US2403485A - Separation of aromatic hydrocarbons from hydrocarbon mixtures - Google Patents

Description

July 9, 1946. H.- V. ATWELL ET AL 2,403,485

SEPARATION 9F AnoMATIc HYnRocARBoNs FROM HYDROCARBON MIXTURES Filed March 8, 1941 INTERFACE LEVEL l soLveNT Y Du Bols EAsTMAN 25 l-lAlzoLovArw/ELL INVENTORS BY l YXMW.

A TTORNEYS Patented July 9, 1946 UNITED STATES PATENT OFFICE SEPARATION OF AROMATIC HYDROCAR- BONS FROM HYDROCARBON MIXTURES Application March 8, 1941, Serial No. 382,328

This invention relates to the separation of aromatic hydrocarbons from hydrocarbon mixtures and particularly from hydrocarbon mixtures such as derived from petroleum and which contain aromatic and non-aroma-tic hydrocarbons including aliphatic and alicyclic hydrocarbons.

The invention broadly contemplates separat-y ing aromatic hydrocarbons from mixtures containing them by extraction with water or with a solvent consisting essentially of water at a temperature considerably above the normal boiling point of water, and under a pressure suicient to maintain the solvent and hydrocarbons substantially in the liquid phase during the extraction. As a result of the extraction a solvent phase is formed comprising aromatic hydrocarbons dissolved in the major portion of the solvent liquid while a hydrocarbon phase is formed comprising non-aromatic and other hydrocarbons mixed with a miner portion of the solvent. The two phases are separated and separately subjected to cooling and settling, without substantial reduction in pressure, so that separation between hydrocarbons and solvent occurs. The separated hydrocarbons are withdrawn,'while the solvent from which the hydrocarbons have been separated is recycled Afor extracting fresh feed mixture.

It is also contemplated that the solvent may comprise water and a small amount of inorganic salts, acids or bases, or organic substances capable of favorably iniiuencing the solvent properties of the water and permitting extraction in the liquid phase at temperatures and pressures lower than otherwise necessary when employing water alone as the extracting medium.

The proportion of added substance which may be employed is such that the mixture of water and added substance Iwill exert substantial solvent action upon aromatic hydrocarbonsat temperatures of 200 F. and above but will exert substantially little solvent action upon aromatic hydrocarbons at ordinary temperatures, for example, from about room temperature to about 159 F. In other words, the proportion of added substance and water in the solvent mixture is such that when the solution of desired aromatic hydrocarbons in the solvent is separated from the extraction zone the desired aromatic hydrocarbons can be substantially entirely separated from the solvent merely by reducing the temperature of the solution to a temperature in the range about 70 to 150 F.

Thus, the proportion of above-mentioned added substance contained in the water may range from a fraction of a per cent to 56% or more by 6 Claims. (Cl. 196--13) weight of the solvent mixture and preferably will range from about 5 to 25% by weight.

The invention has particular application to the extraction of aromatics such as benzol, toluol and xylol from hydrocarbon fractions derived from petroleum or obtained in the thermal or catalytic treatment of petroleum hydrocarbons. For example, it is useful in extracting benzol and toluol from cracked naphtha produced in either catalytic or pyrolytic cracking of hydrocarbons. Other naphtha or hydrocarbon mixtures to which the invention may be applied comprise hydroformed naphtha, pyrolytically or catalytically reformed naphtha, polyformed naphtha and theilike. It is also applicable to the extraction of aromatic hydrocarbons from straight run naphthas or gasolines such as distilled from certain crudes.

While the treatment of hydrocarbon mixtures derived vfrom petroleum sources has been specilically mentioned, nevertheless it is contemplated that the process may be employed for extracting valuable aromatic hydrocarbons from hydrocarbon mixtures derived from other sources.

An object of the invention is to provide a continuous method of extracting aromatic hydrocarbons from a mixture of aromatic and aliphatic or other hydrocarbons with a solvent which can be separated and recovered from the solvent and hydrocarbon phases obtained in the extraction merely by reduction in temperature thereby avoiding resort to fractional distillation.

