US3301915A

US3301915A - Recovery of isoprene from hydrocarbon fractions

Info

Publication number: US3301915A
Application number: US513632A
Authority: US
Inventors: King Ralph William; Shorthouse Barry Owen
Original assignee: International Synthetic Rubber Co Ltd
Current assignee: International Synthetic Rubber Co Ltd
Priority date: 1964-11-06
Filing date: 1965-11-05
Publication date: 1967-01-31
Anticipated expiration: 1984-01-31
Also published as: DE1543090A1; GB1093413A

Description

RECOVERY OF ISOPRENE FROM HYDROCARBON FRACTIONS Filed Nov. 5. 1965 Jan. 31, 1967 R. w. KING ETAL 3 Sheets-Sheet 2 w m mm 5 m mm .y wow \W 1? Mm #m 1 -\\l ww mN/EQ mm QM mom mm ww M535 mm QQJAQ/ :i @238 m QC 6m vow v i. M7 Q U FAWN v 5 53 1 2m @2326 mm. 8N l V \mN mm mwom w mm %m 6w N NON Jan, 31, 1967 R. W. KING ETAL RECOVERY OF ISOPRENE FROM HYDROCARBON FRACTIONS Filed Nov. 5. 1965 3 Sheets-Sheet 3 8m 8m 2% m 4% 2&5 r ma l r wmm ER mm 8N mm C Km Em -E f bm 2 @258 i mm 1% km SN mm M53: 1 @238 U mm 3w 7 X mm m@ 9%? 6w mw United States Patent (3 3,301,915 RECOVERY OF ISUPRENE FROM HYDROCARBON FRACTIONS Ralph William King, London, and Barry Owen Shorthouse, Southampton, England, assignors to The International Synthetic Rubber Company Limited, Southampton, England Filed Nov. 5, 1965, Ser. No. 513,632 Claims priority, application Great Britain, Nov. 6, 1964, 45,358/ 64 11 Claims. (Cl. 260-6815) This invention relates to the recovery of isoprene from hydrocarbon streams consisting wholly or predominantly of C hydrocarbons and particularly from such streams which contain in addition to isoprene appreciable amounts of cyclopentadiene and usually also of piperylenes.

Isoprene-containing hydrocarbon streams are available which are obtained (i) by the chemical or catalytic dehydrogenation of C olefin and/ or parafiin hydrocarbons,

'(ii) by thermal demethenation of specific C hydrocarbons such as 2 methyl pentene 2, and (iii) as a by-product stream of the thermal cracking of naptha and other bydrocarbon feedstocks for the production of ethylene. This by product stream forms a particularly suitable feedstock for the process of the invention. In the last mentioned process other valuable unsaturated gaseous hydrocarbons (propylene, butadiene) are produced simultaneously and usually separated and recovered.

The liquid product from the cracking operations referred to above is normally separated into two or more fractions of different or overlapping boiling range, the main fraction consisting of raw gasoline with a boiling range of approximately to 150 C. This main fraction contains appreciable amounts of C hydrocarbons, both cyclic and acyclic paraffins, mono and di-olefins including cyclopentadiene, piperylenes and isoprene, and smaller amounts of acyclic acetylenes and acetylenic olefins.

When it is desired to recover isoprene, cyclopentadiene or other C hydrocarbons from the main liquid fraction, it is normally first separated by fractional distillation into a stream consisting mainly of C hydrocarbons having an approximate boiling range of to 50 C., and a heavier hydrocarbon stream which may be employed after suitable treatment in motor gasoline.

The C hydrocarbon stream so obtained may vary considerably in composition since this is dependent on the cracking conditions employed, the composition of the cracking feedstock, and the subsequent processing of the cracked product. The concentration of the main components in the mixture generally lie within the following ranges:

Components: Weight, percent Isoprene 8 to 16. Piperylenes 6 to 14. cyclopentadiene 4 to 25. Isopentane 9 to 20. N-pentane 25 to 45. Cyclopentane 2 to 12. Pentenes 12 to 30. Isopropenyl acetylene 0.02 to 0.2. Other acetylenes less than 0.1.

