US3813890A

US3813890A - Process of continuous distillation

Info

Publication number: US3813890A
Application number: US00133913A
Authority: US
Inventors: B Bligh
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-07-02
Filing date: 1971-04-14
Publication date: 1974-06-04
Anticipated expiration: 1991-06-04
Also published as: GB1310514A

Abstract

Method and apparatus for continuous distillation in which products are removed from the ends of a distillation column and are also removed from an intermediate zone of the column, subjected to a refrigeration cycle and returned at least in part to the column, and reflux and reboiling are provided at more than one zone of the column.

Description

United States Patent 1191 Bligh June 4, 1974 [5 1 PROCESS OF CONTINUOUS DISTILLATION 2,627,731 2/1953 Benedict 62/31 2,677,945 5/1954 Miller 62/40 [76] Inventor: Bemafd Ramsay Bhgh, 2,s23,523 2/1958 Eakin 62/26 Jarnes 5 Ave., Hampton H111, 2,919,554 1/1960 Kohler 62/34 Mldd e e Engla 3,339,370 9/1967 Streich 62/40 3,592,015 7/1971 Streich 62/40 [22] 1971 3,605,423 9/1971 Stoklosinski 62/28 [21] Appl. No.: 133,913

- Primary Examiner-Norman Yudkoff Assistant ExaminerA. Purcell [52] US. Cl 62/40, 62/28, 62/31 51 rm. (:1. F25j 3/02, F25j 3/08, F25j 5/00 Age, Haffne [58] Field of Search 62/23, 24, 27, 28, 29,

62/31, 34, 40, 33 [57] ABSTRACT Method and apparatus for continuous distillation in [56] References C'ted which products are removed from the ends of a distil- UNITED STATES PATENTS lation column and are also removed from an interme- 1,594,336 7/1926 Mewes 62/31 diat n of th column, subjected t a rig rati n 2,146,197 2/1939 Twomey 62/28 cycle and returned at least in part to the column, and 2,180,435 11/1939 Schlitt 62/31 reflux and reboiling are provided at more than one 2,469,724 5/1949 Gross 62/28 zone f the column 2,600,110 6/1952 Hachmuth 62/26 2,608,070 8/1952 Kapitza 62/33 7 Claims, 6 Drawing Figures VAPOR I LIQUID PATENTEDJUH 4197 SHEET 1 0F 4 m MEDQE N mmDmv;

INVENTOR B. R.

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PATENIEDJUH 4:914

sum 2 or 4 PATENTEDJUN 41974 SHEU 3 0F 4 mw mzDOI INVENTOR B. '12. B2L L PROCESS OF CONTINUOUS DISTILLATION This invention relates to the art of continuous fractional distillation, (the term continuous is used herein to distinguish this process from batch distillation). It is well-known to those skilled in this art that if a distillation column has reflux added only at the top of the column, and reboiling applied only at the bottom of the column, the thermodynamic efficiency of the process can never approach 100 percent. It has been shown that the thermodynamic efficiency of a distillation column can be increased by having one or more increments of reflux at points between the feed zone and the top reflux zone, and by having one or more increments of heat added between the feed zone and the bottom reboiler, but it has been found that the multiplicity of heaters and coolers make a complexity which outweighs the advantages of high thermodynamic efficiency.

This problem is particularly significant in distillation processes in which the refluxtemperature is below the ambient atmospheric temperature. In these circumstances the process requires a refrigeration system. It follows that if the thermodynamic efficiency of the distillation is to be increased and if conventional refrigeration cycles are used, a number of refrigeration cycles are required, and this requirement adds to the complexity.

An example of this problem is to be found in a process for making ethylene, in which light naphtha is subjected to steam pyrolysis; the products of this pyrolysis are hydrogen, methane, ethylene, propylene and a range of other hydrocarbons. This mixture is separated by low temperature distillation, and at least four distillation columns are required to produce substantially pure ethylene and substantially pure propylene. This well-known process has a poor thermodynamic efficiency which by the prior art can only be improved by a number of refrigeration cycles and a multiplicity of heaters and coolers in some of the aforesaid four distillation columns. There are processes, known to those skilled in the arts, by which ethylene is used to manufacture polyethylene, ethylene glycol and the tere-' phthalate ester of ethylene glycol; the said ester is polymerized to a substance used for making a fibre; propylene is used to manufacture polypropylene and propylene glycol.

