JP2007225143A - Cold supply system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To construct a cold supply system capable of efficiently and maximally utilizing cold of a liquefied natural gas while ensuring a stable operation even in a chemical plant of low cold demand, in utilizing the cold of liquefied natural gas of wide fluctuation band in supply quantity in the chemical plant. <P>SOLUTION: A process of converting a carbon dioxide gas B into a liquefied carbon dioxide gas D by utilizing the cold of liquefied natural gas A, then delivering the liquefied carbon dioxide gas D to an ethylene plant 42, and utilizing vaporization heat generated in converting the liquefied carbon dioxide gas D into a carbon dioxide gas E, as a cold source, is incorporated in this cold supply system utilizing a large-scaled cooling device in the ethylene plant 42, as a part of a cooling system by liquefied propylene H, and the cold supply systems are integrated in the ethylene plant 42 needing the large-scaled cooling device and the other chemical plant 43 conventionally using a compact cooling device, thus the cold is supplied to the other chemical plant 43 besides the ethylene plant 42 in transferring the liquefied propylene H by this cold supply system. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a system that efficiently utilizes the cold energy of liquefied natural gas. Specifically, liquefied carbon dioxide (hereinafter referred to as “liquefied carbon dioxide”) obtained by using the cold heat of liquefied natural gas is converted into carbon dioxide gas (hereinafter referred to as “carbon dioxide”). The present invention relates to a method for effectively utilizing the cold energy generated in the process as a cold heat source for a chemical plant.

  In general, when natural gas is used, liquefied natural gas that has been cooled and liquefied to compress its volume when it is transported from its place of origin is once converted back to gas, and then hydrocarbon chemical materials and refrigerants are used. It is used for various uses such as city gas.

  Normally, when returning this liquefied natural gas to a gas, air or seawater is used as a heat source. However, in this method, the air or seawater is simply cooled, and the liquefied natural gas having a low temperature of −100 ° C. or lower is used. The cold heat of can not be used effectively. Therefore, Patent Document 1 discloses a system that uses the cold heat of the liquefied natural gas as a cold heat source for a chemical plant or the like.

  By the way, in a chemical plant, there are many processes that require cooling and freezing, and a cold energy supply system that supplies cold energy to these processes is provided.

  The cold heat supply system is generally composed of processes such as methane, ethylene, propylene, ammonia, and chlorofluorocarbon, and includes a process of gas compression, liquefaction by cooling, expansion under reduced pressure, and evaporation by supplying cold heat. Much energy is required for gas compression and liquefaction.

  Therefore, in order to reduce energy consumption in a chemical plant, it has been proposed to use the cold heat of liquefied natural gas as a cold heat source of the chemical plant and use liquefied carbon dioxide as the cold medium.

As a specific operation example, the following method shown in FIG.
First, the process in the liquefied natural gas supply plant 11 will be described. A part of the liquefied natural gas (LNG) a is supplied from the LNG storage tank 1 storing the liquefied natural gas to the liquefied carbon dioxide production process 3 through the LNG transfer line 2 and the rest is supplied to the liquefied natural gas evaporator 4. Is done. The liquefied natural gas a supplied to the liquefied carbon dioxide production process 3 performs heat exchange with the carbon dioxide gas b and the like. As a result, the liquefied natural gas a is heated and vaporized to become a natural gas c. The vaporized natural gas c is transferred through the natural gas transfer line 5 to a process using the natural gas c. On the other hand, the carbon dioxide gas b, which is the raw material of the liquefied carbon dioxide gas, is supplied to the liquefied carbon dioxide production process 3 through the liquefied carbon dioxide raw material supply line 6, and is cooled and liquefied by the liquefied natural gas a to become the liquefied carbon dioxide gas d. .

  Next, the obtained liquefied carbon dioxide d is sent to the cold demand plant 12 in the chemical plant through the liquefied carbon dioxide transfer line 7, and the liquefied carbon dioxide cold recovery process provided in the cold supply system 8 therein. 9 is vaporized. The vaporized carbon dioxide e is sent to the process using the carbon dioxide e through the carbon dioxide transfer line 10.