The composition of a C stream from an ethylene pro ducing plant employing a standard hydrocarbon charge stock may show far less variation than has been indicated above.

Many methods have been suggested for the concentration orseparation of isoprene-and other valuable C hydrocarbons from such cracked C hydrocarbon streams.

The following initial operations are a common feature of many of the methods suggested:

(1) The liquid is treated under pressure at a temperature in the range to C., for several hours. The treatment is carried out in a soaking chamber under sufficient pressure to prevent vapourisation. The object of the treatment is to convert the greater part of the cyclopentadiene present in the original mixture to the less volatile compound dicyclopentadiene, which is readily separated from isoprene during subsequent distillation.

(2) The product after heat treatment is fractionally distilled in one or more columns of high efliciency at a high reflux ratio. The main object of this distillation is to separate the mixture into an isoprene rich fraction having a very low piperylene content, and a heavier fraction of low isoprene content and higher piperylene content.

Such treatment is capable of giving an isoprene 'rich fraction containing 25 to 40% by weight of isoprene, and 0.1% by weight or less of piperylenes.

A disadvantage of the treatment described above lies in the relatively high cyclopentadiene content of the isoprene rich fraction, which under the best conditions is usually in the range 0.5 to 1.0% by weight or 2.0 to 4% by weight based on isoprene in the product. cyclopentadiene is a particularly injurious impurity when the product is required to be used without further purification or concentration as feed to catalytic polymerisation processes for the production of polyisoprene, e.g., using lithium butyl as catalyst. Though it may prove possible to reduce the cyclopentadiene content of the isoprene rich fraction to 0.3 wt. percent, it is not practical to reduce it further.

Examination of the subsequent fractional distillation step for the recovery of isoprene in the heat treated feed as an isoprene concentrate of low piperylene content, shows that unconverted cyclopentadiene monomer in the feed is not effectively separated from isoprene in the overhead fraction unless a large proportion of the isoprene is allowed to be lost in the bottoms product. Under conditions of high isoprene recovery in the overhead concentrate, the cyclopentadiene content of the overhead fraction is generally higher than the cyclopentadiene content either of the feed to the column or of the bottoms product.

This behaviour of cyclopentadiene in the fractionating column, which is anomalous when considered in relation to the boiling points of isoprene, cyclopentadiene and cisand trans-piperylene, appears to result from the non-ideal behaviour of the hydrocarbons of different types (cyclic di-olefin, branched and straight chain di-olefins, olefins, paraffins, etc.). In scientific terms, the activity coeflicient of cyclopentadiene in the liquid is significantly higher than the activity coefficient either of isoprene or of piperylene, these being nearly identical in any particular mixture.

It has now been found that in the fractionation of an isoprene and cyclopentadiene containing C fraction such as is referred to above, either before or after the heat treatment, the cyclopentadiene content of the liquid during the fractionation is at 'a maximum value at an intermediate point in the fractionation column. This maximum cyclopentadiene content may 'be many times (between five and twenty) the cyclopentadiene content of the isoprene concentrate which is removed as overhead product, and the peak concentration is generally found near and most commonly slightly above the mid-point in the fracti'onating column.

It has further been found that by withdrawing liquid from the column in the area of high cyclopentadiene concentration, dimerizing the major part of the cyclopentadiene in the withdrawn liquid and recycling the liquid product mixture to an appropriate point in the column, an isoprene concentrate can be obtained as overhead which contains a high proportion (Le. 80 to 96% by weight) of the isoprene in the feed material, but only traces (less than 0.1% by weight and, if necessary, considerably less) of cyclopentadiene and piperylenes.

