Abstract:
A method and system for collecting a plurality of slurries containing solid particles of varying particle size distribution and amount and performing a classification and separation on the slurries is provided whereby a fines-rich stream and a stream containing substantially coarse particles is obtained. The method and system includes separating the fines from the slurry liquid and thickening the separated coarse particles in a settling zone for subsequent use.

Description:
[0001]    Non-Provisional Application based on Provisional Application No. 61/201,067 filed Dec. 5, 2008. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and system for handling slurries that are generated in varying rates and volumes and contain varying solids content. In particular, the present invention relates to a method and system for classifying suspended matter and the suspending liquid in such slurries. 
       BACKGROUND OF THE INVENTION 
       [0003]    There are numerous industrial processes in which fluid streams are generated containing suspended solids which ultimately require separation and classification of the solids from the suspending liquid. An example of such an industrial process is the Fischer-Tropsch three-phase slurry process for the synthesis of hydrocarbons. 
         [0004]    Typically, a Fischer-Tropsch three-phase slurry, hydrocarbon synthesis process is conducted in a bubble column reactor by contacting a stream of synthesis gas (comprising H 2  and CO) with a liquid suspension of solid catalyst. The synthesis gas will have an H 2 :CO molar ratio of from about 1:1 to about 3:1. The dispersing liquid is primarily linear hydrocarbon reaction product. To facilitate contact between catalyst and the synthesis gas, the gas is fed into the bottom of the bubble column through a gas distributor that produces small gas bubbles. As the synthesis gas bubbles rise through the column, they not only disperse the catalyst in the liquid, but they also react to form hydrocarbon products that are mainly liquids under the reaction temperature and pressure conditions. Any gaseous products that are formed rise to the top of the reactor from which they are removed. 
         [0005]    Because it is necessary to maintain the slurry in the reactor at a constant level, liquid products are continuously or periodically removed from the reactor. In doing so, however, it is important to separate dispersed catalyst particles from the liquid being removed to maintain a constant inventory of catalyst in the reactor. Generally, the separation is conducted in a filtration zone located in the slurry bed. The filtration zone typically comprises cylindrical filtering media through which liquid passes from the exterior to the interior of the filtering media where it is collected and removed from the reactor. In some reactor designs, liquid product is filtered in an external filtration system, and separated catalyst is returned to the reactor. 
         [0006]    Over time, the hydrodynamic conditions existing in the bubble column result in some attrition of the catalyst, thereby forming catalyst particles known as “fines”. Fines are those particles having a size less than 10 microns. In contrast, coarse catalyst particles are those particles having a size greater than or equal to 10 microns. The size of the openings in the filtering media will, of course, determine the amount of fines likely to pass through the filter with the liquid product. Thus, the liquid product removed from the bubble column may be subjected to a second stage separation process to provide a substantially fine-free liquid product and a second stage solids-containing stream requiring further liquids-solids separation. The volume and solids content of the second stage stream will vary depending upon the extent of catalyst attrition. Also, over time, fines not passing through the filter will begin to plug the filter. Consequently, filter efficiency is decreased, and remedial action such as backwashing with a liquid stream becomes necessary. Backwashing the filters typically is conducted periodically in a pulsing mode with a liquid. This results in an undesirable accumulation of fines in the slurry. Consequently, it is necessary to control the amount of fines that accumulates in the slurry. Thus, a slip stream containing fines may be removed when necessary, and this fines-laden slip stream may be subjected to separation and classification. 
         [0007]    As is known in the art, the catalyst used in a three-phase slurry reactor is regenerated either continuously or periodically. Typically, the catalyst is regenerated when its activity decreases to the point where reactant conversion cannot be maintained. In the regeneration process, the first step generally requires the removal of wax from the catalyst. In one method, this is accomplished by washing the catalyst with a solvent. The solvent typically is separated from the catalyst by decanting, thereby providing a liquid stream containing solids, primarily fines. These solids also require classification and separation from the liquid. 
         [0008]    Obviously, the rate of flow of catalyst-containing stream used to control the slurry fines concentration will be different from that generated in the regeneration step as will their solids content and particle size distribution. So too will it be different from the solids-containing stream generated in the above mentioned second stage separation process. 
         [0009]    Other sources of solids-containing liquid streams requiring solids classification and separation are those obtained from occasional flushing of the bubble column reactor cone, unit purges, process equipment flushes, etc. These, of course, will be generated at different times, at varying rates, and will have different solids content. 
