EP2137475B1 - Method for cooling a cryogenic exchange line - Google Patents

Description

The present invention relates to a method of cryogenic separation, refrigeration or liquefaction of a fluid by means of an exchange line.

It is known to use cryogenics to fractionate a gas stream into at least two fluids of different composition, generally into a fluid said to be light, that is to say composed essentially of the most volatile constituents and into a so-called heavy fluid consisting essentially of the most easily condensable constituents. To do this, the mixture to be fractionated is cooled in an exchanger or in a set of exchangers called the exchange line until obtaining a diphasic liquid / vapor mixture extracted from said exchange line and separated into a liquid-vapor separation device. The steam can be cooled again until a new diphasic state is obtained and fractionated a second time.

By way of example, mention may be made of the fractionation of a hydrocarbon stream (C1, C2,..., Ci, Ci + 1,... Cn) into a fluid consisting essentially of the lightest hydrocarbons (methane C1 , C2 ethane, ..., Ci) and a second fluid consisting essentially of heavier hydrocarbons (Ci + 1, ... Cn). The term "essentially" is used to mean that a small fraction of the lighter compounds will generally be found in the heavy fraction and conversely a small portion of the heavier compounds in the vapor fraction.

This separation can be improved by placing trays in the two-phase separation system and by adding a reboiling and / or a stripping section in order to eliminate the lighter from the liquid fraction and / or a condenser and / or or by increasing the reflux to remove heavy from the vapor fraction. These methods are known to those skilled in the art and are not developed in the present invention. Thereafter, the term liquid-vapor separator will be used to encompass all equipment generating at least one liquid outlet and one gas outlet from at least one two-phase supply. This equipment can be of the vertical or horizontal gravity separator type, equipped or not with a mist eliminator, of the cyclone type, or of a distillation column ...

The liquid outlet may contain a small amount of bubbles driven by the speed of the liquid as the vapor outlet may contain liquid droplets or aerosols without departing from the scope of the invention.

Other applications include recovering a methane-rich fluid and a methane-depleted fluid from a source rich in various hydrocarbons. In this way, it is also possible to obtain several fluids such as a fraction rich in methane, a fraction rich in ethane or ethylene, and a C3 + fraction. This type of process makes it possible in particular to recover hydrogen with a purity of approximately 95% from a mixture of hydrogen and hydrocarbons, to eliminate a part of the nitrogen contained in gas rich in hydrocarbons. hydrocarbons. It also makes it possible to recover a very rich fraction of CO ₂ and a residual gas containing lighter constituents such as N ₂ , Argon, O ₂ ...

This fractionation may not be a goal in itself but only a means to provide cooling capacity for liquefying another fluid such as natural gas. In this case, the various separated fluids are recombined after heating, recompressed and reinjected into the exchange line. This is called a refrigeration cycle.

These applications have led to many developments in both process and technology. In particular, the exchangers may be of the coil type, tube exchanger and shell or preferably of the plate heat exchanger type. In the latter case, numerous improvements have been made to the exchange waves and the introduction of fluids, in particular two-phase fluids, into these exchangers in order to optimize the heat transfer.

All the percentages mentioned in the following are molar percentages.

An example of these units will now be described in relation to the Figure 1 . This example relates to obtaining hydrogen under pressure at a purity of 95% from a gaseous mixture under pressure containing about 70% hydrogen, 18% methane and 12% heavier hydrocarbons.

The mixture to be separated 1 is introduced at ambient temperature and under a pressure of 40 bar absolute into the plate heat exchanger 10 to be cooled via the exchange passages 11. At a first temperature level depending on the composition of the hydrocarbons, heavier and pressure, generally from -40 to -120 ° C, the fluid 1, then two-phase, is extracted from the exchanger and separated into its vapor fraction 2 and its liquid fraction 3 in the liquid-gas separator 30. The liquid fraction 3 is released via the expansion valve 50 to low pressure and revaporized in the exchange line via the exchange passages 13.

The vapor phase 2 enriched in hydrogen and methane is again cooled in the exchanger 20 via the passages 22, partially condensed and extracted to - 160 ° C. The vapor fraction 4 from the separator 40 constitutes the production of hydrogen at a content of 95 mol%. It is then reheated in the passages 24 and 14 of the exchangers 20 and 10.

The liquid fraction 5 consisting mainly of methane is expanded at low pressure in the valve 60, revaporized in the exchanger 20 (passages 25) and heated in the exchanger 10 (passages 15). The fluids 6 and 7 respectively associated with the exchangers 20 ( passages 26) and 10 (passages 17) may optionally be used as a refrigerating auxiliary. It may be external fluids such as liquid nitrogen from a storage or a nearby air separation device, or a fluid internal to the process, such as a fraction of the hydrogen produced, partially heated and then expanded in an expansion turbine and reinjected at the cold end of the exchanger 20.

It is also possible to promote the vaporization of methane 5 by injecting a small fraction of the hydrogen production. This is what the potential circuit comprising the expansion valve 70 represents.