Ar further object is to provide a continuous method of extraction in a substantially closed circuit through which the solvent medium circulates and from which the separated hydrocarbons can be separately withdrawn without substantial reduction in the pressure within the circuit or extraction system.

In order to describe the invention in more detail reference will now be made to the figures of the accompanying drawing showing methods of practicing the extraction process of the invention. y

As shown in Figure 1 a hydrocarbon feed such as cracked naphtha or a fraction of cracked naphtha containing aromatic, alicyclic and aliphatic hydrocarbons is conducted from a source not shown through a pipe I communicating with branch pipesl 2 and 3 leading to the lower portion of an extraction tower 4.

The extraction tower 4 advantageously comprises a vertical vessel containing a suitable packing or contact `material `5 such as Raschig rings.

A stream of water is continuously introduced to the upper portion of the extractionV tower through a pipe 6. The water flows downwardly over the packing material within the tower, while the feed hydrocarbons ow upwardly therethrough, so that countercurrent contact between water and hydrocarbon is realized.

The water is introduced to the tower at an elevated temperature of about 40G to 600 F. and preferably about 475 to 525` F.

The hydrocarbon feed is likewise introduced to the lower portion of the tower in a heated condition, for example, at a temperature substantially the same as that at which the water solvent is introduced. In some cases it may be advantageous to introduce the feed hydrocarbon at a somewhat lower temperature, for example, 25 or 50 below the temperature at which the solvent is introduced, thereby maintaining a temperature gradient within the tower.

As indicated at the outset the extraction tower is operated under a pressure suiiiciently elevated to maintain the solvent and hydrocarbons undergoing treatment substantially in the liquid phase. Thus, the pressures will depend upon the particular temperatures maintained within the tower as well as upon the volatility of the liquids within the system and may range from 1000 to 5500 pounds per square inch gauge, for example.

As a result of the countercurrent contact between solvent and hydrocarbons there will be formed a solvent rich phase or solution which will comprise aromatic hydrocarbons dissolved in the bulk of the solvent. The undissolved hydrocarbons or hydrocarbon rich phase will comprise aliphatic and alicyclic hydrocarbons mixed with a small amount of solvent.

The hydrocarbon rich phase is removed from the top of the tower while the solvent rich phase is removed from the bottom of the tower.

As indicated in Figure 1 the interface level between solvent and hydrocarbon phases may be maintained near the bottom of the tower, provision being made forv observing the level with either a sight glass or a differential float control not shown in the drawing.

The solvent phase accumulating in the bottom of the tower is drawn off through a pipe Iii and a cooling coil II and from there conducted to a settling vessel I2.

In passing through the cooling coil Il the temperature of the solvent phase is reduced to a temperature below 200 F. for example in the range about 70 to 150 F., so that upon introduction to the settler I2 separation into layers or phases occurs due to the relative insolubility of the aromatic hydrocarbon in the solvent at the reduced temperatures.

The upper layer or phase in the settler I2 will comprise aromatic hydrocarbons which are drawn off through a Valve controlled pipe I3. The lower layer or phase will comprise water satue rated with, or containing a very low content of aromatic hydrocarbons at the temperature prevailing within the settler I2, and this water is drawn oi through a Valve controlled pipe I4.

Likewise the hydrocarbon phase accumulating in the top of the extraction tower is drawn off through a pipe and passed through a cooling coil 2| to a settler 22 similar to the settler I2. In passing through the cooler 2| the temperature of the hydrocarbon phase may be reduced to about 70 F.

Likewise in the settler 22 a small amountY of phase separation will occur as a result of reduction in temperature, the upper phase comprising aliphatic and naphthenic hydrocarbons which are discharged from the system through a valve controlled pipe 23. The lower layer in the settler 22 will comprise Water which is drawn oli" through a valve controlled pipe 2d. Since the amount of water present in the mixture entering the settler 22 will usually be quite small, the water separated at this point may be discharged from the system. On the other hand instead of passing the hydrocarbon phase to settler 22 it may be passed directly to a still or other means wherein the pressure is reduced and the hydrocarbons recovered for such further treatment as may be desired.