The invention thus consists in a process for recovering an isoprene-enriched fraction with a relatively low cyclopentadiene content from a hydrocarbon stream containing isoprene and cyclopentadiene and consisting predominantly of C hydrocarbons, which process comprises fractionally distilling the hydrocarbon stream to obtain an isoprene-enriched overhead fraction and during the course of the distillation withdrawing liquid from an intermediate point in the fractionation system at which a relatively high cyclopentadiene concentration exists, subjecting the' withdrawn liquid to dimerisation conditions for the dimerisation of cyclopentadiene, and returning the resulting liquid product to an intermediate point in the fractionation system, which will normally be close to the point from which liquid is withdrawn.

The feed material will usually contain piperylenes (cisand trans-) as well as cyclopentadiene and the piperylenes will be separated in the bottoms fraction.

In the dimerisation treatment during which higher polymers of cyclopentadiene may also be formed, suitable conditions are a temperature of 100 to 160 0, preferably 120 to 140 C., a pressure of from to 30 atm., preferably to 25 atm., at least sufficient to maintain the cyclopentadiene in the liquid phase and a contact time of from 10 minutes to 6 hours, preferably minutes to 3 hours.

By dimerising a liquid containing an increased amount of cyclopentadiene as compared with the starting material, the rate of dimerisation is increased and the size of the dimerisation reactor can be reduced. Also the possibility of reaction between the cyclopentadiene and isoprene is much reduced, thereby largely avoiding an appreciable loss of isoprene.

The process of the invention may be carried out as a continuous, batch or semi-continuous operation in suitably designed equipment.

The feed material to which the process of the invention may be applied is a hydrocarbon stream containing the hydrocarbons isoprene, cyclopentadiene, and usually cisand trans-piperylenes. Preferably the feed material is a fraction of relatively narrow boiling range to 50 C.) and consisting predominantly of C hydrocarbons.

If the cyclopentadiene content of the feed material is high or if a product with a very low cyclopentadiene content is required, the feed material may first be subjected to a heat treatment for several hours in the liquid phase at an elevated temperature preferably in the range 100 to 140 C., and under pressure as in the known method previously described. Suitable conditions are a contact time of 3 to 6 hours, a temperature of 125 C., and a pressure of at least 10 atmospheres absolute, preferably in the range 15 to atmospheres, to prevent vapourisation of the material during heat treatment. This pretreatment is desirable when the feed has more than 10% of cyclopentadiene, and may advantageously be employed when the cyclopentadiene content is over 5%.

In cases where the feed material has an excessively high cyclopentadiene content, it is preferably subjected additionally to a simple distillation step before the further treatment described below, to separate the major part of the C hydrocarbons in the feed from cyclopentadiene polymers formed by the heat treatment, and so lessen the degree and incidence of fouling in the subsequent processing equipment. When such a simple distillation step is carried out, the maximum temperature during distillation should not exceed 140 C., and should preferably be below 100 C., to reduce or prevent thermal depolymerisation of the cyclopentadiene dimers separated from the bulk of the C hydrocarbons during the distillation.

In carrying out the process of the invention, the feed material, with or without the pretreatments described above, may be fractionated in an efficient fractional distillation plant or column containing the equivalent of between 60 and 200 theoretical plates-preferably between 100 and 150 theoretical plates. The column is provided with a means of withdrawing part or all of the liquid refiux passing down the column from an intermediate point in the column which is preferably situated slightly above the mid tray of the column. The column is also provided with a point for returning the liquid removed for processing to an intermediate point in the column, which may be above or below the point from which liquid is removed. In a plant designed for continuous operation, the point of liquid return is preferably located on the plate immediately below the point from which liquid is removed. In a plant designed for batch operation, the point of liquid return is preferably several plates above the point from which liquid is removed. The material is usually returned to a point at which the isoprene/ piperylenes ratio substantially corresponds to that of the returned material.

By subjecting the withdrawn liquid to dimerisation condit-ions for the dimerisation of cyclopentadiene, e.g. in a soaking vessel, and returning the liquid product to the column, the maximum cyclopentadiene content of the liquid in the column may be reduced to 1 wt. percent or less.