JP 2003-161574 A

  However, chemical plants have many processes that require cooling and refrigeration, and each plant is equipped with a cold heat supply system, but there are also facilities with a small refrigeration system scale. On the other hand, the amount of chilled natural gas that can be supplied with cold energy varies depending on the amount of liquefied natural gas used. Therefore, when the cold heat of liquefied natural gas is used as a cold heat supply source in each chemical plant that requires a small amount of cold heat relative to the amount of liquefied natural gas used, the cold heat supply amount and the cold demand amount will not match. However, there was a problem that the amount of refrigerated natural gas used for cooling could not be increased.

  In addition, the chemical plant requires cooling and cooling supply to the refrigeration process in accordance with the operation load, so that it is indispensable to stabilize the operation of the cooling supply process. For this reason, the smaller the amount of cold heat demand, the greater the influence of fluctuations in the amount of cold supplied from the liquefied natural gas, making it impossible to make maximum use of the cold heat of the liquefied natural gas.

  In the process of FIG. 3 described above as a specific example, the amount of liquefied natural gas a supplied to the liquefied carbon dioxide production process 3 through the LNG transfer line 2 is liquefied carbon dioxide in order to avoid the generation of excess liquefied carbon dioxide. The amount of liquefied natural gas necessary to produce the amount of liquefied carbon dioxide that can be vaporized in the cold heat recovery step 9 is limited, and the remainder is supplied to the liquefied natural gas evaporator 4 and supplied to the seawater line 13 through the seawater f and heat. The gas is exchanged and vaporized, and transferred to the process using the natural gas c through the natural gas transfer line 5.

  In a general chemical plant, a centrifugal gas compressor is often used for a cold heat supply system, and the operation range is about 60 to 100% of the maximum output. However, it is preferable to avoid sudden load fluctuations in terms of stable operation. Therefore, it has been demanded that the fluctuation range of the cold energy recovery accompanying the change in the amount of liquefied carbon dioxide gas is suppressed to about 30% of the cold energy supply capacity of the cold energy supply system. As a result, depending on the amount of natural gas used, the entire amount of liquefied natural gas cannot be supplied to the liquefied carbon dioxide production process 3, and the surplus is supplied to the liquefied natural gas evaporator 4 to waste cold energy. In some cases, the cold energy of the liquefied natural gas a cannot be used to the maximum extent.

  Therefore, the present invention provides a cold energy supply system that can efficiently use the cold energy of liquefied natural gas even in a chemical plant with a small amount of cold energy demand, and can ensure stable operation even when the amount of liquefied natural gas used varies. The purpose is to provide.

  This invention uses the cold heat of liquefied natural gas to convert carbon dioxide gas into liquefied carbon dioxide gas, then sends the liquefied carbon dioxide gas to a chemical plant, and generates heat of vaporization when the liquefied carbon dioxide gas is converted into carbon dioxide gas. Incorporating the process of using methane as cold energy into a large-scale cold energy supply system using propylene used in an ethylene plant, and supplying cold energy from a large-scale cold energy supply system using propylene to chemical plants with low demand for cold energy As a method, the above-mentioned problems were solved by integrating the cooling / heating supply systems of a plurality of chemical plants.

  The cold heat supply system using propylene gas generally used in an ethylene plant has a very large amount of cold heat supplied compared to the cold heat supply system used in other chemical plants. The amount of cold supplied may be about 2 to 10 times higher than the amount of cold supplied by liquefied carbon dioxide converted from carbon dioxide by the cold heat of liquefied natural gas. In many cases, the amount of liquefied carbon dioxide produced due to changes in the amount of liquefied natural gas used, and even if the amount of cold heat recovered due to vaporization of liquefied carbon dioxide varies, the amount of cold supplied by adjusting the operation of the propylene gas compressor is required. It is possible to correspond to the quantity, and it is possible to supply cold heat to the ethylene plant and continue to supply cold heat to other chemical plants.