The upper layers withdrawn from the settlers I2 and 22 may be distilled separately to distil oil? the small amount of retained water and also to eiect separation between benzol and toluol or the individual aromatic constituents as the case may Y be. If desired these hydrocarbon layers may be subjected to drying by filtration.

'Ihe water drawn oir from the settling vessels through the pipe Iii, and including that from pipe 24 if so desired, is conducted through the pipe '(25 by means of a pump 25 to a heater 2l', wherein the temperature of the water is raised to the desired point, and thereafter conducted from the heater through the previously mentioned pipe 6 for return to the upper portion of the extraction tower. Makemp Water or solvent can be added to the pipe 25 through a pump 28.

Thus, as indicated in the drawing the system comprising the extraction tower, the coolers I I and 2l, settlers I2 and 22, the heater 2l' and the pipes affording uid communication therebetween provides a closed circuit through which the solvent medium circulates continuously. Advantageously the pressure is substantially equalized throughout the entire circuit and which involves effecting the cooling and settling of the phases withdrawn from the top and bottom of the extraction tower without reduction in the pressure.y

In carrying out the extraction the volume of water circulated through the extraction tower is usually greater than the volume of feed hydrocarbons passing therethrough. Thus, the ratio of water to hydrocarbon feed may range from l to 12 parts or more of water'to l part of hydrocarw bon feed.

Figure 2 illustrates an alternative method of operation wherein the extraction tower is maintained substantially iilled with a body of water and circulation of water through the system is effected by thermo-Siphon action.

Thus, the tower 30 is substantially similar to the tower 4 described in Figure 1 and may likewise be provided with packing material 3 I.

The water or solvent consisting essentially of water is introduced to the upper portion of the tower through pipe 32 while the naphtha or mixture of hydrocarbons containing aromatics, after passage through a heater 33, is introduced to the lower portion of the tower through a pipe 34.

The operation is regulated so that the interface level 35 is maintained near the top of the tower. Thus, the heated naphtha after introduction to the lower portion of the tower bubbles upwardly through the body of water contained within the tower. The unabsorbed hydrocarbons accumulate as a layer or hydrocarbon rich phase in the top of the tower and are removed therefrom through a pipe 35. From the pipe 3S the withdrawn hydrocarbons are passed through a cooler 31 and conducted to a separating vessel 38 similar to the vessel 22 referred to in Figure 1.

A stream of water containing the dissolved aromatic hydrocarbons is continuously drawn off from the bottom of the tower through a pipe 39 leading to a heater 40.

The mixture after heating in the heater 40 rises through a vertical pipe 4I leading to a cooler 42 placed at an elevation somewhat higher than the top of the tower.

The mixture passing through the previously mentioned heater 40 is sufficiently heated so that the density is reduced thereby causing the liquid to rise through the pipe 4|.

In the cooler 42 the temperature is reduced to some point in the range about 70 to 150 F. so that upon introduction of the cooled mixture to a separator 43 separation into phases occurs. The upper phase will comprise aromatic hydrocarbons which were extracted from the naphtha feed while the lower phase will comprise water from which the aromatic hydrocarbons have been separated.

This water phase is continuously drawn off from the bottom of the separator 43 through a pipe 44 and passed through a heater 45 wherein the water or solvent is heated to the desired temperature for eifecting extraction within the extraction tower. The so heated and recycled water or solvent is returned to the upper portion of the tower through the previously mentioned pipe 32.

The separated aromatic hydrocarbons are discharged from the top of the separator 43 through a pipe 46.

As previously mentioned in connection with Figure 1 the water drawn off from the bottom of the separator 38 may be discharged from the system since it is relatively small in amount. In any case it is necessary to continuously add a small amount' of makeup water to the system and this can be done by means of a pump 41, by which means additional water is injected into the pipe 39.

