GB2358145A - Dissolving a mixture of gases in a beverage - Google Patents

Abstract

A process for providing separately-adjustable solution of two gases eg CO<SB>2</SB> and N<SB>2</SB>, in a dispensed beverage eg beer, uses gas permeable hollow fibre membrane 2 with a valve assembly 4,5 operated in a sequence that sequentially feeds the gases from respective supplies 8,9 to the bore side of the membrane with the beverage fed from supply 1 to the shell side. The first gas is vented from the bore side of the membrane through an orifice 6 before applying the second gas.

Description

2358145 PROCESS FOR DISSOLVING GASES IN A LIQUID

TECHNICAL FIELD

This invention relates to a process for dissolving gases in liquids, for example beverages, the liquid being dispensed in discrete volume amounts.

BACKGROUND

The taste and presentation of a beverage such as beer depend critically on the levels of dissolved gas or gases it contains as it is dispensed. In order to achieve good and reproducible control over the dispense process, the.dissolved levels of these gases must be consistent.

This invention relates principally to the dispense of beverages containing at least two species of dissolved gases at the point of dispense. Usually these two gas species will be carbon dioxide and nitrogen.

The use of non-flooding gas-permeable hollow fibres and associated control means for effecting bubble-less transfers of gases into solution in liquids is described in the prior patents US5,565,149 and US6,138,995. That technology has been commercialised in a range of CellarstreamC Dispense Systems, which are manufactured and marketed by Headmaster Ltd. and Permea, a division of Air Products and Chemicals Inc. Cellarstream is a registered Trademark of Air Products and Chemicals Inc.

In some countries, mixed gases, with certain ratios of carbon dioxide and nitrogen are readily available to the drinks dispense industry. In many other countries, where the beverage dispense industry has traditionally used only carbon dioxide gas, such mixed gases are either un-available or relatively expensive.

In the following description, distinction is made between the term "gas species" (which denotes a known chemical molecule such as carbon dioxide, nitrogen etc.) and the term "gas types" (which denote a gas applied to a process, and may be either a pure gas or a mixture of gases).

BACKGROUND TO THE INVENTION

In the systems described in US5,565,149 and US6,138,995 the process for transferring a gas to (or from) the liquid is principally controlled by the adjustment of the partial pressure of that gas when it is applied to the bore side of non-flooding hollow fibres contained within a gas/liquid contactor module.

Furthermore, the systems described therein include means for employing either one ot two gas types, which may be either pure or mixed gases, and controls for setting the pressures at which they are applied in the gas/liquid contactor module.

Additionally, the systems described therein include control means designed to fully change the gas in the fibre bore volume by completely venting the bore-side content to atmosphere at specific stages of the dispense process, namely at the beginning and at the end of each dispense.

In certain instances, there are practical difficulties associated with the systems described in US5,565,149 and US6,138,995 if precise levels of two different gas species such as nitrogen and carbon dioxide are both required to be dissolved in a beverage which iffitially does not already contain the desired level of one of these gas species.

The difficulty is particularly severe when working with liquids which initially contain no, or very little, dissolved gas. This is because the two gases commonly concerned, namely nitrogen and carbon dioxide, have remarkably different soiubilities. When a pre-mixed gas type containing these two gas species is simply applied to the bore side of a gas/liquid contactor of the type disclosed in US5,565,149 and US6,138,995, then the local gas content in the fibres will rapidly become nitrogen-rich as the highlysoluble carbon dioxide gas is consumed through solution into the liquid if that liquid initially contains a low amount of dissolved carbon dioxide. The gas addition performance of those systems will therefore vary significantly even over the time taken to dispense a single drink, and even if attempts are made to compensate for the differing solubilities by employing special gas rixes. Furthermore, since such special gas mixes are not generally available as standard, they are expensive.