  As a result, the entire amount of liquefied carbon dioxide gas generated corresponding to the amount of liquefied natural gas used can be converted to carbon dioxide gas, and its cold energy can be used without waste, and the cold heat of liquefied natural gas can be utilized to the maximum. Thus, the energy required for the cold supply system can be saved throughout the plant.

  Furthermore, by integrating the cooling / heating supply systems of a plurality of chemical plants, the cooling / heating supply system can be simplified and the number of equipment can be reduced, compared to the preparation of the cooling / heating supply system in each chemical plant.

The present invention will be described in detail below.
The present invention is a cooling and heating system that sequentially repeats the steps of (1) gas compression of propylene, (2) liquefaction and / or supercooling by cooling, (3) expansion under reduced pressure, and (4) evaporation by supplying cold energy. There,
Using the cold heat generated when vaporizing the liquefied carbon dioxide gas absorbed by the carbon dioxide gas by absorbing the cold heat of the liquefied natural gas, as a part of the cold heat for cooling in the step (2) performed in the ethylene plant,
A part of propylene liquefied by cooling is transferred to another chemical plant outside the ethylene plant, and the step (4) is performed as a refrigerant of a cooling system in the other chemical plant. The propylene gasified by the cooling system is returned to the ethylene plant, and the steps (1) to (4) are repeated. Here, provision of cold heat means taking heat, and a cold heat supply system using a refrigerant is a system that cools an object by transferring heat to a lower temperature medium.

  Each process of the said cold-heat supply system is a process of a general refrigerating cycle, Comprising: Propylene is used for the refrigerant | coolant cooled in an ethylene plant, and is specifically as follows.

The compression in the step (1) is adiabatic compression of propylene in a gas state. The propylene in the gas state is generally used by circulating the propylene evaporated in the step (4). However, if it is insufficient, it may be newly added from the outside. In addition, in order to reduce the energy required for compression, it is more preferable to perform multistage compression rather than compressing in one stage, and in accordance with the difference in propylene cold temperature required in various places in the plant, It is preferable to employ a multistage compressor having about four stages. Most of the energy consumption required for the cold energy supply system according to the present invention is the amount of energy required for the compressor power required for gas compression in the step (1), in order to achieve the optimum effective use of energy. It is important to minimize the amount of gas to be compressed and the compression ratio. In addition, when using a cooling source that does not require energy such as seawater at the time of cooling described below, the energy consumption required for the cold heat supply system according to the present invention is the energy consumption required for the compressor power described above. It becomes only.

  In the cooling in the step (2), the propylene gas is liquefied to form liquefied propylene, or the liquefied propylene is further cooled to a supercooled state. The propylene gas to be liquefied may be propylene gas evaporated by donating cold in the step (4) described later, in addition to the propylene gas compressed in the step (1). In the present invention, liquefied carbon dioxide utilizing the liquefied natural gas is used as at least a part of this propylene gas or a refrigerant for supplying cold heat used for cooling the liquefied propylene.

  Due to the reduced pressure expansion in the step (3), the liquefied propylene becomes wet steam in which the temperature and the liquid and gas are mixed. This step is preferably performed by an adiabatic expansion valve with as little heat as possible.

  In the evaporation by the provision of cold in the step (4), heat exchange is performed between the liquefied propylene and the object to be cooled, and the object to be cooled is cooled. On the other hand, as a result of the warming of the liquefied propylene, it evaporates and becomes propylene gas again. The propylene in the gas state returns to the step (1) again, and the above steps are repeated in order. In the present specification, providing cold energy means cooling the other party and taking heat away.

  The cooling target in the step (4) is a cooling target in the ethylene plant having the cooling heat supply system according to the present invention, as well as a fluid that requires cooling of another chemical plant. When the step (4) is performed in the other chemical plant, the liquefied propylene is sent from the ethylene plant to the other chemical plant, and the fluid to be cooled in the other chemical plant. Heat exchange is performed between the two, the evaporated propylene gas is sent back to the ethylene plant, and the process returns to step (1).

