WO2008051079A1

WO2008051079A1 - Method for separating a medium mixture into fractions

Info

Publication number: WO2008051079A1
Application number: PCT/NL2007/050518
Authority: WO
Inventors: Raichel Elton Taciano Leito
Original assignee: Romico Hold A.V.V.
Priority date: 2006-10-27
Filing date: 2007-10-26
Publication date: 2008-05-02
Also published as: NL2000292C2

Abstract

The invention relates to a method for separating a medium mixture into at least two fractions with differing mass density. The method provides a mixture for separating, subjects the medium mixture to a volume force, and discharges at least one of the separated fractions, with the proviso that, prior to being subjected to the volume force, the medium mixture is cooled to below the temperature associated with the critical point of the phase diagram of the medium mixture. After cooling the difference in mass density of the fractions of the mixture for separating is increased by expansion of the mixture to a determined final pressure and final temperature, whereby at least one of the fractions for separating present in the medium mixture changes phase.

Description

Method for separating a medium mixture into fractions

The invention relates to a method for separating a medium mixture into at least two fractions with differing mass density. Such a method generally comprises the processing steps of A) providing a mixture for separating, B) subjecting the medium mixture to a volume force, and C) discharging at least one of the separated fractions.

The separation of a (flowing) medium mixture has many diverse applications. A medium mixture is here 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 for instance known from various applications of liquid cleaning, (flue) gas cleaning and powder separation. Separation of fractions with a great difference in particle size and/or a great difference 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 chemical separating techniques and/or separating techniques such as sedimentation and centrifugation. Certainly when processing large volumes of medium mixture, chemical separating techniques 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. Another per se known technology makes use of the differences in mass density of the fractions for separating by applying a centrifugal force to the mixture by causing the mixture to rotate in a centrifuge or a cyclone. This technique is not usually sufficiently selective to realize a separation of the desired level in a short time.

The present invention has for its object to provide a method which has an increased selectivity for the fractions for separating, and which also enables a rapid separation.

The invention provides for this purpose a method according to the preamble which is further characterized in that prior to step B) the medium mixture is cooled in a step D) to a temperature lower than the critical point of the phase diagram of the medium mixture, and in a step E) the difference in mass density of the fractions of the mixture for separating is then increased by expansion of the mixture to a final pressure and final temperature at which at least one of the fractions for separating present in the medium mixture changes phase. Surprisingly, by applying both measures a higher selectivity for at least one of the fractions for separating is achieved compared to the known method, in other words the said at least one fraction is separated in purer form, wherein the fractions desired in the purified medium mixture remain present in greater measure. The decrease in temperature to below the critical point can for instance be obtained by feeding the medium mixture to active or passive cooling means. Although not essential according to the invention, it is advantageous when the temperature is decreased at almost constant pressure. Because the temperature is reduced according to the invention to below the critical point, the medium mixture as a whole will undergo a phase separation. Some examples of possible applications of the present invention are the separation of an air/nitrogen mixture, de-aerating or degassing of water, dehydrating of air and the cleaning of natural gas. The method is preferably applied for the purpose of separating gas/gas mixtures, such as for instance natural gas, into fractions. The temperature decrease to below the critical point will in this case result in a phase change from the gaseous state to the liquid state. By first re-cooling the medium mixture before beginning the expansion in order to then reach an even much lower final temperature (and final pressure) as a result of the expansion, at least one of the fractions for separating will undergo a phase change from liquid to gas, in this case the most volatile fractions. This results in a medium mixture with a liquid matrix incorporating gas bubbles, in other words a bubbled structure. It has been found that such a mixture can be separated with enhanced selectivity, wherein the selectivity is higher than the selectivity which can be achieved by a method wherein the gas mixture is expanded from the gaseous phase.