A number of embodiments of the invention are described below, by way of example, with reference to the accompanying drawings in which:

FIGURE 1 is a flow diagram illustrating the continuous treatment of a C fraction containing 5 wt. percent or less cyclopentadiene,

FIGURE 2 is a flow diagram illustrating a batch operation according to this invention, and

FIGURE 3 is a flow diagram illustrating a modified form of batch operation.

It should be understood that feed streams having a cyclopentadiene content higher than 5 weight percent are best subjected to the pretreatment steps of heat treatment and in certain cases simple distillation as described previously to reduce the cyclopentadiene content of the feed before being treated by the processes described below.

The feed material may also be subjected to a further pretreatment before entering the continuous version of the process shown in FIGURE 1. This pretreatment consists of a topping distillation in a high efiiciency fractionating column, e.g. equivalent to 40 to 100 theoretical plates. The object of this topping operation is to remove C bydrocarbons and other light ends, and particularly isopropenyl acetylene which is an active catalyst poison in many polymerisation processes employing isoprene. At the same time, some of the other low boiling C hydrocarbons in the feed, e.g., isopentane, may be partially removed without significant loss of isoprene. This results in a higher concentration of isoprene in the end product from the process.

Referring to FIGURE 1, the isoprene and cyclopentadiene containing feed stream, subjected if necessary or if desired to one or more of the treatments described above, passes as liquid in line 1 to the heat exchanger 101 where it is preheated to a suitable temperature (30 to C. depending on the column pressure) before entering the fractionating column 102 at an intermediate point via line 2. The preheater 101 is not essential and may be dispensed with. The fractionating column 102 is a high efliciency liquid vapour counter-current contacting device fitted with a number of trays or filled with a suitable type of packing, in known fashion. The column efficiency is preferably not less than 60 theoretical plates, and desirably not less than theoretical plates.

Part of the liquid passing down to the base of the column leaves by line 3 to the reboiler 103 where it is partially vapourised by indirect heat exchange with steam or other convenient heating medium, and returns to the column by line 4. The rest of the column base product,

substantially free of isoprene, passes via line to pump 104 and thence via line 6 to heat exchanger 101 in which it is cooled, and then by line 7 to storage or further treatment before utilisation, egg. in the production of motor gasoline.

Overhead vapour from the column passes via line 8 to the condenser 105, from which condensed hydrocarbon liquid passes via line 9 to the reflux drum 106. The pressure in the column, reflux drum and other processing equipment is controlled by admitting or withdrawing gas or. vapour to or from the reflux drum via line 14a. The pressure in the reflux drum may be controlled at any suitable pressure from atmospheric to 20 atmospheres, but a pressure of 2 to 3 atmospheres absolute is generally found most convenient.

, Liquid passes from the reflux drum 106 via line 10 to the pump 107, from which it is discharged by line 11. Part of the pump discharge is returned as reflux to the top of the column via line 12.

The remainder passes via line 13 to the cooler 108 where it is further cooled, and thence via line 14 to product storage. This is the isoprene rich concentrate containing only traces of cyclopentadiene and piperylene which i the final product from the process.

A special liquid draw-off tray 102a is provided in the column, usually a few trays above the feed tray. Part and preferably all of the liquid descending on to this tray is removed via line 15 and passes to pump 109, which raises its pressure by at least 10 atmospheres and preferably- 15 to 30 atmospheres. Liquid passes from the pump via line 16 to the heat exchanger 110 where it is preheated to a temperature in the range 70 to 120 C. The liquid leaves the heat exchanger via line 17, passing to the additional heater 111, where it is heated by indirect heat exchange with steam or other suitable medium to a' temperature in the range 100 to 150 C.

