IL24353A - Pure aromatic hydrocarbons and their recovery by extraction or extractive distillation with solvent mixtures - Google Patents

Description

PURE AROMATIC ASB RECOVERY BY EXTRACTION OR EXTRACTIVE WITH MIXTURES The separation of aromatic in particular toluene and xylene from hydrocarbon mixtures by extraction with selective solvents or by extractive distillation has been used industrially for several o from the fact that new solvents for this process are continuously proposed and patented it may be concluded that the ideal solvent for this process has not yet been Along with pure solvent mixtures are very often The addition of certain proportions of water to the solvent is a very common method and offers the following Increase in the solvent mainly of the higher the extract is possible even for small boiling point differences between the aromatic components and the since the aromatics form mixtures vrith water and may therefore be removed from the extract below The sump temperature of the distillation column separating the aromatics from the solvent is on the one whereby thermal decomposition of the product is avoided on the other the thermal energy required for heating the solvent to the sump temperature of the column is case of extraction there arises the the further advantage of existences of the system is so that higher concentrations of aromatics in the feed mixture are while the system is On the other hand the following disadvantages have to be taken into account when water is added to the solven The the extent to which the solvent can be Water evaporates azeotropioally from the solvent together with the aromatics to be whereby the required heat energy is considerably In some cases the water produces undesirable chemical mainly There are also known a number of solvent mixtures for the extraction of mixtures of ethyleneglycol and or mixtures of a containing glycol derivatives with a like ethanol and Such solvent mixtures the disadvantage that part of the solvent has a lower boiling point and the other part a higher boiling point than the aromatics to be This leads to high capital investments and operational costs for the separation of the aromatics from the which are generally not offset by the advantages of such a Other known proposals for the contains one or two hydroxyl groups in the molecule and the second contains two or more such groups in the or ethylene carbonate with addition of ol formic ol hydroquinone as or solvents such as alkane o of diethyleneglyool or with or or mixtures of any of these solvents that are selective for aromatics No advantage has ever been alleged for using any of these and other known mixtures of selective solvents of higher boiling point than that of the aromatics to be recovered either by extraction or by extractive over the use of pure nor have any specifications been made as to the amounts or relative proportions in which the various components of the mixture are to be used for the extraction or extractive distillation of in order to attain any advantages over pure solvents without any The state of prior art concerning the possibilities of using solven mixtures for extraction and extractive distillation of aromatics may therefore be summed up in stating that the effect of water addition to a solvent is qualitatively known and that there exists some knowledge on the properties of various combinations of glycol Against this the by which solvents are to be mixed in order to obtain good characteristics for the extraction and extractive distillation of aromatics are not described in the scientific and patent According to the invention it has now been that in spite of the very large number of solvents for this purpose and the fact that these solvents diffe chemically to a very considerable there exist certain rules whose knowledge enables to select from the extremely large number of possible combinations those from which optimal properties may be Selective solvents selected for the recovery of aromatics by extraction or by extractive distillation should possess above all the following physical A high degree of A high A range of the capability of forming a two phase system even at a high concentration of The term of is used the herein to concentration of aromatics in the ternary system at which a system still which separation into two phases will no longer occur when this concentration is Low solubility of the solvent in the hydrocarbon Low High Boiling point above the boiling range of the aromatics to be Thermal and chemical stability at the boiling Melting point below the ambient It has been found that these different physical properties do not occur in the various solvents at and that certain regularities can be observed which are of special importance for the selection of a favourable combination of such Prom among the above listed properties it is in particular the third the range of which is suitable as a leading feature for classification of the solvents into certain If the solvents are classified by increasing range of they may be divided into three Solvents of low aromatic Solvents of high aromatic Solvents whose with aromatics is not In the following the qualification aromatic will be used for a concentration of less than and preferably less than aromatics as the maximum concentration for a Two typical phase diagrams for two pyrrolidone and of the first group aromatic with indication of the distribution eciuilibrium in the system are given in t and The solvents of this group are characterised by a high low selectivity and high solubility of the solvent in the hydrocarbon Because of their low selectivity the solvents of this first group are usually employed in admixture with with the result that above mentioned disadvantages must be taken into According to the invention it has been ound that most of the solvents belonging to this group have two further important and favourable namely low viscosity and stability at the boiling This group includes mainly compounds with heterocyclic ring ketodioxane and also and all of which have an aromatic range below and also with an aromatie range of between 35 and The solvents of the second of which two representatives sulfolan and ethylenecarbonate are given in 3 and comprise such with a high aromatic that is with an aromatic range of at least maximum concentration in a They are therefore at the miscibility limit with aromatics for completely miscible with benzene in any hile the miscibility with xylene is not Among these there are some combining a very high selectivity with a good the latter lower than with solvents of group The compounds of this group would therefore be ideal solvents for extraction of aromatics as far as their selectivity and capacity are but it is most peculiar that substances have such unfavourable as to constitute serious In all representatives of this group have the disadvantage of instability at their boiling point and of possessing a high In addition of them melt above the ambient This group includes mainly compounds with oxygen and with dinitrile sulfolan ethylene and and tetraethyleneglycol and The solvents of the third of which the two representatives diethyleneglyool and propyleneglyeol are represented in 5 and 6 have a miscibility gap with there used from this group members which dissolve less than by weight of