MXPA00006665A - Ethylene plant refrigeration system - Google Patents

Abstract

A refrigeration system for an ethylene plant uses a low pressure demethanizer (12) and a binary refrigerant (20) comprising a mixture of methane and ethylene or methane and ethane. The refrigeration composition may be constant throughout the system or separators (88, 108, 120, 128) may be used to divide the refrigerant into a methane-rich binary refrigerant (140) and an ethylene- or ethane-rich binary refrigerant (142).

Description

ETHYLENE PLANT REFRIGERATION SYSTEM Background of the Invention The present invention relates to a cooling system for providing the cooling requirements of an ethylene plant. More particularly, the invention is directed to the use of a binary refrigerant comprising a mixture of methane and ethylene for cooling in an ethylene plant. Ethylene plants require refining to separate desired products from a cracking heater effluent. Typically, a C3 refrigerant usually propylene, and a C2 refrigerant typically ethylene, are employed. Often, particularly systems using demetallizers with low pressure where lower temperatures are required, a separate methane cooling system is also employed. In this way, three separate cooling systems are required, in cascade from the lowest temperature to the highest. Three compressor and driver or exciter systems, complete with suction drums, separate exchangers, pipes, etc., are required. Also, a methane cooling cycle often requires reciprocating compressors that can partially fuel any savings in capital costs that result from the use of low pressure demethanizers.

