Tetraethylene GlycolA Superior Solvent for Aromatics Extraction

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Tetraethylene Glycol—A Superior Solvent

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 17, 2016 | http://pubs.acs.org Publication Date: June 1, 1970 | doi: 10.1021/ba-1970-0097.ch014

for Aromatics Extraction G. S. SOMEKH and B. O. FRIEDLANDER Union Carbide Corp., Chemical and Plastics, P. O. Box 65, Tarrytown, Ν. Y. 10591 The separation of benzene, toluene, and C aromatics (BTX) from gasoline fractions is reviewed from a fundamental and historical point of view, and the disadvantages of the old distillation processes are presented. The Udex process, which has been the dominant liquid extraction and extrac tive distillation process for effecting this separation, is being used more and more. Phase equilibrium data show the dif ferences between the various glycol solvents that can be used in this process. These data have shown that tetraethyl ene glycol is superior to other solvents. There is a significant trend in the industry to employ tetraethylene glycol. 8

^nphe benzene-toluene-Cg aromatics fraction is presently the principal raw material used to manufacture petrochemicals. This fraction (usually referred to as BTX) is even more important than ethylene as a raw material. Reformed gasolines contain these aromatics in concentions ranging from about 30 to 60%. BTX is also available from hydrogenated coke oven light oils, cracked gasolines, or hydrogenated dripolenes in which the concentrations can vary from about 70 to 97%. The earliest large scale process used to separate BTX from aliphatics was straight distillation. However, it is impossible to obtain high-purity benzene by straight distillation, as can be seen from Figure 1. This figure shows the large number of homogeneous, binary azeotropes that exist between aliphatics and benzene. The boiling point of the pure aliphatic is shown on the left ordinate, and a line is drawn to connect each aliphatic to the composition and boiling point of its binary azeotrope with benzene. Thus, for each binary (aliphatic-benzene pair) there are three points on the vapor-liquid equilibrium curve: the boiling point of each of the two pure components and the boiling point of the azeotrope. A

228 In Refining Petroleum for Chemicals; Spillane, L. J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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Tetraethylene Glycol

What happens if benzene is recovered b y batch fractional distillation in the presence of aliphatics? First, pure light aliphatics—branched hexanes and all the C s — w o u l d be removed overhead since they do not form azeotropes with benzene. Then, aliphatics i n the 7 0 ° - 8 0 ° C boiling range would come off and would contain roughly 2 5 % benzene. Thus, it is immediately apparent that one disadvantage of this process is that a good deal of benzene is lost i n the light ends. The benzene that is distilled next is only fairly pure and comes off with the aliphatics that boil i n the 8 0 ° - 9 5 ° C range. W i t h luck, the gasoline fraction might not have too many of these components. However, a substantial amount of high quality benzene cannot be obtained by this process because ali phatics that have boiling points even as high as 100 °C are very difficult (if not impossible) to separate from benzene by fractional distillation. Furthermore, the whole benzene fraction ( aliphatics a? well as benzene ) had to be distilled to get part of the benzene at only a good purity.


3

Benzene

Ο

20

40

60

80

100

Weight Per Cent Benzene Figure 1.

Azeotropy in nonaromatic binary systems

hydrocarbon-benzene

Attempts to improve benzene purity by changing the pressure i n hopes of altering the relative volatilities w i l l not be very successful, as shown i n Figure 2. The slopes of the vapor pressure curves of benzene and aliphatics are essentially parallel. Thus, the relative volatilities are almost unaffected, and the compositions of the binary azeotropes w i l l not change much with change in pressure. In the 1940s extractive distillation was used to increase aliphaticbenzene relative volatilities, thereby increasing benzene recovery and

In Refining Petroleum for Chemicals; Spillane, L. J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

230

REFINING PETROLEUM FOR CHEMICALS


I 00,000

,

0

0

I _ L _ ]

300

1

1

1

1

1

1

1

200

1

1

100

60

Temperature °C (*e Scale) Figure 2.