Additionally, the target levels of dissolved gases for certain beverages may demand that a particular gas be applied within the systems at an accurate pressure, say within 1 or 2 psi of a desired nominal value. The systems disclosed in US5,565,149 and US6,138,995 show means for regulation of gas pressures applied to the contactor module and such regulation devices are incorporated within the existing commercialised systems, However, in practical operation it is difficult to achieve a precise setting of the regulator which controls the gas pressure applied during dispense. This is because the dispense process imposes unusual demands on the characteristics of this pressure regulator. Initially, i.e. before dispense flow starts, its rate of delivery of gas is zero. After dispense commences, and the venting of boreside gas has completed, this regulator must refill the bore-side of the contactor and re-establish the new bore-side pressure within the minimum possible time. Typically, the required average delivered gas flow rate for this step exceeds 5 litres/minute. Then, having re-filled the bore-side, the demand for gas flow until completion of dispense may be higher than 3 litres per minute. Only relatively expensive pressure regulator devices can achieve and maintain constant the delivered pressure under such varying operating conditions. In those circumstances which demand more accurate pressures to be maintained it is necessary for the user to by-pass the control of the existing internal regulators and to purchase additional pressure regulators which then are used to control the pressure of the external gas feeds to the systems disclosed in US5,565,149 and US6,138,995.

THE INVENTiON We have now discovered that it is possible to avoid these practical difficulties by employing novel additional principles in the controls fitted to the gas/liquid contactor modules of the type used in the systems disclosed in US5,565,149 and US6,138,995. A main characteristic of those modules is their high surface area of hollow fibres providing contact between gas contained within the fibres and liquid contained outside the fibres. The internal diameter of the fibre bores is generally below 0.3mm.

The present invention provides a process for controlling the quantity of each of two gas species dissolved in a liquid by placing the liquid under controlled pressure and 2 transporting the liquid under pressure through a shell side feed port of a gas/liquid contactor module containing a bundle of gas permeable hollow fibre membranes which are sealed at one end and communicate at their common open ends with the bore side gas feeding means of the module, and wherein any liquid contacts only the membrane outer surfaces. A first gas is selected and applied to the bore side of the hollow fibre membranes at a predetermined pressure to effect dissolution to a predetermined amount of the first gas in the liquid. The first gas is vented from the bore side of the hollow fibre membranes through an orifice, to leave a predetermined amount of residual gas in the bore side of the hollow fibre membranes. A second gas is selected and applied to the bore side of the hollow fibre membranes at a predetermined pressure to effect dissolution to a predetermined amount of the second gas in the liquid. The second gas is then vented from the bore side of the hollow fibre membranes to leave a predetermined amount of residual gas in the bore side of the hollow fibre membranes. The sequence of selection and application of the first and second gases is responsive to the starting or stopping of the transport of the liquid through the shell side of the gas/liquid contactor module.

The gases are generally nitrogen or carbon dioxid or may be oxygen or other suitable gas which will dissolve in a liquid in a gas permeable hollow fibre membrane system. The liquid is water, beer, wine, spirits, or other suitable liquid, preferably for human consumption.

In contrast to the systems disclosed in the earlier US5,565,149 and US6, 138,995, wherein the control means are designed to fully change the gas in the fibres' bore volume by completely venting the bore-side content, i. e. substantially to atmospheric pressure, at specific stages of the dispense, the present invention provides control means which limit and provide adjustment for the.amount of gases vented from, and also the amount of gas re-admitted to, the module's bore-side at those same stages of dispense operation.

In addition to achieving improved control over the addition of two gas species to liquids, further practical benefits stem from this invention, as will now be described with reference to Figures attached, and by reference to the example of the use of carbon dioxide and nitrogen gases. Other gases and combinations of gases may alternatively be used.

The format of a typical module according to the US5,565,149 and US6,138, 995 and containing gas-permeable hollow fibres is shown in Figure 1, where the liquid is contained in the shell volume, i.e. external to the fibres, and gas is contained within the bore side of the fibres. The fibres are closed at one end and open at the other end, communicating with a port for gas feed to the bore-side. Liquid, in the shell side, only contacts with the outer surface of the fibres, and ports are provided for entry and outlet of the liquid. The total bore-side volume is generally smaller than the shell-side volume of the liquid. Because of the ability of the permeable fibres to transfer gases to, and from, solution in the liquid, it will generally be the case that the bore side will contain mixtures of carbon dioxide and nitrogen at each stage of the dispense process.

Furthermore, when the system is used to dissolve two gas species in the liquid, any difference in solubility between those gases will, after only a brief period of use, result in depletion in the concentration of the more soluble gas contained within the boreside.