  Next, the liquefied carbon dioxide gas used as a refrigerant when the propylene is cooled and liquefied in the step (2) will be described. This liquefied carbon dioxide gas is obtained by liquefying carbon dioxide gas by utilizing the cold heat of the liquefied natural gas when the liquefied natural gas is gasified. Examples of the liquefaction method include a method of cooling the carbon dioxide gas itself and a method of separating the liquefied carbon dioxide gas liquefied by cooling a mixed gas containing carbon dioxide such as air. Here, the use of carbon dioxide gas is not only easy to transfer, but because carbon dioxide gas can be a raw material in an ethylene plant, carbon dioxide gas that has been transferred to an ethylene plant and gasified can be used as a raw material on the spot. It is. Here, the term “easily transported” means that the liquefied natural gas liquefier and the ethylene plant are generally separated from each other. When liquefied natural gas having a low temperature of −100 ° C. or lower is directly transported, a heat dissipation loss occurs. However, the temperature of the liquefied carbon dioxide gas is higher than the boiling point of natural gas, indicating that the heat dissipation loss during transfer can be kept low.

  As a cold heat transfer medium when using the cold heat of liquefied natural gas in an ethylene plant, not only liquefied carbon dioxide gas directly cooled by the cold heat of liquefied natural gas as described above but also liquefied natural gas and chemicals are used. Even when an intermediate refrigerant other than carbon dioxide, which is a temperature between the temperature of propylene, which is a necessary refrigerant in the plant, is used, the effect of utilizing the cold of liquefied natural gas can be obtained. As this intermediate refrigerant, a substance whose boiling point is between the boiling point of methane, which is the main component of natural gas, and the boiling point of propylene to be cooled can be used. Examples thereof include fluorinated hydrocarbons such as trifluoromethane and tetrafluoromethane, and ethane. These intermediate refrigerants are preferably selected in consideration of the distance between the liquefied natural gas gasifier and the ethylene plant. However, among these intermediate refrigerants, use of carbon dioxide is most preferable because the conditions of the boiling point temperature and the pressure for liquefaction are optimal and easy to handle.

  The pressure of the liquefied carbon dioxide gas when using the liquefied carbon dioxide gas as a refrigerant for cooling propylene in the step (2) needs to be 0.52 MPa or more and 3 MPa or less as a gauge pressure. Hereinafter, the pressure indicates a gauge pressure. The saturation temperature of liquefied carbon dioxide is determined by the pressure, but if it is less than 0.52 MPa, carbon dioxide can only exist in a solid or gaseous state, and uses the latent heat when gasifying the liquefied liquefied carbon dioxide. You can no longer use cold energy in the way you do. On the other hand, when it exceeds 3 MPa, the gasification temperature of the liquefied carbon dioxide gas becomes close to normal temperature, and the effect as a refrigerant is reduced.

  The temperature of the liquefied carbon dioxide gas when using the liquefied carbon dioxide gas as a refrigerant for cooling propylene in the step (2) needs to be −55 ° C. or higher. This is because if it is lower than −55 ° C., carbon dioxide can exist only in a solid or gas state depending on the pressure, and liquefied carbon dioxide gas may not be used as a refrigerant. On the other hand, it is preferably −10 ° C. or lower, and more preferably −30 ° C. or lower. When it exceeds −10 ° C., the gasification temperature of liquefied carbon dioxide gas is high, and the effect as a refrigerant is reduced.