It is the object of the present invention to provide a process for continuous distillation which is thermodynamically more efficient than a conventional distillation process (this latter being one in which reflux is added only at the top of a column and reboiling is applied only at the bottom of the column).

It is a further object of the present invention to provide the said efficient distillation process in which the coldest zone of the distillation process is below the ambient atmospheric temperature.

It is a particular object of the invention to provide a process for separating pyrolysis gas into ethylene and other constituents.

Another object of the invention is to provide a process for separating natural gas into a methane-rich fraction and an ethane-rich fraction.

It will be appreciated by those with a wide knowledge of chemical engineering that this invention will find other applications, for example if a source of natural gas has appreciable quantities of propane in it this invention can be applied to the process for obtaining a substantially pure propane fraction.

According to the present invention there is provided a distillation column with a means for withdrawing a stream from a zone between the ends of the column, a means for passing the said stream through a refrigeration cycle, a means for returning at least some of the said stream to the distillation column, a means for providing reflux at more than one zone of the column and a means for providing reboiling at more than one zone in the column.

understood by those skilled in the arts and require no further description herein.

The first form of the invention is described with reference to FIG. 1. A distillation column I, most suitably constructed with plates 24 and downcomers 25, has its coldest zone 26 below 5C; line 2 is the feed pipe; line 3 comes from a reservoir of liquid which is fed from a downcomer and which is below line 2. This liquid is a mixture of the components being distilled and is called the refrigerant herein. Part of this stream passes through line 4, is cooled in heat exchanger 5, and is let down in pressure at valve 6, to the main reflux condenser 7 where the refrigerant vaporizes at least partially. The condensate so created passes via vessel 18 to pump 19. Pump 19 delivers the liquid partly to the top of the distillation column and partly to line 20 as top product. From the main reflux condenser 7'the refrigerant vapor, which may contain some liquid, passes via line 8 to heat exchanger 5 where it is warmed and vaporized completely. This refrigerant vapor is then warmed nearly to atmospheric temperature in heat exchanger 9 prior to entering compressor 10.

A second portion of the liquid refrigerant from line 3 passes through line 11, is cooled in heat exchanger 12, and is let down in pressure at valve 13 by such an amount that the refrigerant temperature falls to a level appropriate for carrying out some condensation at a zone in the distillation column between line 2 and the top of the column. The refrigerant vaporizes at least partially in heat exchanger 14 which is suitably a number of tubes or coils erected in the said zone. From heat exchanger 14 the refrigerant vapor, which may contain some liquid, passes via line 15 to heat exchanger 12 where it is warmed and vaporized completely. This refrigerant vapor is then warmed nearly to atmospheric temperature in heat exchanger 9 prior to entering compressor 10.

v The two refrigerant streams are combined in the compressor and delivered at such a pressure that the vapor condenses in heat exchanger 16 which is cooled by water near atmospheric temperature. The liquid refrigerant is then'cooled in heat exchanger 9 and passes back to the distillation column via valve 17. The entry point 23 of this refrigerant is just above the reservoir mentioned above. Because some of the refrigerant flashes off at valve 17, this zone of the distillation column acts like an intermediate reboiler.

The heat for the main reboiler 21 may come either from steam or from any other appropriate source; line 22 takes the bottom product.

In this plant description and in all the descriptions which follow, all heat exchangers are of the indirect type. Heat exchangers such as 5 and '9, which are depicted in FIG. 1 as rectangles, are preferably, but not essentially, of the plate and corrugation type. Heat exchangers depicted as circles, such as 7 and 16, are preferably, but not essentially, of the tube in shell type. This convention, which is well-known in the art of chemical engineering, is used in all the figures inthis patent specification.

There are many minor variants on the first form of the invention. For example, if the low pressure gas streams leaving heat exchangers 5, 12, are near to atmospheric temperature then heat exchanger 9 is omitted.

If the distillation column has a number of feed pipes entering at different zones, heat exchanger 14 (herein called the intermediate condenser must be above at least one of these feed zones, and the intermediate reboiling zone 23 must be below the intermediate condenser and below at least one of the feed zones.