In order to keep conditions constant for each successive dispense process, the control schemes described in US5,565,149 and US6,138,995 provide for the gas 3 contained in the bore-side volume to be substantially vented at the start and end of the liquid dispense flow. Immediately after such venting actions, the bore-side is refilled with that gas type which is appropriate for the relevant stage of the dispense. In order to maintain the effectiveness of gas addition throughout the dispense, it is important that the duration of these venting stages should be small in comparison to the time taken to dispense the beverage drink. It is preferred that the time taken should be no longer than 2 seconds.

In contrast, the method according to this invention provides for control over the amount of bore-side gas which is released at each vent stage. When a controlled vent stage is complete, the next gas type is adMitted to the bore-side of the contactor module. The effect of this is to create, along the length of each hollow fibre, a predetermined change in concentration of each of the two gas species concerned, as will next be explained.

The effective volume of gas contained within the module at any stage comprises the sum of the volumes of the bores of all fibres, (Vf), plus the volume between the face of the seal at the fibres' open ends and the gas feed port of the module, (Vc) as shown in Diagram 1. The volume of gas contained in the control valves connected to the contactor module is ignored here for simplicity.

We have discovered that when only a proportion of the gas initially contained in this total volume (Vf + Ve) is released back through the gas feed port by venting through orifice such that the bore-side gas pressure changes from a first internal pressure P1 to an intermediate lower pressure P, and then the second gas type is admitted until a second internal pressure P2 is reached, this process results in a non-uniform concentration of the gas species along the length of the fibres.

The reason for this is that when the second gas type is selected and begins to be admitted (i.e. as the bore-side pressure starts to rise again from the intermediate pressure P), there is substantially no gas- phase mixing of the second gas type with the gas initially contained within the fibre volume (Vf) because diffusion mixing of gas species is very slow within the bore section of the small-diameter fibres. Mixing of the second gas type with the gas in the module's bore-side substantially only takes place within the volume (Vc). As the bore-side pressure continues to rise from intermediate value P towards its final value P2, the composition of gas entering the open ends of the fibres is therefore progressively changing from that of the gas initially contained in the bore-side towards that of the second feed gas type.

If the time taken for completion of these steps is fixed, e.g. by the type of controller fitted to the systems as described in US5,565,149 and US6,138,995, then these gas concentrations along the fibres can be accurately and reproducibly established by adjustment of the size of the orifice which determines intermediate vent pressure P. An example of the concentration profile derived from this model is shown in Figure 2 for the case of a 30% carbon dioxide gas (the initial gas) and pure carbon dioxide (the second gas type).

It is also possible achieve the same effects by employing a fixed size of orifice and uing a variable timer in the system's controls to change the duration of the vent stage of the process.

Although these two control options have equivalent effects, we have found that for the following reasons it is preferred to use a controller which delivers a vent duration which can be adjusted simply by rotation of a dial or screw. Firstly, it is difficult to achieve both mechanical stability and a sensitive, smooth adjustment in inexpensive 4 orifice devices. Secondly, once a system is installed, there may be a need for the user to make a further small correction to the intermediate vent pressure P in order to suit local conditions which differ from standard specifications (e.g. a different beverage temperature or a different preference for dispense presentation); it is simpler in practice for the user to alter a timer control than it is to adjust an orifice.

It has been found that the controller should preferably provide a range of adjustment duration of the vent stage between 1.0 and 2.0 seconds, i.e. within a time which is short compared with the time taken for dispensing the typical volume of liquid. Fixed orifices of different sizes can be fitted, thus extending the range of the process in discrete increments and allowing the process to be adjusted accurately for any application yet still providing for small subsequent adjustment if desired. In selecting the size of orifice it is important also to take into consideration the size of the gas paths within the valve assembly which conduct the bore-side gas to that orifice.

Considering the composition of gas contained within the bore of each element of a fibre's length, the net activity of each contained gas species for transfer into, or out of, solution in the liquid which is in contact with the buter surface of the element is determined by the difference between the partial pressure of that gas species and its dissolved concentration in that local element of liquid.