It should be noted that the expansion valves 50 and 60 serve to relax liquids with a high pressure, here 40 bar abs., Until low pressure. It is therefore small valves.

It is common to use the notion of CV in talking about the size of the valves. Many books or documents give on the one hand the methods of calculation and on the other hand the CV of the valves available on the market. For the latter, it is conventional to indicate the full-open CV which makes it possible to determine the maximum flow rate that can pass through the valve under given operating conditions. By way of illustration and without going into the calculations, for a gas feed rate of the order of 10 000 Nm ³ / h, the CV of these valves would be less than 1.

It is the same for the optional valve 70 which serves to relax a tiny fraction of the hydrogen produced (a few percent maximum).

In a conventional way the cold setting of such a separation unit is either by free expansion of the gas to be treated or more generally by using an external cooling supply.

The process which makes it possible to obtain the normal operating conditions, here a first cut-off temperature between the exchangers 10 and 20 of -80 ° C. for example and a cold end temperature of -80 ° C., is called the cold-setting of the exchange line. 160 ° c to obtain the required purity from equipment at room temperature or ambient, if the exchange line has not had time to reach the ambient temperature.

The problem of a cooling using the only free expansion of the gas to be treated in the expansion valves 50, 60 and possibly 70 is that the total flow expanded is very low and therefore the cooling capacity obtained is itself very low. However, this cooling capacity is intended to cool the exchange line, ancillary equipment such as separators, to compensate for heat losses, ie exchanges with the outside environment ... Such cooling can take place. dozens of hours and even possibly not to achieve the desired operating point.

This occurs especially in the case where the thermal losses become, at a certain temperature level reached at the cold end, equal to the cooling capacity produced by free expansion. At this point, the cold setting stops and it is not possible to go beyond.

For this reason, it is common to use the cooling supply circuits 6 and 7 for example to hasten the cold setting. These passages 26 and 27 can be used permanently or only temporarily during the cooling phases. As indicated above, it is conventional to use liquid nitrogen at low or preferably medium pressure to accelerate the achievement of target temperature levels.

However, it appeared that no more than the simple free expansion in the process relief valves (here 50, 60 and possibly 70), the use of the external cooling supply was a satisfactory solution.

Indeed, the supply of cold in a still hot exchange line and where in particular it circulates few fluids in the normally very stressed passages that are the revaporization passages of the liquids (here passages 13 and 25 in particular) causes thermal shocks and significant stresses between exchange passages and at the input / output boxes. These shocks and stresses are likely to quickly cause mechanical problems in the brazing or welding between components of the exchanger.

This is particularly true for the technology of brazed aluminum plate heat exchangers, which to date constitute the bulk of the exchange lines of cryogenic gas separation or liquefaction units.

EP-A-0091830 discloses a method according to the preamble of claim 1.

According to the invention there is provided a method according to claim 1.

• le deuxième moyen de détente est une vanne ;
• le premier et le deuxième moyen de détente sont installés en parallèle ;
• on envoie de la vapeur provenant du troisième moyen de détente au liquide provenant du séparateur de phases ;
• le CV du deuxième moyen de détente égale 3 fois le CV, de préférence 5 fois le CV du premier moyen de détente ;
• le CV du deuxième moyen de détente égale 3 fois le CV, de préférence 5 fois le CV du troisième moyen de détente ;
• pendant la mise en froid on commande le deuxième ou quatrième moyen de détente manuellement ou on régule la pression du gaz d'alimentation ;
• la séparation cryogénique est un procédé de séparation d'hydrocarbures ou de production d'hydrogène, de préférence de 90 à 98 % de pureté ou de production de CO₂, de préférence de pureté supérieure à 95 %, encore plus préférentiellement supérieure à 98 % ou un procédé d'élimination d'azote ou argon d'une fraction plus lourde ou la liquéfaction est une liquéfaction de gaz naturel.

Optionally:

• the second detent means is a valve;
• the first and the second detent means are installed in parallel;
• steam from the third expansion means is supplied to the liquid from the phase separator;
• the CV of the second expansion means equals 3 times the CV, preferably 5 times the CV of the first expansion means;
• the CV of the second expansion means equals 3 times the CV, preferably 5 times the CV of the third expansion means;
• during cooling, the second or fourth expansion means are manually controlled or the pressure of the supply gas is regulated;
• cryogenic separation is a process for separating hydrocarbons or producing hydrogen, preferably of 90 to 98% purity or production of CO ₂ , preferably of greater than 95% purity, still more preferably greater than 98% or a process for removing nitrogen or argon from a heavier fraction or the liquefaction is liquefaction of natural gas.

The solution recommended in the present invention will now be explained via the Figure 2 .

This figure shows the modifications made to the cold end of the exchange line described above. These modifications can also be made at the first separator pot 30 and more generally at each expansion of a liquid fraction.

The invention consists in adding to the diagram corresponding to the normal operation in steady state mode, a so-called chilling expansion valve used only (or mainly) during start-up of the unit.

