TECHNICAL FIELD

The invention relates to a device for separating a flowing medium mixture into fractions with differing mass density, comprising separating means for separating the flowing mixture into fractions. The invention also relates to a method for separating a flowing medium mixture into fractions with differing mass density.

BACKGROUND ART

The separation of a (flowing) medium mixture has many diverse applications. A medium mixture is understood to mean a mixture of solid and/or liquid and/or gas particles of micron or submicron size dispersed in at least one liquid or gas. Examples are a gas/gas mixture, a gas/liquid mixture or aerosol, a liquid/liquid mixture, a gas/solid mixture, a liquid/solid mixture, or such a mixture provided with one or more additional fractions. The separation of a medium mixture is known from various applications of liquid lighting, (flue) gas lighting and powder separation for instance. Separation of fractions with a relatively large difference in particle size and/or in mass density is relatively simple. Large-scale use is made for this purpose of processes such as filtration and screening. In the separation of fractions with a smaller difference in mass density, as is for instance the case for gas/gas mixtures, use is made of physical or chemical absorption techniques and/or separating techniques such as sedimentation and centrifugation. When large volumes of medium mixtures need to be processed, chemical separating techniques, such as amine separation for instance, are less economic and usually also less environmentally-friendly. Separating fractions by means of sedimentation requires time and, when processing larger volumes of medium mixture, makes it necessary to make use of voluminous reservoirs, which is, among other things, expensive. A compact device for separating a flowing medium mixture into fractions with differing mass density is described in NL 1026299. This device is much smaller than known distillation towers and comprises a rotor of substantially parallel feed channels for the flowing mixture, a feed for the medium mixture to be separated connecting to the rotor, and outlets connecting to the rotor for discharging the fractions of the separated medium mixture. In a rotating feed channel the heavy fraction of the mixture for separating is moved further outward towards the wall thereof than the light fraction under the influence of the centrifugal force, thus resulting in a separation. The separated fractions are discharged through heavy and light fraction outlet lines.

Separating a medium mixture into fractions involves at least a partial separation of the fractions. Indeed, in most cases a separated fraction may still comprise a part of another fraction of the mixture, although the presence of the other fraction will be significantly smaller than its presence in the original mixture. In other words, a concentration of 100% in the separated fractions is difficult to achieve and it is generally desirable to increase selectivity without increasing energy requirements.

The present invention has for its object to provide a device and method capable of providing an increased selectivity for the fractions to be separated. Another object is to increase the efficiency of the separation, i.e. to keep energy consumption low and avoid loss of mixture fractions. DISCLOSURE OF THE INVENTION

The invention provides for this purpose a device according to claim 1. In particular a device for separating a flowing medium mixture into fractions with differing mass density is provided, the device comprising a feed for the mixture to be separated, outlets for final light and heavy fractions, and in between interconnected first, second and third separating means that each separate an inlet mixture in heavy and light fractions that are each discharged through a heavy and light fraction outlet of each separating means, wherein the first and second separating means are configured such that the heavy fraction outlet of the first separating means connects to an inlet of the second separating means, and the light fraction outlet of the second separating means reconnects to an inlet of the first separating means to provide a feedback loop, and wherein an inlet of the third separating means and/or the heavy fraction outlet of the third separating means connects to the first separating means.

According to the present invention the separation selectivity of the known device is increased by providing the device with second separating means that connect to an outlet of the first separating means, and reconnect to the inlet thereof, in combination with third separating means. By providing the first, second and third separating means in accordance with the invention, the concentration of one fraction in another separated fraction is significantly reduced when compared to the state of the art device. An additional advantage of the invented device is that it can be given a compact form. Moreover, treatment times are generally short. The separating means of the invented device separate an inlet mixture into two fractions that, in the context of the present application are denoted as a final light fraction and a final heavy fraction. This terminology should not be construed narrowly and does not imply that the light fraction is light per se and the heavy fraction is heavy per se. It rather implies that the light fraction is lighter than the heavy fraction (or the heavy fraction is heavier than the light fraction). In purifying a gas/gas mixture for instance, the final light fraction exiting from the device may be the purified gas fraction, whereas the final heavy fraction exiting from the device may be the contaminated fraction. In the device according to the invention, the final light fraction preferably exits from the light fraction outlet of the third separating means, whereas the final heavy fraction preferably exits from the heavy fraction outlet of the second separating means.

The invention offers several ways in which the combination of the first and second separating means can be combined with the third separating means, as long as an inlet, and/or the heavy fraction outlet, of the third separating means connects to the first separating means. Some combinations however provide particularly useful results.

In one useful embodiment of the invention, a device is provided wherein the first and third separating means are configured such that the feed of the device connects to the inlet of the first separating means, and the light fraction outlet of the first separating means connects to the inlet of the third separating means.