The expansion can be performed according to the invention in any per se known expansion means suitable for the purpose. By means of expansion the temperature of a medium can be decreased within a very short period of time. Expansion is preferably realized by applying an expansion cooler of the "joule thomson" type. The medium mixture is cooled isenthalpically in such an expansion cooler, whereby the pressure can be decreased relatively independently of 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 be thus brought to the desired low temperature level. The expansion is preferably performed isentropically (or adiabatically) using a turbine. In such a cooling pressure and temperature are decreased together. The advantage of working with a separate cooling medium, compared to expansion of the medium for separating, is for instance that this separate cooling medium can be optimized for the desired cooling action. As already stated above, the temperature decrease is responsible for affecting the density of the fractions. Particularly favourable effects can thus be achieved if the mixture consists of fractions with the same phase (for instance 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). This phenomenon of phase change of a substance as a result of temperature change is of course a generally known phenomenon. The present invention is however based on the realization that a very advantageous separation becomes possible when the expansion is performed after the medium mixture has first been cooled to below the critical point, and is therefore a liquid. The combination of the described phase change (or in any case change in the difference in mass density of fractions for separating) and the subsequent subjecting of the medium mixture to a volume force (for instance by rotation) provides for a very advantageous separation of the mixture into at least two fractions.

In a preferred embodiment of the method according to the invention the medium mixture is brought in step D) to a temperature and pressure at which the medium mixture is substantially liquid. By then preferably also performing the expansion in step E) such that the medium mixture is brought to a final pressure and final temperature at which the medium mixture is a gas/liquid mixture, a separation is obtained in step B) with further increased selectivity. A medium mixture is more particularly obtained which can be separated into purer fractions than has heretofore been the case. Each fraction will in other words contain less of another fraction.

In a preferred embodiment of the method according to the invention the cooling and the expansion are performed such that the final temperature is a maximum of 50⁰C higher than the temperature which corresponds at the final pressure to the transition of a mixture fraction to substantially solid phase. A phase 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 high pressure and temperature 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 temperature which at the final pressure corresponds to the transition of a mixing fraction to substantially solid phase is thus the temperature as according to the intersection of the final pressure line and the solid line. The method according to the invention is even more preferably characterized in that the final temperature is a maximum of 20⁰C higher than the temperature which at the final pressure corresponds to the transition of a mixing fraction to the solid phase. The final temperature is most preferably substantially equal to the temperature which at the final pressure corresponds to the transition of a mixture fraction to the solid phase. By selecting the cooling and expansion such that the end point (the combination of obtained final pressure and final temperature) on the phase diagram is located as closely as possible to the solid line, a separation is obtained with further improved selectivity. It thus becomes even possible in principle to apply any separation technique, even those which do not provide high selectivity per se, such as for instance separation by means of gravitational force and/or a cyclone. The separation efficiency of the rotation means is increased according to the present invention by influencing the mass density of at least a part of the mixture before the medium reaches the rotation means such that the differences in mass density of the fractions for separating are increased. Increasing the difference in mass density of the fractions for separating can for instance take place by changing the temperature (heating or cooling subject to circumstances) of the mixture. It is thus simpler to separate the fractions from each other by means of rotation (as a result of the increased difference in centripetal forces exerted on the fraction). It is noted here that the separation of the fractions is understood to mean at least partial separation of two fractions such that a significant difference in the average mass density of the two fractions results; a complete (100%) separation will be difficult to realize in practice. As a consequence of the rotation of the mixture, now with increased differences in the mass density of the fractions for separating, the lighter fraction will migrate at least substantially to the inner side of the rotation and the heavier fraction will migrate at least substantially to the outer side of the rotation. This is a separation which increases the possible uses of at least one of the fractions compared to the mixture. Even after separation, this usable ("cleaned") fraction may still comprise a part of another undesired fraction (be contaminated with another fraction), although this other fraction will be significantly smaller than the presence of this undesired fraction in the original mixture. The successive steps of the method according to the invention result in an unexpectedly high separation efficiency without bulky equipment being required for this purpose (i.e. the device can be given a very compact form) and wherein the medium need only be treated for a short period. A device can be given an even smaller form (with a smaller volume) if the medium mixture comprises at least one gas fraction and the medium mixture is carried under higher pressure through the device.

