Patent Publication Number: US-2004040893-A1

Title: Stripping process and apparatus

Description:
BACKGROUND  
       [0001] This invention relates to the field of hydrocarbon refining. More particularly, this invention relates to the purification of hydrocarbon streams.  
       [0002] It is well known in the art to strip hydrocarbon feedstocks with a gas. However, there are ever-present incentives for the development of new and/or more effective processes for stripping hydrocarbons since heavy components in hydrocarbon feedstocks can clog equipment.  
       [0003] In many refining processes, the hydrocarbon feedstocks are mixed with hydrogen and heated to a high temperature using heat exchangers and furnaces. Heavy components contained in some hydrocarbon feedstocks can cause coke formation in areas where a dry point occurs. The coke can plug the tubes in heat exchangers, furnaces, or other equipment. Therefore, it is necessary for heavy components to be removed from these hydrocarbon feedstocks to avoid problems with clogged equipment.  
       [0004] In addition, incomplete vaporization can cause problems in refining processes that do not tolerate liquids.  
       [0005] Therefore, a novel stripping process and apparatus that effectively strips hydrocarbon feedstocks and also provides for a substantially vaporized feedstock for refining processes would be a significant contribution to the art and economy.  
       SUMMARY OF THE INVENTION  
       [0006] It is an object of the present invention to provide an improved process for removing heavy components from a hydrocarbon feedstock which is economical and efficient.  
       [0007] A further object of the present invention is to provide an improved apparatus to be used in removing heavy components from a hydrocarbon feedstock which is economical in construction and reliable and efficient in operation.  
       [0008] In accordance with the first embodiment of the present invention, a process is provided for removing heavy components from a hydrocarbon feedstock. The process comprises, consists of, or consists essentially of:  
       [0009] (a) heating a hydrocarbon feedstock wherein at least a portion of the hydrocarbon feedstock is vaporized so as to form a first vapor stream and a first liquid stream;  
       [0010] (b) separating the first vapor stream and the first liquid stream;  
       [0011] (c) stripping the first liquid stream with a counter current flow of a gas so as to form a second vapor stream and a second liquid stream; and  
       [0012] (d) recovering the second vapor stream and the second liquid stream from the stripper column.  
       [0013] In accordance with the second embodiment of the present invention a system or apparatus is provided for removing heavy components from a hydrocarbon feedstock comprising, consisting of, or consisting essentially of:  
       [0014] (a) a vaporization zone; and  
       [0015] (b) a vessel defining a liquid vapor separation zone and a stripping zone, the vessel being in fluid flow communication with the vaporization zone.  
       [0016] The third embodiment of the present invention is an apparatus comprising, consisting of, or consisting essentially of;  
       [0017] (a) a vaporization zone;  
       [0018] (b) a liquid-vapor separation vessel in fluid flow communication with the vaporization zone; and  
       [0019] (c) a stripping vessel in fluid flow communication with the liquid-vapor separation vessel.  
       [0020] The fourth embodiment of the present invention is an apparatus comprising, consisting of, or consisting essentially of:  
       [0021] (a) A first vessel defining a vaporization zone and a liquid-vapor separation zone;  
       [0022] (b) a second vessel defining a stripping zone wherein the second vessel is in fluid flow communication with the first vessel.  
       [0023] Other objects and advantages of the invention will be apparent from the detailed description of the invention and the appended claims.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0024] The drawing is a partially cut away elevation of an apparatus representing one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0025] The process of this invention involves the removal of heavy components from a hydrocarbon feedstock.  
     [0026] The hydrocarbon feedstock can be any hydrocarbon feedstock containing heavy components. This can include hydrocarbon streams in refineries such as naphtha, straight run naphtha, coker naphtha, catalytic gasoline, visbreaker naphtha, alkylate, isomerate, reformate, and the like and combinations thereof. This can also include gasoline such as, but not limited to, coker gasoline, thermally cracked gasoline, visbreaker gasoline, fluid catalytically cracked gasoline, heavy oil cracked gasoline, and the like and combinations thereof. Diesel fuels can also be used. Suitable diesel fuels include, but are not limited to, light cycle oil, kerosene, jet fuel, straight-run diesel, hydrotreated diesel, and the like and combinations thereof.  