The kinetics for gas transfer in gas/liquid contactors as disclosed in US5,565,149 and US6,138,995 are determined mainly by diffusion in the liquid boundary layer which is in intimate contact with the fibre surfaces. In a dispense application, where the liquid is either at rest or flowing, these kinetics will vary accordingly.

As the gas species dissolve in the liquid and are thus removed radially from the fibres' bore-side, further amounts of the currently-selected process gas, i.e. the gas type selected by the controls according to the current stage of the dispense process, may enter the bore-side and restore its total pressure.

Operation of the process of this invention is simplified by arranging for the bore-side pressures, P1 and P2, to be substantially the same.

The adjustment of the intermediate vent pressure P provides the new and important ability to control the bore-side concentration gradient of each gas type along the length of the fibre bundle, and hence control over the amount of each of the two gas species which dissolve for a given total bore-side gas pressure. At a given total boreside applied pressure, an increase in the dissolved level of one gas species will result in a corresponding decrease in the dissolved level of the other gas species.

Because gas solution levels in the gas/liquid contactor module obey Henry's Law, adjustment of the total bore-side applied pressure (P1, P2) provides a further degree of freedom of the process of this invention in the ability to change the total dissolved amounts of both gas species.

Therefore, with the ability to control both the total amounts of both dissolved gases and to control the ratio between those amounts, the process of this invention provides control over the individual amounts of each dissolved gas species.

The step producing the controlled venting of gas according to this invention and described above, i.e. reduction of bore-side gas pressure to a pre-determined intermediate pressure P befor re-pressurising again with the next selected gas, is executed both at the start and at the end of each dispense event.

Upon completion of the vent step at the end of each dispense, when liquid flow stops, the first gas type is used to re-pressurise the module's boreside; upon completion of the vent step at the start of each dispense, when liquid flow starts, the second gas type is used to re-pressurise the module's bore-side.

The first gas type is always chosen to contain a lower concentration of carbon dioxide, or the more highly-soluble gas species, than does the second gas type. Accordingly, when liquid is at rest within the module between dispense events it is exposed to fibres containing an average lower partial pressure of carbon dioxide compared to the situation when the liquid is flowing during dispense. Since the liquid output from the module at all practical dispense'flow rates does not reach full saturation by a dissolved gas, this choice of the relative carbon dioxide contents in the two gas types enables the process to be adjusted to deliver liquid with uniform dissolved concentrations of both gas species.

Although, as explained, there is little mixing of gas types by gas-phase diffusion within the fibre bores, there will nevertheless be soMe mixing of the gas species during dispense flow by a sequence involving initial radial transfer of carbon dioxide into the liquid at the fiber sections rich in carbon dioxide; transport of that liquid along the module in its shell-side to fibre sections containing a gas which is more rich in nitrogen; and transfer of some dissolved carbon dioxide there from solution radially back into the fibre bores while, simultaneously, nitrogen transfers out into solution in that liquid.

After an initial few dispense events, this sequence leads to identical gas concentration profiles being reproduced in the fibres at every corresponding stage of subsequent dispense events. Provided that the condition of the feed liquid remains constant, then the dissolved gas levels in the output liquid are also accurately reproduced at every dispense event.

It will be apparent that, while the gas vented at the end of dispense will have a composition close to that of the second gas type, the gas contained in the fibre section most remote from the bore-side feed port will contain more carbon dioxide than the first gas type but less than the second gas type. The actual compositions will be dictated by the adjustment of the process' vent steps, the process pressure, the liquid flow rate during dispense, and its temperature.

This new ability to control, in a pre-determined manner, the activity for transfer of each gas species to, or from, solution in the shell-side liquid has additional significant. practical benefits.

Firstly, it permits close control over the levels of each of two species of gases simultaneously dissolved in the liquid using feed gas types which need not be premixed gases (which would be necessary with the systems disclosed in US5,565,149 and US6,138,995) but can instead be pure gases, e.g. carbon dioxide and nitrogen. Such gases are commonly available, and nitrogen can also be generated on-site using an air separator system.

Secondly, it allows the process to be operated with equal pressures of both gas types applied, when selected by the qontrols, to the gas/liquid contactor. This simplifies the means for controlling the gas pressures applied in the process. Thus the system needs only to be fitted with means for controlling the pressure of one of the two gas types. It is preferred to control the pressure of the first gas type, and to use a pilot- 6 operated valve to control the pressure of the second gas type applied to the module, i.e. when it is selected by the controls.

