US2862983A - Sulfuric acid refining of benzene - Google Patents

Description

Dec. 2, 1958 S. NORD SULFURIC ACID REFINING OF BENZENE Filed May 17, 1957 United States Patent O SULFURIC ACID REFINING F BENZENE Sol Nord, East Orange, N. `J.

Application May 17, 1957, Serial No. 659,829 3 Claims. (ci. 26o-674) This invention relates to the refining of benzene, i. e. to the production of high purity benzene from mixtures of benzene with toluene, olens, sulfur compounds, and other impurities. More specifically, it relates to a process of producing high purity, high freeze-point, low bromine number, low sulfur-content benzene from coke oven light oil, coke oven light oil distillates, petroleum fractions, or other mixtures containing bezene, olens, and sulfur compounds such as carbon disulde and/or tiophene.

Prior art In the rening of benzene-containing mixtures such as coke oven light oil or coke oven light oil distillates, it is conventional practice to treat the mixture with concentrated sulfuric acid before distillation. The sulfuric acid reacts with the unsaturated compounds present in the mixture, forming a sludge which settles out with the spent acid. The result is a decrease of unsaturation in the nal benzene product, as indicated by its acid wash test and bromine number. It also makes it possible to obtain freeze-points in the nal benzene product which approach the freezing point of pure benzene (5.4 C.) more or less closely. It is also known that unsaturated sulfur compounds such as thiophene are partially removed by this treatment, and that the sulfur content of the final benzene product is accordingly also decreased.

In certain cases, it is difficult to obtain sufficient puriiication by this treatment, e. g. it may be difhcult to meet specifications of acid wash test, bromine number, freezepoint, or sulfur content. In general, all that can be done Aabout this is to increase the amount of sulfuric acid used in the acid washing treatment. This, however,

does not always solve theproblem, and is expensive in Furthermore, no unusual or complicated equipment is required. Conventional acid washing and distillation equipment are used.

Objects An object of my invention is to provide a process for the production of benzene of improved purity from benzene-containing mixtures.

A second object of my invention is to provide a process for the production of benzene of improved purity from benzene-containing mixtures, without the use of novel equipment or chemicals, and with very little increase in sulfuric acid consumption or decrease in final yield of product.

A third object of my invention is to provide a process for improving the acid wash test, sulfur content, bromine number, and freeze-point of benzene over that obtained by conventional processes, without the use of novel equip- 2,862,983 Patented Dec. V2, 1958 ICC v Advantages An advantage of my invention is that no novel or expensive equipment is required.

A second advantage of my invention is that no novel or expensive chemicals are required.

A third advantage of my invention is that the increase in acid consumption is very small.

A fourth advantage of my invention is that there is substantially no decrease in final yield of product.

In the accompanying figure, I have shown a schematic process flow-sheet of a preferred embodiment of my invention. For the purposes of illustration, I have used coke oven light oil as the material to be treated, but I may use other benzene-containing matereals instead.

Description The starting point of my process is a conventional acid washer 10, equipped with suitable means for agitation 11, which may for example consist of a mechanically driven agitator or a recirculating pump. Concentrated sulfuric acid, preferably 66 B. HZSO.,= is added at 12 to a mixture of coke oven light oil 13 and recirculated B-T distillate, the source and composition of which is described below. Spent acid and acid sludge are withdrawn at 14, and the acid-washed light oil is withdrawn at 15. This acid-washing step can be performed batchwise or continuously, and as it is well known in the art, is not described further here.

The acid-Washed light oil is then pumped by pump 16 to a neutralizerr 17, which contains suitable means for agitation 18, of a type similar to that used in the acid washer 10. Dilute NaOH solution is added at 19, in order to neutralize acid entrained in the oil. Spent alkali is withdrawn at 20, and neutralized oil at- 21. VThis neutralization step can also be performed batchwise or continuously, and as it is well known in the art, is not described further here.

The acid-washed, neutralized oil is pumped by pump 22 to an intermediate section of a forerunner fractionating tower 23, of a conventional type. Benzene forerunner vapors are taken overhead at 24 and condensed in condenser 25. A portion of the condensate is returned as reflux at 26, and the remainder is withdrawn as benzene forcrunners at 27. Bottoms are withdrawn at 28, dropping into a conventional reboiler 29. Vapors 30 are returned to the bottom of the -tower 23, and forerunnerfree bottoms are withdrawn at 31. In the operation of the forerunner column 23, a sufficient fraction is taken overhead, with suicient fractionation, so that the bottoms are substantially free of benzene forerunners, i. e. of material boiling below benzene. Since this operation is conventional, it is not described further here.

