Patent Publication Number: US-2010126215-A1

Title: Method For Cooling A Cryogenic Exchange Line

Description:
The present invention relates to a heat exchange line and to a method of cooling down such a heat exchange line. 
     It is known to use cryogenics to fractionate a gas stream into at least two fluids of different composition, generally into what is called a light fluid, i.e. one essentially composed of the more volatile constituents, and what is called a heavy fluid essentially consisting of the more easily condensable constituents. To accomplish this, the mixture to be fractionated is cooled in a heat exchanger or in a number of heat exchangers, called a heat exchange line, until a liquid/vapor two-phase mixture extracted from said heat exchange line and separated in a liquid/vapor separator is obtained. The vapor may be further cooled until a new two-phase state is obtained and fractionated a second time. 
     To cite an example, a stream of hydrocarbons (C 1 , C 2 , . . . , C i , C i+1 , . . . C n ) is fractionated into a fluid essentially consisting of the lighter hydrocarbons (methane C 1 , ethane C 2 , . . . , C i ) and a second fluid essentially consisting of the heavier hydrocarbons (C i+1 , . . . C n ). The term “essentially” is used to indicate that a small fraction of the lighter compounds will in general be found in the heavy fraction, and conversely a small portion of the heavier compounds will in general be found in the vapor fraction. 
     This separation may be improved by inserting trays into the two-phase separation system and by adding a reboiling section and/or a stripping section in order to remove the light components from the liquid fraction and/or a condenser and/or by increasing the reflux in order to remove the heavy components from the vapor fraction. These processes are known to those skilled in the art and are not discussed in the present invention. Consequently, the expression “liquid/vapor separator” will be used to cover all equipment generating at least one liquid output and at least one gaseous output from at least one two-phase feed. Such equipment may be of the horizontal or vertical gravity separator type, whether or not equipped with a devesiculator, of the cyclone or distillation column type, etc. 
     The liquid output may contain a small amount of bubbles entrained by the speed of the liquid, likewise the vapor output may contain liquid aerosols or droplets, without in any way departing from the scope of the invention. 
     Other applications consist in recovering a methane-rich fluid and a methane-depleted fluid from a source rich in various hydrocarbons. Thus, several fluids may also obtained, such as a methane-rich fraction, an ethane- or ethylene-rich fraction and a C 3+  fraction. This type of process makes it possible in particular to recover hydrogen with a purity of about 95% from a mixture of hydrogen and hydrocarbons and to remove a portion of the nitrogen contained in gases rich in hydrocarbons. It also makes it possible to recover a fraction very rich in CO 2  and a waste gas containing lighter constituents, such an N 2 , argon, O 2 , etc. 
     This fractionation may not be an objective per se, but only a means of delivering the refrigerating power intended for liquefying another fluid, such a natural gas. In this case, the various separated fluids are recombined after being warmed, recompressed and reinjected into the heat exchange line. This is then referred to as a refrigeration cycle. 
     These applications have given rise to many developments both in processes and in technology. In particular, the heat exchangers may be of the coiled type, such as a tube/shell heat exchanger, or preferably of the plate heat exchanger type. In the latter case, many improvements have been named regarding heat exchange corrugations and regarding the injection of the fluids, in particular two-phase fluids, into these heat exchangers so as to optimize the heat transfer. 
     In what follows, all the percentages mentioned are molar percentages. 
    