The heated liquid passes via line 18 into the soaking vessel 112, which is preferably provided with internal baffles to reduce back mixing and short circuiting within the vessel. The vessel is freed of gas or air before start up. The volume of the vessel 112 is so arranged as to produce a liquid residence time in the vessel in the range 10 minutes to 6 hours, preferably between minutes and 3 hours, depending on the temperature in the vessel and the degree of cyclopentadiene removal required. Liquid leaving the soaking vessel 112 via line 19 is cooled by heat exchange with incoming liquid in 110 and returned to the column, preferably to the tray below the liquid draw-off, by line 20. When all liquid is withdrawn from tray 102a, the returning liquid should desirably enter the tray below the tray 102a. Line 20 is provided with a throttling or back-pressure control valve 20a, preferably of the automatic type, to prevent loss of pressure and vapourisation in the soaking vessel 112.

Preheater 111 may be by-passed or dispensed with when the amount of cyclopentadiene in the feed is high, since the dimerisat-ion of cyclopentadiene occurring in 112 raises the temperature of the liquid and so causes an adequate temperature difference across the heat transfer surface of the heat exchanger 110.

FIGURE 2, shows a flow diagram of the process in which the invention is applied in a batch manner. This has advantages over continuous processing where the scale of operation 'is relatively small, particularly in so far as it The batch charge in the kettle 201 is heated and vapour"- ised in the normal manner, vapour passing through the high efliciency fractionating column 202, leaving v-ia line 22 and passing to the condenser 203. Condensed liquid passes from the condenser via line 23 to the reflux drum 204, which is preferably designed to have a very low liquid capacity in relation to the capacity of the kettle 201. The pressure in the equipment described is controlled by admitting or withdrawing gas or vapour to or from the reflux drum via line 40.

Condensed liquid passes from the reflux drum 204 via line 24 to the reflux/product pump 205, from which it is discharged via line 25. The major part of this stream returns as reflux to the top of the column 202 via line 26, whilst the smaller part passes as product via line 27 to the cooler 206, which it leaves vialine 28.

In the batch process, several other overhead fractions may be separated from the charge in addition to the isoprene rich concentrate, thus eliminating the need for additional equipment generally required in conjunction with the continuous process. Product line 28 is connected to

branch lines

29, 30, 31 and 32 for removal of the various overhead fractions.

Line 29 is used for removal of the first fore-running fraction which consists of C hydrocarbons, isopropenyl acetylene, isopentane and other low boiling C hydrocarbons, but is substantially free of isoprene. This fraction may be employed in the manufacture of motor gasolene. Line 30 is used for removal of the isoprene rich concen trate, substantially free from cyclopentadiene, piperylenes and isopropenyl acetylene. This is the main product from the process. Line 31 may be used for removing an intermediate fraction containing both isoprene and piperylenes after the main product has been removed.

This intermediate fraction may advantageously be added to the charge of a subsequent batch and so be reprocessed.

Line 32 is used for withdrawing a heavy C distillate fraction containing piperylenes, pentenes, normal and cyclopentane and higher boiling hydrocarbons. This fraction, which is substantially free of cyclopentadiene and dicyclopentadiene, may be used in the manufacture of motor gasoline with the minimum of further treatment.

Part of the liquid descending the fractionating column 202 is withdrawn from a specially provided draw-off tray 202a via line 33 to pump 207, which raises its pressure to at least 10 atmospheres and preferably to 15 to 30 atmospheres. The liquid passes via line 34 to the heat exchanger 208 where it is heated to a temperature in the range 70 to C. and thence via line 35 to auxiliary heater 209 which raises the temperature of the liquid further to 100 to C. before entering the soaking vessel 210, via line 36, provided with internal baflles as in the continuous process and designed to give a liquid residence time which is preferably within the range of 30 minutes to 3 hours. Liquid leaving the soaking vessel via line 37 is cooled by heat exchange with incoming liquid in heat exchanger 208 and returned to the column via line 38. This line is provided with a throttling valve 380, preferably of the automatic type, to prevent loss of pressure and liquid vapourisation in the soaking vessel 210-.

