Abstract:
Method and process plant for liquefaction of gas, particularly natural gas with multicomponent refrigerant, suited for small and medium sized scale, where the plant solely is based on conventional two-flow plate heat exchangers and conventional oil lubricated compressors. By the arrangement of the heat exchangers and the compressors according to the invention it is avoided that oil from the compressors, that to some extend will follow the flow of refrigerant, may reach the coldest parts of the plant. Any freezing of oil and plugging of conduit etc. is thus avoided.

Description:
The present invention relates to a method for liquefaction of gas, particularly natural gas, using multicomponent refrigerant. 
     BACKGROUND OF THE INVENTION 
     Liquefaction of gas, particularly natural gas, is well known from larger industrial plants, so called “baseload” plants, and from peak shaving plants. Such plants have the property in common that they convert a substantial quantum gas pr time, so they can bear a significant upfront investment. The costs pr gas volume will still be relatively low over time. Multicomponent refrigerants are commonly used for such plants, as this is the most effective way to reach the sufficiently low temperatures. 
     Kleemenko (10th International Congress of Refrigeration, 1959) describes a process for multicomponent cooling and liquefaction of natural gas, based on use of multiflow heat exchangers. 
     U.S. Pat. No. 3,593,535 describes a plant for the same purpose, based on three-flow spiral heat exchangers with a an upward flow direction for the condensing fluid and a downward flow direction for the vaporizing fluid. 
     A similar plant is known from U.S. Pat. No. 3,364,685, in which however the heat exchangers are two-flow heat exchangers over two steps of pressure and with flow directions as mentioned above. 
     U.S. Pat. No. 2,041,745 describes a plant for liquefaction of natural gas partly based on two-flow heat exchangers, where the most volatile component of the refrigerant is condensed out in an open process. In such an open process it is required that the gas composition is adapted to the purpose. Closed processes are generally more versatile. 
     There is however, a need for liquefaction of gas, particularly natural gas, many places where it is not possible to enjoy large scale benefits, for instance in connection with local distribution of natural gas, where the plant is to be arranged at a gas pipe, while the liquefied gas is transported by trucks, small ships or the like. For such situations there is a need for smaller and less expensive plants. 
     Small plants will also be convenient in connection with small gas fields, for example of so called associated gas, or in connection with larger plants where it is desired to avoid flaring of the gas. In the following the term “product gas” is used synonymously with natural gas. 
     For such plants it is more important with low investment costs than optimal energy optimization. Furthermore a small plant may be factory assembled and transported to the site of use in one or several standard containers. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to provide a method and a process plant for the liquefaction of gas, particularly natural gas, that is adapted for small and medium sized scale liquefaction. 
     It is furthermore an object to provide a plant for the liquefaction of gas for which the investment costs are modest. 
     It is thus a derived object to provide a method and a small scale process plant for cooling and liquefaction of gas, particularly natural gas, with a multicomponent refrigerant, where the plant is solely based on conventional two-flow plate heat exchangers and conventional oil lubricated compressors. It is furthermore a derived object to provide a small scale plant for the liquefaction of natural gas, which plant may be transported factory assembled to the site of use. 
     With the plant according to the invention there is obtained a small scale plant for cooling and liquefaction, where the plant costs is not prohibitive of a cost-effective operation. By the way with which the components of the plant are combined, it is avoided that oil from the compressors, which to some extent will contaminate the refrigerant, follows the flow of refrigerant to the coldest parts of the plant. It is thus avoided that the oil freezes and plugs conduits etc., which is an essential part of the invention. 
     To obtain this it has been necessary to include equipment for distribution of refrigerant between pairs of heat exchangers in separate rows, where the heat exchangers that cool the product flow is denoted primary heat exchangers and the heat exchangers that cool/heat different components of the multicomponent refrigerant are denoted secondary heat exchangers. The primary and secondary heat exchangers may be of same type and have similar dimensions, but the number of plates will depend upon the flow rate through the heat exchangers. 