It is contemplated that inert gas may be introduced to the pipe 39 or the pipe 4l to serve as a gas lift or to reduce the density of the fluid rising through the pipe 4l so that positive and rapid flow of fluid through the pipe 4l is secured. In such case provision is made for separately discharging the gas from. the separating vessel 43.

Instead of introducing extraneous gas it is contemplated that the heating in the heater 40 may be such as to generate a small amount of steam or vapor thereby serving substantially the same function of reducing the density of the fluid in the pipe 4|. The resulting vapors would be substantially condensed in the cooler 42.

While a single extraction tower has been described above nevertheless it is contemplated that the extraction may be carried out in a multistage operation employing a plurality of extraction towers. Also it is contemplated that the hydrocarbon layers removed from the separating vessels of Figures 1 and 2 may be subjected to subsequent and separate re-extractions in separate stages.

The following example illustrates the results obtained in a continuous countercurrent tower extraction such as described in Figure 1, when continuously charging to the lower portion of the tower naphtha comprising a mixture of toluene, 20% olens and 60% paramns by weight. The toluene in the mixture has a specific gravity v pose of removing the olens.

6 V of about 0.8613 relative to water at 68 F. The olefins used in the mixture comprise a fraction having a boiling range of about 200 to 250 F. separated from polymer naphtha obtained in the catalytic polymerization of normally gaseous oleiins. The parains comprise a fraction having a boiling range from 200 to 250 F. separated from a gasoline obtained by catalytically alkylating gaseous isoparains and olens.

Water is continuously charged to the upper portion of the tower in the proportion of about 6 parts of water to 1 part of naphtha charge.

The temperature at the top of the tower is maintained at about 530 F. while the temperature at the bottom of the tower is about 519 F. The average pressure maintained within the tower is about 2500 pounds.

The extract phase comprising aromatic hydrocarbons dissolved in the water is continuously withdrawn from the bottom of the tower while the raflinate phase comprising non-aromatic hydrocarbons is continuously withdrawn from the top of the tower.

The water free extract will have an average specific gravity of .8142 relative to water at 60 F., this extract will contain an average of about 66% by weight of toluene, which amounts to about 46% by volume of the toluene contained in the charge mixture.

Where it is desired to obtain toluene sub- I stantially free from olefns the extract hydrocarbons may be subjected to treatment with a suitable agent such as sulfuric acid for the pur- The acid treatment may also be employed for the purpose of removing sulfur compounds such as mercaptans as well as other impurities. Other reagents may be employed for this purpose including solid adsorptive material Such as acid treated clay,

In extracting aromatic hydrocarbons such asv toluene from naphtha the light naphtha fraction may be subjected to extraction with water followed by distillation of the resulting extract hydrocarbons to segregate a fraction rich in the desired aromatic constituents. As an alternative ,procedure the naphtha may be fractionated to segregate a fraction rich in the desired aromatic constituents, i. e., toluene or consisting essentially of hydrocarbons having a boiling range of about 200 to 250 F. and this fraction then subjected to solvent extraction in order to separate the toluene.

It is contemplated that the naphtha or naphtha fraction rich in the desired aromatics may be subjected to preliminary treatment to remove gum forming bodies and sulfur compounds prior to extraction with water. For example, the naphtha or suitable fraction thereof may be passed directly from the fractionating tower of the conversion unit in which the naphtha is vproduced to a conventional clay treating tower for the removal of diolens or gum forming bodies. This clay treatment or a separate clay treatment may be carried out at temperatures sufciently elevated to effect desulfurizing of the hydrocarbon mixture. Thereafter, the treated hydrocarbon mixture is subjected to extraction with water to remove the aromatic constituents in a manner similar to that already described. The naphtha hydrocarbons from which the desired aromatics have been extracted can be run to gasoline or motor fuel production.

While the invention has been described in connection with the recovery of aromatic hydrocarbons from naphtha it is contemplated that the invention may be applied in the extraction of other hydrocarbon fractions such as the higher boiling portions of cracked or reformed gasoline and also to kerosenes and lubricating oils.

