Abstract:
This invention is a process modification to enable maximization of benzene production from intermediate refinery streams and virgin condensate or other streams having benzene precursors. The modified process or a unique sequence of refinery units also concentrates benzene and xylene precursors if so desired. This concentrated precursor stream is processed by a specific low severity, high yield naphtha reformer. The modified process results in maximizing benzene or xylene volume and reducing overall operating costs especially for refineries producing aromatics and high quality gasoline simultaneously and producing additional benzene from toluene.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a new concept to maximize benzene production using refinery and aromatics extraction processes in a unique operating sequence and mode tailored to maximize the amount of benzene produced and minimize the operating and capital costs to produce benzene.  
         BACKGROUND OF THE INVENTION  
         [0002]    For the most part, refinery aromatics are produced from a naphtha reformer unit. Depending upon the naphtha feed composition, its inherent Naphthenes plus Aromatics percent; the naphtha reformer produces aromatics including benzene, toluene and xylenes in various concentrations. An aromatics extraction unit recovers aromatics either from the whole reformate stream or a fractionated portion of reformate. Both benzene and a portion of the xylenes, para-xylene, can be used directly in the production of petrochemical derivatives. Toluene on the other hand has a limited market in terms of petrochemical derivatives and TNT precursors. Consequently, toluene and xylenes are processed in a hydro-dealkylation unit at a considerable cost and production of low value fuel gas from an expensive feedstock to produce additional benzene. Similarily, toluene can be processed in a toluene disproportioning unit to benzene and lower value products. A direct route to benzene from benzene precursors is more cost effective in these times of competitive commodity business and minimization of costs.  
           [0003]    The benzene content of gasoline has been regulated to a low value in gasoline in developed nations including Canada and the USA. Consequently, refiners have chosen four methods to reduce benzene in the gasoline product as follows:  
           [0004]    Remove benzene precursors before the naphtha reformer to preclude benzene production  
           [0005]    Hydrotreat or saturate the benzene fraction of the reformate  
           [0006]    Develop new catalysts that selectively do not react with the benzene precursors.  
           [0007]    Remove benzene by solvent or extractive distillation.  
           [0008]    Each of these processes except the last one reduce the net amount of benzene available for benzene derivative production thereby increasingly moving the industry to expensive methods to produce benzene. Consequently, what is needed is a lower cost method to produce benzene directly rather than expensive further processing of other aromatics such as toluene and xylenes.  
         SUMMARY OF THE INVENTION  
         [0009]    The genesis of this process idea is to fractionate benzene precursors from the naphtha feed to the naphtha reformer and also collect benzene precursors from condensate or natural gasoline utilizing extractive distillation and feed the resulting concentrated benzene precursor stream to a specified low severity high yield naphtha reformer. At the same time, recover the hydrogen co-product to hydrotreat the condensate and benzene precursor concentrated stream. 
       
    
    
     DESCRIPTION OF THE INVENTION  
       [0010]    The cost of this process is substantially lower than that of a reformer at high severity to yield BTX aromatics and meet the gasoline octane specification. High severity reforming results in a significant volume loss across the naphtha reformer and may crack some benzene precursors to lower value products. Furthermore, the toluene and xylenes produced in the reformer need to be processed in a high cost hydro-dealkyation unit to produce benzene or feed toluene to a toluene disproportioning unit to produce benzene.  
         [0011]    The production of aromatics from benzene precursors will reduce the size of the aromatic extraction process, significantly reducing capital and operating costs. Precursors will be an important source of benzene without the high cost of further processing of toluene and xylenes.  
         [0012]    Alternative sources of benzene such as Pygas and Coker by-products involve high operating costs. These sources although significant in volume are not sufficient to satisfy the world benzene market.  
         [0013]    Another high cost process is the UOP/BP Cyclar process that uses propane and butane to make benzene and aromatics. There is only one world scale unit at a BP refinery in the UK. This process has the limitation of not producing a significant portion of the aromatics as benzene.  
         [0014]    Another option envisaged by this process idea is to not only concentrate die benzene precursors but also the xylene precursors prior to the naphtha reformer. Additional xylene precursors would originate from condensate or other refinery streams high in benzene and xylene precursors. In this mode of operation, the naphtha reformer will produce xylenes for the para xylenes market.  
         [0015]    The operation of the specified naphtha reformer on benzene as well as xylene precursors would be a less severe and hence of a lower cost than the normal refinery reformate complex.  
       DETAILS OF THE INVENTION  
       [0016]    The normal refinery configuration is shown on the figure labelled “Present Refinery Configuration”. In this configuration, crude is processed by a crude distillation unit labelled crude unit. Crude can be a combination of crudes and intermediate refinery feedstocks resembling crude oil. The overheads for the crude unit, whole range naphtha, is processed in a naphtha splitter producing an overhead stream normally called Light Straight Run or LSR and a bottoms stream of heavy naphtha. This material, after hydrotreating to remove Sulphur, along with other intermediate refinery sweet naphtha streams is fed to a naphtha reformer. The purpose of the naphtha reformer is to produce re-formulate naphtha components in to those that constitute gasoline product. For the most part, these are aromatics including benzene, toluene and xylenes as well as other aromatics that allow this product; reformate, to have an octane quality sufficient to meet gasoline octane specifications. In addition, the proper boiling point and all the gasoline specifications are met by blending LSR and reformate and other gasoline components such as butane, alkylate, methyl tertiary butyl ether, ethanol and catalytic cracker gasoline.  