In this embodiment, the mixture to be separated into fractions is first treated in the assembly of the first and second separating means, and the third separating means are adapted to further separate a light fraction exiting from the combination of the first and second separating means in a third light and heavy fraction.

In another useful embodiment of the invention, a device is provided wherein the first and third separating means are configured such that the heavy fraction outlet of the third separating means reconnects to the feed to provide a feedback loop. This embodiment reduces the loss of final light fraction in the device, particularly when the third separating means comprise a membrane separator. Indeed, membranes separators tend to loose an important amount of light fraction in the permeate flow (the heavy fraction outlet) which is undesirable. In yet another embodiment of the invention, a device is provided wherein the first and third separating means are configured such that the feed connects to the inlet of the third separating means and the heavy fraction outlet line of the third separating means reconnects to the inlet of the first separating means to provide a feedback loop. The light fraction outlet of the first separating means then preferably connects to the inlet of the third separating means. This embodiment also improves efficiency by reducing the loss of the third heavy fraction.

All embodiments described above may provide synergy between the first, second and third separating means, as will be elucidated further below in more detail.

An embodiment that may increase selectivity and efficiency further is provided by a device that comprises feed means for feeding a separation enhancing agent to an inlet of one or more of the separating means of the device. A separation enhancing agent supports separation in two or more fractions and may for instance comprise an agent that prevents actual solids formation ('freezing out') of one of the fractions in the mixture from occurring. This allows separating means of the device to operate at optimized conditions of pressure and temperature, where separation has the highest possible selectivity, and yet avoid freezing of one of the fractions. The agent may be an external additive, or in the alternative, may be one or more recycled components from a heavy fraction exiting from the device.

An embodiment of the invention providing a device wherein the feed means are configured for feeding a separation enhancing agent to an inlet of the third separating means is preferred.

Another embodiment of the invention relates to a device wherein the third separating means and a fourth separating means are configured such that a heavy fraction outlet of the third separating means connects to an inlet of the fourth separating means, and a light fraction outlet of the fourth separating means reconnects to the feed to provide a feedback loop for the light fraction exiting the fourth separating means. In an embodiment comprising feed means for a separating agent, the third and the fourth separating means are preferably configured such that the heavy fraction outlet of the fourth separating means reconnects to the inlet of the third separating means to provide a feedback loop for the separating agent. The separating means of the device in accordance with the invention may be any means known in the art. It is for instance possible that one or more of the separating means comprise chemical separating means, such as an amine treatment separator. Amine treatment separators as stand alone unit typically consume a lot of energy and require capital intensive and huge installations. Another possibility is to use membrane separators. Membrane separators as stand alone unit may yield an acceptable selectivity but loose quite some light fraction through its permeate flow. Yet another possibility is to use distillation columns. These also involve large installations that operate at high pressure and low temperature. Distillation may allow producing a heavy fraction with high selectivity. The lighter fraction however usually contains an important amount of heavy fraction.

A useful embodiment of the invention provides a device wherein at least one of the separating means comprise rotating means for rotating the flowing mixture such that it separates in a light and a heavy fraction. The at least one separating means preferably comprises the first, the second and optionally the fourth separating means. Suitable rotating means comprise a cyclone, or a cascade of cyclones. In the case of a cyclone it is possible to give the separating means a stationary form and to set the medium mixture only in rotation. Baffles can optionally be placed in a cyclone, for instance for the purpose of causing a determined fraction to condense on the baffles and for controlling the cyclone. Cyclones are in particular useful separating means when separating a flowing medium mixture at relatively low flows, such as would typically occur when purifying biogas mixtures.

A particularly preferred embodiment in accordance with the invention provides a device wherein the rotating means comprise a rotating assembly of substantially parallel feed channels, such as the one disclosed in NL 1026299. Such rotating separators have the advantage that the average distance of a medium particle to a wall (in radial direction) is limited, whereby a desired degree of separation can be achieved in a relatively short time (which corresponds to a relatively limited length of the rotating separator in axial direction). The operation of such a rotating assembly of feed channels is further positively influenced if a laminar flow of the medium is maintained in the channels. Conversely, it is also possible for the medium to be carried through the channels with turbulent flow. The flow speeds to be applied can be varied or optimized according to the situation.

Another preferred embodiment of the invention relates to a device wherein the third separating means comprise a membrane separator. A membrane separator is known per se and typically comprises a filter unit in a casing. The filter unit comprises a membrane which surface is selectively permeable for some fraction of the medium mixture only. The medium mixture to be separated is fed under pressure to an inlet of the casing and carried through the filter unit, which unit effectuates retention of some of the fractions and transmittance of other fractions. The retentate (also termed light fraction in the context of this application) is carried to a light fraction outlet, whereas the permeate (or heavy fraction) is led to a high fraction outlet and discarded or treated further. A filter unit membrane typically comprises hollow fibers.

