IE47225B1 - Process for extracting one or more aromatic hydrocarbon from a mixture containing the same - Google Patents

Description

This invention relates to a process for extracting one or more aromatic hydrocarbon from a mixture containing the same. More particularly, but not exclusively, the present invention relates to a process for extracting a mixture of benzene, toluene, xylenes and Cg and Cg+ aromatic hydrocarbons from mixtures which contain them.

A number of processes are known for the extraction of aromatic hydrocarbons, which use liquid-liquid extraction stages followed by an extractive distillation stage, and which use as the extraction agent and extractive distillation agent a high boiling organic solvent in TO admixture with water.

The high-boiling organic solvents which are particularly suitable for these extractions and extractive distillations are morpholine, the alkyl derivatives of morpholine, and the aldehydic and ketonic derivatives of morpholine; among the latter, good results have been obtained with N-formyl morpholine in admixture with water, up to 30% by weight of the combined weight of N-formyl morpholine and water being water.

However, the conventional extraction and extractive distillation techniques as outlined above, and also that which is particularly described in British Patent Specification No. 1,271,596 are, as a general rule, somewhat expensive as regards heat consumption, especially in the operations of extractive distillation and solvent stripping to recover the solvent to be reused in both the extraction and extractive distillation runs.

Our Patent Specification No. 1585/78 claims a process for extracting one or more aromatic hydrocarbon from a hydrocarbon mixture containing the one or more aromatic hydrocarbon and at least one other hydrocarbon, which process comprises :subjecting the hydrocarbon mixture to liquid-liquid extraction in one extraction column or in two extraction columns arranged in series, using an extraction agent comprising an organic solvent, to produce a raffinate as an overhead product and an extract of the aromatic hydrocarbon(s) in the extraction agent as the bottoms product; subjecting the extract from the liquid-liquid extraction to extractive distillation to produce an overhead product ' and a bottoms product; splitting the overhead product from the extractive distillation into a relatively high boiling fraction and a relatively low boiling fraction; recycling the relatively low boiling fraction to the bottom region of the one liquid-liquid extraction column or to the bottom region of the second of the two liquid-liquid extractive columns; recycling the relatively high boiling fraction to the one liquidliquid extraction column or to the first of the two liquid-liquid extraction columns, at a point above that at which the hydrocarbon mixture is fed; subjecting the bottoms product from the extractive distillation to rectification to separate the aromatic hydrocarbon(s) from the organic solvent; isoenthalpieally expanding the organic solvent separated off in the rectification to produce a liquid phase comprising organic solvent having a lower content of water and aromatic hydrocarbon(s) than that of the organic solvent separated off in the rectification, and a gaseous phase formed predominantly of water with a low content of organic solvent and aromatic hydrocarbon(s); and condensing the gaseous phase and employing some or all of the resulting condensate in one or more of the following ways :(i) recycling the condensate to a base region of the one liquid-liquid extraction column or to the base region of the second of the two liquidliquid extraction columns, (ii) mixing the condensate with the bottoms produce o-f the extractive distillation, before that product is subjected to rectification, and (iii) mixing the condensate with the extract from the liquid-liquid extraction, before the extract is subjected to extractive distillation.

Preferably the liquid phase is recycled to the one liquid-liquid column or to the first of the two liquid-liquid extraction columns at a point above that at which the hydrocarbon mixture is fed.

According to the present invention, there is provided a process for extracting one or more aromatic hydrocarbon from a hydrocarbon mixture containing the one or more aromatic hydrocarbon and at least one other hydrocarbon, which process comprises:subjecting the hydrocarbon mixture to liquid-liquid extraction in one extraction column or in two extraction columns arranged in series, using an extraction agent comprising an organic solvent, to produce a raffinate as an overhead product and an extract of the aromatic hydrocarbon(s) in the extraction agent as the bottoms product; subjecting the extract from the liquid-liquid extraction to extractive distillation to produce an overhead product and a bottoms product; splitting the overhead product from the extractive distillation into a relatively high boiling fraction and a relatively low boiling fracti on; recycling the relatively low boiling fraction to the bottom region of the one liquid-liquid extraction column or to the bottom region of the second of the two liquid-liquid extraction columns; recycling the relatively high boiling fraction to the one liquidliquid extraction column or to the first of the two liquid-liquid extraction columns, at a point above that at which the hydrocarbon mixture is fed; subjecting the bottoms product from the extractive distillation to rectification to separate the aromatic hydrocarbon(s) from the organic solvent; separating off aromatic hydrocarbon(s) from the aqueous distillate head product obtained in the rectification; and using the resulting aqueous distillate to wash the raffinate and then sending the aqueous distillate to the bottom region of the one liquidliquid extraction column or to thebottom region of the second of the two liquid-liquid extraction columns.