The second form of the invention is described with reference to FIG. 2. A distillation column 31 has its coldest zone 26 below 5C; line 32.is the feed pipe; line 33 takes refrigerant from a reservoir of liquid which is below line 32. The refrigerant is cooled in

heat exchangers

34, 35, and is let down in pressure at valve 36 to the main reflux condenser 37 where the refrigerant vaporizes partially. The refrigerant liquid and vapor are disengaged in vessel 38; the vapor is used to cool heat exchanger 35; the liquid is passed to the intermediate condenser 44 where it is vaporized at least partially. The refrigerant from the intermediate condenser 44 rejoins the vapor from vessel 38 at line 46. The combined streams are warmed in

heat exchangers

34, 39, and enter compressor 40. After the compressor the refrigerant is condensed in the water cooled heat exchanger 41 and cooled in heat exchanger 39 before being let down at valve 42 and returned to the distillation column at a point just above the reservoir.

The main reflux pump is item 47; the top product is withdrawn via line 43, and the bottom product via line 45.

This form of the invention has many minor variants comparable with the variants of the first form. If the refrigerant contains a substantial proportion of a perma-' nent gas such that the mixture cannot be condensed by cooling water in heat exchanger 41, then an auxiliary refrigeration system is required; (an example of this case is given later in this specification).

If the distillation column has a number of feed pipes entering at different zones, the intermediate condenser 44 must be above at least one of these feed zones, and the re-entry point at valve 42 must be below the intermediate condenser and below at least one of the feed zones.

The third form of the invention is described with reference to FIG. 3. A distillation column 531 has its coldest zone 26 below 5C; line 532 is the feed pipe. Via line 533 a mixture (herein called the refrigerant) is withdrawn from the vapor space in a zone 544, which is above the feed zone and below the top reflux zone.

Line 533 passes to a disengagement vessel 545 where the greater part of any spray is separated from the vapor; the liquid collects at the bottom of vessel 545 and runs back to the distillation column by gravity via line 546, which contains a U-bend in order to keep this line primed with liquid. Vapor from vessel 545 passes along pipe 534 to

heat exchangers

535, 539, where it is warmed successively to atmospheric temperature nearly.

The vapor enters compressor 540 and after delivery it is cooled in heat exchanger 553 by water near to atmospheric temperature. The high pressure gas is divided in two parts; the first part is cooled in heat exchanger 539 and then condensed in heat exchanger 547, which is an intermediate reboiler in a zone of the distillation column between the feed zone and the main reboiler 551. The intermediate reboiler is suitably a tube bundle fitted into a downcomer which is sized for the purpose. The refrigerant leaving heat exchanger 547 is cooled in heat exchanger 535 and then let down in pressure via valve 543 into zone 544; this stream provides intermediate reflux.

The second high pressure stream from heat exchanger 553 is cooled successively in

heat exchangers

538, 541, 542, at which point the refrigerantis liquid. The refrigerant is let down in pressure via valve 536 to the reflux condenser 537 where it vaporizesat least partially. The reflux so created passes via vessel 548 to pump 549. Pump 549 delivers liquid partly to the distillation column top and partly to line 550 as top product. From the main reflux condenser 537 the refrigerant vapor, which may contain some liquid, passes through

heat exchangers

542, 538, where it is warmed nearly to atmospheric temperature before entering compressor 540.

The manner in which liquid refrigerant is produced in this cycle depends on the thermodynamic properties of the refrigerant. If it contains a substantial proportion of a permanent gas, then some auxiliary refrigeration is required for heat exchanger 541. Alternatively if the refrigerant at a high pressure is condensable at atmospheric temperature, heat exchanger'54l can be omitted.

If the distillation column has a number of feed pipes entering at different zones, the intermediate reflux zone 544 must be above at least one feed pipe, and the intermediate reboiler 547 must be below the intermediate reflux zone and below at least one feed zone.

in another variant of this form of the invention there are a number of intermediate reboilers at different zones of the distillation column, each of these reboilers is subject to the stipulations of the previous paragraph.

in the description which follows temperatures are 9 given in degrees centigrade and pressures in atmospheres absolute where 1.000 atmospheres X 1.013 dynes per square centimetre.

