be made within the scope of the invention. For example, the alternative flow spaces may be in a "parallel" rather than a "series" arrangement.

Claims (8)

1. A method of ultra high temperature treatment of a process-liquid in which preheated process-liquid is brought to sterilisation temperature in indirect heat-exchange with a service-fluid and briefly held at sterilisation temperature prior to cooling, in which the effective heat transfer area of the heat-exchanger is reduced, on turndown, by causing the processliquid or the service-fluid to by-pass a proportion of the flow spaced in the heat-exchanger.

2. A method as claimed in claim 1, in which the heat-exchanger is a plate heat-exchanger.

3. A method as claimed in claim 1 or 2, in which the process-liquid is milk.

4. A method as claimed in claim 1,2 or 3, in which the portion of the flow spaces through which the service-fluid is diverted is a continuous terminal distal portion of the flow spaces.

5. A method as claimed in claim 1, 2, 3 or 4, in which the portion of the flow spaces through which the service-fluid is diverted is one quarter of the flow spaces.

6. A method as claimed in any preceding claim, in which the ratio of the flow rates of the servicefluid to the process-liquid is maintained substantially at unity.

7. A method of indirect ultra high temperature treatment substantially as hereinbefore described with reference to the accompanying drawings.

8. Apparatus for the indirect ultra high temperature of a process-liquid comprising a preheater, a heat-exchanger having two sets of flow spaces for raising the preheated processliquid to sterilisation temperature in indirect heatexchange with a service-fluid, means for circulating the service-fluid through one set of flow spaces of the heat-exchanger and means for circulating the process-liquid selectively through all or a part only of the second set of flow spaces.

9, Apparatus for indirect ultra high temperature treatment of a process-liquid comprising a preheater, a heat-exchanger having two sets of flow spaces for raising the preheated processliquid to sterilisation temperature in indirect heatexchange with a service-fluid, means for circulating the process-liquid through one set of flow spaces of the heat exchanger and mean for circulating the service-fluid selectively through all or a part only of the second set of flow spaces.