It has been found that with the process of the present invention it is possible to reduce the consumption of heat by using, in the extractive distillation and rectification stages, an organic solvent with a quantity of water which is higher (in terms of percentage with respect to the organic solvent) than that utilized in the liquid-liquid extraction column(s).

The percentages of water in the liquid-liquid extraction columns is preferably from 0.5% to 12% by weight, and, in the other stages, ’ 47225 is preferably from 2% to 15% by weight; generally the percentage of water in the extractive distillation and solvent rectification (aromatics stripper) is higher than that in the liquid-liquid extraction stages.

For a better understanding of the present invention and to show hpw the same· may be carried into effect, reference will nov/ be made, by way of example, to the accompanying drawing which shows diagrammatically 3 plant for carrying out the process of the present invention. The plant includes a pipe 17 for feeding the mixture to be separated to a lower portion of a liquid-liquid extraction column 1, wherein counter-flow contact takes place with a solvent mixture fed to the top portion of the extraction column via a pipe 20. From the head of the extraction column 1, a raffinate emerges, which is sent, via a pipe 18, to a scrubbing column 3 for removal of any traces of entrained organic solvent. This washing is carried out under counterflow conditions using water coming from a decanting apparatus 12.

From the top of the scrubbing column 3 the raffinate is obtained, which is now devoid of organic solvent, and which is sent via a pipe 19 to the exterior of the plant.

The water and the other compounds entrained therein, as recovered 20 from the bottom of the scrubbing column 3, are sent, via a pipe 33, to a lower region of a second liquid-liquid extraction column 2. The aromatic-enriched solvent mixture which leaves the bottom of the extractor 1, is fed, via a pipe 21, onto the head plate of the extraction column 2.

From the head plate of the extraction column 2 itself, a raffinate is obtained, which is sent via a pipe 41 to the bottom of the extraction column 1, in order that a portion of the aromatic hydrocarbons still contained in that raffinate may be recovered.

To a lower region of the extraction column 2 are sent, via respective pipes 33 and 26, the water used for washing the raffinate in scrubbing column 3, and a stream composed of hydrocarbons and water coming from a second stage (9) of condensation of the overhead product of an extractive distillation column 5.

One of the preferred features of the present invention is to have available a stream composed predominantly of water but also containing organic solvent and aromatic hydrocarbons, as obtained from the condensation in a condenser 15 of the vapour phase leaving an evacuated flash-drum 14. Via a pipe 40 a portion of such stream is fed to the base of the extraction columm 2 until the optimum water content is attained.

The bottom stream from the extraction column 2 is sent via a pipe 22 to a heat exchanger 4, wherein heat recoverery is carried out at the expense of an impoverished organic solvent coming from the bottom of an aromatics stripper 10, and the heated bottom stream is fed in a pipe 23 to the extractive distillation column 5. Prior to its introduction into the column 5, the enriched solvent in pipe 23 is caused to be supplemented with water via a pipe 38, the water coming from the condenser 15.

The addition of water via the pipe 38, acts favourably by facilitating the separation of the non-aromatic hydrocarbons, thus permitting a high purity aromatic extract to be obtained, with reduced heat consumption relative to the cases in which less water is used.

Bearing in mind that the organic solvents used in the aromatics extraction stages are high-boiling in nature, and that the higher the water content, the lower is the boiling point of the mixture concerned, the addition of water via pipe 38 as indicated above has also the result of further reducing the heat consumption required by the extractive distillation column 5.

As a matter of fact, if it is only necessary to heat the stream 27, composed of the solvent mixture and the extracted aromatic hydrocarbons, to a lower temperature, that quantity of heat is saved, i Which corresponds to the temperature differential, which is attributable to different levels of water content, the performance of the extractive distillation column 5 being the same..

The head product of the extractive distillation column 5 is sent via pipe 24 to a partial condenser 6 wherein a minimum fraction of water is caused to condense along with a fraction of the heavier aromatic hydrocarbons, of the high-boiling saturated hydrocarbons and of the non-aromatic hydrocarbons which are characterised by an anomalous polarity which is such as to encourage their solubility in the organic solvent. The condensate is separated in a separator 7 and the separated condensate is fed via a pipe 25 to an intermediate region of the extraction column 1 above the feed pipe 17, so as to facilitate the venting into the raffinate of the anomalous polarity non-aromatic compounds, thus avoiding both the accumulation of these compounds in the cycle and any deterioration in the purity of the desired aromatic extract.