The process is described with reference to FIGS. 4A, 4B and 4C. Feed gas enters the plant at about 1.2 atm and about 25C via line 61 and vessel 62. The gas is compressed in a three-

stage compressor

63, 64, 65, to a pressure most suitably 22.4 atm. The compressor is installed with

water coolers

66, 67, 68, and

disengagement vessels

69, 70, 71, which collect organic and aqueous condensates. The

water coolers

66, 67, cool the gas to about 28C and cooler 68 to 26C. The aque ous liquid is rejected via

lines

72, 73, 74. The vapor from vessel 71 is passed through tower 79 for removal of carbon dioxide by sodium hydroxide solution and through tower 80 containing water for removing traces of sodium hydroxide. The vapor is then cooled in

heat exchangers

81, 82, and the mixed vapor and condensate pass to vessel 84. The cooling is effected by the condensate streams 76, 77; stream '76 is mostly C C and C hydrocarbons and about 0.1 percent molar ethylene glycol injected via line 83. The ethylene glycol prevents the formation of solid hydrates. The mixture is let down to 1.3 atm at valve 85 and it partially vaporizes in heat exchanger 82; this liquid-vapor mixture is returned via line 87 to vessel 62 where the liquid is withdrawn via line 89 and the vapor is recompressed.

Stream 77 is mostly C C, and C hydrocarbons and ethylene glycol is injected via line 78 to the proportion of about 0.1 percent molar. The mixture is let down to 3.5 atm at valve 86 and it partially vaporizes'in heat exchanger 81; this liquid-vapor mixture is returned via line 88 to vessel 69 where an aqueous liquid is withdrawn via line 72, an organic liquid is withdrawn via line 75, and the vapor is recompressed.

Heat exchangers

81, 82, may suitably be like dephlegmators erected such that aqueous liquid can be withdrawn from the bottom. Aqueous liquid is also withdrawn via line 90 from vessel 84; hydrocarbon condensate is withdrawn via line 91 to the distillation column 130 to be described later. Vapor from vessel 84 passes via line 92 to tower 93 containing alumina for drying. The vapor is then cooled in

heat exchangers

94, 96, and the condensate collects in vessel 97 and passes to distillation column 101 via pump 98 and heat exchanger 94. The cooling in heat exchanger 96 is done by the products, hydrogen, methane, ethylene and ethane. The vapor from vessel 97 passes to distillation column 100.

The

distillation columns

100 and 101 together make up the primary de-propanizer; the

condensers

102, 103, provide the main reflux, which is returned to the top of column 100 via vessel 104 and pump 99; intermediate reflux is provided by

coolers

105, 95; cooler 105 operates with cooling water at about 22C; heat exchanger 95 is cooled by the products. Column 100 operates at 21.0 atm and column 101 at 2 I .2 atm. Pump 106 takes liquid from the bottom of column 100 to the top of column 101. Part of the condensate from cooler 105 returns to column 101 via lines 107, which contains a U-bend to maintain a vapor-free stream; the remaining part of this condensate is fed to a zone near the middle of column 100 by means of pump 148.

The main reboiler 108 is heated by steam; the bottom product, which has a major proportion of C hydrocarbons and a minor proportion of C is-sent via line 109 to the secondary de-propanizer 130.

The secondary de-propanizer, which operates at about 10.7 atm, has three feeds, line 109, line 91, and pump 129 from line 75. Item 131 is a water cooled partial condenser, 132 is a condensate drum, 133 is a reflux pump, 134 is a steam heated reboiler. The top product 137 contains mostly C hydrocarbons with smaller amounts of others such as ethylene; it is returned to compressor stage 65 by way of vessel 70. The bottom product 135 contains C and C hydrocarbons, and goes to the de-butanizer 136. I

The top product of the

primary de-propanizer

100, 101, contains hydrogen, light hydrocarbons and about 0.3 percent molar C,; it leaves vessel 104 as vapor at about 20C. It is warmed by

heat exchangers

111, 112, 113, leading to vessel 114 containing catalyst for the selective hydrogenation of acetylenes. Heat exchanger 113 is heated by steam. The process gas is cooled again by

heat exchangers

112, 115, 111; heat exchanger 115 is water cooled and after it comes vessel 116 for removing condensable impurities.

Cold process gas leaving heat exchanger 111 passes along line 117 andis' then split into

streams

118 and 141. Stream 141 is partially condensed in heat exchanger 119 by cold product streams consisting of hydrogen and methane. Stream 118 is partially condensed in

heat exchangers

120, 121, 122, 123, 124, and the process gas and condensate pass to vessel 125 which is at about -56C. The gas from vessel 125 is warmed to 42C in heat exchanger 123 and leaves as stream 127. The liquid from vessel 125 is warmed and partially vaporized in heat exchanger 121 and leaves as stream 128. Heat exchanger 124 is cooled by boiling ethane product at 61C; heat exchanger 122 is cooled by boiling ethylene product at -55C, and exchanger by gaseous ethylene at -55 to 22C.