benzene at Their capacity is lower than that of the solvents of the other two Their selectivity is higher than that of the first group and mostly lower than that of the second Most of them have high viscosities and boiling points Only some of them are thermally stable at the boiling This group includes mostly compounds with a configuration and with one or hydroxyl groups in the combined in many instances with amino and malondinit hydrazine and many In accordance with the it has now been a solvent of the third group there are obtained mixtures with considerably better properties than would have been expected from the mixing the selectivity is in all cases higher than would be expected from the mixing rules and is often even higher than the selectivity of each of the individual The receptivity for aromatics of solvents of group is limited by their low aromatic range and with the solvents of group by their low dissolution capacity for By combination of a solvent of group a solvent of group there results a receptivity for aromatics which is always greater than that of each individual and in many cases exceeds that of the more component by and An undesirable property of the solvents belonging to the third group their high But even in this respect a favourable property of the was discovered in that the viscosity of the solvent mixture is always lower than that calculated according to the mixing According to the it has further been found that the most favourable mixing proportions for individual solvent mixtures may be established quickly by the following special ternary solvent A solvent C benzene produced by admixture and the solubility limits as well as the critical point the point at which the ligh and heavy phases become equal to each are The composition of the solvent mixture corresponding to this In case of a feed mixture wherein the components consist mainly of or in case a paraffinic antisolvent is the proportion of solvent belonging to group A may be 5 to higher than defined On the other hand if the portion of the feed mixture has a high content of olefins h naphVenee it may often be preferred to increase the proportion of the solvent of group by 5 These possibilities are explained in the following with reference to 7 11 referring by way of example to the solvent mixture glycol pyrrolidone in the following It can be seen from 7 that the receptivity for aromatics of a mixture containing a solvent of group A and a solvent of group C is greater than the receptivity corresponding to the mixing which is represented and even be by the dotted line A generally is higher than the receptivity of each of the individual pure components of the In the example represented in the figure the receptivity is achieved by a mixture consisting of glycol and 4 and is by weight of benzene in the heavy The evolution of the selectivity may be seen in the curves of In this figure the selectivity a logarithmic linear is the composition of the a for mixtures respectively 10 and of benzene in the light curves have a maximum corresponding to 70 of The maximum of the receptivity lies thus at another composition of the and optimal mode of finding the optimum between selectivity and capacity may be seen from this figure the capacities are plotted on the abscissa and the selectivities on the ordinate for various mixing proportions of and The resulting curve depends on the content of In this diagram an aromatic concentration of in the light phase was taken as a basis in order to cover the entire It may be seen that the points corresponding to various mixtures are at a considerable distance from the straight line A C of the pure and are displaced towards the upper right side towards the values of higher capacity and higher It can be seen that the right and uppermost region which corresponds to the optimal economic lies between 50 of This value of approximately glycol is also found if the diagram of the two solvents with benaene is measured and the critical point is established as described shows this diagram for the system NMP glycol At the oritical point P the ratio glycol s is 55 Finally it may be seen from 1 that the viscosity of a mixture of NMP and glycol is considerably lower than was to be expected according to the mixing rule A in particular in the median Other solvent mixtures of optimal composition for the Components Of the solvent mixture weight Diethanolamine Aniline Butyrolactone Phenol Glycol Aniline Glycol Phenol 50 Glycerine Dimethylformamide Aniline Phenol Furfural Butyrolactone Dimethylsulfoxide Butyrolactone alonni Malonnitrile Monoethanolamine Dimethylformamide Malonnitrile Dimethylsul oxide Malonnitrile Glycerine Glycol Glycerine Qlycol Butyrolactone Aniline Furfural Furfural Phenol The process of the invention is generally applicable to the recovery of aromatics from hydrocarbon mixtures containing by extraction or extractive also for obtaining extracts wherein concentration of aromatics only been enriched as compared to their concentration in the feed A special advantage of the process according to the invention consists in that it is also suitable for the recovery of highly pure aromatics which have gained lately in importance as starting materials for chemical By the process according to the invention aromatics containing less than and less than of substance can easily be Even more severe purity for concentrations of that can no longer be detected can be met by this The process of the is by way of example in of the accompanying drawings and by one specific For the sake of clarity the compositions of the different production stages are given in the figures directly in percentage by In this B benzene T toluene X xylene P paraffinic hydrocarbons h Since the solvent mixture has a boiling point much higher than the boiling range of the aroiaatics to be its composition does not change during the so tnat a separation into glycol and P is not necessary in the EXAMPLE of feed mixture having the indicated compositio through line to the stage of a extractor in which it is extracted with 7085 of a solven mixture containing by weight of glycol and by weight of In the extractor 8810 of an extract having the indicated composition are obtained and are sent through line to the preliminary distillation column This column has 40 plates and works with a reflux ratio of 2 725 kg h of the top fraction containing roughly h of the balance being mainly is recycled through pipe as reflux to the 389 of a raffinate free from aromatics leave the extractor through line The sump fraction of column consists of 8085 of an extract consistin exclusively of solvent and while all the contained initially in the extract have been distilled off at the top of column The extract is fed through line to a solvent stripper The stripper contains 20 real plates and works with a reflux ratio of 1 000 kg of pure aromatics of the indicated composition are se arated at the to and leave throu h line The bottom fraction which practically consists of pure solvent is in an amount of 7085 through line to the extractor The mixture of aromatics distilled off at the top of coluinn is then separated by known methods into toluene and The benzene thus obtained had a melting point of the toluene had a 20 refraotive index of and in the xylene fraction no could be detected by 17 insufficientOCRQuality