In yet another preferred embodiment of the method according to the invention the medium mixture is brought in step D) to a temperature and pressure at which the medium mixture is substantially a liquid/solid mixture. By then preferably also performing the expansion in step E) such that the medium mixture is brought to a final pressure and final temperature at which the medium mixture is a gas/liquid/solid mixture, a separation is also obtained in step B) with further enhanced selectivity.

In a further preferred embodiment of the method according to the invention the cooling and the expansion are performed here such that the final temperature is at least 20⁰C lower than the temperature which at the final pressure corresponds to the transition of a mixture fraction to substantially solid phase. The method according to the invention is still more preferably characterized in that the final temperature is at least SO⁰C lower than the temperature which at the final pressure corresponds to the transition of a mixture fraction to the solid phase.

In a preferred variant of the method according to the invention of the medium mixture is subjected to gravitational force during processing step B). Such a separation technique is very simple and requires little energy and investment. In a further preferred variant the medium mixture is subjected to a centrifugal force during processing step B) by feeding the medium mixture to rotation means. The rotating means can for instance be formed by at least one cyclone (vortex), or alternatively by an assembly of a plurality of cyclones. In the case of a cyclone it is possible to give the rotating means a stationary form and to set only the medium into rotation. The application of a plurality of (smaller) cyclones has an advantage relative to a single cyclone which is comparable to the advantage of a rotating assembly of feed channels. 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.

A particularly favourable method according to the invention is characterized in that during processing step B) the medium mixture is set into rotating flow in rotation means provided for this purpose and comprising a rotating assembly of feed channels. Such rotary separators have the advantage that the average distance of the medium from a wall (in radial direction) remains limited, whereby a desired degree of separation can be achieved in a relatively short time (which corresponds to a relatively limited length of the rotary separator in axial direction). The operation of such a rotating assembly of feed channels is further influenced positively if a preferably 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. A particularly suitable rotary separator of the present type is described for instance in EP 028616OA, the content of which is expressly incorporated in the present application.

In particular preference the method according to the invention is applied by subjecting the medium mixture during processing step B) to gravitational force and/or to a centrifugal force, and/or setting the medium mixture into rotating flow in rotation means provided for this purpose and comprising a rotating assembly of feed channels. The combination of separation techniques can result in a further enhanced selectivity of the fraction(s) for separating and/or purity of the medium mixture with the contaminating fractions at least partially removed.

The method according to the invention can be performed with a relatively small throughflow 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 inventive combination of processing steps D), E) and S B) provides the unexpected advantage of a much simpler fraction separation than was possible according to the prior art. A simple method for increasing the difference in mass density of the fractions of the mixture for separating is based on causing the mixture to expand. The fall in temperature resulting herefrom provides the desired effect of increasing the difference in mass density of the fractions for separating within0 a very short time; this effect can be obtained in less than 0.1 or less than 0.05 second while making use of extremely simple means. The effect of increasing the difference in mass density of the fractions for separating is influenced further in positive manner according to the invention by cooling the mixture in step D) to below the critical point of the medium mixture before separating the mixture during processing step B). 5

A particular preferred application of the method according to the invention is characterized in that natural gas is provided during processing step A), that prior to processing step B) the temperature of the natural gas is lowered in a step D) to a temperature which is lower than the critical point of the phase diagram of the medium0 mixture, and the temperature of the natural gas is subsequently further lowered by expansion to a determined final pressure and final temperature, whereby at least one of the contaminating fractions present in the medium mixture, such as for instance CO₂ and H_∑S, changes phase, which fractions are separated from the fraction of hydrocarbons during processing step B) such that the fraction of hydrocarbons with5 contaminants at least partly removed is discharged during processing step C). The reserves of natural gas which can be recovered in economically cost-effective manner are limited since a significant part of the technically recoverable natural gas is contaminated with unwanted gases. Certainly when they occur in the natural gas in tens of percents, it has heretofore not been possible to separate these contaminating gases to0 a sufficient extent from the hydrocarbons in economically cost-effective manner. The method according to the invention does not have this drawback.