Mixed refrigerant systems have been well known in the industry for many decades. In these systems, multiple components are used in a simple • cooling system to provide cooling over a wider range of temperatures, allowing a mixed cooling system to replace multi-component pure cascade cooling systems. These mixed cooling systems have found wide use in natural gas plants based on liquid cargo. Articles have also been written in the application of mixed cooling systems to the design of ethylene plants, but they are complicated in operation due to the multiplicity of components in the refrigerant. Also, they are less efficient in the propylene refrigeration compressor cycle temperature range at -40 ° C or hotter. SUMMARY OF THE INVENTION It is an objective of the present invention, therefore, to provide a simplified refining system for an ethylene plant having a low pressure demethanizer using a mixture of methane and ethylene, or alternatively methane and ethane as a binary refrigerant in cascade against a cooling system of propylene or in an alternate form of propane. This system replaces the separate cooling systems of methane and ethylene, which are used in conjunction with a propylene refrigeration system in conventional plants and saves a compressor system. The refrigerant composition may be constant throughout the system or separators may be employed to partially evaporate and split the binary refrigerant into a methane-rich stream and a methylene-rich stream for separate circulation in one or more heat exchangers. The objectives, arrangement and advantages of the cooling system of the present invention will be apparent from the description that follows. Brief Description of the Drawings Figure 1 is a schematic flow diagram of a portion of an ethylene plant illustrating one embodiment of a cooling system of the present invention. Figure 2 is a schematic flow chart similar to Figure 1, but illustrating an alternative embodiment of the invention. Figure 3 is a schematic flow diagram illustrating a variation of the embodiment of Figure 2. Description of Preferred Modes The present invention involves an ethylene plant in which a pyrolysis gas is first processed to remove methane and hydrogen and then it processes in a known manner to produce and separate ethylene as well as propylene and some other by-products. The separation of gases in an ethylene plant through condensation and fractionation at cryogenic temperatures requires cooling over a wide range of temperatures. The cost of capital involved in the cooling system of an ethylene plant can be a significant part of the cost of the total plant. Thus, capital savings for the refining system will significantly affect the total plant cost. Ethylene plants with high pressure demethanizers operate at pressures above 2,758 MPa (400 psi), which can cause reflux of overhead products when condensing against a pure component ethylene refrigeration. The demethanizer head product temperatures of these systems are typically in the range of -85 ° C to -100 ° C. Ethylene refrigeration at approximately -101 ° C, typically used to cool the overhead condenser. At pressures below 2758 MPa (400 psi), the temperature of overhead products is typically too low to use ethylene refrigeration unless vacuum suction was employed. But that is not desirable due to the increase in the cost of capital and the consideration of safety due to potential air leakage to the system. The present invention involves the use of a low pressure demethanizer and a binary coolant system. For purposes of the present invention, a low pressure demethanizer is one that operates below about 2.41 MPa (350 psi) and generally in the range of 0.345 to 1.034 MPa (50 to 150 psi) and with temperatures of overhead products in The range from -200 ° C to -235 ° C. The advantage of the low pressure demethanizer is the lower total plant power requirement and a lower total capital cost of the plant while the disadvantage is the lower cooling temperature required and therefore the need to date for a compressor of cooling with separate methane. The binary refrigerant of the present invention comprises a mixture of methane and ethylene. The ratio of methane to ethylene will vary depending on the ethylene plant cracking feed material, cracking severity, cooling train pressure and the nature of the coolant, among other considerations, but will normally be in the range of 10:90 a 50:50 and more likely in the range of 20:80 to 40:60. The use of the cooling system of methane and ethylene or methane and ethane, together with a propylene or propane cooling system, allows the refrigeration load and temperatures required for an ethylene plant that has a low pressure demethanizer while avoiding the need for three separate refrigerants of methane, ethylene and propylene. A binary refrigerant with a high pressure demethanizer will not be used, because there is no need to provide that level of refrigeration. There is no need to use a simple substitute common binary refrigeration system for a pure component ethylene refrigeration system. It would only be more expensive and complex. Mixed refrigerant systems to replace both the ethylene and propylene refrigeration systems have been proposed but require at least one component lighter than ethylene such as methane. Therefore it is at least a ternary system. It is usually also more economical to use heavier components than propylene such as C4 components, such that the system is usually at least one quaternary coolant system. The purpose of the present invention is to provide the necessary cooling for the charge gas (pyrolysis gas) in general to separate the hydrogen and methane and to provide a feed for the demethanizer. With reference to the embodiment of the invention shown in Figure 1, the charge gas feed 2 which is the conditioned pyrolysis gas as required and cooled, is typically at a temperature of about -35 ° C to -37 ° C. and at a pressure of approximately 3.45 MPa (500 psi) and is typically already partially liquefied. The charge gas 2 is progressively cooled by the cooling system of the present invention in the heat exchangers 4, 6, 8 and 10 and is separated to produce demethanizer feeds as will be explained later. Heat exchangers 4, 6, 8 and 10 are typically aluminum, soldered solder exchangers, also referred to as core or finned plate exchangers and can be physically combined as fewer units or expanded in a greater number of units. In demethanizer 12, the Cx and lighter components, primarily methane and hydrogen, are separated from the C2 and heavier components. The net head products 14 of demethanizer 12 are used as a cooling stream in the cooling system as will be explained below. The bottoms 16 of the demethanizer can also be used as a cooling stream in another portion of the cooling system as will also be explained below. Turning now to the cooling system per se, the binary refrigerant as previously identified as a mixture of methane and ethylene, is compressed by the refrigeration compressor 18 to a pressure in the range of about 3.0 to 4.0 MPa. In the table below, specific pressures and temperatures are listed for a specific example of the invention. The compressed binary refrigerant 20 is cooled in 22 and 24 such as with cooling water or other cold stream and further cooled in 26 such as by a propylene refrigerant at a temperature in the range of about -30 to -40 ° C. . The liquid binary refrigerant is collected in the receiver or accumulator 28. The refrigerant 30 of the receiver 28 can also be cooled in 32 by heat exchange with the bottoms 16 from the demetallizer 12 or another cold stream that is heated, which will reduce the temperature. The demethanizer bottoms leaving the heat exchanger 32 in 34, are sent to the deethanizer for the conventional production and separation of ethylene, propylene and other by-products. The binary refrigerant 36 of the heat exchanger 32 is then passed to the first of the series of heat exchangers 4, 6, 8, 10 and 11. The heat exchangers 4 to 10 are the heat exchangers that provide the cooling of the charge gas from the pyrolyzer. The heat exchanger 11 provides reflux to the demethanizer.

First with reference to the heat exchanger 4, the binary refrigerant 36 is passed through the heat exchange coil 46 and cooled. A portion of the binary refrigerant is then removed at 48 and the temperature abated by reducing the pressure through the expansion valve 50. This portion of cooled binary refrigerant is then passed back through the heat exchange coil 52. The valve of expansion 50 is controlled in response to the temperature of the charge gas stream 54 cooled in the heat exchanger 4, thereby controlling the temperature of the refrigerant in the heat exchange coil 52. The binary refrigerant in the heat exchange coil 52 absorbs heat and vaporizes and overheats to a temperature range of 1 to 5 ° lower than the inlet stream 36. The vaporized binary refrigerant 56 of the coil 52, passes to the suction drum 58 from which the refrigerant vapor stream 60 it supplies the binary refrigeration compressor 18. The suction drum 58 as well as other suction drums 84, 102 and 130 referred to below, are pres- Only to separate any liquid that may be present in an unbalanced condition to avoid potential damage to the compressor. It is not required for the normal operation of the system.