A membrane separator generally incurs limited energy loss but is relatively large in size.

Membrane separators also tend to lose quite some amount of separated light fraction through the permeate flow. The device according to the invention in which a particular combination of first, second, third and optionally fourth separating means is used limits or even completely eliminates loss of separated fractions.

Particularly preferred according to one embodiment is a device wherein the first and second separating means comprise a rotating assembly of substantially parallel feed channels, and wherein the third separating means comprise a membrane separator. This embodiment allows to substantially reducing the volumetric size in m ³ of the membrane separator, preferably with 10%, more preferably with 20% and most preferably with 30% relative to the size of a stand alone membrane separator.

Yet another particularly preferred embodiment comprises a device wherein the first, the second, the third and the fourth separating means comprise a rotating assembly of substantially parallel feed channels, and the device optionally further comprises feed means configured for feeding a separation enhancing agent to the inlet of the third separating means.

To optimize the separation, an embodiment of the invention provides a device comprising means for physically influencing the difference in mass density of the fractions to be separated, which mass density influencing means connect upstream of at least one of the separating means. The separation efficiency of the separating means is increased further according to these preferred embodiments by influencing the mass density of at least a part of the mixture before the medium reaches the separating means such that the differences in mass density of the fractions to be separated are preferably increased.

Increasing the difference in mass density of the fractions to be separated can take place in an embodiment that provides a device wherein the mass density influencing means comprise a cooler and/or a heater. Changing the temperature of the mixture may for instance cause a phase change of one of the fractions in the medium mixture. It is then easier to separate the fractions from each other, for instance by means of rotation, as a result of the increased difference in centripetal forces exerted on the fraction.

Another embodiment relates to a device wherein the mass density influencing means comprise an expander and/or a compressor. An expander allows decreasing the temperature of a medium within a very short period of time, whereas a compressor effectuates the opposite. Expansion is preferably realized by providing an expansion cooler of the "Joule Thomson" type. In such an expansion cooler, the medium mixture is cooled isenthalpically, whereby the pressure can be decreased substantially independent from the temperature. Another option is that the cooling is brought about by a cooling medium, which is for instance expanded in a separate circulation system so as to bring it to the desired low temperature level. The expansion may also be performed isentropically (or adiabatically) using a turbine. In an expander pressure and temperature are usually decreased together. Working with a separate cooling medium, compared to expansion of the medium for separating, allows optimizing the separate cooling medium in view of the desired cooling action.

The ensuing temperature decrease is responsible for affecting the density of the fractions.

Particularly favourable effects can be achieved if the mixture consists of fractions with the same phase (for instance a gas/gas mixture or a liquid/liquid mixture), at least one fraction of which undergoes a phase change due to the temperature change such that the phases of the fractions for separating differ from each other (whereby for instance a gas/liquid mixture, a gas/solid mixture or a liquid/solid mixture results). It is noted however that it is not necessary to create a phase difference between the components for separating for the device to operate properly. The device is equally applicable for separating fractions which are and remain in the same phase (for instance liquid/liquid mixtures such as a dispersed liquid and gas/gas mixtures).

The active or passive cooling means can be deployed directly to increase the difference in mass density of the fractions for separating. In a particularly advantageous application, the cooling means are disposed upstream of the expansion means in the direction of flow of the medium for the separating means. The mixture is thus first cooled down before the expansion begins and then reaches a much lower temperature level as a result of the expansion, thereby creating extra options for separating the fractions. Pre-cooling may also take place, for instance by dissipation into the environment.

The invention also relates to a method for separating a flowing medium mixture into fractions with differing mass density. The invented method advantageously uses the device described herein and comprises the steps of supplying a medium mixture to be separated in final light and heavy fractions to an inlet of one of interconnected separating means; treating an inlet mixture in each of first, second and third separating means such that it separates in light and heavy fractions for each separating means; discharging said light and heavy fractions for each separating means; wherein the heavy fraction exiting from the first separating means is treated in the second separating means, and the light fraction exiting from the second separating means is fed back to the inlet of the first separating means for further separation; the method further comprising treating the light fraction exiting from the first separating means in the third separating means and/or treating the heavy fraction exiting from the third separating means in the first separating means. As already stated above, it has advantages when the difference in mass density of the fractions to be separated in the medium mixture is physically increased prior to causing separation thereof in the first, second and/or third, and optionally fourth separating means. Such increase is preferably effectuated by causing an expansion of the medium mixture, or decreased by causing a

compression thereof. Other preferred embodiments comprise increasing or decreasing the difference in mass density of the fractions in the mixture and/or of fractions to be further separated by cooling or by heating. Cooling is preferably performed before expansion.