An example of this configuration is shown in Figure 3. It should be noted that in the following description the formats of all control valves used are given as examples only and in practice the process is not restricted to utilise these formats.

Liquid to be processed is supplied from pressurised container 1 (the means of pressurisation is not shown) to the shell-side input connector 21 of gas/liquid contactor module 2 via flow switch 11. The output liquid is connected via output connector 22 to tap 3. The bore-side feed port23 of module 2 is connected to sole noid-op erated valves 4,5 which are 3- port, 2-way design; the valves' internal gas paths are drawn so that, when un-powered, the valve assembly's internal gas paths are from 23 to 41 and from 42 to 51 respectively. Orifice 6 is fitted to port 51 of valve 5. Pressurised gas source 8 and pressure regulator 81 deliver this first gas type at a set pressure to port 41 of valve 4. Pressurised gas source 9 and its pressure regulator 92 deliver this as the second gas type. The setting of 92 is kept higher than that of 82, and the second gas type is connected to the inlet port 102 of balancing valve 10. The pilot port 101 of valve 10 is connected in common with port 41 of valve 4, and the second gas type outlet, its pressure reduced to the same pressure as the pilot port of valve 10, is supplied via outlet port 103 to port 52 of valve 5. Operators 43 and 53 of valves 4 and 5 are wired to terminals 74 and 75 of controller 7, which is supplied with electrical power source 12 at terminals 712 and connected to flow switch 11 at terminal 711. When no liquid flows, terminals 74 and 75 are not powered. When liquid flow starts, the signal from flow switch 11 causes terminals 74 to be powered immediately and then terminals 75 to be powered after a delay time set by controller 7. When liquid flow next stops, the signal from 11 causes terminals 75 to be un-powered immediately and then terminals 74 to be un-powered after a delay time set by 7.

The effects of these control actions are; when liquid is not being dispensed, the first gas type 8 is applied to the module; when dispense starts, an amount of gas is vented from the module's bore side, that amount being determined by the size of restrictor 6 and the delay time between power being applied to operator 43 and to operator 53; in the period between power being applied to operator 53 and dispense flow being stopped, application of the second gas type 9 to the module; when dispense flow stops, venting an amount of gas from the module's bore side, that amount being determined by the size of restrictor 6 and the delay time between power being applied to operator'43 and to operator 53; after venting, and until the next dispense event, when terminals 43 are unpowered, re-application of the first gas type 8 to the module.

EXAMPLES

EXAMPLE1. A dispense system according to this invention was connected as shown in Figure 4, which uses the same numbers for parts as Figure 3. The liquid processed was plain city water 1 containing no dissolved carbon dioxide, delivered to the system using a 7 Rojet electric beverage pump 13, and delivered from the system to a conventional dispense tap 3 via 14, a remote beverage cooler together with a 1 0-meter length of 318" diameter tubing cooled by contact with aflow and return tube containing recirculating cold water from the remote cooler. The delivery tube and coolant tubes were insulated. The temperature of the feed water and the system was 11 Celsius. Dispense flow rate was 1.5 litres per minute, and the dispense volumes were single Imperial pints.

A side connection was made in the delivery tube immediately before the dispense tap, allowing a small needle valve 15 to withdraw samples of liquid from the delivery line. The system was connected to nitrogen gas (first gas type) at a pressure of 3,45 bar via an external pressure regulator 81, and to carbon dioxide (second gas type) at 5.52 bar via regulator 92. The pilot port 101 of balancing valve 10 was connected to the pressure-regulated nitrogen feed gas.

The valves 4,5 used for the test each had an effective internal orifice size of 1.6mm, and the system's controller 7 was set to deliver a vent stage duration of 1.4 seconds, i.e. the time interval between power being applied to valve operator 43.and to valve operator 53.

The vent port 51 of valve 5 was fitted in turn with a range of commercially available orifices 6 produced by laser machining of sapphire discs. The system was operated in single pint (0.568 litre) dispenses via tap 3 for each orifice in turn to determine the correlation between orifice 6 diameter and carbonation levels in the dispensed liquid using fixed gas feed conditions for the system.