Streams

127, 128, are feed lines to the de-ethanizer, which is operated according to the first form of the invention. The de-ethanizer is conveniently installed as three

vessels

200, 201, 202, each containing plates and downcomers; the bottom section of vessel 201 serves as a reservoir for liquid. The de-ethanizer operates in the range 19'to 20 atm.

From vessel 201 liquid refrigerant at about 3C which is predominantly C5 and C hydrocarbons passes along line 207and divides into two streams; part is cooled in heat exchanger 208 and then divides again; one fraction is let through valve 224 to a pressure of about 6.3 atm and cools heat exchanger 225 which is the intermediate condenser for the de-ethanizer; the refrigerant vapor with some liquid passes to heat exchanger 208 where the refrigerant vaporizes completely; this stream continues to heat exchanger 215 where it warms up to 22C and enters compressor 217; another fraction of the cooled refrigerant passes along line 226 to valve 227 where it is let down to about 6.5 atm for the reflux condenser 103 of the primary depropanizer; the vapor boiling off together with some liquid passes along line 228 to heat exchanger 208.

The second main refrigerant stream from line 207 is cooled in heat exchanger 209 and then divides; one fraction is let through valve 246 to a pressure of about 2.1 atm and provides refrigeration successively to

heat exchangers

210, 211. The refrigerant boiling off and some liquid pass along line 229 to heat exchanger 209 where the refrigerant vaporizes completely; this stream continues to heat exchanger 215 where-it warms up to 22C and enters compressor 216. Another fraction of the cooled refrigerant from heat exchanger 209 passes along line 230 to valve 231 where it is let down to about 2.1 atm for cooling heat exchanger 233, which is part of the refrigeration system for the de-methanizer. The refrigerant boiling from heat exchanger 233 and some liquid pass along line 232 and enter heat exchanger 209.

Refrigerant compression takes place in three

stages

216, 217, 218, with

water coolers

219, 220, 221, the final delivery pressure being 38 atm which is adequate to bring about condensation in cooler 221 at about 28C. The condensate passes into vessel 222 and thence it is cooled in heat exchanger 215 and returned to vessel 201 via valve 223.

The main reboiler 234 of the de-ethanizer is heated by steam, and the bottom product, which is almost entirely propylene and propane, is withdrawn via line 235, and may be separated by another distillation column (not shown). 1

The overhead vapor from vessel 200 is partially condensed in heat exchanger 211, the condensate collects in vessel 212 and is returned as reflux via pump 214.

Vapor from the top of vessel 201 passes via line 203 to the intermediate reflux condenser 225 and thence the liquid-vapor mixture passes to the bottom of vessel 200; liquid from the bottom of vessel 200 is transferred to the top of vessel 201 via pump 204. Between

vessels

201, 202, the vapor transfer line is 205, and the liquid transfer line is 206. The feed line 127 enters the deethanizer above the first plate from the bottom of vessel 200; the feed line 128 enters at about two plates above the reservoir in vessel 201.

From the de-eth'anizer the top product which contains about 0.4% C, hydrocarbons is withdrawn as vapor from vessel 212 via line 236 at 52C. This stream is partially condensed in heat exchanger 210 and the condensate at 66C collects in vessel 213, and is withdrawn via line 237 as feed to the de-methanizer.

The vapor from vessel 213 passes along line 238 to the cooling train consisting of

heat exchangers

239, 240, 241, 242, and'thence to vessel 243 at about 1 19C. The liquid is withdrawn from vessel 243 via line 248 to heat exchanger 240 where it partially vaporizes; this mixture is fed to the de-methanizer via line 249.

The vapor from vessel 243 is cooled to l40C in heat exchanger 244 and passes to vessel 245; condensate from vessel 245 is fed via line 250 to the demethanizer top section 300. From vessel 245, vapor which is mostly hydrogen is warmed to -l25C in heat exchanger 244; the gas is then put into an expansion engine, must suitably a turbine 255, where the gas is cooled to about -146C at a discharge pressure of 6.2 atm; the discharge gas is put through heat exchanger 244 and then through a second expansion engine 254, where the gas is cooled to about l46C. at a discharge pressure of 2.1 atm; this discharge gas makes another pass through heat exchanger 244, and then it warms up successively through

heat exchangers

239, 119, 96, 95. For the equipment below 1 19C the material of construction is metal free from iron and chromium.