Vapor pressure vs. temperature benzene-n-heptanemethylcyclohexane

purity. Phenol was a favorite solvent for this purpose. However, the whole benzene fraction still had to be distilled. Furthermore, for these distillation procedures to be practical, a separate facility was needed to treat each aromatic-containing fraction—C«, C , and C . 7

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The Udex Process In the early 1950s the Udex extraction process came into commercial existence and began to take over this separation. L i q u i d extraction can be used to separate hydrocarbons by type. Thus, benzene as well as toluene and C aromatics could be recovered from aliphatics in one process. B y extracting the aromatics selectively and then distilling them from the solvent, distillation of the aliphatics is avoided so that operating costs are lower than in the fractional and extractive distillation processes. H i g h purity aromatics can be obtained because the aromatics are not only purified i n the extraction step but in the distillation step as well. The Udex process was, therefore, a significant breakthrough, and it has been used widely ever since its introduction. 8


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Diethylene glycol ( D E G ) was the first solvent employed in the Udex process. The structural formula of D E G , as well as those of the other presently used glycols, are shown in Table I. Diethylene glycol is ethyl ene glycol with an extra ethylene oxide group. Triethylene and tetraethylene glycols merely have additional ethylene oxide groups. A description of the l i q u i d - l i q u i d equilibria in the benzene-heptane system with glycols is given now using D E G as the model glycol. Figure Table I.

Glycols Employed in the Udex Process

HO [CH2CH 0]iCH2CH OH

Diethylene glycol

DEG

HO [ C H C H 0 ] C H C H O H

Triethylene glycol

TEG

HO [ C H C H 0 ] C H C H C H

Tetraethylene glycol

TETRA

Dipropylene glycol

DPG

2

2

2

2

H HOC*-CH

2

2

2

2

3

2

2

2

3

H CH OCH C\

*

0 20 Benzene

Figure 3.

Ri i g i o

40

60 80 Diethylene

100 Glycol

Temperature-composition data for the sys tem: benzene-diethylene glycol


232



3 is a temperature-composition diagram; benzene is the left ordinate, D E G is the right ordinate. The curve is the miscibility or solubility curve. The area inside the curve represents the compositions at which two liquids exist i n equilibrium. The area outside the curve represents the compositions of the two components which form one liquid. The

Diethylene Glycol

1 0 0 % DEG

Figure 4.

Ternary temperature-composition diagram

critical solution temperature ( C S T ) is the temperature above which only one liquid phase is formed, no matter what the composition. This sort of curve is typical for a l l aromatics and actually with a l l solvents. Below the C S T the solubility of the aromatic i n the solvent tends to be greater



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Tetraethyletie Glycol

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than the solubility of the solvent in the aromatic. A t 45 °C, over-all com position A (containing about 20 wt % D E G ) , separates into upper phase Β (containing about 95 wt % benzene) and lower phase C (containing about 68 w t % D E G ) . W i t h aliphatics the curve is much higher, and the solubility of the aliphatic i n the solvent is much less than that of the aromatic. This is illustrated better i n Figure 4. This is a ternary temperature-composition diagram—a triangular prism. It is easy to read. Each corner represents 100% of one compo nent, and the side opposite 0 % . Lines parallel to the base represent the various percentages of that component. The 25, 50, and 7 5 % lines are shown for heptane. Point Y has the composition of about 8 0 % benzene, 15% heptane, and 5 % D E G . A feel for ternary diagrams can be obtained by realizing that the closer the composition is to a particular corner, the more of that component there is i n the solution or mixture. Thus, Y is near the benzene corner and far away from the heptane and D E G corners. Benzene

A

C T

i )

T

2 >

T

3

Figure 5. Liquid-liquid equilibria in the benzene-nonaromatic solvent system at various temperatures Referring back to the prism, the b e n z e n e - D E G binary is that pre sented i n the previous figure. The h e p t a n e - D E G binary has a similar curve, except that it is shifted much higher. Thus, at any temperature below the critical solution temperatures, the mutual solubilities of ben zene and D E G are much higher than those between heptane and D E G . The equilibria at any particular temperature can be determined by taking a horizontal slice, which can be either one solubility curve (slice near the top) or two separate solubility curves (slice near the bottom). Figure 5 shows some possible configurations. T is at a higher tem perature than T , and T> is at a higher temperature than T . Referring r

2

a


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Benzene

n-Heptane

9 2 wt. % D E G 8wt.%Water

B. Recovery

C. Purification

-Heptane Benzene, (HeptaneXDEG, Water

DEG, Water

Reflux:__

Feed Β Benzene, Heptane

Figure 6.