    
     One example of these units will now be described in relation to  FIG. 1 . This example relates to the production of pressurized hydrogen with a purity of 95% from a pressurized gas mixture containing about 70% hydrogen, 18% methane and 12% heavier hydrocarbons. 
     The mixture  1  to be separated is injected at ambient temperature and under a pressure of 40 bar absolute into the plate heat exchanger  10  to be cooled therein via the heat exchange passages  11 . At a first temperature level dependent on the composition of the heaviest hydrocarbons and on the pressure, generally between −40 and −120° C., the fluid  1 , then a two-phase fluid, is extracted from the heat exchanger and separated into its vapor fraction  2  and its liquid fraction  3  in the liquid/gas separator  30 . The liquid fraction  3  is expanded via the expansion valve  50  down to a low pressure and revaporized in the heat exchange line via the heat exchange passages  13 . 
     The vapor phase  2  enriched in hydrogen and in methane is again cooled in the heat exchanger  20  via the passages  22 , partially condensed and extracted at around −160° C. The vapor fraction  4  coming from the separator  40  constitutes the production of hydrogen with a 95 mol % content. It is then warmed in the passages  24  then 14 of the heat exchangers  20  and  10 . 
     The liquid fraction  5  mainly consisting of methane is expanded down to a low pressure in the valve  60 , revaporized in the heat exchanger  20  (passages  24 ) and warmed in the heat exchanger  10  (passages  14 ). 
     The fluids  6  and  7  associated with heat exchangers  20  and  10  respectively may optionally be used as refrigeration top-up. They may be external fluids, such as for example liquid nitrogen coming from a storage tank or from a neighboring air separation unit, or a fluid internal to the process, such as for example a fraction of hydrogen produced, which is partially warmed, then expanded in an expansion turbine and reinjected into the cold end of the heat exchanger  20 . 
     It is also possible to promote vaporization of the methane  5  by injecting a small fraction of the hydrogen production. This is shown by the optional circuit that includes the expansion valve  70 . 
     It should be noted that the expansion valves  50  and  60  are used to expand liquids from a high pressure, here 40 bar abs, down to a low pressure. They are therefore small valves. 
     It is common practice to use the notion of HP (horsepower) when speaking of the size of the valves. 
     Many works or documents give, on the one hand, methods of calculation and, on the other hand, the HP of commercially available valves. In the latter case, it is conventional to indicate the HP in the fully opened position, which makes it possible to determine the maximum flow rate of a fluid that can pass through the valve under given operating conditions. As an illustration, but without wishing to go into the calculations, for a gas feed flow rate of the order of 10 000 Nm 3 /h, the HP of these valves will be less than 1. 
     The same applies to the optional valve  70  that serves to expand a very small fraction (a few percent at most) of the hydrogen produced. 
     Conventionally, such a separation unit is cooled down either by free expansion of the gas to be treated or, more generally, using an external refrigeration top-up. 
     What is called here the cooling-down of the heat exchange line is the procedure for obtaining the normal operating conditions, here a first cut-off temperature between the heat exchangers  10  and  20 , for example −80° C., and a temperature at the cold end of −160° C. in order to obtain the required purity from equipment operating at ambient of sub-ambient temperature if the heat exchange line has not had the time to reach the ambient temperature. 
     The cooling-down problem using just the free expansion of the gas to be treated in the expansion valves  50 ,  60  and optionally  70  is that the total expanded flow is very small and therefore the refrigeration power obtained is itself very low. However, this refrigeration power is intended to cool the heat exchange line and the ancillary equipment, such as the separators, and to compensate for the thermal losses, such as the heat exchange with the external medium. Such a cooling-down procedure may take several tens of hours and may even possibly not reach the desired operating point. 
     This occurs in particular if the thermal losses become, at a certain temperature level reached at the cold end, equal to the refrigeration power produced by free expansion. At this point, the cooling-down stops and it is not possible to cool down further. 
     For this reason, it is common practice to use for example the refrigeration top-up circuit  6  and  7  in order to hasten the cooling-down. The passages  26  and  17  may be used permanently or only temporarily during the cooling-down phases. As indicated above, it is conventional to use low-pressure or preferably medium-pressure liquid nitrogen to speed up the process for obtaining the intended temperature levels. 
     However, it is apparent that no more than the simple free expansion in the process expansion valves (here  50 ,  60  and possibly  70 ), the use of external refrigeration top-up is not a satisfactory solution. 
     This is because refrigeration top-up into a heat exchange line which is still warm, and in which in particular little fluid flows in the normally heavily used passages, i.e. the liquid revaporization passages (here passages  13  and  25  in particular), causes thermal shocks and high stresses between heat exchange passages and in the inlet/outlet boxes. These shocks and stresses are liable to rapidly cause mechanical problems at the brazed or welded joints between constituent components of the heat exchanger. 
     This is particularly the case for brazed aluminum plate heat exchangers—the technology used at the present time for most heat exchange lines for cryogenic separation or gas liquefaction units. 
     According to another aspect of the invention, what is provided is a process for the cryogenic separation, refrigeration or liquefaction of a fluid by means of a heat exchange line comprising:
         the extraction of at least one two-phase fluid from said heat exchange line;   the separation of said two-phase fluid into at least a vapor fraction and a liquid fraction in a phase separator;   the expansion of at least one portion of said liquid fraction by means of a first expansion means; and   the reinjection, warming and at least partial vaporization of said expanded liquid fraction in the heat exchange line,       

     characterized in that:
         the first expansion means is a valve;   as said heat exchange line is being cooled down, at least one fraction of the fluid extracted from the heat exchange line and/or from the phase separator is expanded in a second expansion means in parallel with the first expansion means; and   in normal operation, the second expansion means is essentially closed.       