The de-methanizer is a variant of the second form of the invention and is conveniently installed as three

vessels

300, 301, 302, operating at about 18.7 atm. Vessel 300 is most suitably a column with plates and downcomers; and has the main reflux condenser built into the top; feed line 250 enters vessel 300 at a zone about four plates from the top. Vessel 301 has a reservoir at the bottom for the liquid refrigerant, about six fractionating plates in the middle and the intermediate reflux condenser at the top; vapor passes up the tubes of this condenser and partial condensation takes place; the boiling refrigerant is in the space surrounding these tubes; the uncondensed vapor leaving the top of these tubes passes to the bottom of vessel 300 via line 303; liquid from the bottom of vessel 300 passes to the top plate of vessel 301 via line 304 which contains a U- bend to maintain a vapor-free stream. Stream 249 is fed onto a plate near the middle of vessel 301. An overflow line 306 from the reservoir provides reflux for the bottom vessel 302, which contains suitable plates and downcomers; vapor from the top of vessel 302 passes up to vessel 301 via line 305.

From vessel 301 liquid refrigerant, which is a mixture of methane, ethylene and ethane at about C, passes along line 322 and is cooled in

heat exchangers

310, 311. This stream is let down to 1.5 atm at valve 312 into the space round the tubes of the main reflux condenser. The vapor boiling off at about l44C passes through

heat exchangers

311, 310, 317, and is warmed to 22C before entering

compressor

318, 319. A liquid purge 313 from the refrigerant of the main reflux condenser is split into two streams; stream 314 goes to the intermediate reflux condenser and thence via line 323 and

heat exchangers

310, 317, to the compressor; the second purge fraction 315 goes to heat exchanger 241 and thence via line 316 and heat exchanger 317 to the compressor. The compressor delivers the refrigerant at 21 atm and has

water coolers

320, 321. The high pressure gas is cooled in heat exchanger 317, partially condensed in the intermediate reboiler 309, and completely condensed in heat exchanger 233 at about 66C. The condensed refrigerant then goes to the reservoir via line 307 and valve 308. The feed line 237 enters near the top of vessel 302, and the reboiler 309 is below this feed zone. Reboiler 309 consists of tubes immersed in liquid either in a downcomer or on a plate.

Reboil for the bottom of vessel 302 is provided by withdrawing liquid at line 324, and sending it to the reflux'condenser 102 of the primary de-propanizer by pump 325 and line 326; the vapor so produced is returned to the bottom of vessel 302 via line 328.

Line 327 is a branch from line 324 and it takes the mixture of liquid ethylene and liquid ethane to the

C splitter

400, 401, which operates at about 15.5 atm. Ethylene vapor at about 37C is withdrawn from the top of vessel 400 and is warmed in

heat exchangers

402, 403, 404, to 22C before entering compressor 405. Part of the ethylene is delivered at 23.5 atm and is' cooled in water cooler 406; it is cooled further in heat exchanger 404 and passes via line 408 to intermediate reboiler 409 where it condenses. The liquid ethylene then takes line 410 to heat exchanger 402 where it is cooled; part of this stream is withdrawn as product via line 416, and part enters the top of vessel 400 as main reflux via valve 411.

The second stream from compressor 405 is delivered at 30.9 atm and cooled in water cooler 407; this stream is cooled in heat exchanger 404 and condensed in the main reboiler 413. The liquid ethylene is cooled in heat exchanger 403 and enters the top of vessel 400 via valve 415. i

Liquid from the bottom of vessel 400 enters the intermediate reboiler 409 and the partially vaporized mixture (424) enters the disengagement vessel 418. Thence the vapor returns to vessel 400 and the liquid runs into vessel 401 via valve 419; line 425 is a liquid overflow pipe between vessels4l3 and 400.

The liquid ethylene product 416 is divided into two parts; one part is withdrawn as liquid via line 417; the other part goes via line 420 to valve 426, it is let down to 8.9 atm and used to cool the process gas in

heat exchangers

122, 120, 96, and used to cool the intermediate condenser 95.