ι—•Benzene, |^ Heptane

Sr^ |

Benzene ^

Benzene, (Heptane), DEG, Water

TU

Benzene, DEG,Water

Liquid-liquid equilibria in the system: benzene-heptane92 wt % DEG-8 wt % water at 125°C volume %

to the curve at temperature T the three components are completely mis cible in the area above curve A B C . Inside the miscibility curve two liquid phases are formed, and the compositions of these phases lie at the ends of tie lines. Curve A B represents compositions of a l l possible hydrocarbon raffinate phases, and curve C B represents compositions of the solvent extract phases. A t point Β (the Plait point) both phases have the same composition and density. As the temperature is lowered below the benz e n e - D E G critical solution temperature, two separate solubility curves are formed, as is illustrated at the low temperature ( T ) . Figure 6 shows an actual extraction system—benzene, heptane, 9 2 % D E G - 8 % water at 125 °C under pressure. Water is included to reduce u

3



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the boiling point of the D E G solvent to about 140°C as well as to i n crease selectivity. If water had been excluded, one continuous solubility curve would have been obtained at 125°C. However, in this system there are two separate curves. The tie lines are shown in solid lines. A singlestage extraction is depicted on the ternary diagram. The feed contains 30% benzene. Extracting with fresh solvent at a solvent/feed ratio of roughly 8/1 produces a hydrocarbon raffinate containing only 15% ben zene in just one extraction stage. The benzene content of the raffinate can be reduced to 0 % by countercurrent, multistage extraction, depicted i n flow diagram B. Referring back to the ternary diagram, if the extract of the one-stage extraction were distilled, the hydrocarbon distillate ob tained would be almost 7 0 % benzene. This composition (point A ) is indicated at the end of the dilution line that goes through the extract composition. The dilution line represents all possible compositions of the extracted hydrocarbons and glycol. The ternary diagram also indicates that pure benzene can be ob tained in the extract b y treating it countercurrently with additional pure benzene—i.e., reflux. If the benzene content of the extract were built up to about 30%, there would be no heptane i n the extract. This multi stage, countercurrent purification is depicted i n flow diagram C . Column Β can be put on top of column C , as depicted i n Figure 7. The benzene-heptane raffinate of the purification section is merely addi tional feed for the recovery section. ^ Solvent: DEG, Water Recovery

Raffinate: Heptane

Section

Feed: Benzene, Heptane

DEG,Water-^T Impure B e n z e n e

Purification Reflux: Pure Benzene

Benzene-Heptane

Section

• Extract' -•DEG,Water Pure B e n z e n e

Figure 7. Schematic of extractor with recovery and purification sections


236


Figure 8 is the actual flow diagram of the Udex system (2, 4). The extract is transferred to the top of the distillation column. The distillate obtained overhead is condensed and decanted. The bottom layer, containing water and a small proportion of glycol, is recycled to the bottom of the distillation column. T h e top layer consists of benzene along with some light aliphatics ( C s and C s ) and is recycled to the bottom of the extractor as reflux. The aromatics (along with some water) come off as a vapor side stream. These vapors are condensed and decanted. B T X is removed, and the water phase is recycled. As mentioned earlier, water is included in solvent primarily to reduce the boiling point at the bottom of the distillation column.


5

6

Extractor Reflux-^

Raffinate JL^

Glycol (Water)

Recovery Section

Aromatic Product

Feed

Extractor Reflux Extractor

Figure 8.

Distillation

Column

Schematic flow diagram of Udex process

Both vapor distillates also contain some glycol. In the décantation steps most of the glycol goes into the water phases, and hence a separate distillation column is not needed and B T X reflux is not needed. The raffinate and B T X are washed further with small proportions of water to remove the traces of glycol. Increasing the Aromatics Capacity W i t h the growing demand for petrochemicals and high octane gasoline, more and more extraction capacity has been and w i l l be needed throughout the years. It has been the trend i n the industry to employ solvents of increasing solvency to achieve this capacity increase. The



14.

SOMEKH AND FRiEDLANDER

Figure 9.


237

Liquid-liquid equilibria in the system: benzene-heptane-92 wt % DEG-8 wt % water at 125°C volume %

industry started with diethylene glycol. The l i q u i d - l i q u i d equilibria i n the benzene-heptane system with D E G were presented above and are shown again i n Figure 9. The data are at use conditions: 125°C with the solvent containing 8 % water. A t this water concentration the solvent has a boiling point of about 140 °C. Diethylene glycol solvent is very selective i n that it does not dissolve much aliphatics. However, it does not dissolve much benzene either. The limited solubility with benzene reduces extraction capacity, so that benzene distributes i n only about 1 to 4 ratio betwen extract and raffinate. The distribution coefficients with toluene and C aromatics (which determine the effective capacity of the solvent) are substantially lower. W i t h triethylene glycol at 121.5°C (Figure 10) the results are better. In this case the solvent contains 5 % water, and its boiling point is about 140°C. This solvent is also very selective, and the extracts have low heptane solubilities. Benzene distributes i n a ratio just under 1 to 2 between extract and raffinate. Triethylene glycol has nearly twice the 8


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extraction capacity of diethylene glycol. It is also important to realize that even highly aromatic feeds can be treated. However, tetraethylene glycol is the best of the glycols studied (Figure 11). T h e solvent contains 3.9% water so that it also has a boiling point of about 140 °C. Benzene distributes quite favorably i n tetraethylene glycol. The tie lines are rather flat. The data are at 100°C only. Highly aromatic feeds can be treated also.