The point at which the liquid is returned to the column is in the case of the batch process preferably several plates higher in the column than the point of liquid offtake. The reason for this is that the composition of the liquid at any point in the column is continuously changing during batch distillation, so that by the time the liquid leaving the take-off tray has passed through the time tank, the concentrations'of isoprene and piperylenes in the liquid correspond to those of a point in the column higher than the point from which the liquid has been removed from treatment.

The liquid charge introduced into the kettle for batch operation should be large in relation to the capacity of the soaking vessel 210, and the period of the total batch cycle should be at least 24 hours, preferably 72 hours or more for eflicient operation of the process described.

The operation is started at total reflux and sufficient time is given for the concentration gradient to be established in the column before off-take of fore-runnings via line 29 is commenced. The soaking vessel 210 is conveniently left full of liquid from the previous batch, and flow of liquid through the soaking vessel is commenced as soon as reflux liquid is descending the column. The reflux ratio is preferably increased progressively during withdrawal of each fraction, but lowered on commencement of a fresh fraction.

The liquid left in the kettle at the end of the distillation will contain 95% or more of the cyclopentadiene in the feed in the form of dimer and higher polymers and co-polymers. It is not advisable to attempt to remove all heavy C hydrocarbons from the kettle by raising the temperature too far at the end of the batch distillation, since this can cause depolymerisation of di-cyclopentadiene in the kettle. The kettle temperature at the end of the distillation should not exceed 150 C., and should preferably not exceed 100 C.

Liquid remaining in the kettle at the end of the distillation is removed via line 39 by pump 212, by which it is pumped to separate storage. This liquid may be utilised for the manufacture of cyclopentadiene and its derivatives.

Control valves

29a, 30a, 31a and 32:; are suitably provided in the

lines

29, 30, 31 and 32 respectively. Valve 38a, a back pressure valve similar to valve 20a of FIG- URE 1, is provided in line 38.

As previously stated, a modified batch process which may be referred to as a semi-continuous process, is illustrated in FIGURE 3. In place of the kettle in the batch process, the charge is contained in a tank or suitably constructed vessel 301 which is maintained at a lower temperature than that in the kettle for the batch process. The charge is fed to vessel 301 by line 41 and is circulated at a high rate via line 42 by pump 302, thence via line 43 through heat exchanger 303, thence via line 44 through the vapouriser 304 and finally via line 45 into the base of the column 305 above the liquid level in the column. Liquid is removed from the base of the column 305 via line 46 by pump 306, thence via line 47 to heat exchanger 303, thence via line 48 to cooler 307, and back into the charge vessel 301. The rest of the operation and equipment is as described for the batch process, the same reference numerals being used to designate the rest of the equipment as are used in FIGURE 2, with the exception that the plate from which liquid is withdrawn is designated 305a. Liquid left in the charge vessel 301 at the end of the distillation is pumped away via line 50.

The main advantage of the semicontinuous process is that it enables very large batches of material to 'be processed over a long batch cycle, (i.e. a week or more) without subjecting the material to prolonged residence times at the kettle temperature, and consequent loss of isoprene and fouling, which would be a disadvantage of the batch process under similar circumstances.

The semi-continuous process further enables isoprene in the feed to be recovered in the product concentrate at yields in excess of 95%, with a cyclopentadiene content of less than 0.05 wt. percent.

The fractionation column is operated at a high reflux ratio. In the case of continuous distillation, the preferred reflux: feed ratio lies within the range :1 and :1. In the case of batch distillation the reflux ratio is preferably increased continuously as distillation of the isoprene rich concentrate proceeds, e.g., from an initial reflux: product ratio of 5:1 to a final reflux: product ratio of 1.

Whilst a single column has been shown in the processes described above, it should be understood that this may consist of two or more columns placed in series with intermediate vapour lines, liquid transfer lines and pumps. Such an arrangement is frequently used where a large number of theoretical plates are required.

The process of the invention and the advantage obtained thereby is illustrated by the following example.