The reason that the binary refrigerant is first passed through the heat exchanger 4 to cool before flash evaporation at 50 is to decrease the vapor percent evaporated instantaneously to a fixed instantaneous evaporation pressure. In this way, the evaporated liquid will instantly be cooler and can provide more cooling at lower temperatures. For a pure component refrigerant, the evaporated liquid temperature is instantaneously set for any instantaneously evaporated evaporated liquid pressure and there would be no net gain of cooling before instantaneous evaporation. This same principle applies to the other heat exchangers 6, 8, 10 and 11. Additional cooling in the heat exchanger 4 as well as in the other heat exchangers 6, 8 and 10, is provided by currents 62, 64 and 66 which are common at low temperature of hydrogen, methane at low pressure and methane at high pressure, respectively. These low temperature streams 62, 64 and 66 come from the cryogenic hydrogen / methane separation system 68 and head products 14 from the demetallizer 12. The net head product stream 66 also provides cooling for the heat exchanger 11 which serves as a condenser reflux of the demethanizer.

The cooled charge gas 54 can also be cooled to 70 and fed to the next heat exchanger 6. The cooling in exchanger 70 can be reboiled and interwoven from demethanizer 12. The remaining binary cooled refrigerant 72 of heat exchanger 4 is also fed to the next heat exchanger 6. This heat exchanger 6 is operated in the same way as the heat exchanger 4 except that all the relevant temperatures are now lower including the temperatures of the incoming binary refrigerant stream 72, the output binary refrigerant stream 74, the binary refrigerant stream 76 after the expansion valve 78, the vaporized binary refrigerant stream 80 of the coil 81 and the output charging gas stream 82. The evaporated binary refrigerant 80 is fed to the suction drum 84 and then fed into the compressor 86 binary refrigeration 18. The charge gas stream 82 is fed to the s eparator 88 wherein the cooled charge gas is separated into a less volatile demethanizer feed stream 90 and a more volatile overhead product stream 92 which is now more concentrated in methane and hydrogen. The head products 92 and the binary refrigerant 74 are passed to the next heat exchanger 8 where the cooling process continues in the same way producing the cooled charge gas 94 and the additional binary 96 refrigerant. Again, a portion of the binary refrigerant passes through the expansion valve 98 and the coil 100 to the suction drum 102. The steam 104 is then fed to the binary refrigerant compressor 18. The heat exchanger 8 can also be further cooled by the cooling stream. vaporized binary 106 from the heat exchanger 10. The charge gas 94 from the heat exchanger 8 is fed to the separator 108 where the more volatile components are removed from overhead products at 110 and fed to the heat exchanger 10. These overhead products are now even more concentrated in hydrogen and methane. The bottoms from the separator 108 are fed into 112 of the demetallizer 12. The continuous cooling process in the heat exchanger 10 by the expansion of an additional portion of the binary refrigerant through the expansion valve 114 and the evaporation of the coil 116 to produce the aforementioned binary refrigerant stream 106. The outlet charge gas 118 is fed to the separator 120 with the overhead products 122 now that it is primarily hydrogen and methane. The overhead products 122 are fed to the hydrogen / methane 68 separation system where the hydrogen and methane are cryogenically separated to produce the hydrogen stream 62 and the low pressure methane stream 64. The bottoms from the separator 120 are fed at 124 to the dematerizer 12. The remaining binary refrigerant stream 126 is further cooled in the heat exchanger 11 by the net head products of demethanizer 66. The binary refrigerant stream 126 expands at 133 and passes back through the coil 135 in the heat exchanger 11 to mix with the refrigerant from the valve 114. The stream of raw head products 14 from the demetallizer 12 passes to the heat exchanger 11 where it is partially condensed. This partially condensed stream 127 circulates to separator 128. Liquid 129 of separator 128 flows back to demethanizer 12 as reflux. The head products 66 of the separator 128 are now the overhead products of the net demethanizer comprising primarily methane, which is reheated as it passes back through the heat exchangers 11, 10, 8, 6 and 4. The demethanizer column 12 has the typical reboilers and interweavers between stages that have not been shown. The demethanizer bottoms 16 comprise C2 and heavier components. Retrievers and interboxers are typically provided by cooling the charge gas such as by heat exchanger 70. Stream 106 passes to suction drum 130 and then 132 to binary refrigerant compressor 18. Although Figure 1 illustrates four heat exchangers 4, 6, 8 and 10 the number of these heat exchangers may vary depending on the particular needs for any particular ethylene process and in particular on the particular charge gas. The following table lists temperatures and some pressures for the binary refrigerant and for the charge gas (process gas) including the demethanizer system at various sites in the process flow scheme of Figure 1 for a specific example: Some of the advantages of the binary refrigerant system of the present invention have been mentioned previously and include a reduction in the number of compressor systems and in the capacity to use all centrifugal or axial compressors instead of a methane refrigerant compressor. An additional advantage is that the binary refrigerant composition is easier to maintain than a more complicated mixed refrigerant containing 3 or more components. This is most evident in the case of an imbalance or system instability that results in refrigerant venting. The ventilation process results in the loss of more of the light components of the coolant than the heavier components. This changes the proportion of the components that must be corrected when restarting. The more complicated the composition of the refrigerant, the more difficult it is to correct the ratio. In the process of the present invention illustrated in Figure 1, the refrigerant composition remains constant throughout the process. However, in the alternate embodiment of the invention illustrated in FIG. 2, there is a separation of binary refrigerant in a binary methane-rich stream and a stream rich in binary ethylene. "In figure 2, which generally shows only the portion of Figure 1 being modified, an expansion valve 136 is located on line 36. The pressure of the binary refrigerant drops and a portion evaporates. The liquefied portion and the vapor portion are separated in the instantaneous vaporization tank 138, whereby the vapor portion 140 will be rich in methane and the liquid portion 142 will be rich in ethylene or ethane. In this embodiment of Figure 2, the methane-rich stream 140 passes through all heat exchangers 4, 6, 8 and 10 and a portion then expands at 144 and passes back as stream 146 through all heat exchangers 10 , 8, 6 and 4. Another portion 126 of the stream 140 leaving the exchanger 10, is cooled in the exchanger 11, expands at 133 and passes back through the exchanger 11 and gathers the stream 146 at the inlet of the exchanger 10. The output methane-rich binary refrigerant stream 146 will then be passed back to the first stage of the compressor 18. The ethylene-rich stream 142 is handled somewhat like the binary refrigerant stream in Figure 2, in which a portion is it removes after each of the first three heat exchangers at 148, 150 and 152 and expands at 154, 156 and 158. The expanded portions are then passed back through one or more of the heat exchangers to produce the corr binary coolant rich in ethane or ethylene outlet 160, 162 and 164 which are fed back to the appropriate compressor stages.