The forerunner-free bottoms from fractionating tower 23 are pumped by pump 32 to an intermediate section of a benzene fractionating tower 33 of a conventional type. Pure benzene product vapors are taken overhead at 34 and condensed in a condenser 35. A portion of the condensate -is returned as reflux 36, and the remainder is withdrawn as pure benzene product at 37. Bottoms are withdrawn at 38, dropping into a conventional reboiler 39. Vapors 40 are returned to the bottom of tower 33, and benzene bottoms are withdrawn at 41. The operation of this tower 33 is conducted in a novel manner, and constitutes a critical part of my invention. Instead of taking all -the benzene voverhead as product, a predeterfeed tothe benzene tower 3-,and the amount of those impurities boiling in .the benzene range which normally cause high wash tests, sulfur content, and bromine numbers, and low freeze-points in the benzene. This optimum concentration should thereforebe determined by experi- .ment in any given case.

The benzene bottomsfrom the benzene tower 33 are `now pumped by pump 42 to an intermediate section of a B-T fractionating tower '43 of a conventional type.

B-T (i. e. a mixture consisting of benzene, benzene impurities, and toluene) vapors are taken overhead at 44 and condensed in a condenserAS. A portion of the condensate is returned as refluxA, and the remainder is withdrawn as B-T distillate at 47. Benzene-free bottoms are withdrawn at 48, dropping into a conventional reboiler49. Vapors 50 are returned to the bottom of the tower 43, and benzene-free bottoms are withdrawn at 51, The operation of this tower is such that no substantial amount of benzene remains in the bottoms, and that substantially all the benzene and benzene impurities are taken overhead, withas little toluene as possible. Any toluene going overhead beyond the amount necessary to insure that all the benzene and impurities are taken overhead, .is unnecessarily recycled. Thus, the toluene concentration of the overhead will depend largely on .the fractionating eflciency of the tower 43. The benzene-free bottoms are lthen available for working up into pure toluene and heavier fractions by conventional means. The B-T distillate, on the other hand, is pumped by pump 52 back to the washer 10 for re-washing, along with a fresh batchof coke oven light oil (or other charge material).

I have found thatif thefbenzene contained in the B.*T fraction so produced isdirectly recovered by fractional distillation in the conventional way, without re-washing,

tent, and bromine number, and a low-freezepoint. For example, this benzenemay have a bromine number over 500, and often over 1500. It may have a sulfur contient of 0.1%, and often as high as 0.3%. The acid wash test is -generally ,worse than number 6, and the freeze point is generally down to 2-3 C. These results are contrary to what one would expect, since there is no reason to suppose, `nor any literature or plant experience to indicate that the benzene impurities which cause these results can be :fractionated out of benzene. On the contrary they generally require doubling ortripling the amount of sulfuric acid used in the washing, vsimply because they cannot be removed in any other known way.' Alternatively, very expensive types of benzene purification plants have been proposed, including solvent extraction and other expensive operations. However, I have found that these impurities concentrate in the intermediate B--T fraction, produced by; the method described above. In a sense, they seem to be entrained with the toluene and heavier bottoms from tower33, as if the toluene and heavier were fsolvents in anextractive distillation process. Whatever the reason may be, we have found that4 the benzeneimpurities can be more or less isolated in the -Bf-T distillate fraction so produced, and thus kept out of both thepbenzene and the toluene fractions. I have also found that rc-washing this B--T distillate fraction, along with the regular charge material, eliminatesthese impurities with virtually a neglligible increasein acid-consumption or decrease in yie d.

Thus, it will be seen that I have provided an extremely .such benzene has unusually high wash test, sulfur consimple and inexpensive method of improving the purity of benzene, i. e. 4wash test, sulfur content, bromine number, and freeze-point, without the use of any novel equipment or chemicals.

Other modifications in the details of operation may also be made without departing from the spirit of my nvention, the principal novel features of which are set forth below.

I claim:

l. In a process for producing benzene of high purity, in which a benzene-containing charge mixture is contacted with concentrated sulfuric acid to react with impurities, treated with alkali to neutralize the remaining acid, and then subjected to fractional distillation to produce pure benzene and other products, the novel steps comprising: producing by fractional distillation an overhead fraction comprising primarily benzene and being substantially free of `heavier components, and a residual comprising substantially all the components heavier than benzene together with a predetermined quantity of benzene which is sufficient to contain the major portion of the remaining impurities boiling in the benzene range; producing by further fractional distillation of said residual an intermediate overhead fraction containing substantially all the remaining material .boiling in the .benzene range ytogether with some higher-boiling material, and a residualsubstantially comprising components heavier than benzene; and recycling said intermediate overhead fraction for further contacting with sulfuric acid and re-fractionation.