Preferably, separation of the flowing medium mixture and/or of a fraction thereof is carried out by causing rotation thereof in rotating means, more preferably in a rotating assembly of substantially parallel feed channels. The third separation is in another preferred embodiment carried out in a membrane separator.

Particularly preferred embodiments relate to a method wherein the mixture to be separated is supplied to the first separating means and the light fraction exiting from the first separating means is supplied to and treated in the third separating means; and/or wherein the heavy fraction exiting from the third separating means is fed back to the flowing mixture to be separated; and/or wherein the heavy fraction exiting from the third separating means is fed back to the flowing mixture to be separated; and/or wherein the mixture to be separated is supplied to the third separating means and the heavy fraction exiting from the third separating means is fed back to the inlet of the first separating means.

Another embodiment of the method provides supplying a separation enhancing agent to an inlet of a separating means, preferably the third separating means, which separation enhancing agent is more preferably recycled and re-fed from a heavy fraction outlet of a separating means to an inlet of a separating means. In other preferred embodiments of the method, the heavy fraction exiting from the third separating means is supplied to an inlet of a fourth separating means, and a light fraction exiting from the fourth separating means is fed back to the flowing mixture to be separated; and/or the heavy fraction exiting from the fourth separating means and comprising an amount of separating agent is fed back to the inlet of the third separating means to recover said amount.

Separation of a medium mixture may be enhanced by providing the right conditions of pressure and temperature for a particular mixture composition. A (p, T) diagram of the medium mixture (a diagram of the pressure against the temperature) is generally characterized by a range where the fractions of the medium mixture form one phase (the mixing range) and a more or less closed range where at least a part of the fractions form a distinct phase (demixing range). A gaseous range, a liquid range and a solid range are generally further distinguished, wherein the gaseous range is located on average at the higher temperatures and the solid range, conversely, at low pressure and temperature. A number of lines demarcate these ranges, in particular a liquid line which indicates the boundary between combinations of pressure and temperature under which (in addition to other phases) a liquid phase also occurs, and a solid line which indicates the boundary between combinations of pressure and temperature under which (in addition to other phases) a solid phase also occurs. The position of a medium mixture in a (p, T) diagram, and therefore its phase, can be influenced by cooling/heating means and/or by expansion/compression means, as has been described above. A preferred target point prior to separating the medium mixture in a rotating assembly of substantially parallel feed channels is located below the liquid line of the phase diagram, and relatively close to the intersection of the solid and the liquid line.

The method according to the invention can be performed with a relatively small through-flow device since the separate processing steps can be carried out within a very short period of time, for instance individually in less than 1 second, usually in less than 0.1 second or even in less than 10 or less than 5 milliseconds. This makes lengthy processes, with associated devices which are dimensioned such that they can contain large volumes, unnecessary. The device and method according to the invention are particularly useful for cleaning natural gas, in particular for separating methane (CH4, the light and useable fraction) from carbon dioxide (C02, the heavy and contaminated fraction). Gas streams obtained from natural gas wells often contain relatively high amounts of carbon dioxide which lowers the heating value of the gas and is highly corrosive. In such situations, carbon dioxide must be removed from the gas stream in order to meet the carbon dioxide specification for saleable gas products. Additionally, carbon dioxide needs to be separated from gas mixtures caused by injecting carbon dioxide -containing gases into oil wells for enhanced oil recovery.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be further elucidated on the basis of the non-limitative exemplary embodiments shown in the following figures. In the figures:

Figure 1 shows a schematic view of a device according to an embodiment of the invention;

Figure 2 shows a schematic view of a device according to another embodiment of the invention; Figure 3 shows a schematic view of a device according to yet another embodiment of the invention;

Figure 4 shows a schematic diagram of the required volume of a membrane separator versus C02 feed concentration; and

Figure 5 shows a schematic diagram of the CH4 fraction lost in separating a natural gas using a state of the art device, and using embodiments of the invented device.