Additional tests were made using a calibrated needle valve as orifice 6. A test was also made with no orifice fitted to valve 5, a condition corresponding to an estimated equivalent orifice size of 1.5mm due to the internal pathways for gas flow through valves 4, 5 during the venting condition.

The carbonation samples were taken from valve 15 and analysed according to the well-established procedures for a Coming 965 Analyser. That instrument was calibrated with laboratory-prepared standard solution for 2.0 vols.

Eight samples were analysed for each setting, and it was found that the sampling procedures gave a maximum deviation between individual readings and the series average of only 0.03 vols. in all cases.

The results are plotted in Figure 5., where it can be seen that there is a direct and smooth correlation between orifice diameter and resultant in the liquid outlet from the module, i.e. prior to the dispense tap 3. The nitrogenation level cannot be measured using these techniques, but the series of tests showed a distinct visual correlation between carbonation and nitrogenation; at low carbonation the dispensed liquid was white signifying high level of dissolved nitrogen. Since the system was operated at constant overall pressure of feed gases, this balance between the respective concentrations of dissolved carbon dioxide and nitrogen is as predicted.

EXAMPLE2. Plain water 1, containing no dissolved carbon dioxide, was processed using the system shown schematically in Figure 6. The configuration of this system corresponds to systems disclosed in US5,565, 149 and US6,138,995.The contactor 8 module 2 used here was the same unit used in the series of tests described in Example 1.

The feed gas 8 was a mixed gas containing 50.3% carbon dioxide and 49.7% nitrogen, supplied to port 41 of valve 4 at a pressure of 3.45 bar via pressure regulator 81 and measured by gauge 811. Valve 5 outlet port was not fitted with a restrictor, i.e. it was left open allowing substantially complete venting of the bore-side gas at both start and end of each dispense.

The dispense temperature and flow rate conditions used here were the same as used in Example 1.

The carbonation obtained using the same sampling methods as used inExample 1 was 1.08 vols. When using a tap 3 designed for nitrogenated drinks, the dispensed liquid was heavily whitened by liberation from solution of very small bubbles of nitrogen.

This example demonstrates that it is possible, using the systems described in US5,565,149 and US6,138,995 with a pre-mixed 50% carbon dioxide 150% nitrogen gas as input, to process an initially "flaf' liquid to a dissolved gases content which is characterised by one level of carbon dioxide and another level of dissolved nitrogen. or the system according to this invention (but using pure nitrogen and pure carbon dioxide gases as inputs). This is in contrast to the results from Example 1 where it was demonstrated that the present invention enables a range of these dissolved gas levels to be achieved under the same operating conditions EXAMPLE 3. It is well known that the dispense presentation of beverages containing either dissolved carbon dioxide or both dissolved carbon dioxide and nitrogen is significantly affected by the liquid temperature. It is also well known that the height of the foam on a dispensed drink is strongly influenced by, among other factors, the level of dissolved carbon dioxide.

Therefore one application of the system according to this invention is to modify the balance between the levels of dissolved carbon dioxide and nitrogen in order to increase the height of the foam, allowing the standard product to be dispensed at colder temperatures without compromising its presentation.

A 50-litre keg 1 containing a beverage with dissolved gases at the levels 1.05 vols. of carbon dioxide and 50 ppm of nitrogen (units are equivalent to mg. of gas per litre of liquid) was connected to the system shown in Figure 7. A supply of pre-mixed gas 8 containing 29.8% carbon dioxide and 70.2% nitrogen was connected to the keg 1 at a constant pressure of 2.15 bar via regulator 81 which was also used to displace the product from 1 via the gas/liquid contactor 2 to the dispense tap 3 in 30' diameter tubing. The same pre-mixed gas, at the same pressure, was connected as the first gas type to valve 4, and also to the pilot port 101 of the balancing valve 10. The second gas type 9 was carbon dioxide, delivered at a pressure of 60psi from regulator 92 to the inlet port 102 of valve 10. The outlet port 103 of valve 10 was connected to valve 5 as shown.

The temperature of both the keg and the module was 10 Celsius.