     Use of multicomponent refrigerant is known per se, while achieving the benefits inherent with being able to reach very low temperatures in a simple plant, based on conventional components, is not. With the plant according to the invention is also obtained a natural flow direction in the plant, namely so that evaporating fluid moves upward while condensing fluid moves downward, avoiding that gravity negatively interferes with the process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a flow diagram of a process plant according to the invention, 
     FIG. 2 shows an alternative embodiment of the plant of FIG. 1, 
     FIG. 3 shows a section of the plant of FIG. 1, with a preferred embodiment of a distribution device for the refrigerant, 
     FIG. 4 shows the same section as FIG. 3, with a different embodiment of the distribution device for the refrigerant, 
     FIG. 5 shows the same section as FIGS. 3 and 4, with a still different embodiment of the distribution device for the refrigerant, 
     FIG. 6 shows the same section as FIGS. 3,  4  and  5 , with a still different embodiment of the distribution device for the refrigerant. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A feed flow of gas, e.g. of natural gas is supplied through conduit  10 . This raw material is supplied with a temperature of e.g. approximately 20° C. and with a pressure as high as allowable for the plate heat exchanger in question, e.g. 30 barg. The natural gas has been pre-dried and CO 2  has been removed to a level where no solidification (freezing) occurs in the heat exchangers. The natural gas is cooled in the first primary heat exchanger  12  to about −25 to −75° C., typically −30° C., by heat exchanging with low level (low pressure) refrigerant that is supplied to the heat exchanger through conduit  92  and departs from the heat exchanger through conduit  96 . The cooled natural gas flows further through conduit  14  to the next primary heat exchanger where it is cooled again, condensed and undercooled to about −85 to −112° C. by heat exchange with low level refrigerant that is supplied to the heat exchanger through conduit  84  and departs from the heat exchanger through conduit  88 . If required low volatile components of the natural gas may be separated from the rest of the product flow between heat exchanger  12  and  16 , by introducing a phase separator (not shown). From heat exchanger  16  the condensed natural gas flows through conduit  18  to still another heat exchanger  20  where the condensed natural gas is cooled to a temperature low enough to ensure low or no vaporizing in the subsequent throttling to the pressure of the storage tank  28 . The temperature may typically be −136° C. at 5 bara or −156° C. at 1.1 bara in the storage tank  28 , and the natural gas is led to the tank through throttle valve  24  and conduit  26 . The low level refrigerant supplied to heat exchanger  20  through conduit  78  is at its coldest in the process plant, and comprises only the most volatile parts of the refrigerant. 
     Low level refrigerant in conduit  96  from heat exchanger  12  is joined with low level refrigerant in conduit  94  from heat exchanger  64 , where it is used for cooling high level refrigerant, and from this point led through conduit  40  to at least one compressor  46  where the pressure increases to typically 25 barg. The refrigerant then flows through conduit  52  to a heat exchanger  54  where all heat absorbed by the refrigerant from the natural gas in the steps described above, is removed by heat exchange with an available source, like cold water. The refrigerant is thereby cooled to a temperature of typically about 20° C. and partly condensed. From here on the refrigerant flows through conduit  58  to a phase separator  60  where the most volatile components are separated out at the top through conduit  62 . This part of the refrigerant constitutes the high level refrigerant to secondary heat exchanger  64  arranged in parallell to primary heat exchanger  12 . In heat exchanger  64  the high level refrigerant from conduit  62  is cooled and partly condensed by the low level refrigerant that is supplied to heat exchanger  64  through conduit  90  and departs from the same through conduit  94 . From this point the high level refrigerant flows through conduit  66  to a second phase separator  68 . Again the most volatile fractions are separated into a high level refrigerant through conduit  70 , and supplied to secondary heat exchanger  72  arranged in parallel with primary heat exchanger  16 . In heat exchanger  72  the high level refrigerant from conduit  70  is cooled and partly condensed by low level refrigerant that is supplied to heat exchanger  72  through conduit  82  and departs from the same through conduit  86 . 
     From heat exchanger  72  the partly condensed high level refrigerant flows through conduit  74  to a throttle valve  76  for throttling to a lower pressure, and flows from this point as low level refrigerant through conduit  78  to the last heat exchanger  20  where the last step of undercooling of the at this point liquefied natural gas takes place. The refrigerant in conduit  78  is thus at the lowest temperature of the entire process, typically in the range −140° C. to −160° C. In FIG. 1 heat exchanger ( 20 ) represents the third step of cooling of the product gas. 
     Alternatively the partly condensed high level refrigerant in conduit  74  may be directed to an additional heat exchanger  114 , cf. FIG. 2, where high level refrigerant from  74  is undercooled by low level refrigerant supplied to heat exchanger  114  through conduit  120  subsequent to having been throttled to low pressure through a throttle valve  118 . 
     From the first phase separator  60  the less volatile part of the refrigerant flows through conduit  100 , is throttled to a lower pressure through valve  102 , is mixed with flows of low level refrigerant from conduits  86  and  88  leaving heat exchangers  72  and  16  respectively, whereafter the joined flow of low level refrigerant flows on to heat exchangers  12  and  64  and is distributed between these in a way to be further described below with reference to FIGS. 3-5. Together with the less volatile fraction of the refrigerant in conduit  100  there will always be some contaminations in the form of oil when ordinary oil cooled compressors are used. It is thus an important feature with the present invention at this first, non-volatile flow  100  of refrigerant from the first phase separator  60  only is used for heat exchange in the pair of heat exchangers  12 / 64  that is least cold, as heat exchanger constitutes the first cooling step of the product gas. 
     From the second phase separator  68  the low volatile part of the refrigerant flows through conduit  108 , is throttled to lower pressure through valve  110 , is mixed with low level refrigerant  80  from heat exchanger  20  and thereafter supplied to heat exchangers  16  and  72 , between which the refrigerant is distributed in a way that is further described below with reference to FIGS. 3-6. 