EXAMPLE A high efliciency continuous pilot plant fractionating column equipped with special gauze packing was set up with ancillaries as shown in FIGURE 1. An internal overflow weir allowed all liquid to fiow down the column when the soaking vessel (112) was not in use and no liquid was being withdrawn. The efllciency of the column was tested at a total reflux :at a top pressure of 15 p.s.i.g. using a mixture of isoand normal pentanes as test mixture. Analyses of samples taken from the reflux drum and column base showed that the column had an average efficiency of 78 theoretical plates.

The feed point to the column corresponded to 36 theoretical plates above the reboiler. The liquid draw-off tray was located immediately above the feed point, and the liquid from the soaking vessel was returned to the feed point in the column.

The column was fed with 65 pounds. per hour of a cracked C hydrocarbon feed at 50 C. from which part of the cyclopentadiene had been removed by heat treatment and distillation.

The composition of the treated feed, analysed by gasliquid chromatography was as follows:

Weight percent Isopentane 13.4 n-Pentane 22.6 2-methyl butene-l 9.4 Pentene-l 8.6 Pentene-2 (cisand trans-) 7.3 2-methyl butene-2 3.8 Isoprene 10.6 Piperylenes (cisand trans-) 11.5 Cyciopentane 11.9 cyclopentadiene 0.7 Unidentified 0.2

The column was operated at a top pressure of 15 p.s.i.g.

and a top temperature of 49 C. The liquid boil up rate and reflux return were adjusted to give an internal reflux:feed ratio of 16:1, i.e., an internal reflux rate of 1040 pounds per hour. The soaking vessel 112 had a capacity of 20 cubic feet, corresponding to a liquid residence time of approximately 40 minutes, and was operated at a temperature of C. and a pressure of 290 p.s.i.g.

The column was first operated with the soaking vessel out of commission, valves on

line

15 and 20 being closed, and all liquid on the take-off tray overflowing down the column. a

After careful adjustment of the temperature and flow rates, there were obtained 26 pounds/hour of overhead product from line 14 containing 25.5%. isoprene, 0.2% piperylenes and 1.1% cyclopentadiene.

The bottoms product from line 7 contained 0.5% isoprene, 18.7% piperylenes and 0.4% cyclopentadiene as monomer.

The column continued to operate on the same feed and at the same feed rate as before: the valves on

lines

15 and 20 were opened, and the pump, heat exchangers and soaking vessel were brought gradually into commission. A period of 28 hours was allowed to reach steady conditions from the time the soaking vessel was full of liquid.

25 pounds per hour of overhead product were withdrawn from line 14, containing 26.4% isoprene, 0.15% piperylenes and 0.08% cyclopentadiene.

The bottoms product from line 7 contained 0.4% isoprene, 18.2% piperylenes and 0.05% of cyclopentadiene as monomer.

These runs showed that operation inaccordance with the invention as compared with a normal fractional dis tillation resulted in a 14-fold reduction in the cyclopentadiene content of the overhead product without loss of isoprene.

All percentages referred to in the example are percentages by weight.

We claim:

1. A process for the recovery of an isoprene fraction with a very low cyclopentadiene content as an overhead fraction by fractional distillation from a hydrocarbon stream containing isoprene and cyclopentadiene and consisting predominantly of C hydrocarbons, in which process liquid is withdrawn during the course of the fractional distillation from an intermediate point in the fractional distillation system in an area of relatively high cyclopentadiene concentration, the withdrawn liquid is subjected to dimerisation conditions for the dimerisation of cyclopentadiene and the resulting liquid product is returned to an intermediate point in the fractional distillation system.

2. A process as claimed in claim 1, in which the hydrocarbon stream contains piperylenes and in which an isoprene-enriched fraction is recovered as overheads and a piperylene-enriched fraction is also recovered, the isoprene-enriched fraction having a considerably reduced cyclopentadiene and piperylenes content.

