Abstract:
A method for optimizing the production of nitration grade toluene from a solvent extraction process that produces an aromatic rich extract and a saturate rich raffinate, comprising adding an effective amount of the raffinate back to the extract.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the production of a nitration grade toluene product. More particularly, this invention relates to the production of a nitration grade toluene product from the process of thermal cracking hydrocarbons. 
     2. Description of the Prior Art 
     Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In an olefin production plant, a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. 
     This mixture, after preheating, is subjected to hydrocarbon thermal cracking using elevated temperatures (1,450 to 1,550 degrees Fahrenheit or F.) in a pyrolysis furnace (steam cracker or cracker). This thermal cracking is carried out without the aid of any catalyst. 
     The cracked product effluent of the pyrolysis furnace (furnace) contains hot, gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule, or C1 to C35 inclusive, both saturated and unsaturated). This product contains aliphatics (alkanes and alkenes), alicyclics (cyclanes, cyclenes, and cyclodienes), aromatics, and molecular hydrogen (hydrogen). 
     This furnace product is then subjected to further processing to produce, as products of the olefin plant, various, separate and individual product streams such as hydrogen, ethylene, propylene, and fuel oil. After the separation of these individual streams, the remaining pyrolysis gasoline contains essentially C4 hydrocarbons and heavier. This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while a C5 and heavier stream is removed as a bottoms product. 
     The C4 stream can contain varying amounts of n-butane, isobutane, 1-butene, 2-butenes (both cis and trans isomers), isobutylene, acetylenes, and diolefins such as butadiene (both cis and trans isomers). 
     The C5 and heavier stream contains primarily (in major proportion, greater than 50% by weight) C6 through C8 hydrocarbons, both aromatic and non-aromatic (alkanes and alkenes). Aromatics are separated from this stream by way of a solvent extraction process, and the aromatics in the extract are themselves separated to recover individual plant product streams such as a benzene product, a toluene product, and the like. The non-aromatics are typically passed to the gasoline pool. 
     There are several marketable grades of toluene product that can be recovered from the aromatic rich extract that is recovered from the solvent extraction process. The grades of toluene product are “commercial” which contains at least 95 weight percent (wt %) toluene, “nitration grade” which contains at least 98.5 wt % toluene, and “TDI grade” which contains at least 99.9 wt % toluene, all wt % based on the total weight of the product. 
     This invention is directed toward producing an optimum amount of nitration grade toluene product from the aromatic (primarily benzene and toluene) rich extract of a solvent extraction. 
     SUMMARY OF THE INVENTION 
     Pursuant to this invention, the quantity of nitration grade toluene product recovered from the aforesaid extract is optimized by first recovering benzene from the extract followed by blending with the remaining extract an effective amount of raffinate that was generated by the solvent extraction process. 
     This invention is particularly effective if the aforesaid blending takes place upstream of the typical benzene splitter column that is conventionally employed upstream of the distillation column that produces the nitration grade toluene product of the plant. 
     In this description, a distinction is made between a distillation column and a splitter column and that is a splitter column makes a cleaner (finer, narrower) cut of the desired product than does a distillation column. For example, with a benzene distillation column whose primary goal is to remove a benzene stream from a mixture of hydrocarbons containing benzene, more benzene, on a comparative weight basis, will be left in the bottoms (non-benzene) product of that column than would be left in the bottoms (non-benzene) product of a benzene stripper column whose primary goal is the same. Put another way, the benzene splitter column will come nearer to recovering all the benzene present in that column as a separate overhead product than would a benzene distillation column. This is accomplished in one way, as an example, by employing more distillation trays in a splitter column than in a distillation column. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified flow diagram of a typical thermal cracking plant integrated with the C6 and heavier section of its hydrocarbons processing unit. 
         FIG. 2  shows a modification of the C6 and heavier section of  FIG. 1  in accordance with this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a thermal cracking plant  1 . In plant  1  hydrogen, ethylene, propylene, fuel oil, pyrolysis gasoline and a C4 stream (not shown) have been separated from the cracked product stream of the pyrolysis furnaces in plant  1 , thereby leaving a C5 and heavier stream  2  to which this invention is directed. 
     Stream  2  is typically processed in a de-pentanizing column  3  from which is taken a C5 stream  4  which is processed (not shown) elsewhere in the hydrocarbons processing unit. 
     The remainder of stream  2 , containing primarily C6 and heavier hydrocarbons, is removed as stream  5  and passed into a hydrotreating unit  6  wherein alkenes and thiols are selectively saturated to alkanes thereby producing a stream  7  that is primarily composed of aromatics and saturated non-aromatics. 
     Stream  7  is passed to a degassing tower  8  for the removal from stream  7  of sulfur and acid gas that was formed in hydrotreaters  6 . The remainder of stream  7  forms stream  10 . Stream  10  is composed primarily of C6 and C7 hydrocarbons both aromatic and alkane with very minor amounts, from about 0.3 to about 4 wt % based on the total weight of stream  10 , of C5 and C8 hydrocarbons (aromatic and non-aromatic). 
     Stream  10  is passed to a conventional solvent extraction process which uses a solvent that preferentially removes aromatic hydrocarbons from stream  10  to form an extract  13 , and leaves the saturates (C6 and C7 non-aromatics) in a separate raffinate stream  12  together with minor amounts, from about 0.3 to about 13.0 wt % based on the total weight of stream  12 , of toluene. 