         [0010]    An object of this present invention is to provide a method and system for classifying and separating solids suspended in liquid streams that are generated at varying rates and which contain differing amounts of solids. 
         [0011]    Another object of the invention is to provide an integrated system for particle size classification and for solids thickening, which system is capable of functioning under differing load conditions. 
         [0012]    These and other objects of the invention will become apparent from the description of the invention below. 
       SUMMARY OF THE INVENTION 
       [0013]    Broadly stated, the invention features a method and system for collecting a plurality of slurries from one or more sources that contain solid particles of varying particle size distribution and amount. A classification separation is performed on the collected slurries to provide a fines-rich stream and a stream containing substantially coarse solid particles. 
         [0014]    In particular embodiments, one or more of the following features may be included. The slurry collected may be held in the collection system for a selected period of time and the solids maintained in suspension by mechanical agitation. The coarse fraction may be collected and thickened in a settling zone before subsequent treatment or use. The fines-rich stream may be sent to a solids concentration unit for separation of the fines from the slurry liquid. 
         [0015]    The method and system of the present invention is particularly useful in collecting various slurry streams, other than the primary product stream, from a three-phase slurry process. The collected streams are then subjected to a classification separation. 
         [0016]    Specific embodiments and other features of the method and system of the invention will be described in the detailed description of the invention below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic diagram depicting one embodiment of the present invention. 
           [0018]      FIG. 2  is a schematic diagram illustrating another embodiment of the invention. These embodiments are described in detail below. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present invention is applicable to industrial processes which generate slurry streams that require the separation and classification of the solid matter in those streams. Indeed, the invention is particularly applicable to three-phase slurry processes. A specific example of such a process is the Fischer-Tropsch synthesis process. For convenience, the invention will be described by specific references to the Fischer-Tropsch hydrocarbon synthesis process. 
         [0020]    Suitably, the reactor for the Fischer-Tropsch synthesis process is a bubble column reactor comprising a vertical vessel for containing a catalyst suspended in a liquid phase through which synthesis gas is bubbled. 
         [0021]    Also suitably, the reactor will include one or more gas disengaging vertical downcomers which assist in the circulation of slurry in the reactor. 
         [0022]    As indicated previously, the synthesis gas comprises H 2  and CO in the molar ratio of 1:1 to 3:1. 
         [0023]    The slurry liquid comprises those linear hydrocarbon reaction products that are liquids at reaction conditions. 
         [0024]    Suitable Fischer-Tropsch hydrocarbon synthesis catalysts include effective amounts of Co and one or more of Re, Ru, Fe, Ni, Th, Zr, Hf, Mg and La on a suitable inorganic support. 
         [0025]    The Fischer-Tropsch reaction is carried out at temperatures, pressures and hourly gas space velocities in the range of about 320° F. to 850° F., 80 to 600 psi and 100 to 40,000 V/hr/V, expressed as standard volumes of the syngas mixture (60° F., 1 atm) per hour per volume of catalyst, respectively. 
         [0026]    In the operation of the Fischer-Tropsch process liquid products will be removed from the reactor through internal filters to provide a primary product stream. Optionally, the slurry liquids may be sent to a filter system located outside the reactor for the separation of liquid products from catalyst solids to provide the primary product stream. 
         [0027]    Also during the operation of the Fischer-Tropsch process, a slurry slip stream will be processed to generate a fines-laden secondary stream, either in a continuous or batch mode, to control the slurry fines content. This secondary liquid stream that will require separation and classification of the solids. 
         [0028]    Other sources of solids-containing secondary streams that may be generated include the liquid collected from flushing the reactor cone, the decant liquid obtained from washing catalyst before regeneration, reactor purges, and equipment processing flushes. 
         [0029]    In the present invention any one or all of these secondary streams may be treated in the system of the invention. Thus in one aspect of the invention, a Fischer-Tropsch hydrocarbon synthesis plant is provided which includes the solids classification and separation system described herein. 
         [0030]    A first embodiment of the system of this invention and its integration into a Fischer-Tropsch hydrocarbon synthesis process will now be described by reference to  FIG. 1 . As shown, various sources  1 ,  2 ,  3  and  4  of slurry liquids from a Fischer-Tropsch process are provided with conduits  5 ,  6 ,  7  and  8  for transfer of their respective slurries for collection in upper section  11  of collection drum  10 . The upper section  11  of collection drum  10  includes mechanical mixing means  12  for keeping the solids suspended in the slurries collected in drum  10 . Heating means also is provided in drum  10  to prevent the cooling and solidification of wax product in the slurry. One such heating means is a coiled conduit  14  through which steam from a steam source  15  is circulated. This heating means also includes means  16  for receipt of condensate. Other heating means may include hot oil, electric heating jackets and the like. 