9 The tubing arrangement between the module and the dispense tap were similar to that used in Examples 1 & 2, but an additional product cooler 16 was installed just prior to the dispense tap 3 in order to reduce the dispensed drink temperature to between 1.5 and 3.0 Celsius.

A series of tests was conducted for each of a range of orifice 6 sizes using 1 Imperial pint (0.568 litre) poured in a single step, through the tap 3 which was fitted with a conventional "creame' disc and flow straightener device. After pouring and settling, the foam height and temperature of each drink was measured.

These tests were conducted over a period of 14 days using the same keg. It was confirmed that performance was stable throughout.

The results are shown in Figure 8. The results include measurements made with the vent port 51 of valve 5 sealed shut. In that condition no gas venting step can take place, enabling the presentation of the standard (unprocessed) product to be determined for comparison.

It can be seen that the system according to this invention affords an accurate method of controlling the presentation of the beverage when dispensed at a temperature much colder than is normally used.

EXAMPLE4 A dispense system according to the present invention was connected as shown in Figure 9. The 50-litre keg 1 contained a beer which had previously been processed to contain no dissolved carbon dioxide or nitrogen gases. The beer was delivered to the gas/liquid contactor module 2 via a Flojet G56 gas-operated beer pump. The first gas type, 8, was pure nitrogen gas delivered to valve 4 via regulator 81. The output from this regulator was set at 3.58 bar. This regulated line of the nitrogen gas was connected in common with the feed gas for the gas pump 13. The second gas type, 9, was carbon dioxide, and its feed pressure to valve 10 was set to 6.0 bar by regulator 92. A fixed restrictor 6 was fitted to port 51 of valve 5. The size of this restrictor was 0.8 mm. The controller 7 used in these tests was manufactured with an adjuster screw which enabled the time delay between power being applied to valve operators 43 and 53 to be varied from 1.0 to 2.0 seconds. An additional connection point 24 was fitted to port 23 and special equipment (not shown) was used to measure the process intermediate venting pressure P, i. e. at the instant that power was applied to valve 5 operator 53. Dispensed volumes were set at 1.0 and 0.5 imperial pints (0.568 and 0.284 litres), using a flow rate of 1.53 litres/second through the tap3 which was fitted with a "creamer disd'and flow straightener. Dispense temperature was 6 Celsius.

In a series of measurements conducted over a range of settings for the venting time, the level of dissolved carbon dioxide in the dispensed drink was measured as a function of the intermediate vent pressure P using the Coming 965 Analyser as before.

Ko quantitative measurements of dissolved nitrogen levels were made. However, qualitative observations of the concentration and time required for settling of the very small bubbles of nitrogen in such a series of controlled dispenses are known by those familiar with nitrogenated, beer dispense presentations to be a reliable guide to variations in the dissolved nitrogen levels.

In this series it was clearly seen that the level of dissolved nitrogen decreased as the measured level of dissolved carbon dioxide increased.

It is to be noted that under these dispense conditions the loss of carbon dioxide due to gas breakout in the tap 3 naturally increases as the liquid's carbonation level increases.

The results are shown in the graph of Figure 10. It can be seen that the adjustable time function of controller 7 provides excellent and smooth control over the in-glass level of dissolved carbon dioxide and dissolved nitrogen.

EXAMPLE5 The dispense system described in Example 4 was operated using the same dispense conditions and at the same temperature, and with the same feed gas types. The first gas type, 8, was pure nitrogen gas delivered to valve 4 via pressure regulator 81. This was set to deliver an output pressure of 3.00 bar. This regulated nitrogen gas was connected in common with the feed gas for the gas pump 13, as in the previous example.

In a series of measurements conducted over a range of settings for the venting time, the level of dissolved carbon dioxide in the dispensed drink was measured as a function of the intermediate vent pressure P using the Corning 965 Analyser as before.

The results are shown in the graph of Figure 11 which also includes the results from the previous Example 4 to permit a direct comparison.

It can be seen that, at given values for intermediate vent pressure P, the effect of changing the nitrogen feed pressure, which here also determines the pressures at which both feed gases are applied to the module, is to change the amount of carbon dioxide which is dissolved into the beer. This relationship naturally demonstrates that the gas solution mechanisms taking place within the module act in accordance with Henry's Law. The dissolved levels of nitrogen produced in the process also have the same dependency on applied pressure.