The present invention will be further elucidated on the basis of the non-limitative exemplary embodiments shown in the following figures. Herein: figure 1 shows a schematic view of a device according to the invention, figure 2 shows a schematic view of an alternative embodiment variant of a separating device according to the invention, and figure 3 shows an example of a phase diagram of a natural gas mixture to be separated with the method according to the invention.

Figure 3 shows a phase diagram of a contaminated gas such as for instance natural gas which can be cleaned with die invented method. This is more particularly the phase diagram of a CH₄/CO₂/H₂S mixture. The y-axis shows the pressure 100, while the temperature 200 is shown along the x-axis. The phase diagram further comprises a range (designated with G or V) where the fractions of the medium mixture form one phase (the mixing range) and a more or less closed range (designated with G+V, V+VS and G+V+VS) where at least a part of the fractions form a distinct phase (demixing range). In range G the medium mixture is gaseous, in range V the medium mixture is liquid. In range G+V a mixture is present of liquid and gas, wherein in the present case CO₂ and H₂S are in the liquid phase and CH₄ in the gaseous phase. Present in range G+V+VS is a mixture of gas, liquid and solid, more particularly CH₄ being in the gas phase, H₂S in the liquid phase and CO₂ in the solid phase. Although not indicated in figure 3, it is also possible for H₂S to change to the solid phase when the temperature falls further. A number of lines demarcate the relevant ranges, in particular a dew point line 110 which indicates the boundary between combinations of pressure 100 and temperature 200 below which (in addition to other phases) a liquid phase L also occurs, and a solid line 120 which indicates the boundary between combinations of pressure 100 and temperature 200 below which (in addition to other phases) a solid phase VS also occurs. The phase diagram shows a critical point 140, a concept generally known to the skilled person, at which the gaseous phase and liquid phase are in equilibrium with each other. The critical temperature is indicated in the phase diagram of figure 3 with Tent- It will be apparent that the phase diagram shown in figure 3 is given only by way of example, and that the method is likewise applicable for separating medium mixtures with more fractions, and therefore a more complicated phase diagram.

Referring to figure 1, a device 1 is shown for cleaning a contaminated gas such as for instance natural gas, in which device 1 the method according to the invention can be performed. The contaminated gas is supplied as according to arrow P₍ by a feed 2 under a pressure of between 100 and 300 Bar (usually a typical pressure of about 250 Bar) and at a temperature of fer instance more than 100⁰C. The gas supplied as according to arrow Pi is then cooled in a heat exchanger 3, for instance by means of cooling into the atmosphere. The cooling is such that the natural gas is brought to a temperature which is lower than the critical temperature T_cπt thereof, for instance to the temperature Ti indicated in figure 3. The gas is preferably cooled to temperature Ti at almost constant pressure. At temperature Ti the gas is in the liquid phase. For a typical natural gas containing a CH₄/CO₂/H₂S mixture the critical temperature will amount to about -50⁰C. A suitable temperature Ti is then for instance -6O⁰C. The thus cooled liquid flows from heat exchanger 3 as according to arrow P₂ to a throttle valve 4. The liquid supplied as according to arrow P₂ is expanded by means of throttle valve 4, preferably in isentropic manner, to a lower pressure of between for instance 5 and 20 Bar. This isentropic pressure and temperature decrease is indicated in figure 3 by means of broken line 130. As a result of the sudden fall in pressure the temperature of the liquid will fall back to a final temperature T₂ (and a corresponding final pressure p₂) such that a part of the fractions present in the liquid changes phase. According to the invention the final temperature T₂ is preferably relatively close to, for instance a maximum of 50⁰C higher than, the temperature which corresponds at the final pressure P₂ with the transition of the medium mixture to the solid phase which, referring to figure 3, is the temperature Ts. More particularly, at least a part of the main constituent CH₄ present in the liquid natural gas will enter the gaseous phase due to the expansion. The contaminating fractions of CO₂ and H₂S remain in the liquid phase. As a result a medium mixture is created with a liquid matrix incorporating gas bubbles, in other words a bubbled structure 5. This liquid/gas bubble mixture 5 is carried through channels 6 of a rotor 7 whereby, as a result of the rotation R of rotor 7, the gas bubbles condense against the sides of channels 6 of rotor 7 which are directed toward a rotation shaft 8. The condensed gas bubbles leave rotor 7 on the side remote from throttle valve 4 as a gas bubble flow 12 which will form substantially centrally due to the difference in mass density. The liquid matrix 9, which consists substantially of liquid CO₂ and H₂S, is collected in a basin 10 which can be emptied by means of activating a pump 11 such that the liquid CO₂ and H₂S are discharged as according to arrow P₃. The gas bubble flow 12 with the CO₂ and H₂S at least partially removed is extracted and leaves device 1 as according to arrow P₄ as cleaned gas. If desired, the liquid/gas bubble mixture 5 created by the method according to the invention can be separated beforehand by being subjected to gravitational force.