         [0031]    The slurry collected in drum  10  is classified and separated when appropriate by withdrawing slurry from drum  10  and transferring it to a hydrocyclone  17 . 
         [0032]    Hydrocyclones are devices in which a liquid feed is subjected to centrifugal forces by a rotating movement of the feed caused by a tangentially directed inlet. During the rotating movement, liquid with coarse catalyst particles concentrate at the bottom of the hydrocyclone while liquid with catalyst fines concentrate at the head of the hydrocyclone. 
         [0033]    As shown in  FIG. 1 , slurry liquid is withdrawn via conduit  18  from the side of drum  10  and pumped by pumped by pump  20  via conduit  19  into hydrocyclone  17 . Although a centrifugal pump  20  is shown schematically in the Figure, it is preferred that the pump used be a positive displacement pump and especially a progressive cavity pump to minimize attrition of the catalyst. 
         [0034]    As shown in  FIG. 1 , hydrocyclone  17  has a top conduit  21  for removal of an overflow stream and a bottom conduit  22  for removal of an underflow stream. The overflow stream may be transferred to a solids concentration unit  23 , such as a cross-flow type separation device, in which the fines are collected and removed via line  24  for disposal or reclaiming and liquid product is removed via line  25  for further processing. 
         [0035]    The underflow stream removed via line  22  from hydrocyclone  17  is transferred to a settling zone shown as the bottom section  26  of drum  10 . Bottom section  26  is encompassed by a cylindrical part  27  and a bottom cone-shaped part  28 . Also, bottom section  26  is separated from the top section  11  of drum  10  by a plurality of stilling baffles  29 . 
         [0036]    The bottom section  26  operates much like a thickener with coarse catalyst solids settling in the cone  28 . In a traditional thickener, clarified liquid is separated from solids using an overflow weir or draw-off nozzle. In this embodiment of the present invention, liquid is fed into section  26  and then rises into the upper section  11  of drum  10 . Preferably, the liquid used is a portion of the overflow stream of hydrocyclone  17 . 
         [0037]    The coarse catalyst collected in section  26  of drum  10  is removed via line  30  and may be sent, for example, to a catalyst regeneration unit dewaxing drum (not shown) for dewaxing and then regeneration. 
         [0038]    In the first embodiment just described, only one hydrocyclone is shown; however, optionally a bank of hydrocyclones may be used. 
         [0039]    Also, in the first embodiment just described, only one collection drum is used; however, optionally a second collection drum may be employed and operably connected to sources  1 ,  2 ,  3  and  4  and having the elements described in connection with the  FIG. 1  single drum embodiment. 
         [0040]    Whether one or two drums are used in the practice of the invention, it is preferred that they have a sufficient capacity to allow for variability in slurry flow rate based on the design capacity and output of the Fischer-Tropsch process with which they are integrated. For example, each drum preferably is sized to have a one day hold up capacity. In this way, the classification and separation system of the invention may be operated independently of the operation of the hydrocarbon synthesis process. It also allows for short maintenance periods on downstream equipment without impacting the ability to receive slurry feeds from the synthesis process. 
         [0041]    Another and preferred embodiment of the invention is depicted schematically in  FIG. 2 . In this embodiment, instead of a coarse catalyst settling zone being integral with the collector drum  10 , a separate external settling zone, i.e., vessel  126 , is used as a coarse catalyst collection section. As shown in  FIG. 2 , vessel  126  has a cylindrical part  127  and a bottom cone-shaped part  128 . Optionally, vessel  126  may also be provided with a mixing device  129 . In operation, slurry liquid is withdrawn from collection drum  10  via conduit  18  and is sent to hydrocyclone  17 . The underflow stream  22  from hydrocyclone  17  is sent via conduit  22  to the coarse catalyst collection vessel  126 . Liquid separated from the solids in vessel  126  is sent to drum  10  via line  132 , while collected solids may be removed via line  130 . In this embodiment, vessel  126  preferably is operated at elevated pressure, thereby eliminating the need to provide pumps to transfer the separated liquid to drum  10 . As with the  FIG. 1  embodiment, optionally a second collection drum may be provided, if desired, and operated substantially similarly to the single drum embodiment.