DETAILED DISCLOSURE OF THE INVENTION

Referring to figure 1 , a device 1 is shown for purifying a contaminated gas such as for instance natural gas, in which device 1 embodiments of the method according to the invention can be performed. A heavily contaminated gas, for instance comprising 50 mole CH4 and 50 mole C02, is supplied according to arrow PI to a feed 2 at a pressure of between 50 and 150 Bar for instance, and at a temperature of typically more than 100°C. The gas may have been pre -treated prior to entering the feed 2, if desired. The gas supplied according to arrow PI is then cooled in a heat exchanger 3, for instance by means of cooling into the atmosphere. The cooling will typically be such that the natural gas is brought to a temperature which is lower than the critical temperature of the gas, and in which the gas is in the (supercritical) liquid phase. A suitable temperature is for instance around -30°C. The cooled gas then flows through feed 2 according to arrow P2 to an expander 4, provided downstream of the cooler 3. The liquid gas is isentropically expanded by means of expander 4 to a lower pressure, for instance between 25 and 45 Bar. As a result of the sudden fall in pressure, the temperature of the liquid gas will reduce further to about -50°C, such that a part of the fractions present in the liquid gas changes phase. In particular, at least a part of the useable constituent CH4 of the liquid natural gas will enter the gaseous phase due to the expansion, and the methane vapor fraction in the liquid/vapor gas mixture is maximized. The contaminating C02 fraction remains in the liquid phase. As a result, a mixture is created comprising CH4 gas bubbles in a liquid C02 matrix, or, alternatively, comprising C02 liquid droplets in a gaseous CH4 matrix.

This medium mixture is then supplied through feed 2 according to arrow P3 to first rotating means in the form of a rotor 5, comprising g a plurality of substantially parallel feed channels. The liquid/gas bubble (or gas/droplet) mixture is carried through the channels of the rotor 5 whereby, as a result of the rotation of rotor 5, the heavier liquid phase (the droplets or the liquid matrix) is forced radially outwards towards the sides of the feed channels, whereas the gaseous phase (the bubbles or gaseous matrix) remains closer to the centerline of the channels. The gaseous CH4 leaves rotor 5 on the side remote from expander valve 4 and is discharged through a light fraction outlet line 5c of the rotor 5 according to arrow P4 in the form of a useable fraction (cleaner natural gas). This useable fraction exiting from the first separating means comprises about 85 mole CH4 and 15 mole C02 in a typical example. The light fraction exiting from the first rotor 5 ('the first light fraction') is then fed to third separating means in the form of a membrane separator 6 that connects to the light fraction outlet 5c of the first separating means 5. The first light fraction is further purified in the membrane separator 6 and leaves the separator 6 as a third light fraction (or retentate) in accordance with arrow P6 through light fraction outlet line 6c of the third separating means 6. The light fraction leaving the membrane separator 6 (the 'third light fraction') is the final light fraction in the form of purified CH4 gas. The third heavy fraction (or permeate) leaves the separator 6 through heavy outlet line 6w in accordance with arrow P5. The retentate typically comprises 97-99 mol CH4, which illustrates the high selectivity achievable with the device according to the invention. This high selectivity may be achieved by providing a feedback loop between the heavy outlet of the rotor 5 to the inlet feed 2 of the rotor 5, preferably combined with another feedback loop provided between the heavy outlet 6w of the membrane separator 6 and the feed 2, in accordance with arrow P5.

As shown in figure 1 , the first feedback loop comprises the heavy outlet 5w of the first rotor 5 and second separating means in the form of second rotor 9, the light outlet 9c of which reconnects to feed line 2 through a three-way valve 12. The first heavy fraction which typically comprises 65 mole of liquid C02 is discharged through a heavy fraction outlet 5w according to arrow P7, optionally cooled in a heat exchanger 7, and expanded by means of expander 8, before entering the second rotor 9 of substantially parallel feed channels according to arrow P8. The liquid/gas bubble or gas/droplet mixture is carried through the channels of the rotor 9 whereby, as a result of the rotation of rotor 9, the heavier liquid fraction preferentially moves radially outwards towards sides of the feed channels. The light fraction (gaseous methane CH4) leaves rotor 9 on the side remote from the expander 8 and is discharged as a second light fraction according to arrow P9. The C02- rich contaminated liquid mixture is discharged as second heavy fraction according to arrow P10 through a heavy fraction outlet 9w of the second rotor 9. The device allows obtaining a liquid C02-rich fraction which is advantageous since it can be easily discharged or transported to other equipment for storage for instance. According to the invention, the second light fraction is re -fed according to arrow P9 to the first rotor 5, optionally after having been compressed in compressor 10 and heated in heat exchanger 11 to bring the second light fraction in optimal pressure and temperature conditions for separating in the first rotor 5.