     The low level refrigerant flowing upwards through the pairs of heat exchangers arranged in parallel, denoted primary heat exchangers for cooling of the product gas and secondary heat exchangers for cooling of high level refrigerant, will be heated and partly evaporated by the heat received from the natural gas and from the high level refrigerant. The flow of low level refrigerant is for each pair of heat exchangers  16 / 72  and  12 / 64  respectively split in to partial flows which are thereafter joined again. It is convenient that the two flows of low level refrigerant leaving any pair of heat exchangers have equal temperature, i.e. that the temperature of low level refrigerant in conduit  86  is approximately the same as the temperature of low level refrigerant in conduit  88 . There is a corresponding situation for the temperature in conduits  94  and  96 . In order to obtain this situation, there is arranged a distribution device at the inlet side of each pair of heat exchangers. 
     FIG. 3 shows a section of the plant of FIG. 1, comprising a first phase separator  60 , two pairs of primary and secondary heat exchangers  12 / 64  (also called first cooling step) and  16 / 72  (also called second cooling step), as well as the conduits connecting these components. In addition FIG. 3 furthermore shows a jector shaped distribution device  106  receiving the flows of refrigerant from conduits  86 ,  88  and  104 , cf. FIG. 1, in which the velocity energy from the pressure reduction from a high to a low pressure level in conduit  104  is used to overcome the pressure loss in a mixer for fine dispersion of the liquid in the two-phase flow. On its downstream side the distribution device  106  splits the flow and distributes it between the two conduits  90  and  92  leading to the primary  12  and the secondary  64  heat exchanger constituting the next pair of heat exchangers, in a ratio conveniently determined by a correct area-ratio in the distributing device. FIG. 4 shows an alternative way for controlling the distribution of refrigerant between conduits  90  and  92 . On the downstream side of heat exchangers  12  and  64 , and more precisely on the conduits  96  and  94  respectively, there are arranged temperature controllers (TC) so that the temperature may be registered. This way it is possible, continuously or periodically to adjust the inertia valve  118  so that the temperatures within the conduits  94  and  96  become as equal as possible, since this is the most rational way to operate the plant. The adjustment of the distributor  106  may be performed manually, though it is preferred that it is performed automatically by means of a processor controlled circuit. 
     A corresponding arrangement (not shown) for distribution/controlling is preferably arranged also to the inlet side of the heat exchangers  16  and  72 , with a temperature control of conduits  86  and  88 . 
     FIGS. 3-6 also show controlling means interconnected between the phase separator  60  and the throttle valve  102 , which is continuously controlled in a way that ensures that the level of condensed phase in the phase separator is maintained between a maximum and a minimum level. 
     FIG. 5 shows an alternative way of controlling the distribution of the refrigerant between conduits  90  and  92 , by which only one inertia valve  118  is used, and the degree of opening of this valve is controlled by the temperature controllers TC. In this case it is convenient to use a mixing device  124  of suitable type, schematically indicated with a zig-zag line. 
     FIG. 6 shows a still further embodiment of the distribution device. The principle is generally the same, but a mechanically different solution is applied, as the device comprises two separate valves  120 ,  122  connected to each of the conduits  90 ,  92 , the degree of opening again being controlled by the temperature controllers TC. 
     For the liquefaction of natural gas it is preferred that the plant has two phase-separators  60  and  68  as shown in FIG. 1, and as a consequence of this a three step cooling/condensing of the product flow. For other purposes it may be sufficient with one step less, and only one phase separator. The cooling ability will then be somewhat less. It is also possible to use more than three steps, but this is usually not convenient for relatively small plants from economical and operational points of view. 
     While FIG. 1 only shows one compressor, it is often more convenient to compress the refrigerant in two serial steps, preferably with interconnected cooling. This has to do with the degree of compression obtainable with simple, oil lubricated compressors, and may be adapted in accordance with the relevant need by a skilled professional. 
     Again with reference to FIG. 1 it may be convenient to include an additional heat exchanger as explained hereinbelow. Since the low level refrigerant in conduit  40  normally will have a temperature lower than that of the high level refrigerant in conduit  58 , it may be convenient to heat exchange these against each other (not shown), thus lowering the temperature of said high level refrigerant further prior to its introduction into phase-separator  60  via conduit  58 . 
     By the method and the plant according to the invention it is provided a solution by which gas, like natural gas may be liquefied cost-effectively in small scale, as the processing means utilized are of a very simple kind. The controlling and adaption of the process ensures that oil from the compressors contaminating the product gas can not freeze and plug conduits or heat exchangers, as the oil do not reach the coldest parts of the plant. 
     The method and the plant as described above, constitutes preferred embodiments, while the invention in its general form only is limited by the enclosed claims.