3. A process according to claim 2 in which the said resulting liquid product is returned to a point in the fractional distillation system at which the isoprene/piperylene ratio corresponds approximately with that of the said resulting liquid product.

4. A process as claimed in claim 1, in which the hydrocarbon stream contains more than by weight of cyclopentadiene and is subjected to a preliminary treatment for the dimerisation of cyclopentadiene to reduce the cyclopentadiene content below 5% by weight.

5. A process as claimed in claim 4, in which the hydrocarbon stream contains more than 5% by weight of cyclopentadiene and is subjected to the said preliminary treatment.

6. A process as claimed in claim 4, in which the hydrocarbon stream after said preliminary treatment is distilled to separate the polymers formed and then fractionally distilled to recover the overhead isoprene fraction.

7. A process as claimed in claim 1, in which the hydrocarbon stream is subjected to a preliminary distillation to obtain a stream of increased isoprene content, which is subjected to said fractional distillation.

8. A process as claimed in claim 1, in which the withdrawn liquid is heated to a temperature of from to 160 C., under a pressure of 10 to 30 atmospheres sufficient to maintain the cyclopentadiene in the liquid phase and for a period of from 10 minutes to 6 hours.

9. A process as claimed in claim 8, in which the withdrawn liquid is heated to a temperature of from to C., under a pressure of from 15 to 25 atmospheres and for a period of from 20 minutes to 3 hours.

10. A process for the recovery of an isoprene fraction with a considerably reduced cyclopentadiene and piperylenes content from a hydrocarbon stream containing isoprene, piperylenes and more than 5% by weight of cyclopentadiene and consisting substantially completely of C hydrocarbons, which process comprises subjecting the hydrocarbon stream to a preliminary treatment for dimerisation of cyclopentadiene to reduce the cyclopentadiene content below 5% by weight, distilling the hydrocarbon stream to separate the polymers formed therefrom, fractionally distilling the hydrocarbon stream from which the polymers have been separated, withdrawing during the course of the fractional distillation liquid from an intermediate point in the fractional distillation system in an area of relatively high cyclopentadiene concentration, subjecting the withdrawn liquid to dimerisation conditions for the dimerisation of cyclopentadiene, returning the resulting liquid product to an intermediate point in the fractional distillation system, and recovering an isopreneenriched fraction as overheads product and a piperylenesenriched fraction as bottoms product.

11. A process as claimed in claim 10, in which the withdrawn liquid is heated to a temperature of from 120 to 140 C., under a pressure of from 15 to 25 atmospheres and for a period of from 20 minutes to 3 hours.

References Cited by the Examiner UNITED STATES PATENTS 2,704,778 3/1955 Maisel 260-681.5 2,768,224 10/1956 Page et al 260-6815 DELBERT E. GANTZ, Primary Examiner.

G. E. SCHMITKONS,Assistant Examiner.

Claims

1. A PROCESS FOR THE RECOVERY OF AN ISOPRENE FRACTION WITH A VERY LOW CYCLOPENTADIENE CONTENT AS AN OVERHEAD FRACTION BY FRACTIONAL DISTILLATION FROM A HYDROCARBON STREAM CONTAINING ISOPRENED AND CYCLOPENTADIENE AND CONSISTING PREDOMINANTLY OF C5 HYDROCARBONS IN WHICH PROCESS LIQUID IS WITHDRAWM DURING THE COURSE OF THE FRACTIONAL DISTILLATION FROM AN INTERMEDIATE POINT IN THE FRACTIONAL DISTILLATION SYSTEM IN AN AREA OF RELATIVELY HIGH CYCLOPENTADIENE CONCENTRATION, THE WITHDRAWN LIQUID IS SUBJECTED TO DIMERISATION CONDITIONS FOR THE DIMERISATION OF CYCLOPENTADIENE AND THE RESULTING LIQUID PRODUCT IS RETURNED TO AN INTERMIEDCIATE POINT IN THE FRACTIONAL DISTILLATION SYSTEM.