Abstract:
A process is presented for the purification of 1,3 butadiene. The process is for treating a butadiene stream from an oxidative dehydrogenation unit, where a butane stream is dehydrogenated, generating a butadiene rich stream. The butadiene rich stream is fractionated and passed through a butadiene recovery unit. Additional C4 compounds recovered from the fractionation bottoms stream are further processed for increasing yields of butadiene.

Description:
FIELD OF THE INVENTION 
       [0001]    The field of the invention relates to the dehydrogenation of hydrocarbons. Specifically, the invention relates to the dehydrogenation of light hydrocarbons using an oxidative dehydrogenation process. 
       BACKGROUND OF THE INVENTION 
       [0002]    Butadiene is an important monomer for the production of high molecular weight polymers. It is used extensively in the production of tires and other products in the automotive industry. It is also used extensively in the production of synthetic rubbers, such as styrene-butadiene rubber, nitrile-butadiene rubber, and styrene-butadiene latex. Polymer grade butadiene requires a high purity, and needs to be essentially free of active chemicals such as acetylenes, carbonyls, and contaminants such as sulfur and other heavy components. 
         [0003]    Butadiene is usually a by-product recovered from the steam cracking of naphtha during the production of ethylene and propylene. Naphtha cracking, either through steam or catalytic cracking, produces a range of products, and is not optimized for the production of chemicals such as butadiene. Another route for the production of butadiene is through catalytic oxidative dehydrogenation of n-butene, which yields a higher concentration of butadiene, but also yields some undesirable products that must be removed. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is for the production of a purified stream of 1,3 butadiene, where the butadiene is generated from an oxidative dehydrogenation process. The oxidative dehydrogenation of a butene stream generates a crude butadiene stream. While the process generates relatively high levels of oxygenates, including aldehydes and furan, the process does not generate isobutylene or isobutane, thereby simplifying the separation and purification process. 
         [0005]    The process includes passing the crude butadiene stream to a butadiene fractionation unit generating an overhead stream comprising 1,3-butadiene and 1-butene, and a bottoms stream comprising oxygenates, other C4 compounds and heavier hydrocarbons. The overhead stream is passed to a butadiene recovery unit for generating a purified butadiene stream and a second stream comprising 1-butene. 
         [0006]    The bottoms stream generated from the butadiene fractionation unit can be further processed to increase butadiene yields. The first step in processing the bottoms stream is to separate C4 hydrocarbons and recycle the C4 hydrocarbons back to the dehydrogenation unit. The C4 hydrocarbon stream can also be separated and processed to pass butenes to a dimerization unit for generating octenes. The octenes can be further processed to generate p-xylene. 
         [0007]    In another embodiment, the bottoms stream includes vinyl acetylene. The vinyl acetylene can be passed to a selective hydrogenation unit to convert vinyl acetylene to n-butenes. The resulting process stream from the selective hydrogenation unit is then passed to the butadiene fraction unit to recover the additional butene-1 and butene-2 produced for further processing. 
         [0008]    Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following detailed description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]    The FIGURE is a process for the production of polymer grade 1,3-butadiene. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    The production of butadiene can be performed with oxidative dehydrogenation of n-butene. The oxidative dehydrogenation process provides an attractive route for the production of butadiene as there are lower concentrations of undesirable compounds that need to be removed, such as isobutylene or isobutane. This reduces separation costs due to fewer similar boiling point compounds. 
         [0011]    Oxidative dehydrogenation is a process of dehydrogenation of n-butenes through contacting the paraffin with a catalyst. Oxidative dehydrogenation is an exothermic process that avoids some of the thermodynamic constraints of non-oxidative dehydrogenation processes by forming water as a by-product. In addition, carbon deposition is minimized, or eliminated, for a more stable catalytic process. 
         [0012]    Oxidative dehydrogenation utilizes a catalyst such as vanadium oxide, molybdenum oxide, chromium oxide, or a vanadium-magnesium oxide catalyst, or even a combination of the metal oxides on a support. Supports can include aluminas, zirconia, titania, magnesia, and other refractory materials. 
         [0013]    Oxidative dehydrogenation of butenes generates a crude butadiene stream which has a relatively high concentration of 1,3 butadiene, but also includes higher levels of oxygenates, such as aldehydes, over a typical process stream from a steam cracker crude C4 stream. In addition to the high levels of aldehydes, a major contaminant is furan, and can be as high as 2500 ppm by weight. The furan will not be removed with a sodium bisulfate wash, but will require additional processing by solvent extraction or fractionation. The oxygenates need to be removed to avoid negative impacts on downstream processing units. The use of oxidative dehydrogenation for the production of butadiene generates a crude product stream that lacks isobutylene, isobutane or C3 and lighter hydrocarbons in the starting feed. This allows for fractionation of the feed through a single fractionation unit and without the use of a selective hydrogenation process, such as UOP&#39;s KLP™ process. Thus providing a savings in energy and equipment. 
         [0014]    The present invention comprises a process, as shown in the FIGURE, for purifying a crude butadiene stream. The process includes passing a crude butadiene stream  10  to a butene-2 column  20 . The butene-2 column generates an overhead stream  22  comprising 1,3 butadiene, and a bottoms stream  24  comprising oxygenates and heavier components. The overhead stream  22  is passed to a butadiene extraction unit  30  to generate a purified 1,3-butadiene product stream  32 . The process can, optionally, include a wash to remove residual oxygenates in the overhead stream  22 , where the overhead stream  22  is passed to a wash  26 , to generate a washed butadiene stream  28 . The washed stream  28  is then passed to the butadiene extraction unit  30 . 
         [0015]    The butene-2 column  20  is a super-fractionator, and is designed in a single shell. A super-fractionator is a system where there is separation of a multicomponent mixture, and in this case, the overhead condenser serves as a super-fractionation stage, and the overhead stream is partially condensed with the product stream remaining in the vapor phase. Super-fractionators are used to separate streams with close relative volatilities between key components, and have high internal vapor and liquid flow rates. This generates a purified overhead stream with the bottoms stream comprising the bulk of the remaining components. 