  The cold heat supply system used in the present invention gasifies liquefied carbon dioxide gas as part of a refrigerant supply system having a large gas compressor used in an ethylene plant that requires a large-capacity cold heat supply source. Even if it is used as a part of the refrigerant used for cooling, the ratio of the amount of cold to the whole amount of cold supplied by the cold heat supply system is small. In general, a centrifugal gas compressor used in an ethylene plant has an operation range of 60 to 100%. Therefore, in order to make the best use of the cold heat of liquefied natural gas, it can be obtained at the maximum use of liquefied natural gas. It is preferable to gasify the liquefied carbon dioxide gas in a cold heat supply system that is equal to or greater than the amount of cold heat of the liquefied carbon dioxide gas, and more preferably three times or more. As a result, even if the amount of liquefied natural gas used, that is, the amount of liquefied carbon dioxide gas, decreases, sufficient supply of cold heat required by ethylene plants and other chemical plants is provided by adjusting the operation of the gas compressor. And stable operation can be realized. On the other hand, when the amount of liquefied natural gas used increases, the refrigeration of liquefied carbon dioxide bears a part of the cold heat to be supplied by the gas compressor, so the energy required by the gas compressor is saved. I can do it.

  In addition, as a condition of the above-mentioned other plant capable of supplying cold heat with this cold heat supply system, first, a plant that requires a cold temperature close to the supply refrigerant temperature in the cold heat supply system such as a propylene refrigerator of an ethylene plant It is assumed that. For example, in the case of using a propylene refrigerator, cooling of about −35 ° C. or more and 10 ° C. or less is required. Next, it is preferable that the amount of cooling required by the other plant is equal to or less than the amount of cooling by the liquefied carbon dioxide gas. This is because if the cold heat from the liquefied carbon dioxide gas is used directly, not all of the cold heat of the liquefied carbon dioxide gas can be used, but it is the purpose of the invention to use all of the cold heat in the other plant through the cold heat supply system. . In addition, if the amount of cooling required by the other plant is less than one-third of the amount of cooling by the cooling supply system, it corresponds to the fluctuation range of the cooling supply represented when the operation of the other plant is stopped. This is preferable because it is easy to do. In addition to satisfying these conditions, the plant needs to be close to the ethylene plant. This is because if it is far away, the amount of cold heat lost during the transfer of cold heat is too much and the efficiency becomes worse. Specific examples of such a plant include a vinyl chloride monomer plant and a para-xylene plant, and the present invention can be suitably used when these plants are close to an ethylene plant.

  On the other hand, other chemical plants that receive the supply of liquefied propylene by the ethylene plant cold heat supply system have conventionally had the same cold heat supply system as the above-described steps (1) to (4). In the cold energy supply system according to the present invention, the other chemical plant receives a supply of liquefied propylene suitable for the required amount of cold energy from the cold energy supply system of the ethylene plant, and performs heat exchange with a fluid that requires cooling, By returning the gasified propylene gas to the cooling / heating supply system of the ethylene plant, the cooling / heating supply system conventionally used in other chemical plants becomes unnecessary, and the facilities can be simplified. In addition, it is possible to stably supply a necessary amount of cold heat without adjusting the operation according to the supply amount of liquefied carbon dioxide gas.

  A specific flow of the cold energy supply system according to the present invention will be described with reference to FIG. First, the process in the liquefied natural gas supply plant 41 will be described. The liquefied natural gas (LNG) A is supplied from the LNG storage tank 21 to the liquefied carbon dioxide production process 23 through the LNG transfer line 22 and performs heat exchange with the carbon dioxide B and the like. As a result, the liquefied natural gas A is heated and vaporized. The vaporized natural gas C is transferred to a process using the natural gas C through the natural gas transfer line 25. On the other hand, the carbon dioxide gas B that is the raw material of the liquefied carbon dioxide gas D is supplied to the liquefied carbon dioxide production process 23 through the liquefied carbon dioxide raw material supply line 26 and is cooled by the liquefied natural gas A to produce the liquefied carbon dioxide gas D.

  The liquefied carbon dioxide D is sent to the ethylene plant 42 through the liquefied carbon dioxide transfer line 27. The liquefied carbon dioxide gas D supplied to the liquefied carbon dioxide cold energy recovery step 29 provided in the cold energy supply system 28 in the ethylene plant 42 is supplied with cold energy to liquefy the propylene gas G or further cool it. Vaporizes to carbon dioxide E. The carbon dioxide E is sent to the process using the carbon dioxide E through the carbon dioxide transfer line 30. In the liquefaction of the propylene gas G, in addition to the cold heat supplied from the liquefied carbon dioxide gas D, the propylene gas G is also cooled by the cooling capacity that the cold heat supply system 28 originally has.