The vapour phase separated off in the separator 7, is sent via a pipe 26 to a condenser 8 where it is completely condensed, and then collected in the separator 9. This condensate, which consists of water, lighter aromatic hydrocarbons and low-boiling saturated hydrocarbons, is fed to a base region of the extraction column 2 via the pipe 26. 7 225 On taking into due account all the water addition effected at the base of the extraction column 2 via the pipes 33 and 40, the nonaromatic higher-boiling compounds contained in the solvent phase should pass into the raffinate phase, and they should be replaced, partially in the solvent phase, by the lower-boiling non-aromatic hydrocarbons: these can be more readily separated in the subsequent extractive distillation column 5.

The bottom stream in a pipe 27 from the extractive distillation column 5 is supplemented, via a pipe 39, with the remaining fraction of water coming from the.condenser 15 and is subsequently fed to the aromatics stripping column 10.

An important possibility for, but not an essential feature of, the present invention is the addition of water to the enriched solvent mixture prior to the solvent mixture entering the aromatics stripping column 10.

The overhead product from column 10 is sent in a pipe 29 to a condenser 11, the condensate being separated in the decantation apparatus 12 with an aqueous phase and aromatic hydrocarbons, the latter being discharged through a pipe 30.

Bearing in mind that the task of the aromatics stripping column 10 is to separate the aromatic hydrocarbons from the solvent mixture, and that such a separation step is the more burdensome the higher the boiling points of the aromatic hydrocarbons to be separated, it has been ascertained that the addition of water encourages this separation in quite a surprising way. With the process of the present invention, it is possible to operate in the four stages as represented by the 47325 operations carried out in the components numbered 1,2,5 and 10, with those contents of water which are the most suitable for exploiting the effect of the solvent, consistent with the other practical considerations which need to be taken into account.

This impoverished solvent mixture which leaves the stripping column 10, is fed, via the line 34, to the heat exchanger 4 where it preheats the charge to the extractive distillation column 5, and then to the valve 13 where it undergoes an isoenthalpic expansion in accordance with the process of our Patent Specification No. 1585/78, with an appropriate degree of reduced pressure (vacuum) which is a function of the type of feed fed to the plant and of the content of water in the solvent mixture at the bottom of the stripping column 10.

In this way, at the expense of the enthalpy of the stream in pipe 34, which, after the heat exchange in the exchanger 4 is at such a temperature level as not to be any longer susceptible of a profitable exploitation in the plant, the splitting of the stream in pipe 34 into two streams in pipes 35 and 20, which are gaseous and liquid respectively, is achieved by the agency of the isoenthalpic expansion at a degree of vacuum which is sufficiently intense as to attain 40 mmHg of absolute pressure.

The stream in pipe 35 is essentially composed of water, but also contains organic solvent and aromatic hydrocarbons, whereas the stream in pipe 20 has a considerably reduced content of water and hydrocarbons as compared with the stream in pipe 34. Thus, the stream in pipe 20 is made available, which, by virtue of a smaller water content and a smaller content of residual hydrocarbons, allows the liquid-liquid extraction column 1 to operate under better conditions since the dosage of organic solvent can be reduced over that employed in conventional processes, the result being a diminished heat consumption while concurrently obtaining a higher recovery of the aromatic hydrocarbons.

Likewise, there is the stream in pipe 35 available, which, upon condensation in the condenser 15, can be split into the streams 40, and 39 to be used, as outlined above in respect of the process of our Patent Specification No. 1585/78, with the aim of varying the water contents at the base of the liquid-liquid extraction column 2, in the extractive distillation column 5 and in the aromatic-stripping column 10, respectively.

The vacuum in the separator 14 and the condenser 15 is maintained by an ejector 16, via a pipe 36.

The water leaving the decantation apparatus 12 is fed via the pipe 32 to the scrubber, although some may be recycled to the apparatus 10 via a pipe 31.

It is important to note that the degree of vacuum to be adopted is closely dependent on the desire to obtain a complete condensation (with the exception of the uncondensables) of the stream in pipe 35 so as to avoid possible losses of aromatic hydrocarbons. The expedient of introducing an isoenthalpic expansion of the solvent mixture coming from the stripper bottom is such as to permit other advantages to be obtained; these stem from the fact that, contrary to what is experienced with the conventional processes, the solvent mixture obtained at the stripper bottom can have a composition which is different from that of the solvent mixture fed to the extraction column 1. • 47235 One advantage is that the residual content of aromatic hydrocarbons in the stream in pipe 34 is considerably higher than the analogous content in the stream in pipe 20, because in the isoenthalpic expansion (flash) it is possible to separate a portion of these hydrocarbons in the vapour phase. As a result, the degree of fractionation to be obtained in the aromatic stripping column is not so high, whereby considerable savings in heat consumption can be achieved.