In addition to the foregoing the process has application to the extraction of high antiknock aromatic blending stocks from wide boiling range naphthas or naphtha mixtures. It may also be applied to the treatment of high boiling petroleum fractions to obtain raliinates of desired properties, for example, Diesel fuel having a high cetane number, kerosene having superior burning properties or lubricating oils of high viscosity index.

Obviously, many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

l. A continuous process for extracting aromatic hydrocarbons from a hydrocarbon mixture containing aromatic and non-aromatic hydrocarbons by contact with a solvent in continuous countercurrent flow in an extraction tower which comprises maintaining a substantial body of solvent consisting essentially of water within the tower at a temperature sufficiently above the normal boiling point of said solvent and under a pressure surciently high that aromatic hydrocarbons are substantially soluble in the solvent and such that the hydrocarbons and solvent are substantially in the liquid phase, continuously injecting feed hydrocarbon mixture to the lower portion of said tower to rise upwardly therein through the body of solvent, continuously withdrawing a stream of solvent and dissolved hydrocarbons from said tower at a point below the point of feed hydrocarbon injection, heating the withdrawn stream without substantial reduction in pressure to a temperature suicient to reduce its density, discharging the so-heated stream into a vertically disposed conduit having a discharge outlet at an elevation sufciently high to permit iiow by gravity from said discharge outlet into the upper portion of the tower, discharging the heated stream without substantial reduction in pressure from said outlet into a cooling zone, cooling the discharged fluid and passing the cooled duid to a separating zone without substantial recluction in pressure, effecting separation between dissolved hydrocarbons and solvent in said separating zone, discharging the so separated hydrocarbons, reheating and recycling the reheated solvent from which hydrocarbons have been separated to the upper portion of said tower and continuously withdrawing undissolved hydrocarbons from the top of said tower.

2. A continuous process for extracting aromatic hydrocarbons from a hydrocarbon mixture containing aromatic and non-aromatic hydrocarbons by contact with a solvent in continuous countercurrent now in an extraction tower which comprises maintaining a substantial body of solvent consisting essentially of water within the tower under elevated temperature and pressure such that` aromatic hydrocarbons are substantially soluble in the solvent and such that the hydrocarbons and solvent are substantially in the liquid phase, continuously injecting feed hydrocarbon mixture to the lower portion of said tower to risel upwardly therein through the body of solvent, continuously withdrawing a stream of solvent and dissolved hydrocarbons from the bottom of said tower, heating the withdrawn stream without substantial reduction in pressure to a temperature suii'icient to reduce its density, discharging the so heated stream into a vertically disposed conduit having a discharge outlet at an elevation substantially above the top of the tower, discharging the heated stream without substantial reduction in pressure into a ,cooling zone, cooling the discharged uid and passing the cooled fluid to a separating zone without substantial reduction in pressure, effecting separation between dissolved hydrocarbons and solvent in said separating zone, discharging the separated hydrocarbons, reheating the substantially hydrocarbon free solvent without substantial reduction in pressure, and returning the reheated solvent to the upper portion of said tower.

3. A continuous process for extracting aromatic hydrocarbons from a hydrocarbon mixture containing aromatic and non-aromatic hydrocarbons by contact with a solvent in continuous countercurrent flow in an extraction tower, which comprises maintaining a substantial body of solvent consisting essentially of water within the tower ata temperature in the range about 475 to 525 F. and under a pressure such that the hydrocarbons and solvent are substantially in the liquid phase, continuously injecting a naphtha hydrocarbon mixture of aromatic and non-aromatic constituents to the lower portion of said tower to rise upwardly therein through the body of solvent, continuously withdrawing a stream of solvent and dissolved aromatic hydrocarbons from the lower portion of said tower at a point below the point oi hydrocarbon injection, heating the withdrawn stream without substantial reduction in pressure to a temperature sufficient to reduce its density, discharging the so-heated stream into a verti- `cally disposed conduit having a discharge outlet at an elevation suiiiciently high to permit ow by gravity from said discharge outlet into the upper portion of the tower, discharging the heated stream without substantial reduction in pressure from said outlet into a cooling zone, cooling the discharged fluid and, without substantial reduction in pressure, effecting separation between aromatic hydrocarbons and solvent as a result of said cooling, discharging the separated aromatic hydrocarbons, reheating the substantially hydrocarbon-free solvent without substantial reduction in pressure, and returning the reheated solvent to the upper portion of said tower by gravity flow.