Referring to figure 3, it is also possible according to the invention to cool the gas to a temperature T₄. At this temperature T₄ the gas is in the liquid phase, wherein at least one fraction is in the solid phase. For a typical natural gas containing a CH₄/CO₂/H₂S mixture the temperature T₄ will for instance amount to -8O⁰C and lower. The thus cooled liquid/solid mixture flows from heat exchanger 3 as according to arrow P₂ to a throttle valve 4. The liquid/solid mixture supplied as according to arrow P₂ is expanded by means of throttle valve 4, preferably in isentropic manner, to a lower pressure of between for instance 5 and 50 Bar, preferably between 10 and 40 Bar. This isentropic pressure and temperature decrease is indicated in figure 3 by means of broken line 150. As a result of the sudden fall in pressure the temperature of the liquid/solid mixture will fall back to a final temperature T3 (and a corresponding final pressure P3) such that a part of the fractions present in the liquid/solid mixture changes phase, and more specifically becomes gaseous. More particularly, at least a part of the main constituent CH₄ present in the natural gas mixture will enter the gaseous phase due to the expansion. The contaminating fractions of CO₂ and H₂S will remain in the liquid and solid phase. As a result a medium mixture is created with a liquid matrix incorporating gas bubbles and solid particles. Referring to figure 1, this 3-phase mixture 5 is carried through channels 6 of a rotor 7 whereby, as a result of the rotation R of rotor 7, the gas bubbles condense against the sides of channels 6 of rotor 7 which are directed toward a rotation shaft 8. The condensed gas bubbles leave rotor 7 on the side remote from throttle valve 4 as a gas bubble flow 12 which will form substantially centrally due to the difference in mass density. The liquid matrix 9, which consists substantially of liquid and solid particles (CO₂ and H₂S) incorporated therein, is collected in a basin 10 which can be emptied by means of activating a pump 11 and discharged as according to arrow P₃. The gas bubble flow 12 with the CO₂ and H₂S at least partially removed is extracted and leaves device 1 as according to arrow P₄ as cleaned gas.

Figure 2 shows a separating device 20 to which a gas mixture for separating is supplied by a feed 21 as according to arrow Pj₀. In a connecting turbine 22 the mixture is compressed in order to then enable more efficient cooling of the gas mixture in a connecting heat exchanger 23. The pressure increased by compressor 22 also makes it possible to have the whole device 20 operate at a higher pressure level (for instance 10 to 50 Bar), whereby it can be given a more compact form than if this compression step were to be omitted. After cooling of the mixture in heat exchanger 23 to below the critical point T_ent, and more particularly to the range where a liquid/solid mixture results, this mixture is fed to a turbine 24. Owing to the pressure-decreasing effect resulting from turbine 24 the temperature of the mixture will decrease such that the most volatile fraction of the mixture changes to the gaseous phase. In this way a gas/liquid/solid mixture is created. According to the invention the final temperature T3 is preferably at least 50⁰C lower than the temperature which corresponds at the final pressure P₃ to the transition of the medium mixture to the solid phase. In the cyclone 25 connected to turbine 24 the gaseous fraction will move in the form of bubbles in opposite direction to the centrifugal force and collect in the centre of cyclone 25. The liquid flowing from baffles 26 with solid crystals therein is collected in a drip tray 27, and further discharged therefrom as according to arrow Pn. The gas fraction leaves the cyclone through a central outlet 28 as according to arrow Pn. A typical flow speed of the mixture is 1 to 15 m/s, more in particular 10 m/s, this subject to the pressure and the mixture ratios. If desired, the mixture can be further purified after leaving the cyclone, for instance by being fed to the channels of a rotor, as has been elucidated in the exemplary embodiment described in figure 1.