The liquid ethane product passes along line 422 and is cooled in heat exchanger 126; it is let down at valve 421 to 3.5 atm and it cools the process gas in heat exchanger 124; it is then warmed in

heat exchangers

126, 96, 95.

From the top of the de-methanizer liquid methane is withdrawn via line 329; it is let down to 9.4 atm at valve 330 and it is used to cool

heat exchangers

242, 239, 119, 96, 95. A gaseous purge stream 331 from the top of the de-methanizer is mostlymethane with a small proportion of hydrogen; it is warmed to -7lC in heat exchanger 239 and then put through expansion engine 247 most suitably a turbine; the delivery gas 332 is at about l4lC and 2 atm and is used to cool

heat exchangers

239, 119, 96, 95.

The aqueous liquid residues from

vessels

62, 69, may be treated for the recovery of ethylene glycol, for example by distillation.

lt is to be understood that in the example given the conditions of temperature and pressure are open to variation depending on the composition of the feed gas and on the temperature of the cooling water available; alternatively, air coolers can be used.

TABLE 1 Typical composition of gas to he separated Mole The three basic forms of the invention are shown in F168. 1, 2 and 3 with the top product delivered as liquid; there are some variants in which the main reflux condenser is a partial condenser and the top product is delivered partly or entirely as a gas; examples of such variants respectively are the de-methanizer in FIG. 4C and the de-ethanizer in FIG. 48.

I claim:

1. A process for the continuous fractional distillation of a fluid containing two or more substances at least one of which has a boiling point below ambient temperature in which the main liquid reflux is provided by condensing at least part of the vapors leaving the top of the distillation column and in which the main reboiling is provided at a zone near the bottom of the distillation column, characterized in that:

a. a refrigeration cycle is used to provide the main reflux at the coldest zone of the column by indirect heat exchange and the refrigerant used in the refrigeration cycle is a fluidwithdrawn from an intermediate zone of the column;

b. the refrigeration cycle includes the stages of compression, condensation, expansion and evaporation, and at least part of the refrigerant is returned to a zone of the column near to the intermediate zone of withdrawal; I

c. in addition to the main reflux condenser at the top of the column and the main reboiler at the bottom of the column, there is at least one other zone at an intermediate level in which intermediate reflux is applied and at least one other zone at an intermediate level in which intermediate reboil is applied;

d. the refrigeration cycle, which provides the main reflux by indirect heat exchange, is common with the cycle which provides intermediate reflux and intermediate reboil.

2. A process as claimed in claim 1 wherein a stream of liquid is withdrawn from an intermediate zone below the zone into which the fluid being fractionated is fed and is divided into two further streams one of which, after cooling, is passed through a condenser in the column between the said feed zone and the top of the column causing condensation of vapor and refluxing of the resulting liquid down the column, and the other of which after cooling passes to the main reflux condenser at the top of the column where the condensed liquid formed is returned in part at least to the top of the column as reflux, and both streams after passing through the reflux condensers are warmed near to ambient temperature, are subjected to compression, condensation and further cooling and returned via an expansion valve to the column at a zone just above that from which the original stream leaves, the flash-gas at said expansion valve providing intermediate reboiling.

3. A process as claimed in claim 1 wherein a stream of liquid is withdrawn from an intermediate zone below the zone into which the fluid being fractionated is fed, and after cooling is passed to the main reflux condenser which provides reflux to the top of the column, the said stream boils partially in the said main reflux condenser and the vapor and liquid paits of the said stream are separated, the liquid part passing to one or more reflux condensers between the top of the distillation column and the feed zone, the liquid part vaporizing at least partially and then combining with the above vapor part, the recombined streams are warmed near to ambient temperature, are subjected to compression, condensation and further cooling and returned via an expansion valve to the column at a zone just above that from which the original stream leaves, the flash-gas at the said expansion valve providing intermediate reboiling.

4. A process as claimed in claim 1 wherein a stream and top reflux are obtained, after which this part of the stream is warmed near to ambient temperature, compressed and joined with the other compressed vapors, the combined stream being cooled and divided into two parts as mentioned above, one part of which goes through a heat exchanger in the column and re-enters the column near the zone from which the original stream was withdrawn.

5. A process according to claim 1 in which there is more than one process fluid introduced by way of more than one feed stream to the fractional distillation column for the purpose of fractionation.