  The liquefied propylene H obtained by liquefying propylene gas G in the cold heat supply system 28 in the ethylene plant 42 is sent to the cold heat demand target in the ethylene plant 42, and part of the cold propylene demand is supplied through the liquefied propylene transfer line 31. It is sent to another chemical plant 43 that it has. In another chemical plant 43 to which the liquefied propylene H is sent, in the liquefied propylene cold heat supply step 33, the cold heat of the liquefied propylene is supplied to the object to be cooled and gasified to become propylene gas G. This propylene gas G is returned from the other chemical plant 43 to the liquefied carbon dioxide cold heat recovery step 29 of the ethylene plant 42 through the propylene gas return line 32, and becomes liquefied propylene H again to be recycled. As a result, the cooling / heating supply system 8 for each cooling / demand plant 12 that is necessary in the conventional method shown in FIG.

  FIG. 2 shows a more detailed example of the cold heat flow of the cold energy supply system in which the liquefied carbon dioxide cold energy recovery step 29 and the liquefied propylene cold energy supply step 33 are incorporated. The propylene gas G ′ compressed by the gas compressor 51 in the ethylene plant 42 is supplied to the discharge gas condenser 53 through the compressor discharge line 52 and exchanges heat with the coolant I supplied from the coolant supply line 54. To cool and liquefy to become liquefied propylene H. The cooling liquid I is selected depending on the pressure of the propylene gas G ′, but generally seawater or cooling water is used. The liquefied propylene H is decompressed and expanded through the decompressor 55, so that the temperature is lowered to the saturation temperature of the pressure after decompression to become a mixture H ′ of liquefied propylene and propylene gas. It is supplied to a certain liquefied carbon dioxide evaporator 56.

  In the liquefied carbon dioxide evaporator 56, heat exchange is performed between the mixture H ′ and the liquefied carbon dioxide D, and the cold heat of the liquefied carbon dioxide D is supplied to the mixture H ′ to liquefy propylene gas contained in the mixture H ′. Alternatively, the temperature is further lowered by supercooling the liquefied propylene. Further, the cooled mixture H ′ is supplied to the propylene evaporator 57 except for a part thereof. In the propylene evaporator 57, heat exchange is performed with the plant fluid J that is supplied from the plant fluid supply line 58 and needs to be cooled in the ethylene plant 42, and is evaporated to completely become propylene gas G. In the cold energy supply system according to the present invention, the propylene evaporator 57 cools the plant fluid J that requires cooling, thereby fulfilling the role of cold energy supply. The propylene evaporator 57 is composed of at least one or more, and it is preferable to cool a plurality of plant fluids with a plurality of evaporators, similarly to an evaporator conventionally used in a general chemical plant. The propylene gas G evaporated by the propylene evaporator 57 is supplied to the gas compressor 51 through the propylene gas line 59 at the outlet of the evaporator, and is compressed again to recycle the cold supply system.

  In the cold energy supply system according to the present invention, the amount of cold energy that can be supplied to the plant fluid J is determined by the amount, temperature, and pressure conditions of propylene to be supplied to the propylene evaporator 57. The required exchange heat amount in the propylene evaporator 57 is It is necessary to adjust the flow rate to be constant according to the request on the process side. By adjusting the flow rate of propylene with the gas compressor 51, the necessary heat amount on the process side is satisfied.