Benzene

95 Vol. % TEG - 5 Vol.% Water

Heptane

Figure 10.

Liquid-liquid equilibria in the system: benzene-heptane-95 vol % TEG-5 vol % water at 121.5°C volume %

The data are summarized in Figure 12, which depicts the distribution coefficient as a function of benzene concentration i n the raffinate. T h e average distribution coefficient with diethylene glycol is about 0.25 and that with triethylene glycol is nearly 6 0 % higher at about 0.4. However, toluene and C aromatics are so much more easily extracted with triethylene glycol that about half as much solvent is needed compared with D E G . Some refiners have increased capacity b y adding dipropylene glycol to their diethylene glycol solvent. A t about 2 5 % D P G , the solvent is similar to triethylene glycol. However, individual solvents are pre8


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Benzene

96.1 V o l . % T E T R A 3.9 Vol. % Water

Heptane

Figure 11.

Liquid-liquid equilibria in the system: benzene-heptane-96.1 vol % TETRAS.9 vol % water at 100°C volume %

A TETRA, 3.9Vol.7.H 0,IOO C • TEG. 5.0 Vol. %H 0J2I.5*C • PEG, 8.0W1 % H0> I25*C 2

e

2

2

0

10 20 30 40 50 60 Volume % Benzene in Raffinate

Figure 12. Distribution of benzene between extract and raffiinate for various glycols


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ferred over mixtures to avoid having to control solvent composition. The distribution coefficient with T E T R A is about 0.6 compared with the 0.4 with triethylene glycol. Here again the comparison is that the solvent/ feed ratio with T E T R A would be nearly half that with triethylene glycol. A l l of these results have been confirmed in our pilot plant (Figure 13). This pilot plant is a fully integrated, continuous system. It can be used to simulate any extraction and distillation process using one or more extractions and one or more distillations.

Figure 13.

Pilot plant

The pilot plant was operated for long periods on various feed stocks, both synthetic and real, and the results confirmed all of the laboratory bench scale findings. The superiority of tetraethylene glycol has been shown by low S/F ratios, low R / F ratios, high recoveries of all aromatics, high purity of benzene and other aromatics and low operating costs. Processes using the more recent solvents, sulfolane ( 1 ) and N-methylpyrrolidone (3), have been studied. A simplified flow diagram of the processes using these solvents is shown in Figure 14. The distillations are done in two columns, not one, and distillation reflux is usually employed. The extractor reflux is distilled in the first distillation column,


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241


Raffinate (Nonaromatics)


Reflux Distillation

BTX Distillation

BTX to Fractionation

Reformate or Dripolene (Gasoline Fraction)

t

Extract

Solvent

7 Refux (light nonaromatics-** benzene)

Figure 14.

Generalized schematic flow diagram of "conventional" process to separate light aromatics from gasoline fractions

and aromatics are distilled from the solvent i n the second distillation column. Though these solvents are beneficial i n the extractor (high capacity for aromatics), there are significant disadvantages i n the distillations (difficult to purify aromatics and difficult to separate aromatics from solvent). O n the other hand, the authors have found that the tetraethylene glycol Udex exhibits superior economics. Conclusion A significant trend has developed recently i n the industry toward using tetraethylene glycol. Plant capacities have been increased considerably. Recoveries of C aromatics are very high. Aromatic purities are even better than they were with the lower glycols. Furthermore, operating costs have been cut by up to half. The commercial performance of tetraethylene glycol has indeed been excellent. 8

Literature Cited (1) Deal, G. H. etal.,Petrol Refiner Sept. 1969, 185-192. (2) Grote, H. W., Chem. Eng. Progr. Aug. 1958, 43-48. (3) Hydrocarbon Processing Nov. 1965, 181. (4) Read, Davis, API Meeting, San Francisco, Calif., May 14, 1952. RECEIVED January 12, 1970.


Tetraethylene GlycolA Superior Solvent for Aromatics Extraction

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