The advantage of the scheme of Figure 2 where the binary refrigerant is separated is that it allows a higher pressure in the suction of the compressor for any composition of the binary refrigerant determined at the outlet of the compressor. The suction pressure is higher because the coolant composition is richer in methane and therefore for a fixed coolant temperature, the pressure will be higher. This means that the compression ratio of the compressor is lower and this can result in a reduced compressor cost. A variation of figure 2 has no valve 136 on line 36. On the contrary, the pressure on line 36 is reduced such that the stream does not completely liquefy and remains in a vapor portion. The separator 138 separates the condensed liquid portion from the steam portion enriched with methane. This variation allows the compressor 18 to have an internal discharge pressure for any given composition of methane-ethylene (or methane-ethane) for stream 36. The total compression ratio for the compressor 18 is reduced. The flow rate of stream 36 is increased to compensate for any composition of the determined stream 36. However, compressor costs can be decreased. This scheme is of particular interest for smaller ethylene plants where the current compressor volume at the compressor discharge 18 approaches the lower limit permissible by the centrifugal compressor design. Figure 3 is a still further modification of the present invention similar to the embodiment shown in Figure 2, but with an additional separation step for the binary refrigerant. As shown, there is the first separation at 138 as well as in the embodiment of Figure 2. The methane-rich binary refrigerant vapor stream 140 is passed through the partially exchanged heat exchanger 4 and then passes through line 166 to the additional refrigerant separator 170, wherein the refrigerant is again separated into a second methane-rich steam stream 172 and a second liquid stream rich in ethane or ethylene 174. The methane-rich stream 172 will be richer in methane than the stream 174 and the stream 140. The ethylene or ethane-rich stream 142 passes through the heat exchangers as in the embodiment of Figure 2. Similarly, the second methane-rich stream 172 is passed through the second heat exchanger 6 and then it circulates to the lower temperature heat exchangers as in the other modes where it expands and passes back through the heat exchangers. The second stream rich in ethylene or ethane 174 is passed through the second heat exchanger, expanded at 178 and passed back through the heat exchanger. This figure 3 illustrates only two heat exchangers for simplicity but there may be additional heat exchangers and additional separators similar to separator 170. The advantages of this variant of the process of Figure 3 are that the binary refrigerant pressures are higher than any given temperature level of refrigeration. . This decreases the compression ratios in the binary refrigerant compressor and can reduce compressor capital costs.