6. A process as claimed in claim 1 wherein the fluid being fractionated is derived from pyrolysis gas and the separated products are mainly hydrogen, methane, ethylene, ethane and propylene. v

7. An apparatus for carrying out the process of the continuous fractional distillation of a fluid containing one or more substances, at least one of which has a boiling point below ambient temperature, in which the main liquid reflux is provided by condensing at least part of the vapors leaving the top of the distillation column and in which the main reboiling is provided at a zone near the bottom of the distillation column, characterized in that, said apparatus comprises I a. means for providing at one or more intermediate zones intermediate reflux and means for providing at one or more other intermediate zones intermediate reboil in addition to the said main reflux and main reboiling;

b. means for withdrawing a stream of fluid from an intermediate zone of the column;

0. means for putting said stream through a refrigeration cycle including the stages of compression, condensation, expansion and evaporation, the said cycle providing means for refrigeration by indirect heat exchange at the main reflux condenser and providing means for the said intermediate reflux and intermediate reboil; and

d. means for returning at least part of said stream to the column near to said intermediate zone of withdrawal.

Claims

2. A process as claimed in claim 1 wherein a stream of liquid is withdrawn from an intermediate zone below the zone into which the fluid being fractionated is fed and is divided into two further streams one of which, after cooling, is passed through a condenser in the column between the said feed zone and the top of the column causing condensation of vapor and refluxing of the resulting liquid down the column, and the other of which after cooling passes to the main reflux condenser at the top of the column where the condensed liquid formed is returned in part at least to the top of the column as reflux, and both streams after passing through the reflux condensers are warmed near to ambient temperature, are subjected to compression, condensation and further cooling and returned via an expansion valve to the column at a zone just above that from which the original stream leaves, the flash-gas at said expansion valve providing intermediate reboiling.
3. A process as claimed in claim 1 wherein a stream of liquid is withdrawn from an intermediate zone below the zone into which the fluid being fractionated is fed, and after cooling is passed to the main reflux condenser which provides reflux to the top of the column, the said stream boils partially in the said main reflux condenser and the vapor and liquid parts of the said stream are separated, the liquid part passing to one or more reflux condensers between the top of the distillation column and the feed zone, the liquid part vaporizing at least partially and then combining with the above vapor part, the recombined streams are warmed near to ambient temperature, are subjected to compression, condensation and further cooling and returned via an expansion valve to the column at a zone just above that from which the original stream leaves, the flash-gas at the said expansion valve providing intermediate reboiling.
4. A process as claimed in claim 1 wherein a stream of mainly vapor is withdrawn from a zone between the feed zone and the top of the column and is then warmed near to ambient temperature, compressed, cooled and divided into two parts, one of which is condensed in a heat exchanger in the column below the feed zone in order to provide intermediate reboiling while the other part is cooled and condensed and goes to the main reflux condenser from which top product and top reflux are obtained, after which this part of the stream is warmed near to ambient temperature, compressed and joined with the other compressed vapors, the combined stream being cooled and divided into two parts as mentioned above, one part of which goes through a heat exchanger in the column and re-enters the column near the zone from which the original stream was withdrawn.
5. A process according to claim 1 in which there is more than one process fluid introduced by way of more than one feed stream to the fractional distillation column for the purpose of fractionation.
6. A process as claimed in claim 1 wherein the fluid being fractionated is derived from pyrolysis gas and the separated products are mainly hydrogen, methane, ethylene, ethane and propylene.
7. An apparatus for carrying out the process of the continuous fractional distillation of a fluid containing one or more substances, at least one of which has a boiling point below ambient temperature, in which the main liquid reflux is provided by condensing at least part of the vapors leaving the top of the distillation column and in which the main reboiling is provided at a zone near the bottom of the distillation column, characterized in that, said apparatus comprises a. means for providing at one or more intermediate zones intermediate reflux and means for providing at one or more other intermediate zones intermediate reboil in addition to the said main reflux and main reboiling; b. means for withdrawing a stream of fluid from an intermediate zone of the column; c. means for putting said stream through a refrigeration cycle including the stages of compression, condensation, expansion and evaporation, the said cycle providing means for refrigeration by indirect heat exchange at the main reflux condenser and providing means for the said intermediate reflux and intermediate reboil; and d. means for returning at least part of said stream to the column near to said intermediate zone of withdrawal.