A further advantage is that, inasmuch as the solvent mixture coming from the stripper bottom has a higher content of water and hydrocarbons than the stream in pipe 20, the boiling point of the solvent mixture, the pressure being the same, is lower than that of the stream in pipe . Consequently, it is possible to have a reduced heat consumption which is related to the savings of the sensible heat corresponding to the temperature differential.

Another advantage stems from the realization that, once it has been ascertained that the highest temperature point is at the bottom of the aromatics stripper, a reduction of this temperature produces good results both as regards the stability of the solvent mixture and as regards the suppression of possible corrosion phenomena, with the result that carbon steel can safely be used in the entire installation.

Moreover, as the fractionation is less intense, it is possible to avoid the introduction, at the stripper base, of steam produced by the vaporization of a portion of the water obtained as distillate in the decanting apparatus 12. With the heat supplied by the bottom reboiler being the same, by reducing to a minimum the rate of flow of the distillate in pipe 32, whilst keeping it consistent with the raffinate washing requirements, it is possible to work with high reflux ratios, thus obtaining reduced entrainments of solvent.

Further, by exploiting the isoenthalpic expansion, a stream is produced, which is essentially composed of water, to be used according to the requirements of the plant and thus exploit heat values which could not have been used otherwise, whereas in the conventional process the water to be used in the plant is obtained by distillation of the solvent and subsequently stripped from the hydrocarbons in the separators at the head of the extractive distillation column and the aromatics stripper.

The operations in the several stages of the process according to the present invention, are preferably conducted using the ranges of temperature and water content as specified below:(a) The temperature in the liquid-liquid extraction columns 1 and 2 is preferably from 15°C to 100°C. The water content in the columns 1 and 2 is preferably from 0.5% to 12% by weight, with 0.5% - 8% being the more preferred range for column 1 and with 0.5% to 12% for the column 2. (b) The temperature in the extractive distillation column 5 and the aromatic stripping column 10 is preferably in the range from 50°C to 180°C. The water contents in the columns 5 and 10 is preferably from 2% to 15%, on a weight basis.

The most appropriate water content is selected in each case as a function of the content of non-aromatics and the relative ratios of the aromatics in the feed to the plant.

The present invention is illustrated by the following Example which is not to be construed as a limitation.

EXAMPLE Using a plant as shown and described with reference to the accompanying drawing, the feed which was used was a reforming gasoline which had the following composition: Benzene 10% by weight Toluene 31% by weight Xylenes 21% by weight Cg and Cg+ aromatics 8% by weight Non-aromatics 30% by weight This feed was fed at the rate of 100 kg per hour to the bottom plate of the liquid-liquid extraction column 1 which had 60 foraminous plates.

To the head region of the column 1, at the rate of 370 kg/hour there was sent a solvent mixture of N-formyl morpholine and water, the water constituting 2% of the solvent mixture, which latter had a residual hydrocarbon content of about 1.2% by weight. Onto the 20th plate of the liquid-liquid extraction column 1, with the numbering starting from the bottom of the column, there v/as sent via pipe 25 at the rate of 1.5 kg per hour, of which 0.25 kg per hour was water, the heavier fraction obtained in the separator 7 for the first condensation stage 6 of the head product of the extractive distillation column 5.

The column 1 was maintained at a temperature in the range from 50°C to 60°C. From the head of the column 1 there was withdrawn in pipe 18 the raffinate which after it had been washed in column 3, was discharged in pipe 19 at a rate of flow of 31.9 kg per hour. 47325 As washing water for the raffinate there was used the water in pipe 32 from the decanting apparatus 12, which water was completely devoid of organic solvent; the water was fed to the scrubbing column 3 at the rate of 2.2 kg per hour and subsequently sent to the lower portion of the liquid-liquid extraction column 2.

To the base of the extraction column 2, which contained 20 foraminous plates, was sent in pipe 26 the lighter fraction of the head product of the extractive distillation column 5, at a rate of flow of 28.6 kg per hour, 7.4 per per hour of which were water. ' Also to the base of the column 2 was sent, via the pipe 40, a stream at a rate of flow of 7.8 kg per hour, 4.3 kg per hour of which were water and the remaining fraction was hydrocarbons and organic solvent.

From the bottom of the extraction column 2 was discharged a stream in which there was water in an amount of 4.5% by weight relative to the total mixture, which water constituted 5.7% by weight of the mixture of organic solvent and water.