         [0016]    The butene-2 column utilizes UOP&#39;s MD tray applications for maximum distillation efficiency. The MD trays are multiple downcomer trays for providing high liquid and vapor throughput in a distillation column. The MD trays can include slotted sieve trays, trays having long weir length and large downcomer areas to provide for high liquid handling capabilities. 
         [0017]    The butadiene extraction unit  30  comprises an extractive distillation column  34  and a solvent recovery, also known as a solvent rectifier, column  36 . The extraction unit  30  for producing a purified butadiene product comprises passing the butadiene stream  22  to the extractive distillation column  34  to generate an overhead stream  42  comprising 1-butene, and a bottoms stream  44  comprising butadiene and an extractive solvent. The bottoms stream  44  is passed to the solvent recovery distillation column  36  to generate an overhead stream  32  comprising the purified 1,3-butadiene, and a bottoms stream  46  comprising the extraction solvent. The extraction solvent is recycled to the extractive distillation column  34  as an inlet stream  48 . Extractive solvents used in the extractive distillation of butadiene can include N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl acetamide, and acetonitrile. A common extractive solvent is NMP. 
         [0018]    The process is further designed to increase the butadiene yields by conversion of by-products in the crude butadiene stream. The butene-2 column  20  generates a bottoms stream  24  which comprises other C4 hydrocarbon compounds. The bottoms stream  24  can be passed to a second column (not shown) to generate an overhead stream comprising n-butane and 2-butene, and a bottoms stream comprising vinyl acetylene and oxygenates. One aspect is the issue of separating acetylenes. When there are acetylene rich streams, there is some internal recycle of product, possibly including 2-butenes, into the acetylene rich streams to maintain the acetylene concentration below the explosive limit. The butene-2 column  20  will be designed and operated to ensure a dilution of the acetylene stream to a safe concentration limit, as is known to one skilled in the art of 1,3 butadiene processing. 
         [0019]    In an alternative, a side-draw stream  52  is withdrawn from the butene-2 column to generate a side-draw stream  52  comprising n-butane and 2-butene. The bottoms stream  24  comprising vinyl acetylene is passed to a selective hydrogenation process unit  50  to generate a process stream  54  comprising butenes. The process stream  54  is subsequently processed based upon the content of the process stream and the content of the feeds stream  24 . A first portion of the process stream  62  is passed to a separation column  60  to generate an overhead stream  64  comprising butenes, and a bottoms stream  66  comprising oxygenates and heavier hydrocarbons. The overhead stream  64  can be passed to the butene-2 column  20  for recovery of the butene-1 and butene-2 for further processing. An advantage for passing the overhead stream  64  to the butene-2 column is that it gives the plant operator control over where to maximize net yield by how the selective hydrogenation zone is designed. In addition to the hydrogenation of VA to 1-butene and 2-butene, isomerization of 1-butene to 2-butene can be performed, thus allowing for control of either 1-butene product, or 1,3-butadiene product through further processing of the 2-butene. This allows for shifting of the product streams based upon economics, or production needs. In an alternative, the overhead stream  64  can be passed directly to a dehydrogenation unit for conversion of all 1-butene and 2-butene to 1,3-butadiene. 
         [0020]    In an alternative, the vinyl acetylene dilution can be accomplished through allowing a portion of the heavies in the separation column  60  to leave in the overhead stream  64 . The heavies, comprising C5 and C6 hydrocarbons, then recycle to a higher concentration in the butene-2 column, and therefore in the bottoms stream  24 . In this manner, the VA is diluted with C5/C6 and reduces the amount of C4s dragged into the bottoms stream  24  for VA dilution. This allows for increased C4 recovery for passing to dehydrogenation. 
         [0021]    A second portion of the process stream  56  can be recycled to the selective hydrogenation unit as a diluent to help control the exotherm in the selective hydrogenation reactor. A third portion of the process stream can be used as a purge to control the build up of oxygenates and heavier components. The purge stream  58  can be passed to downstream process units when the separation column  60  is not present for the removal of heavies. 
         [0022]    In an alternative to separating the bottoms stream  24 , C4s can be recovered through a side draw. The side draw stream  52  comprising butenes can be passed to the oxidative dehydrogenation reactor to convert the recovered butenes to 1,3-butadiene. The side draw includes separating out C4 and returning heavies to the butene-2 column. Processing the side draw stream  52  can also include passing the side-draw stream through a wash unit  70  to generate a butene rich stream  72  with reduced oxygenate content. The wash unit  70  removes aldehydes and other oxygenates generated in the oxidative dehydrogenation process. The washed stream  72  can be passed to a dimerization unit  80  to generate a process stream  82  comprising C8 compounds, including octenes, and a raffinate stream  84  comprising C4 compounds. 
         [0023]    The process can further include passing the C8 compounds stream  82  to a reformer. The reformer can process the octenes to generate xylenes, and in particular p-xylenes. P-xylenes are useful precursors for a variety of compounds, including polymers. The raffinate stream  84  from the dimerization unit  80  can be passed back to the dehydrogenation unit. The butadienes generated by the dehydrogenation unit can be passed to the butene-2 column for recovery of the additional 1,3-butadiene. 
         [0024]    As an alternate, the side-draw stream  52 , comprising C4 compounds, can be either washed and passed to the dehydrogenation unit, or directly passed to the dehydrogenation unit, and subsequently passed to the butene-2 column to increase the 1,3-butadiene yield. 
         [0025]    The crude process stream  10  comprising butadiene is full of reactive chemical components. The control and prevention of reactions can include adding an anti-oxidizing agent, or an anti-polymerizing agent to the crude process stream  10 . 
       EXAMPLE 
       [0026]    A crude butadiene stream was generated by an oxidative dehydrogenation process. The crude stream composition is shown in Table 1 as mass %, and the feed rate was approximately 15,000 kg/hr. The butene-2 column generated an overhead stream comprising 1,3 butadiene and 1-n-butene. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Butene-2 column separation of crude butadiene process stream 
               