The purpose of this valve is twofold. It first allows to relax a large flow of gas thus considerably increasing the cooling capacity produced by the unit itself, that is to say that it reduces the cold time and normally allows it alone to achieve the required levels of temperature.

On the other hand, in the case of very fast start-up with external cooling power supply, such as the use of liquid nitrogen, this valve firstly allows the equipment to be partially cooled and the thermal shocks to be limited, but especially to rebalance the exchange line by circulating large flows in the revaporization passages 25 and 13.

This new valve must therefore allow to relax a large fraction of the high pressure gas, here the fluid 2, and to introduce this expanded fluid into the passages 25 devolved normally to the liquid fraction 5.

This valve will preferably be installed in bypass of the expansion valve 60. It will then be about 10 times larger. This is valve 61 of the figure 2 .

It is also possible to add a valve between the fluid 2, that is to say between the outlet of the exchanger and the separator pot 40, and the inlet of the passages 25: this is then the valve 81. According to the invention, a fraction of the flow 4 is also expanded via a valve 71.

In all cases, the additional expansion valve 61, 71 or 81 may pass a flow of an order of magnitude at least 10 times greater than that able to be expanded in the valve 60 or 70.

This additional valve will be gradually closed as and when cold, especially since liquid will appear at the outlet of the exchanger.

It will be totally closed in normal operation.

It will usually be manually controlled (HIC) but can also be driven by the maintenance of high pressure (PIC).

In all these cases, it will be said later that the additional valve (61, 71 or 81) is in parallel with the expansion valve 60.

Note in this regard that it is not possible with the vast majority of commercial valves to have both a valve to pass a large gas flow, ie to have a CV at full opening 10 or more and then regulate with an opening corresponding to a CV of about 0.3. It is conventional to use a valve in an opening range of a factor 5, preferably 3, ie for example with a CV of 0.1 to 0.5 or 0.1 to 0.3 but not beyond. A factor of 5 (or 3) usually makes it possible to perform the nominal run and the reduced (reduced flow) steps without any particular regulation problem. In the case of the example of the figure 1 or 2 in normal operation, the expansion valve 60 maintains the liquid level in the separator pot 40. It therefore controls the liquid flow expanded and revaporized in the exchange line. This flow being the main refrigeration supply of the exchanger 20, it is understood that its regulation is critical. It would be totally impossible with an oversized valve, let alone with a valve 10 times larger than necessary.

As explained above, the additional expansion of a large flow rate of gas to be treated must be via a complementary means that will no longer be used in normal operation or will be at least partially closed to allow satisfactory operation of the unit .

Finally, it will be noted that it is possible, from the moment when a considerable part of the feed gas is expanded via an additional valve in the circuit 25, to also inject into this circuit a cooling auxiliary flow rate such as than liquid nitrogen without creating too much stress. Depending on the case, this extra flow can be suppressed or maintained, at least partially, during normal operation while the additional expansion valve is closed or substantially closed.

Claims (8)

1. Method for the cryogenic separation, the cooling or the liquefaction of a fluid using an exchange line comprising:

   • extracting from said exchange line at least one dual phase fluid (11)

   • separating said dual phase fluid into at least one vapour fraction (4) and one liquid fraction (5) in a phase separator (40)

   • expanding at least one portion of said liquid fraction (5) using a first expansion means (60,)

   • reinjecting, reheating and at least partially vaporising said expanded liquid fraction in the exchange line

   the first expansion means is a valve
   vapour (4) is sent from the phase separator (4') to a third expansion means (70) characterised in that
   during the cooling of said exchange line, at least a fraction of the fluid extracted from the exchange line (5) and/or from the phase separator (40) is expanded in second expansion means (61, 81) parallel to the first expansion means (60) and during a normal operation, the second expansion means is essentially closed,
   and, during the cooling of said exchange line, at least a fraction of the vapour (4) from the phase separator (40) is expanded in a fourth expansion means (71) parallel with the third expansion means (70).
2. Method according to claim 1 wherein the second expansion means (60, 61) is a valve.
3. Method according to one of claims 1 and 2 wherein the first and the second expansion means (60, 61) are installed in parallel.
4. Method according to claim 1 wherein the vapour from the third expansion means (70) is sent to the liquid (5) from the phase separator (40).
5. Method according to one of the preceding claims wherein the CV of the second expansion means (61) equals 3 times the CV, preferably 5 times the CV of the first expansion means (60, 90).
6. Method according to claim 4 wherein the CV of the second expansion means (61) equals 3 times the CV, preferably 5 times the CV of the third expansion means (70).
7. Method according to one of the preceding claims wherein during the cooling the second or fourth expansion means (61, 71) are controlled manually or the pressure of the feed gas is regulated.
8. Method according to one of the preceding claims wherein the cryogenic separation is a method of separating hydrocarbons or the production of hydrogen, preferably with a purity of 90 to 98% or of the production of CO₂, preferably with a purity of 95 %, even more preferably greater than 98% or a method for eliminating nitrogen or argon by a heavier fraction or the liquefaction is a liquefaction of natural gas.

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