     The solvent extraction of aromatics can employ various solvents such as sulfolane, liquid SO2, N-methyl pyrrolidone, and glycols such as diethylene glycol and tetraethylene glycol. Aromatic extraction by glycols, for example, is based on the electrophilic nature of oxygen causing dipoles which exist along the glycol molecule. These dipoles give glycols their affinity for water which is highly polar and aromatics which are non-polar. It would seem that non-polar aromatics would have no attraction to polar glycol molecules, but the pi-electrons of a benzene ring are delocalized over the entire ring, and this allows these electrons freely to travel to each of the six carbons of the benzene ring. Polar solvents interact preferentially with the mobile pi-electrons in the benzene ring of aromatics. This allows the glycol molecule to assume the physical configuration of a bent horseshoe, which brings its positive sites closer to the outside diameter of a benzene ring, the glycol molecule thus essentially surrounds the aromatic molecule, and forms a solvent phase containing primarily aromatic molecules. The solvent molecules reject the non-aromatic molecules. The non-aromatics are transported to the hydrocarbon/solvent interface, and are thereby rejected to the raffinate phase. Solvent extraction is well known in the art, and further description is not needed to inform the art. This process is fully and completely disclosed in detail in U.S. Pat. Nos. 2,773,918 and 3,361,664. 
     Raffinate stream  12  is rich in non-aromatic hydrocarbons, and is normally sent to storage for subsequent processing outside the boundaries of the plant. 
     The aromatic rich extract/solvent complex is processed in unit  11  for recovery of the solvent for re-use in the solvent extraction step thereby leaving an aromatic rich extract stream  13  which contains primarily benzene and toluene and very minor amounts, from about 0.1 to about 0.2 wt % based on the total weight of stream  13 , of C6, C7, and C8 hydrocarbons (aromatic and non-aromatic). 
     Extract stream  13  is introduced into a benzene distillation column  14 , and a benzene stream  15  is recovered overhead therefrom. Stream  15  is marketed as a commercial product of the plant. This leaves a toluene rich stream  16  that is passed to a benzene splitter column  17  for the removal by way of stream  18  of yet more C6 hydrocarbons, i.e., hydrocarbons lighter boiling than toluene. Stream  18  is composed primarily of C6&#39;s (aromatic and non-aromatic) and is typically passed to the gasoline pool. The toluene rich bottoms stream  19  of splitter  17  is passed to a toluene distillation tower  22  for recovery by way of stream  20  of the toluene product of the plant. 
     In this invention stream  20  is the desired nitration grade toluene product of the overall plant. 
     Any remaining C8 hydrocarbons such as xylenes are removed by way of stream  21  and are normally passed to the gasoline pool. 
       FIG. 2  shows the process of  FIG. 1  modified in a manner pursuant to this invention. 
     In  FIG. 2  an effective amount of raffinate  12  is passed by way of stream  30  to be blended with extract stream  16 . The process of this invention is effectively preferably carried out by adding stream  30  to the extract stream upstream of benzene splitter  17  so that C6 hydrocarbons in stream  30  that are lighter boiling than toluene can be removed before the toluene distillation step  22  is performed. 
     Adding raffinate that has just been separated from the extract back to that extract is counter intuitive in the art, and goes against the teachings of the art, but is surprisingly effective in the operation of this invention. 
     Raffinate  12  can contain from about 50 to about 55 wt % hexanes, from about 35 to about 40 wt % heptanes, and from about 7 to about 8 wt % toluene, the remainder, if any, being essentially C8 aromatics (xylenes) and benzene. 
     Pursuant to this invention, an effective amount of raffinate can be employed along with extract stream  16  to produce an amount of nitration grade toluene product  20  that is optimum for the particular operating characteristics of the plant in which this invention is practiced. 
     The effective amount of raffinate added to extract stream  16  can vary widely depending on the particular operating characteristics of the plant in which this invention is employed, but can, for example, vary from about 2 to about 4 wt % of stream  16 , based on the total weight of that stream  16 . 
     EXAMPLE 
     In the process of  FIG. 2 , steam  16  can contain about 90 wt % toluene, the remainder being C6 and C8 hydrocarbons, both aromatic and non-aromatic. 
     Stream  30 , as added to stream  16 , can contain about 53% hexanes, about 38 wt % heptanes, about 7 wt % toluene, less than about 2 wt % xylenes, and less than about 1 wt % benzene, all wt % based on the total weight of the stream. 
     Stream  30  is mixed with stream  16  in the amount of about 3 wt % based on the total weight of stream  30 . 
     The combined stream formed from streams  16  and  30  aforesaid is passed into benzene splitter column  17  which is operated in a manner to produce a C6 stream  18  containing essentially only C6 aromatics and non-aromatics, and a separate stream  19  containing about 95 wt % toluene, the remainder being essentially only C7 and C8 hydrocarbons, both aromatic and non-aromatic. 
     The operation of column  17  also produces a nitration grade toluene plant product  20  which contains at least about 98.5 wt % toluene, the remainder being C7 and C8 hydrocarbons, both aromatic and non-aromatic.