  Here, when the supply temperature of propylene to the propylene evaporator 57 decreases due to the cooling in the liquefied carbon dioxide evaporator 56, the amount of heat of evaporation when the propylene evaporator 57 completely converts the propylene gas to the propylene evaporator increases even at the same propylene flow rate. Therefore, the flow rate of propylene supplied to the propylene evaporator 57 can be relatively reduced, and the amount of propylene gas G generated in the propylene evaporator 57 is also reduced. As a result, less power is required to compress the propylene gas G in the gas compressor 51, and the amount of energy required for gas compression required for the entire cold heat supply system can be reduced, and the liquefied carbon dioxide gas is vaporized. The cold energy when using carbon dioxide can be used effectively. When the flow rate of the liquefied carbon dioxide gas, that is, the amount of cold supplied from the liquefied carbon dioxide gas changes, the amount of propylene gas circulating through the cold heat supply system according to the present invention is adjusted by adjusting the operation of the gas compressor 51. It is possible to change the heat exchange amount in the propylene evaporator 57 to be a predetermined amount. In general, the operation adjustment of the gas compressor 51 is performed by a steam steam turbine or an electric drive motor.

  The liquefied carbon dioxide D supplied from the liquefied carbon dioxide transfer line 27 is subjected to heat exchange with the mixture H ′ of liquefied propylene and propylene gas discharged from the decompressor 55 by the liquefied carbon dioxide evaporator 56 to liquefy the propylene gas. Or supercool the liquefied propylene. As a result, the liquefied carbon dioxide D itself is vaporized to become carbon dioxide E, which is sent from the carbon dioxide transfer line 30 to a process that requires the carbon dioxide E. In the liquefied carbon dioxide evaporator 56, the conditions for vaporizing the liquefied carbon dioxide D are preferably the temperature condition and the pressure condition described above.

  In addition, a part of the mixture H ′ of liquefied propylene and propylene gas exiting the liquefied carbon dioxide evaporator 56 is transferred to the propylene evaporator 61 in another chemical plant 43 having a cold demand through the liquefied propylene transfer line 31. Is done. Here, the mixture H ′ exchanges heat with the process fluid K, which is supplied from the process supply line 62 and requires cooling, and is gasified to become propylene gas G, which is returned to the gas compressor 51 through the propylene gas return line 32. Compressed and recycled. This eliminates the need for a conventional cold energy supply system that separately supplies cold energy at another chemical plant 43 having a cold energy demand, and simplifies the facility.

  Hereinafter, as a specific example implemented for the present invention, in a chemical plant having a small cold energy sharing system, when the cold energy of liquefied natural gas is used as cold energy through liquefied carbon dioxide gas, the conventional method and the cold energy according to the present invention are used. The difference from the method using the supply system will be described.

  As an example, it is assumed that the cold energy supply capacity in a cold energy supply system in an ethylene plant is 10, and the cold energy supply capacity in a cold energy supply system in another chemical plant with a small cold energy demand is 1. Further, the amount of cold heat that can be supplied by liquefied natural gas is set to 0.35 to 1.2 of the cold heat supply capacity in the chemical plant, assuming fluctuations in the amount of liquefied natural gas used during the day and night. Furthermore, in each of the cold energy supply systems, it is assumed that the fluctuation range that can cope with fluctuations in the amount of cold energy used by liquefied natural gas is 35% or less.

  According to the conventional method, even if the refrigeration supply by liquefied natural gas fluctuates, the refrigeration supply system in other chemical plants cannot cope with the fluctuations. Only the lower limit of 0.35 can be used. On the other hand, when the cold energy supply system according to the present invention is used, even if the amount of liquefied natural gas used is maximized, the total amount of cold energy supplied by the large-scale cold energy supply system employed in the ethylene plant is 12%. %, Which is well within the range of fluctuation. Therefore, even if the amount of liquefied natural gas used varies, cold energy derived from liquefied natural gas can always be utilized to the maximum extent.

  As another example, the required amount of cold at an ethylene plant is 1.0, the amount of required cold at a vinyl chloride monomer plant, which is another chemical plant, is 0.15, and each has a cold supply system. An example in which the cold heat supply system according to the present invention is used when the amount of cold heat generated in liquefied natural gas varies between 0.05 and 0.2 day and night is shown.