Claims (8)

1. A process for recovery of aromatic hydrocarbons from hydrocarbon mixtures by extraction and/or extractive distillation by means of a mixture of non-aqueous solvents preferentially dissolving aromatic hydrocarbons, all components of which solvent mixture boil above the boiling range of the aromatic hydrocarbons to be recovered, wherein the solvent mixture contains at least one solvent (A) having an aromatic range (as herein defined) below 50r/e and preferably below 35?o in the system benzene - n-heptane - solvent, and at least one solvent (C) which is of limited miscibility with benzene and preferably dissolves less than by weight of benzene at +20°C.

2. A process according to Claim 1, wherein the solvents (A) and (C) are mixed in relative proportions that correspond to the critical point of the ternary system solvent A - solvent C - benzene.

3. A process according to Claim 1 applied to the recovery of aromatic hydrocarbons from a feed mixture having a high content of paraffin and/or to the case where a paraffinic antisolvent is used, wherein the relative proportion of solvent (C) is up to 10$ lower than stipulated in Claim 2.

4. A process according to Claim 1 applied to the case where the non-aromatic fraction has a high content of naph-thenes and/or olefins, wherein the relative proportion of solvent (C) is up to 10^ higher than stipulatedjin Claim 2.

5. A process according to any one of Claims 1 to 4, wherein the solvent component (A) is N-meth lpyrrolidone, - 18 - 24353/2 formamide, amiline, eth lenediamine, nitromethane, ethylene-glycolmonomethylether, diethyleneglycolmonomethylether and al30 diethylenetriamine, triethylenetetra ine, tetra-ethylenepentamine, or dimethylsulfoxide.

6. A process according to any one of Claims 1 to 5» wherein the solvent component (C) is glycol, diglycol, propyleneglycol, mono-,di- and triethanolamine, glycerine, cyclohexanedimethanol, diglycolamine, formamide, malondi-nitrile or hydrazine.

7. A process according to any one of Claims 1 to 6, wherein a solvent mixture consisting of from 45 to 65 of glycol and 55 to 5 of N-methylpyrrolidone is used.

8. Pure aromatic hydrocarbons containing less than 0.1/*, and preferably less than 0.01$, of non-aromatic substances, when manufactured by the process according to any of Claims 1 to 7. For the Applicants DR. REIHHOLD COHU & PARTNERS PC/rb