The second feedback loop comprises the heavy fraction outlet 6w of the membrane separator 6 , which outlet 6w carries the heavy fraction exiting from the membrane separator 6 back towards the feed 2, in accordance with arrow P5. Feed 2 reconnects the recycled heavy fraction of the membrane separator 6 to the first rotor 5, eventually using a three-way valve (not shown) for further separation. To bring the heavy fraction of the membrane separator in optimal pressure and temperature conditions for separating in the first rotor 5, the outlet 6w is preferably equipped with a compressor 60 and a heat exchanger 61. The second feed-back loop reduces loss of CH4 in the membrane separator 6. Referring to figure 2, another embodiment of the device 1 in accordance with the invention comprises an inlet feed 2 through which a contaminated gas, for instance comprising 80 mole CH4 and 20 mole C02, may be supplied according to arrow PI. The gas supplied according to arrow PI may be cooled in a heat exchanger or expander, but is supplied, as shown, to third separating means in the form of membrane separator 6 that connects to the inlet feed 2. The contaminated gas is purified in the membrane separator 6 and leaves the separator 6 as a third light fraction (or retentate) in accordance with arrow P6 through a light fraction outlet 6c of the membrane separator 6. The light fraction leaving the membrane separator 6 (the 'third light fraction') is the final light fraction in the form of purified CH4 gas. The third heavy fraction (or permeate) leaves the separator 6 through heavy outlet line 6w in accordance with arrow P5. A high selectivity is partly achieved by providing a feedback loop between the heavy outlet 6w of the separator 6 and the feed 2. The third heavy fraction which contains lost CH4 gas in a C02 matrix is fed to the feedback loop according to arrow P5 to first rotating means 5 in the form of a rotor 5 of substantially parallel feed channels. Prior to entering the rotor 5, the third heavy fraction may be cooled in cooler 3 and/or expanded in expander 4 to phase-change the CH4 fraction. In rotor 5, the heavier fraction is forced radially outwards towards the side of the channels whereas the lighter CH4-rich gas phase fraction leaves rotor 5 on the side remote from the expander 4 and is discharged through a light fraction outlet 5c of the first rotor 5 according to arrow P4. As further shown in figure 2, another feedback loop comprises the heavy outlet line 5w of the first rotor 5 and second separating means in the form of a second rotor 9 of parallel feed channels, the light outlet line 9c of which reconnects to feed line 5i of the first rotor 5, optionally through a three-way valve (not shown). The first heavy fraction is discharged according to arrow P7, optionally cooled in a heat exchanger 7, and expanded by means of expander 8 or optionally a Joule -Thompson unit, before entering the second rotor 9 according to arrow P8. After separation in rotor 9, the light fraction (gaseous methane rich gas) leaves rotor 9 on the side remote from the expander 8 and is discharged as a second light fraction according to arrow P9, and re -fed to inlet 5i of the first rotor 5. C02-rich contaminated liquid mixture is discharged as second heavy fraction according to arrow P10 through a heavy fraction outlet 9w of the second rotor 9. Rotor 9 may also be a cyclone. The heavy fraction leaving the rotor 9 (the 'second heavy fraction') is the final heavy fraction in the form of contaminated C02-rich liquid.

Referring to figure 3, a third embodiment of the device and method in accordance with the invention is shown for cleaning a contaminated gas, for instance biogas. The highly contaminated gas, which for instance comprises 50 mole CH4 and 50 mole C02, is supplied according to arrow PI to a feed 2. The gas may have been pre-treated prior to entering the feed 2, if desired. The gas supplied according to arrow PI is then cooled in a heat exchanger 3, for instance by means of cooling into the atmosphere. The cooling will typically be such that the natural gas is brought to a temperature which is lower than the critical temperature of the gas, and in which the gas is in the (supercritical) liquid phase. A suitable temperature is for instance around -30°C. The cooled gas then flows through feed 2 according to arrow P2 to an expansion valve 40, provided downstream of the cooler 3. The liquid gas is expanded to a lower pressure of typically between 25 and 45 Bar, for instance 40 Bar. As a result of the sudden fall in pressure, the temperature of the liquid gas will reduce further to about -60°C, such that a part of the fractions present in the liquid gas changes phase. In particular, at least a part of the useable constituent CH4 of the liquid natural gas will enter the gaseous phase due to the expansion, and the methane vapor fraction in the liquid/vapor gas mixture is maximized. The contaminating C02 fraction remains in the liquid phase. As a result, a mixture is created comprising CH4 gas bubbles in a liquid C02 matrix, or, alternatively, comprising C02 liquid droplets in a gaseous CH4 matrix. This medium mixture is then supplied through feed 2 according to arrow P3 to first rotating means in the form of a rotor 5, comprising g a plurality of substantially parallel feed channels. The liquid/gas bubble (or gas/droplet) mixture is carried through the channels of the rotor 5 whereby, as a result of the rotation of rotor 5, the heavier liquid phase (the droplets or the liquid matrix) is forced radially outwards towards the sides of the feed channels, whereas the gaseous phase (the bubbles or gaseous matrix) remains closer to the centerline of the channels. The gaseous CH4 leaves rotor 5 on the side remote from expander valve 40 and is discharged through a light fraction outlet line 5c of the rotor 5 according to arrow P4 in the form of a useable fraction (cleaner natural gas). This useable fraction exiting from the first separating means comprises about 85 mole CH4 and 15 mole C02 in a typical example, as well as a minor amount of a separating agent, as will be explained further below.