         [0017]    If an aromatics market is available, then reformate is processed in an aromatics extraction process of which there are two basic types, solvent and extractive distillation. The aromatics consisting of benzene, toluene and xylenes are further fractionated usually into benzene and a toluene/xylene mix. Additional benzene can be produced from toluene and xylenes by hydro de-alkylation and/or toluene disproportioning. Alternatively, the xylenes can be further processed to obtain para and ortho xylene for other petrochemical derivative production. Producing benzene from these two processes introduces additional capital and loss of expensive feedstock to lower value fuel gas and hydrogen if there is a need.  
         [0018]    The reformer normally produces benzene and control of benzene in the gasoline can be accommodated by the extraction of benzene in the aromatics extraction unit as detailed above. There are 3 other ways to control the benzene production:  
         [0019]    remove the benzene precursors before the naphtha reformer  
         [0020]    hydrotreat or saturate the benzene produced in the reformer  
         [0021]    use new catalysts that selectively do not react the benzene precursors if included in the feed to the reformer.  
         [0022]    These efforts by the refiner will also limit the amount of benzene available for further petrochemical derivative production.  
         [0023]    Other sources of benzene such as pygas from ethylene crackers and refinery Coker by-products will not be able to meet the world demand for benzene. Consequently, a new process scheme is required to satisfy the demand for benzene without the use of expensive processes involved in converting toluene and xylenes into benzene.  
         [0024]    A process to maximize benzene production is shown in the figure “Modified Refinery Configuration”. The process does not have to be in a refinery setting but may be a “stand alone” benzene producer and/or a benzene precursor concentrator feeding an existing refinery; the splitter, hydrotreater aromatics extraction/fractionation, and specific reformer would constitute an “off site” plant.  
         [0025]    The refinery scheme would be modified to split naphtha into a C6+ bottoms product and C5 top product. The process involves recovering C6 components from the hydrotreater as a separate stream and routing them to the aromatics extraction process. The remainder of the hydrotreated naphtha is routed to the reformer to produce gasoline. Contrary to the present refinery configuration, the light gasoline components are routed to the hydrotreater and the C6s are routed to the aromatics extraction process to recover aromatics comprised of predominately benzene with some small amounts of toluene and xylenes. A further stream of benzene rich condensate or natural gasoline is routed to the naphtha splitter. This material is typically composed of C4-C7s having the majority of components in the C5-C6 ranges. In addition, the stream contains a high proportion of benzene precursors notably methyl cyclo pentane and cyclohexanes. Following hydrotreating, the C6 portion is routed to the aromatics extraction along with other C6 streams to recover benzene  
         [0026]    A separate low sulphur condensate containing no sulphur components and usually a stream from a refinery that is rich in benzene can be processed in the modified refinery. It is usually light reformate as produced in a normal refinery configuration but originating from a refinery or process that has no benzene removal or saturation process. This material can be directed to the aromatics extraction unit to be treated in a smaller fashion to other benzene rich streams.  
         [0027]    The aromatics extraction unit is basically an extractive distillation of benzene that does not recover the benzene precursors with the benzene steam. Small amounts of toluene and xylenes contained in the feed would be recovered as gasoline components.  
         [0028]    Second but a more important function of the aromatics extraction process is to recover a significant portion of the remaining benzene precursors in the raffinate steam. This material can be fractionated from the remaining components by distillation. This material will be routed to a second smaller reformer whose only function is to produce benzene from a stream high in benzene precursors. The benzene product is routed to the same aromatics extraction process to recover additional benzene.  
         [0029]    Precursors not refined to aromatics are recycled to this reformer by the C6 distillation of the raffinate stream in the aromatics extraction process. A sulphur guard bed may be required to remove traces of sulphur prior to reforming or the stream may be recycled to the hydrotreater. Monitoring of the reformer would be undertaken to note whether the amount of C6&#39;s are increasing significantly. If so, a slipstream of this material would be routed to the gasoline pool.  
         [0030]    The operation of the second reformer would be optimized to produce benzene. Production of toluene and xylenes would be minimized. Severity and feedstock variables would be monitored to meet the criteria of maximum benzene production. Older reformer technology may be utilized in this reformer, as the objective is benzene production. It is also assumed that the specific market location may not need other aromatics such as toluene and xylenes.  
         [0031]    Alternatively, the C5s and the C6s leaving a boiling range including iso hexanes and lighter could be routed directly to the gasoline pool. The remaining C6s and C7 to C11s could be routed directly to the aromatics extraction unit rather than a reformer unit to recover all the aromatics from the naphtha fraction.  
         [0032]    All the aromatics are recovered and routed to the benzene fractionation unit to remove benzene and the remaining aromatics are routed to the gasoline pool. The raffinate stream from the aromatics unit, having most of the benzene precursors removed, is also blended into the gasoline pool. Any remaining raffinate not blended to gasoline, is recovered and sold as a feed for steam crackers.  
         [0033]    However a case is possible that recovers the xylene components. A separate splitter can be built on the condensate stream that could be designed to handle a stream of C6&#39;s as overheads and C8&#39;s as the bottom product. This combined material would be routed after hydrotreating to the aromatics extraction process to recover benzene and xylenes. The resulting raffinate stream would be routed to the gasoline pool to meet the octane specification of gasoline. The mid stream C7&#39;s would be routed to the primary reformer. In this way, the benzene precursors are concentrated as in the modified refinery configuration and the aromatics extaction unit can recover the naturally occurring xylenes.  
         [0034]    Alternatively, the xylene precursors could be recovered after the aromatics extraction unit to be processed in the primary reformer to produce additional xylenes.  
         [0035]    The modified process configuration does not include toluene and xylenes feedstock processes to produce additional benzene. These processes are costly and produce products of lower value.