  As in the prior art, when the cold recovery of liquefied natural gas is performed only at the vinyl chloride monomer plant, the amount of cold heat that can be utilized in this plant is within the range of the amount of cold heat that can be handled even if the amount of liquefied natural gas generated varies. Therefore, it becomes 0.05 which is 1/3 of the whole cold energy supply system, and even when the amount of cold energy of the liquefied natural gas is increased from 0.05, the amount of cold energy cannot be used.

  On the other hand, when the cold energy of the liquefied natural gas is recovered at the ethylene plant and the cold energy is supplied to the vinyl chloride monomer plant, the amount of cold energy of the liquefied natural gas that can be recovered at the ethylene plant is 1/3 of the total. 35, which exceeds the maximum amount of cold heat of liquefied natural gas 0.2, so that all of the cold amount of liquefied natural gas can be used. On the other hand, the cold heat supply amount from the liquefied natural gas is temporarily 0, that is, −0.2 is reduced from the maximum value, and together with the necessary cold heat amount 0.15 of the vinyl chloride monomer plant, the cold heat supply system bears. Even if the fluctuation range is maximum, it is -0.35 in total, which is the fluctuation range of the range that can be handled. In addition, when supplying cold heat from an ethylene plant to a vinyl chloride monomer, it is preferable to carry out with -30 degreeC propylene.

  Furthermore, in the conventional method, it is necessary to maintain the cold energy supply system in a chemical plant with a small demand for cold energy. However, the cold energy supply system according to the present invention is not necessary, and the equipment maintenance cost can be saved, and the large-scale system is consolidated. By doing so, it is possible to avoid small energy loss.

The figure which shows the example of a flow of the system concerning this invention The figure which shows the detailed example of a flow of the liquefied propylene of the system concerning this invention The figure which shows the example of a flow of the use of cold in the conventional plant which requires cold

Explanation of symbols

1,21 LNG storage tank 2,22 LNG transfer line 3,23 liquefied carbon dioxide production process 4 liquefied natural gas evaporator 5,25 natural gas transfer line 6,26 liquefied carbon dioxide raw material supply line 7,27 liquefied carbon dioxide transfer line 8,28 Cold supply system 9,29 Liquefied carbon dioxide cold recovery process 10,30 Carbon dioxide transfer line 11,41 Liquefied natural gas supply plant 12 Cold heat demand plant 13 Seawater line 31 Liquefied propylene transfer line 32 Propylene gas return line 33 Liquefied propylene Cold heat supply process 42 Ethylene plant 43 Other chemical plant 51 Gas compressor 52 Compressor discharge line 53 Discharge gas condenser 54 Coolant supply line 55 Depressurizer 56 Liquefied carbon dioxide evaporator 57 Propylene evaporator 58 Plant fluid supply line 59 Evaporation Outlet propylene gas line 1 Propylene evaporator 62 Process supply line a, A Liquefied natural gas b, B Carbon dioxide gas c, C Natural gas d, D Liquefied carbon dioxide e, E Carbon dioxide f Seawater G, G 'Propylene gas H Liquefied propylene H' (Liquefaction Mixture of propylene and propylene gas I Coolant J Plant fluid K Process fluid

Claims (1)

A propylene cold heat supply system that repeats the steps of (1) gas compression of propylene, (2) liquefaction and / or supercooling by cooling, (3) expansion under reduced pressure, and (4) evaporation by supplying cold heat. And
The liquefied carbon dioxide obtained by absorbing the cold heat of liquefied natural gas into carbon dioxide gas, the pressure is 0.52 MPa or more and 3 MPa or less in gauge pressure, and the temperature is −55 ° C. or more and −10 ° C. or less in an ethylene plant. Used as at least a part of the refrigerant used in cooling in the step (2) performed in
A part of propylene liquefied by cooling is transferred to another chemical plant outside the ethylene plant, and the step (4) is performed as a refrigerant of a cooling system in the other chemical plant. The propylene gasified by the cooling system in (2) is returned to the ethylene plant, and the steps (1) to (4) are repeated, and the cold supply system using propylene using the cold heat of liquefied natural gas.

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