2. In a process for producing b enzeneof high purity, in `which a benzene-containing charge mixture is contacted with concentrated sulfuric acid to react with impurities, treated with alkali `to neutralize the remaining acid, and then subjected to continuous fractional distillation in a multiplicity of fractionating towers to produce pure benzene and other products, the novel steps comprising: making a cut between acidfalkali-treated Abenzene and heavier components in which the `overhead fraction is substantially free of heavier Acomponents,.and thebottoms fraction contains a predetermined concentration of benzene which is suicient to contain the major portion of the remaining impurities vboiling in the benzene range; subjecting said bottoms to further fractional distillation to producean overhead containing substantially all the remaining material boiling in the benzene range together with somehigher-boiling material, and a bottom fraction which is substantially free of benzene; and recycling said overhead containing benzene and higher boiling material for further contacting with sulfuric acid, and re-fractionav tion.

3. In a process for producing benzene of high purity, in which a benzene-containing charge mixture is contacted with concentrated sulfuric acid to react with impurities, 4treated with alkali to neutralize the remaining acid, and then subjected to continuous fractional distillation in a multiplicity of fr actionating towers to produce pure benzene and other products, the novel steps comprising: subjectingcharge material which has been contacted with acid and alkali to continuous fractionaldistillation to produce an overhead fraction comprising substantially all the benzene forerunners, and a first bottoms fraction which is ,substantially free ofmaterial boiling below benzene; subjecting said first bottoms fraction to a second continuous fractional distillation to produce an overhead fraction comprising substantially pure benzene product, and a second bottoms fraction comprising heaviercomponents together with a predetermined quantity of .benzene which is sufficient to contain the major portion of the remaining impurities boiling in the benzene range; subjecting said second bottoms fraction to a thirdcontinuous fractional distillation to produce a recycle overhead fraction containing substantially all the remaining material boiling `in the benzene range `together with some higher-boiling material, and a substantially benzene-free bottoms frac- 5 tion; returning said recycle overhead fraction for further acid contacting, along with fresh charge material, and subsequent refractionation; land subjecting said benzenefree bottoms fraction to further fractional distillation t0 produce additional products therefrom.

References Cited in the ile of this patent UNITED STATES PATENTS

Claims (1)

1. IN A PROCESS FOR PRODUCING BENZENE OF HIGH PURITY, IN WHICH A BENZENE-CONTAINING CHARGE MIXTURE IS CONTACTED WITH CONCENTRATED SULFURIC ACID TO REACT WITH IMPURITIES, TREATED WITH ALKALI TO NEUTRALIZE THE REMAINING ACID, AND THEN SUBJECTED TO FRACTIONAL DISTILLATION TO PRODUCE PURE BENZENE AND OTHER PRODUCTS, THE NOVEL STEPS COMPRISING: PRODUCING BY FRACTIONAL DISTILLATION AN OVERHEAD FRACTION COMPRISING PRIMARILY BENZENE AND BEING SUBSTANTIALLY FREE OF HEAVIER COMPONENTS, AND A RESIDUAL COMPRISING SUBSTANTIALLY ALL THE COMPONENTS HEAVIER THAN BENZENE TOGETHER WITH A PREDETERMINED QUANTITY OF BENZENE WHICH IS SUFFICIENT TO CONTAIN THE MAJOR PORTION OF THE REMAINING IMPURITIES BOILING IN THE BENZENE RANGE; PRODUCING BY FURTHER FRACTIONAL DISTILLATION OF SAID RESIDUAL AN INTERMEDIATE OVERHEAD FRACTION CONTAINING SUBSTANTIALLY ALL THE REMAINING MATERIAL BOILING IN THE BENZENE RANGE TOGETHER WITH SOME HIGHER-BOILING MATERIAL, AND A RESIDUAL SUBSTANTIALLY COMPRISING COMPONENTS HEAVIER THAN BENZENE; AND RECYCLING SAID INTERMEDIATE OVERHEAD FRACTION FOR FURTHER CONTACTING WITH SULFURIC ACID AND RE-FRACTIONATION.

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