As shown in figure 3, a first feedback loop aims at an optimal C02 recovery, and comprises the heavy outlet 5w of the first rotor 5 and second separating means in the form of second rotor 9, the light outlet 9c of which reconnects to feed line 2 through a three-way valve 12. The first heavy fraction which typically comprises 65 mole of liquid C02 is discharged through a heavy fraction outlet 5w according to arrow P7, optionally cooled in a heat exchanger 7, and expanded by means of expander 8, before entering the second rotor 9 of substantially parallel feed channels according to arrow P8. The liquid/gas bubble or gas/droplet mixture is carried through the channels of the rotor 9 whereby, as a result of the rotation of rotor 9, the heavier liquid fraction preferentially moves radially outwards towards sides of the feed channels. The light fraction (gaseous methane CH4) leaves rotor 9 on the side remote from the expander 8 and is discharged as a second light fraction according to arrow P9. The C02-rich contaminated liquid mixture is discharged as second heavy fraction according to arrow P10 through a heavy fraction outlet 9w of the second rotor 9. The device allows obtaining a liquid C02-rich fraction which is advantageous since it can be easily discharged or transported to other equipment for storage for instance. According to the invention, the second light fraction is re -fed according to arrow P9 to the first rotor 5, optionally after having been compressed in compressor 10 and heated in heat exchanger 11 to bring the second light fraction in optimal pressure and temperature conditions for separating in the first rotor 5.

The light fraction exiting from the first rotor 5 ('the first light fraction') is then fed to a third separating means in the form of third rotor 30, an inlet of which connects to the light fraction outlet 5c of the first rotor 5. The first light fraction is further purified in the third rotor 30 and leaves the rotor 30 as a third light fraction in accordance with arrow P6 through light fraction outlet line 6c of the third rotor 30. The third rotor 30 acts as a de-methanizer and the light fraction leaving the rotor 30 (the 'third light fraction') is the final light fraction in the form of purified CH4 gas. Supply to the inlet of the third rotor 30 is performed under pressure and temperature conditions that are influenced by a cooler 32 and an expansion valve 33, that are both incorporated in the light fraction outlet 5c of the first rotor 5. Pressure and temperature conditions are such that solid C02 would normally coexist with a vapor and liquid mixture of methane CH4 and C02. At these conditions, C02 would freeze out of solution and could potentially plug up device components, which is undesirable. At higher pressures, where C02 does not freeze out, methane -rich mixtures may become supercritical fluids that are less apt to further purification. Supercritical fluids exist at pressures above the critical point in a phase diagram and may comprise supercritical liquids or gasses.

A process which has become known as the "Ryan/Holmes" process can be employed to separate a feed gas mixture containing methane and carbon dioxide in a distillation column into a methane overhead product and a carbon dioxide bottoms product. The Ryan/Holmes process involves the operation of a distillation column at temperatures, compositions and pressures which produce a solids potential zone for carbon dioxide within the tower. The term "solids potential zone" is employed with the Ryan/Holmes process because, although conditions in the tower are such that carbon dioxide solids would normally occur, the Ryan/Holmes process prevents actual solids formation from occurring. This is achieved by introducing into the upper portion of the distillation column an agent for preventing acid gas solids. The agent can be an external additive, or in the alternative, can be one or more recycled components from the bottoms product taken from the distillation column.

The solution provided by the Ryan Holmes process is used in combination with a device in accordance with the present invention, and this embodiment has proven to give excellent results. To achieve the effect, a separation enhancing agent such as butane for instance is supplied to the gas mixture through tree-way valve 34 according to arrow P20, such that a mixture of 68 mole CH4, 12 mole C02 and 20 mole butane is achieved at the inlet of third rotor 30 at a typical pressure of 25-35 Bar and a temperature of -80°C and lower. As a result of adding the butane, a gas mixture is created with a liquid C02 phase and a gaseous CH4 phase under conditions that C02 would normally freeze out. Although butane has been used as anti-freeze agent, other even lower molecular weight carbohydrates such as ethane and propane for instance may be preferred, among others. The third heavy fraction that contains up to 20 mole of butane or more leaves the rotor 30 through heavy outlet line 6w in accordance with arrow P21and is led to a fourth separating means in the form of a fourth rotor 31. Before entering rotor 31, the third heavy fraction may be subjected to an expansion in expansion valve 35 to influence pressure and/or temperature conditions. Rotor 31 separates its inlet mixture in a heavy fraction that exits through outlet line 1 lw according to arrow P22 and is re-fed to the third rotor 30 according to arrow P22. The fourth heavy fraction comprises butane that is used as a recycled supply of butane to the mixture flow to be separated in rotor 30. The fourth heavy fraction may be led through a compressor pump 38 and a heat exchanger 39 before re-joining the light fraction outlet 5c of the first rotor 5 through three-way valve 50. The light fraction that exits through a light fraction outlet line 1 lc of the fourth rotor 31 according to arrow PI 5 is re-fed to the feed line 2 of the first rotor 5, before which it may be compressed and/or cooled in compression means 36 and heat exchanger 37 to bring the mixture in the desired pressure and temperature conditions. The feed-back loop of the third and fourth rotors (30, 31) allows to recover butane and aims at recovering as much butane as possible, such that the light fraction exiting the fourth rotor 31 comprises minor amounts of butane only, preferably lower than 1 mole of butane.