Prior to entering the extractive distillation column 5, the bottom stream from the extraction column 2 was supplemented via the pipe 38 by a stream which had a rate of flow of 5.9 kg per hour, 3.2 kg per hour of which were water.

The temperature at the bottom of the distillation column 5 was maintained at 146°C and the concentration of water was 3.8% by weight relative to the total mixture present which also included aromatic hydrocarbons and organic solvent; the water was present in an amount of 4.5% by weight as related to the organic solvent alone.

In the present Example shown, the condensate obtained from the condenser 15 was split into two streams only, namely streams 40 and 38; thus water was not passed through the pipe 39. Also there was maintained in the extractive distillation columns the maximum concentration of water consistent with the quantities dealt with.

The bottom stream from the extractive distillation column 5 was sent to the aromatics stripper 10 from the head of which the aromatic hydrocarbons were obtained in a high degree of purity.- The hydrocarbons were condensed in condenser 11, and water separated off in the decanting apparatus 12, The content of non-aromatic hydrocarbons in the stream discharged in pipe 30 was less than 400 parts per million (ppm).

The following recoveries of aromatics were obtained, as a percentage of the quantities introduced in the feed:Benzene 100% Toluene 99.8% Xylenes 97% Cg and Cg+ aromatics 85% The aqueous phase distillate separated off in the decanting apparatus 12 was used for washing the raffinate in the scrubbing column 3.

The bottom product from the aromatics stripper 10 was maintained at the temperature of 160°C and had a residual hydrocarbon content of 2% by weight and a water content of 3.9% by weight. This product was cooled in the heat exchanger 4 and then underwent isoenthalpic 4-7 2 2 5 expansion in accordance with the process of our Patent Specification No. 1585/78 in valve 13 to produce a liquid phase and a gas phase which separated in the separator 14. The liquid phase was mainly organic solvent; it was sent to the extraction column 1 with a content of residual hydrocarbons of 1.2% by weight and a content of water of 2% by weight.

In the separator 14, as well as in the condenser 15, a pressure of 110 mmHg (absolute) was maintained.

As compared with a conventional cycle which does not provide 10 for the isoenthalpic expansion of the solvent leaving the bottom of the aromatics stripper, then, with the purity of resulting aromatic hydrocarbons being the same, the process of the present invention affords heat savings as high as 18% in the extractive distillation column 5 and of the order of 20% in the aromatics stripper 10; higher recoveries of the aromatics hydrocarbons can also be achieved.

Claims (7)

1. A process for extracting one or more aromatic hydrocarbon from a hydrocarbon mixture containing the one or more aromatic hydrocarbon and at least one other hydrocarbon, vihich process comprises:subjecting the hydrocarbon mixture to liquid-liquid extraction in one extraction column or in two extraction columns arranged in series, using an extraction agent comprising an organic solvent, to produce a raffinate as an overhead product and an -extract of the aromatic hydrocarbon(s) in the extraction agent as the bottoms product; subjecting the extract from the liquid-liquid extraction to extractive distillation to produce an overhead product and a bottoms product; splitting the overhead product from the extractive distillation into a relatively high boiling fraction and a relatively low boiling fraction; recycling the relatively low boiling fraction to the bottom region of the one liquid-liquid extraction column or to the bottom region of the second of the two liquid-liquid extraction columns; recycling the relatively high boiling fraction to the one liquidliquid extraction column or to the first of the two liquid-liquid extraction columns, at a point above that at which the hydrocarbon mixture is fed; subjecting the bottoms product from the extractive distillation to rectification to separate the aromatic hydrocarbon(s) from the organic solvent; separating off aromatic hydrocarbon(s) from the aqueous distillate head product obtained in the rectification; and using the resulting aqueous distillate to wash the raffinate and then sending the aqueous distillate to the bottom region of the one liquid-liquid extraction column or to the bottom region of the second of the two liquid-liquid extraction columns.

2. A process according to claim 1, wherein the temperature in the or each liquid-liquid extraction column is from 15°C to 100°C.

3. A process according to Claim 1 or 2, wherein the content of the water in the or each liquid-liquid extraction column is from 0.5% to 12% by weight and the content of water in each of the extractive distillation and rectification stages is from 2% to 15% by weight, the water content in these stages being higher than in the liquid-liquid extraction columns.

4. A process according to any preceding claim, wherein the temperature in each of the extractive distillation and rectification stages is in the range from 50°C to 180°C.

5. A process according to claim 1, substantially as hereinbefore described with reference to the accompanying drawing.

6. A process according to claim 1, substantially as described in the foregoing Examples.

7. An aromatic hydrocarbon whenever extracted by a process according to any preceding claim.