     Optionally:
         at least one of the first and second expansion means is a valve;   the first and second expansion means are installed in parallel;   vapor is sent from the phase separator into a third expansion means and, as said heat exchange line is being cooled down, at least one fraction of the vapor is expanded in a second expansion means in parallel with the third expansion means;   vapor coming from the third expansion means is sent into the liquid coming from the phase separator;   the HP of the second expansion means is equal to three times the HP, preferably five times the HP, of the first expansion means;   the HP of the second expansion means is equal to three times the HP, preferably five times the HP, of the third expansion means;   during the cooling-down, the second expansion means is controlled manually or the pressure of the feed gas is regulated;   the cryogenic separation is a process for the separation of hydrocarbons or for the production of hydrogen, preferably with a purity of 90 to 98%, or for the production of CO 2 , preferably with a purity of greater than 95% and even more preferably greater than 98%, or a process for eliminating nitrogen or argon from a heavier fraction, or the liquefaction is a liquefaction of natural gas.       

     The solution recommended in the present invention will now be explained with the help of  FIG. 2 . 
     This figure shows the modifications made to the cold end of the heat exchange line described above. These modifications may also be made at the first separator pot  30  and more generally at each point of expansion of a liquid fraction. 
     The invention consists in adding, to the scheme corresponding to the normal steady-state operation, an expansion valve, called here a cool-down expansion valve, which is used only (or mainly) when starting up the unit. 
     The purpose of this valve is twofold. Firstly, it allows a large flow of gas to be expanded, thus considerably increasing the refrigeration power produced by the unit itself, that is to say it enables the cool-down time to be reduced and normally makes it possible by itself to reach the required temperature levels. 
     Secondly, in the case of very rapid start-up with an external refrigeration power top-up, such as the use of liquid nitrogen, this valve firstly allows the equipment to be partially cooled and correspondingly to limit the thermal shocks, but in particular to rebalance the heat exchange line by making large volumes flow through the revaporization passages  25  and  13 . 
     This new valve must therefore allow a large fraction of the high-pressure gas, here the fluid  2 , to be expanded and enable this expanded fluid to be introduced into the passages  25  normally reserved for the liquid fraction  5 . 
     This valve will preferably be installed as a by-pass for the expansion valve  60  and will therefore be about 10 times larger—this the valve  61  shown in  FIG. 2 . 
     It is also possible to add instead a valve between the fluid  2 , i.e. between the outlet of the heat exchanger and the separator pot  40 , and the inlet of the passages  25 —this is then the valve  81 . 
     A fraction  4  of the stream may also be expanded via a valve  71 . 
     In all cases, the additional expansion valves  61 ,  71 , or  81  may pass a flow of an order of magnitude at least 10 times higher than that which can be expanded in the valve  60  or  70 . 
     This additional valve will be gradually closed as the cooling-down progresses, in particular as liquid appears at the exchanger outlet. 
     It will a priori be completely closed under normal operation. 
     It will generally be manually controlled (HIC), but may also be controlled by maintaining the high pressure (PIC). 
     In all these cases, the additional valve ( 61 ,  71  or  81 ) may thus be in parallel with the expansion valve  60 . 
     It should be noted in this regard that it is not possible with most commercially available valves to have both a valve for passing a large flow of gas, i.e. one having an HP when fully opened of 10 or more, and then to regulate with an opening corresponding to an HP of about 0.3. It is conventional to use a valve in an opening range with a factor of 5, preferably 3, i.e. for example with an HP of 0.1 to 0.5 or from 0.1 to 0.3, but not beyond this. A factor of 5 (or 3) usually makes it possible to carry out nominal operation or reduced operation (with a reduced flow rate) without any particular regulation problem. In the case of the example shown in  FIG. 1  or  FIG. 2  in normal operation, the expansion valve  60  makes it possible to maintain the liquid level in the separator pot  40 . It therefore controls the flow of liquid that has been expanded and revaporized in the heat exchange line. Since this flow is the main refrigeration feed for the heat exchanger  20 , it will be understood that its regulation is critical. It would be completely impossible with an oversized valve, a fortiori with a valve 10 times larger than necessary. 
     As explained above, the additional expansion of a large flow of gas to be treated must therefore take place via a complementary means, which will no longer be used in normal operation or which will be at least partially closed so as to allow the unit to operate satisfactorily. 
     Finally, it should be noted that it is possible, from the moment that an appreciable portion of the feed gas is expanded via an additional valve in the circuit  25 , to also inject into this circuit a refrigeration top-up flow, such as a flow of liquid nitrogen, without creating excessively large stresses. Depending on the circumstances, this top-up flow may be eliminated or maintained, at least partially, during normal operation whereas the additional expansion valve will be closed or essentially closed.