The advantages of the invention are illustrated by reference to figures 4 and 5. In figure 4, the x- axis exhibits the C02-concentration in mole % in a natural gas mixture to be cleaned. Heavily contaminated gas has C02-concentrations at the right hand side of the x-axis, while cleaner gas has C02 -concentrations at the left hand side of the x-axis. In a conventional membrane separation process, the required membrane volume per feed flow in m3/kg.sec-l, as set out on the yl-axis of the graph of figure 4 rapidly rises with C02-concentration and decreases only slowly after a maximum, as shown by curve 40. The required membrane volume in a combination of a rotating assembly of parallel feed channels in series with a membrane separator is shown by curve 41. The required membrane volume of a embodiment in accordance with figure 2 is shown by curve 42, whereas the required membrane volume of an embodiment in accordance with figure 1 in which an inlet of a membrane separator connects to the light fraction outlet line of a first rotor, and wherein the heavy fraction outlet line of the membrane separator reconnects to the feed line of the first rotor to provide a feedback loop is shown by curve 43. Obviously, the required membrane volume to achieve the same selectivity is much lower for a device in accordance with the invention than for a stand-alone membrane separator, at least for CO-2 concentrations above 18 mole .

A typical feed flow in a stand alone state of the art separator, such as a membrane, is 100 kg.sec \ It is easily inferred form figure 4 that a typical size of a stand alone membrane separator (curve 40) for a C02-concentration of little more than 50 mole for instance is then about 300 m ³ . A device in accordance with the invention allows to reduce the size of the membrane separator for the same C02-concentration to about 200 m ³ , i.e. a size reduction of 33%.

Yet another advantage is illustrated in figure 5, in which figure the C02-concentration in a natural gas is shown along the x-axis, and the fraction of CH4 lost in separating CH4 from the gas mixture is shown along the y2-axis. A stand-alone membrane separator loses up to 13 % of CH4 in the permeate fraction. Embodiments of the device according to the invention do not loose much CH4, and the loss is restricted to about 8% for heavily contaminated gas. The device and method according to the invention allow obtaining a CH4 purity/waste ratio of

98/2. Known separation technologies such as a membrane separator loose a lot of CH4 resulting in a relatively low CH4 purity/waste ratio of typically 84/16. The fraction CH4 lost to waste in the device and method of the invention is typically limited to 1-4 mole%. Process energy consumption is also low and typically around 0,5-1,5% of the gas energy value. A known technology such as amine separation consumes up to 50-60% of the gas. A relatively small process size is yet another advantage. The device according to the invention requires a process size of 1-10 m3/kg.s-l, whereas distillation requires 50 m3/kg.s-l, and amine separation requires even 120 m3/kg.s-l and more. The above advantages are in particular notable for heavily contaminated gas fields, for instance for 50/50 mole% CH4/C02 mixtures.

A device according to the invention yields a high purity CH4 at low waste, with low energy consumption, and a small process size.

The device and method according to the invention can be used for many applications. Any separation of hydrocarbons can in principle be separated, wherein the fractions for separating preferably differ in vapor point. It is thus possible to apply the method for the purpose of purifying natural gas as has been described at length above. It is also possible to apply the invention for cracking naphtha, wherein the above described device can be used as substitute for the usual distillation column. It is also possible to apply the method to separate and purify polyolefins and other polymers. Particularly preferred is to apply the method and device for enrichment of air, i.e. purifying air by augmenting the amount of oxygen in air at the expense of the amount of nitrogen. Such an oxygen-enriched air can be used advantageously as feed in burning installations. Since the amount of oxygen in the purified feed is high, the amount of C02 in the burned mixture will also be high, which facilitates separation thereof. Other preferred applications of the method and device of the invention comprise separating the wet components out of natural or biogas, in particular separating pentane from methane; separating C02 from coal conversion processes; separating C02 from N2; and separating C02 from H2.