4.. A continuous process for effecting fractional separation of oil mixtures by extraction with a solvent in which a component of said mixture is substantially soluble while remaining components are substantially insoluble therein at an elevated temperature and pressure, which comprises continuously maintaining a substantial body of said solvent within the tower under said elevated temperature and pressure, continuously injecting a stream of said oil mixture to the lower portion of said tower to rise upwardly therein through the body of solvent, continuously withdrawing a stream of solvent and dissolved oil from the lower portion of said tower at a point below the point of oil injection, heating the withdrawn stream without substantial reduction in pressure to a temperature sufcient to reduce its density, discharging the so-heated stream into a vertically disposed conduit having a discharge outlet at an elevation sufciently high to permit liow by gravity from said discharge outlet into the upper portion of the tower, discharging the heated stream without substantial reduction in pressure from said outlet into a cooling zone, cooling the discharged iiuid and, without substantial reduction in pressure, effecting separation between dissolved oil and solvent as a result of said cooling, discharging the separated oil, reheating the substantially oil-free solvent without substantial reduction in pressure, and returning the reheated solvent to the upper portion of said tower by gravity flow.

5. A continuous process for extracting toluene from a naphtha hydrocarbon mixture containing toluene and non-aromatic hydrocarbons by contact with a solvent in continuous countercurrent ow in an extraction tower which comprises maintaining a substantial body of solvent consisting essentially of water within the tower at a temperature suiciently above the normal boiling point of said solvent and under a pressure sufficiently high that toluene is substantially soluble in the solvent and such that the hydrocarbons and solvent are substantially in the liquid phase, continuously injecting feed naphtha to the lower portion of said tower to rise upwardly therein through the body of solvent, continuously withdrawing a stream of solvent and dissolved hydrocarbons from said tower at a point below the point of feed hydrocarbon injection, heating the withdrawn stream without substantial reduction in pressure to a temperature suncient to reduce its density, discharging the so-heated stream into a vertically disposed conduit having a discharge outlet at an elevation suiiciently high to permit owby gravity from said discharge outlet into the upper portion of the tower, discharging the heated stream without substantial reduction in pressure from said outlet into a cooling zone, cooling the discharged fluid and passing the cooled fluid to a separating Zone without substantial reduction in pressure, eiecting separation between dissolved toluene and solvent in said separating zone, discharging the so separated hydrocarbons, reheating and recycling 10 the reheated solvent from which hydrocarbons have been separated to the upper portion of said tower and continuously withdrawing undissolved hydrocarbons from the top of said tower.

6. A continuous process for extracting toluene from a hydrocarbon mixture containing toluene and non-aromatic hydrocarbons by contact with a solvent in continuous countercurrent ow in an extraction tower, which comprises maintaining a substantial body of solvent consisting essentially of water within the tower at a temperature in the range about 475 to 525 F. and under a pressure such that the hydrocarbons and solvent are substantially in the liquid phase, continuously injecting a naphtha hydrocarbon mixture boiling in the range about 200 to 250 F. containing toluene and non-aromatic hydrocarbons to the lower portion of said tower to rise upwardly therein through the body of solvent, continuously withdrawing a stream of solvent and dissolved toluene from the lower portion of said tower at a point below the point of hydrocarbon injection, heating the withdrawn stream without substantial reduction in pressure to a temperature sufcient to reduce its density, discharging the soheated stream into a vertically disposed conduit f having a discharge outlet at an elevation suisolvent to the upper portion 0f said tower by gravity ilow.

HAROLD V. ATWELL. DU BOIS EASTMAN,

Images