Claims (9)

1. CLAIMS 1.- In a process for the production of ethylene from a charge gas containing hydrogen, methane, ethylene and other C2 and heavier hydrocarbons, where the process includes a low pressure demethanizer that operates at a pressure of below 2.41 MPa (350 psi) and where the charge gas is cooled by a refrigeration system, a method for cooling the charge gas by the use of a binary refrigerant in the refrigeration system comprising the steps of compressing a mixture of methane and ethylene or methane and ethane to produce binary refrigerant, progressive expansion and cooling of the binary refrigerant through a series of heat exchangers, progressively carry the progressively cooled binary refrigerant and the charge gas in heat exchange contact in the heat exchangers for cooling and in this way separate the hydrogen and a portion of the methane and produce liquid demethanizer feed streams c Concentrated on ethylene and other C2 and heavier hydrocarbons, feed the feed streams from the liquid demethanizer to the low pressure demethanizer and produce a stream of raw demethanizer overhead products that essentially consist of methane, contact in the stream of products from head of the crude demethanizer with the binary refrigerant progressively cooled and remove a reflux stream of demethanizer and a stream of net demethanizer head products and return the reflux stream of demethanizer to the demethanizer.
2. 2. - In a method according to claim 1, characterized in that the stream of net demethanizer overhead products is brought into heat exchange contact with the charge gas in the heat exchangers.
3. 3. - In a method according to claim 1, characterized in that the hydrogen and the methane portion separated from the charge gas by cooling in the heat exchangers are subjected to cryogenic separation to produce a stream of hydrogen and a stream of methane and in where each of the hydrogen and methane streams are brought into heat exchange contact with the charge gas in the heat exchangers.
4. 4. - In a method according to claim 1, characterized in that the stage of progressive expansion and cooling of the binary refrigerant through a series of heat exchangers comprises the steps of passing the binary refrigerant through one of the heat exchangers, subjecting expand a portion of binary refrigerant after passing through the heat exchanger, pass the expanded portion back through the heat exchanger and pass the remaining portion of the binary refrigerant to and through the next of the heat exchangers and repeat the expansion step an additional portion and pass the additional portion back through the heat exchanger.
5. 5. - In a method according to claim 4, characterized in that the portions of the binary refrigerant after passing back through the heat exchangers are passed again to the step of compressing the binary refrigerant.
6. 6. In a method according to claim 1, characterized in that the dematanizer feed stream separated by each of the heat exchangers, each is fed to different steps of the demethanizer.
7. 7. In a method according to claim 1, characterized in that it also includes the step of separating the binary refrigerant in a binary rich coolant in methane and a binary coolant rich in ethane and wherein the step of progressively contacting the cooled binary refrigerant progressively with the charge gas, comprises the step of contacting the charge gas with separate streams of the methane-rich binary refrigerant and the binary refrigerant rich in ethylene or ethane in the heat exchangers.
8. 8. - In a method according to claim 7, characterized in that it also includes the step of separating the methane-rich binary refrigerant in a second binary methane-rich refrigerant and a second binary refrigerant rich in ethylene or ethane and wherein the step of contacting the charge gas comprises the step also of contacting the charge gas with separate streams from the second methane-rich binary refrigerant and the second binary refrigerant rich in ethylene or ethane.
9. 9. In a method according to claim 7, characterized in that the step of compressing the mixture of methane and ethylene or methane and ethane comprises the step of compressing to form a vapor-liquid mixture and wherein the liquid-mixture The vapor separates to form the binary refrigerant rich in methane and the binary refrigerant rich in ethylene or ethane.