             
          
           
               
                 compound 
                 feed 
                 overhead 
                 bottoms 
                 Sidedraw 
                 bottoms 
               
               
                   
               
             
          
           
               
                 n1C4= 
                 3.8 
                 4.8 
                   
                   
                   
               
               
                 1,3 BD 
                 75.3 
                 95.2 
               
               
                 NC4 
                 2.7 
                   
                 12.8 
                 13.5 
               
               
                 t-2C4= 
                 11.1 
                   
                 53.3 
                 55.9 
               
               
                 c-2C4= 
                 6.1 
                   
                 29.2 
                 30.6 
               
               
                 VA 
                 .4 
                   
                 2 
                   
                 42.8 
               
               
                 Oxygenates 
                 0.6 
                   
                 2.7 
                   
                 57.2 
               
               
                 and 
               
               
                 heavies 
               
               
                   
               
             
          
         
       
     
         [0027]    The example shows the single stage separation of butadiene and 1-butene from the other components. The table also shows two alternatives: the composition of the bottoms stream taken as a single bottoms stream, or the alternative sidedraw stream with the remaining bottoms stream comprising oxygenates and heavies. This example also shows an idealized separation for illustrative purposes, such as no C4&#39;s in the bottoms stream. The actual bottoms stream will include sufficient other hydrocarbons, including C4&#39;s, for the safe dilution of the vinyl acetylene (VA). 
         [0028]    The relatively small amount of vinyl acetylene (VA) indicates an advantage for using a small selective hydrogenation process (SHP) unit for the conversion of VA to n-butenes, thus reducing the size of the stream requiring selective hydrogenation, as well as reducing the amount of SHP catalysts. The SHP catalysts are also less sensitive to oxygenates and sulfur over the catalysts used in the usual commercial KLP process. The VA converted to n-butenese can be passed to the butene-2 column for further processing. 
         [0029]    The 1-butene recovered from the butadiene extraction unit is also polymer grade, high value co-product. The sidedraw process stream can be directly recycled to the oxidative dehydrogenation unit, or further processed to make other net products. 
         [0030]    Both the butene-2 column overhead stream and the side-draw stream can be washed with a sodium bisulfate wash to remove any residual carbonyl compounds that are in the overhead stream or the side-draw stream. The wash is added when the residual oxygenates are problematic for downstream process units, or for product quality. 
         [0031]    The butene-2 column bottoms contains mostly 2-butent and n-butane. This can be recycled directly to the dehydrogenation unit. The bottoms stream can also be separated and processed with smaller units. The oxygenates in the bottoms stream comprise a relatively large amount of furan. The high furan control can also be beneficial in that furan competes with the olefins for adsorbed sites on the catalyst. While not being held to any particular theory, it is believed, this can reduce butene hydrogenation and hydrogen consumption to maximize 1-butene selectivity. 
         [0032]    While the invention has been described with what are presently considered the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.