These Examples 4,5 demonstrate that the process of this invention has the ability, through changing the applied gas pressures, to vary both the total amounts of the two species of dissolved gases and, through changing the intermediate vent pressure, to vary the relative amounts of each of the two species dissolved.

Claims (15)

1. A process for controlling the s eparate quantities of two gas species dissolved in a liquid, which process comprises:

(a) placing the liquid under a controlled pressure; and (b) transporting the liquid under pressure through the shell-side feed port of a gas/liquid contactor module containing a bundle of gas- permeable hollow fibres which are sealed at one end and communicate at their common open ends with the bore-side gas feeding means of the module, the liquid contacting only the membrane outer surfaces; (c) selecting and applying a first gas to the bore side of the hollow fibre membranes at a predetermined pressure to effect dissolution to a predetermined amount of the first gas in the liquid; (d) venting the first gas from the bore side of the hollow fibre membranes through an orifice, to provide a predetermined amount of residual gas; (e) selecting and applying a second gas to the bore side of the hollow fibre membranes at a predetermined pressure to effect dissolution to a predetermined amount of the second gas in the liquid; and (f) venting the second gas from the bore side of the hollow fibre membranes to provide a predetermined amount of residual gas, while controlling the sequence of selection and application of the first and second gases to the bore side of the hollow fibre membranes, in response to the starting or stopping of transport of the liquid through the shell side of the gas/liquid contactor module.

2. The process of claim 1 wherein control of the amount of gas vented from the bore side of the hollow fibre membranes is determined by controlling the size of the orifice through which the gas is vented during a predetermined duration for venting of the gas.

3. The process of claim 1 wherein control of the amount of gas vented from the bore side of the hollow fibre membranes is determined by controlling the duration for venting of the gas through an orifice of predetermined size.

4. The process of,claim 1 wherein control of the amount of gas dissolved in the liquid is effected by controlling the duration for venting of the gas from the bore side of the hollow fibre membranes.

12

5. The process of claim 1 wherein control of the amount of gas dissolved in the liquid is effected by changing the pressure at which the selected gas is applied to the bore side of the hollow fibre membranes.

6. The process of claim 1 wherein the liquid is water.

7. The process of claim 1 wherein the liquid is a beverage.

8. The process of claim 1 wherein the beverage is beer.

9. The process of claim 8 wherein the beer contains substantially no dissolved gases before entering the gas/liquid contactor.

10. The process of claim 1 wherein the two gas species are nitrogen and carbon dioxide.

11. The process of claim 1 wherein one of the gas species is carbon dioxide or nitrogen and the other is a mixture of carbon dioxide and nitrogen.

12. The process of claim 1 wherein one of the two gas species contains oxygen.

13. A process for controlling the quantity of each of two gas species dissolved in a beer, which process comprises:

(a) placing the beer under a controlled pressure; (b) transporting the beer under pressure through a shell side feed port of a gas/liquid contactor module containing a bundle of gas permeable hollow fibre membranes which are sealed at one end and communicate at their common open ends with the bore side gas feeding means of the module, the beer contacting only the membrane outer surfaces; (c) selecting and applying a first gas to the bore side of the hollow fibre membranes at a predetermined pressure to effect dissolution to a predetermined amount of the first gas in the beer; (d) venting the first gas from the bore side of the hollow fibre membranes through an orifice, to provide a predetermined amount of residual gas; (e) selecting and applying a second gas to the bore side of the hollow fibre membranes at a 13 predetermined pressure to effect dissolution to a predetermined amount of the second gas in the beer; and (f) venting the second gas from the bore side of the hollow fibre membranes to provide a predetermined amount of residual gas, while controlling the sequence of selection and application of the first and second gases to the bore side of the hollow fibre membranes, in response to the starting or stopping of transport of the beer through the shell side of the gas/liquid contactor module.

14. The process of claim 13 wherein the first gas is nitrogen and the second gas is carbon dioxide.

15. The process of claim 13 wherein the first gas is a mixture of carbon dioxide and nitrogen and the second gas is carbon dioxide.

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