The method according to the invention can be used for many applications. Any separation of hydrocarbons can in principle form the subject matter of the invented method, wherein the fractions for separating preferably differ in vapour 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 method 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.

Claims

1. Method for separating a medium mixture into at least two fractions with differing mass density, comprising the processing steps of: S A) providing a mixture for separating,

B) subjecting the medium mixture to a volume force, and

C) discharging at least one of the separated fractions, characterized in that prior to step B) the medium mixture is cooled in a step D) to a temperature lower than the critical point of the phase diagram of the medium mixture,0 and in a step E) the difference in mass density of the fractions of the mixture for separating is then increased by expansion of the mixture to a final pressure and final temperature at which at least one of the fractions for separating present in the medium mixture changes phase. 5

2. Method as claimed in claim 1 , characterized in that the medium mixture is brought in step D) to a temperature and pressure at which the medium mixture is substantially liquid.

3. Method as claimed in claim 2, characterized in that the expansion in step E) is0 performed such that the medium mixture is brought to a final pressure and final temperature at which the medium mixture is a gas/liquid mixture.

4. Method as claimed in claim 3, characterized in that the final temperature is a maximum of 2O⁰C higher than the temperature which corresponds at the final pressure5 to the transition of a mixture fraction to the solid phase.

5. Method as claimed in claim 4, characterized in that the final temperature is a maximum of 50⁰C higher than the temperature which corresponds at the final pressure to the transition of a mixture fraction to the solid phase. 0

6. Method as claimed in claim 1, characterized in that the medium mixture is brought in step D) to a temperature and pressure at which the medium mixture is substantially a liquid/solid mixture.

7. Method as claimed in claim 6, characterized in that the expansion in step E) is performed such that the medium mixture is brought to a final pressure and final temperature at which the medium mixture is a gas/liquid/solid mixture.

8. Method as claimed in claim 7, characterized in that the final temperature is at least 2O⁰C lower than the temperature which at the final pressure corresponds to the transition of a mixture fraction to the solid phase.

9. Method as claimed in claim 8, characterized in that the final temperature is at least SO⁰C lower than the temperature which at the final pressure corresponds to the transition of a mixture fraction to the solid phase.

10. Method as claimed in any of the foregoing claims, characterized in that the expansion is performed adiabatically and/or isentropically.

11. Method as claimed in any of the foregoing claims, characterized in that the expansion is performed isenthalpically.

12. Method as claimed in any of the foregoing claims, characterized in that the medium mixture is subjected to gravitational force during processing step B).

13. Method as claimed in any of the foregoing claims, characterized in that the medium mixture is subjected to a centrifugal force during processing step B).

14. Method as claimed in any of the foregoing claims, characterized in that during processing step B) the medium mixture is set into rotating flow in rotation means provided for this purpose and comprising a rotating assembly of feed channels.

15. Method as claimed in any of the foregoing claims, characterized in that the medium mixture is subjected during processing step B) to gravitational force and/or to a centrifugal force, and/or the medium mixture is set into rotating flow in rotation means provided for this purpose and comprising a rotating assembly of feed channels.

16. Method as claimed in any of the foregoing claims, characterized in that natural gas is provided during processing step A), that prior to processing step B) the temperature of the natural gas is lowered in a step D) to a temperature which is lower than the critical point of the phase diagram of the medium mixture, and in step E) the natural gas is then brought by expansion to a final pressure and final temperature at which at least one of the contaminating fractions present in the medium mixture, such as for instance CO₂ and FhS, changes phase, which fractions are separated from the fraction of hydrocarbons during processing step B) such that the fraction of hydrocarbons with contaminants at least partly removed is discharged during processing step C).