     [0027] The number of carbon atoms per molecule of heavy components in a particular hydrocarbon feedstock depends upon the boiling range of that feedstock. For example, heavy components will generally be heavier in diesel fuel than in gasoline.  
     [0028] Generally, a heavy component can be any compound having at least 12 carbon atoms per molecule. In gasoline, the heavy components contain in the range of from 12 to 35 carbon atoms per molecule. More often, the heavy components in gasoline contain in the range of from 12 to 25 carbon atoms per molecule.  
     [0029] In the present invention a hydrocarbon feedstock is charged to a vaporization zone where it is heated. Optionally, the hydrocarbon feedstock can be mixed with a diluent gas prior to the heating. Generally, the ratio of diluent gas to the hydrocarbon feedstock is in the range of from about 0.1:1 to about 8:1. For gasoline in particular, the ratio of diluent gas to the hydrocarbon feedstock is preferably in the range of from 0.1:1 to 2:1. The diluent gas is selected from the group consisting of hydrogen, nitrogen, carbon dioxide, methane, ethane, fuel gas, and combinations of any two or more thereof. Preferably, the diluent gas is hydrogen. The hydrocarbon feedstock is preferably heated such that from about 75% to about 99% of the feedstock is vaporized. More preferably, 85% to 90% of the hydrocarbon feedstock is vaporized as a result of such heating. The temperature to which the hydrocarbon feedstock is heated can depend upon a variety of factors. Such factors can include the type of hydrocarbon feedstock, the pressure, and the amount of diluent gas used. Generally, the hydrocarbon feedstock can be heated to a temperature in the range of from about 100° F. to about 800° F. Generally, gasoline can be heated to a temperature in the range of from about 400° F. to about 600° F. More preferably, the gasoline is heated to a temperature in the range of from about 440° F. to about 550° F. Most preferably, the gasoline is heated to a temperature in the range of 460° F. to 510° F.  
     [0030] The hydrocarbon feedstock is vaporized so as to form a first vapor stream and a first liquid stream. These streams pass into a liquid vapor separation zone, which is in fluid flow communication with the vaporization zone. Within the liquid-vapor separation zone, the first vapor stream and the first liquid stream are separated. The first vapor stream leaves the liquid-vapor separation zone overhead, while the first liquid stream flows from the liquid-vapor separation zone into the stripping zone which is in fluid flow communication with the liquid-vapor separation zone, in order to be stripped. In the second embodiment of the invention, the liquid-vapor separation zone and stripping zone are in the same vessel, which is in fluid flow communication with the vaporization zone. In the third embodiment of the invention, the vaporization zone, liquid-vapor separation zone, and stripping zone are all in separate vessels that are in fluid flow communication with each other. In the fourth embodiment, the vaporization zone and liquid-vapor separation zone are in the same vessel. The stripping zone is located in a separate vessel. Both vessels are in fluid flow communication with each other. The stripping zone comprises, consists of, or consists essentially of, in the range of from 2 to 4 theoretical trays.  
     [0031] The first liquid stream is stripped in the stripping zone with a counter current flow of a gas so as to form a second vapor stream and a second liquid stream. The stripping gas is selected from the group consisting of hydrogen, nitrogen, carbon dioxide, methane, ethane, fuel gas, and combinations of any two or more thereof. Preferably, the gas is hydrogen. Preferably, the ratio of the stripping gas to the first liquid stream is in the range of from about 0.05:1 to about 6:1. More preferably, the ratio of the stripping gas to the first liquid stream is in the range of from 0.1:1 to 2.4:1. The type of molecules contained in the second vapor stream depends on the type of hydrocarbon feedstock. The molecules can be heavier in diesel fuel than in gasoline. In diesel fuel for example, the second vapor stream can contain molecules with up to about 25 carbon atoms per molecule. For gasoline, the second vapor stream can comprise compounds having in the range of from 2 to 15 carbon atoms per molecule. The type of molecules contained in the second liquid stream depends on the type of hydrocarbon feedstock. Generally, the second liquid stream can comprise compounds having at least 5 carbon atoms per molecule. Preferably, the second liquid stream comprises compounds having at least 12 carbon atoms per molecule. For gasoline, the second liquid stream preferably comprises compounds having in the range of from 12 to 35 carbon atoms per molecule. More preferably for gasoline, the second liquid stream comprises compounds having in the range of from 12 to 25 carbon atoms per molecule.  
     [0032] In another embodiment at least a portion of the second liquid stream can optionally be reboiled to form a boil-up vapor stream and a bottoms residue stream. The bottoms residue stream passes out of the reboiling vessel, while the boil-up vapor stream returns to the stripper column.  
     [0033] Referring to the drawing, therein is illustrated the second embodiment of the present invention referred to as apparatus  10 , a hydrocarbon feedstock enters vaporizer  100 , defining a vaporization zone, via conduit  102 , which is in fluid flow communication with vaporizer  100 . Optionally, hydrogen can be mixed with the hydrocarbon feedstock via conduit  104 , which is in fluid flow communication with conduit  102 . About 85-90% of the hydrocarbon feedstock is vaporized in vaporizer  100  and passes to vessel  110  via conduit  108 , wherein conduit  108  connects vaporizer  100  and vessel  110  in fluid flow communication. Vessel  110  includes liquid-vapor separation zone  112 , where the vapor is separated from the liquid, thereby forming a first vapor stream and a first liquid stream. The first vapor stream passes out of vessel  110  via conduit  114 , which is in fluid flow communication with vessel  110 . The first liquid stream is stripped in stripping zone  116  also located in vessel  110 , which contains in the range of from 2 to 4 theoretical trays. Hydrogen enters stripping zone  116  via conduit  106 , which is in fluid flow communication with conduit  104  and vessel  110 . The stripping forms a second vapor stream and a second liquid stream. The second vapor stream also passes through liquid-vapor separation zone  112  and leaves vessel  110  via conduit  114 . The second liquid stream leaves vessel  110  via conduit  118  which is in fluid flow communication with vessel  110 . Optionally, the second liquid stream can travel to reboiler  120  via conduit  118 , which is also in fluid flow communication with reboiler  120 . The second liquid stream is reboiled to form a bottoms residue stream, which leaves the reboiler via conduit  122  which is in fluid flow communication with reboiler  120 , and a boil up vapor stream which returns to vessel  110  via conduit  124  which is in fluid flow communication with reboiler  120  and vessel  110 .  
     [0034] The following example is provided to further illustrate this invention and is not to be considered as unduly limiting the scope of this invention.  
     CALCULATED EXAMPLE  
     [0035] This example illustrates the stripping of hydrocarbons using the inventive process.  
     [0036] A computer model was used to simulate the stripping of naphtha. The naphtha composition used as an input to the computer model was obtained from analyses of products from ASTM D-86 distillations of naphtha. The mass flow of the naphtha stream was set at 487,256 pounds per hour. The heavy components for this simulation were mainly modeled as C 24 H 50 . The heavy components in the naphtha stream of this simulation had a mass flow of 2424 pounds per hour. In this simulation, about 87.7% of the naphtha feed was vaporized before stripping. The mass flow of the stream containing hydrogen was set at 6190 pounds per hour. Seventy percent of the hydrogen went to the vaporizer and thirty percent of the hydrogen went to the stripper.  
     [0037] The mass flows of the heavy components are shown in the table for the feedstock before being contacted with hydrogen, before it is vaporized, for the first vapor stream and the first liquid stream, and for the second vapor stream and the second liquid stream.  
                                                       Mass Flow (lb/hr) of Heavy           Stream   Components                                                    Before being contacted with   2424           hydrogen           Before vaporization   2424           1 st  vapor stream   76.4           1 st  liquid stream   2348           2 nd  vapor stream   38.7           2 nd  liquid stream   2309                      
 
     [0038] The second vapor stream has a significantly lower mass flow of heavy components than the first liquid stream has before it is stripped. Therefore, the inventive process is quite useful for removing heavy components.