Patent Description:
Ethylene and propylene are important chemicals for use in the production of other useful materials, such as polyethylene and polypropylene. Polyethylene and polypropylene are two of the most common plastics found in use today and have a wide variety of uses. Uses for ethylene and propylene include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohol.

The great bulk of the ethylene consumed in the production of the plastics and petrochemicals such as polyethylene is produced by the thermal cracking of hydrocarbons. Steam is usually mixed with the feed stream to the cracking furnace to reduce the hydrocarbon partial pressure and enhance olefin yield and to reduce the formation and deposition of carbonaceous material in the cracking reactors. The process is therefore often referred to a steam cracking or pyrolysis.

Steam cracking generates less valuable by-products such as pyrolysis gas (pygas) and fuel oil. Pygas contains large proportions of paraffins and aromatics. Paraffins can be recovered or further processed to prepare useful steam cracking feed. Aromatics are a very poor steam cracking feed because they typically increase the yield of low-value fuel oil.

An efficient process for managing aromatics in a pygas feed is needed for improving the value of steam cracking units. <CIT> relates to a process for producing olefins using aromatic saturation. <CIT> relates to a system and method for producing un-hydrogenated and hydrogenated C9+ compounds. <CIT> relates to a process and apparatus for hydroprocessing and cracking hydrocarbons. <CIT> relates to an improved olefin plant recovery system employing a combination of catalytic distillation and fixed bed catalytic steps. <CIT> relates to hydrodesulfurization of gasoline fractions. <CIT> relates to apparatuses and methods for hydrotreating coker kerosene.

The invention is in accordance with the appended claims. The disclosed process provides for saturation of a pyrolysis stream while managing the resulting exotherms. The process splits the pyrolysis stream into at least two feed streams for at least two saturation reactors. The process splits the hydrogen stream into at least two feed streams for the at least two saturation reactors. A recycle stream is also provided to manage the exotherm. The feed may comprise at least <NUM> wt% and preferably at least <NUM> wt% aromatics.

Additional details and embodiments of the disclosure will become apparent from the following detailed description of the disclosure.

The term "communication" means that fluid flow is operatively permitted between enumerated components, which may be characterized as "fluid communication". The term "communication" may also mean that data or signals are transmitted between enumerated components which may be characterized as "informational communication".

The term "downstream communication" means that at least a portion of fluid flowing to the subject in downstream communication may operatively flow from the object with which it fluidly communicates.

The term "upstream communication" means that at least a portion of the fluid flowing from the subject in upstream communication may operatively flow to the object with which it fluidly communicates.

The term "direct communication" means that fluid flow from the upstream component enters the downstream component without passing through any other intervening vessel.

The term "indirect communication" means that fluid flow from the upstream component enters the downstream component after passing through an intervening vessel.

The term "bypass" means that the object is out of downstream communication with a bypassing subject at least to the extent of bypassing.

The term "column" means a distillation column or columns for separating one or more components of different volatilities. Unless otherwise indicated, each column includes a condenser on an overhead of the column to condense and reflux a portion of an overhead stream back to the top of the column and a reboiler at a bottom of the column to vaporize and send a portion of a bottoms stream back to the bottom of the column. Feeds to the columns may be preheated. The top pressure is the pressure of the overhead vapor at the vapor outlet of the column. The bottom temperature is the liquid bottom outlet temperature. Overhead lines and bottoms lines refer to the net lines from the column downstream of any reflux or reboil to the column. Stripper columns may omit a reboiler at a bottom of the column and instead provide heating requirements and separation impetus from a fluidized inert media such as steam. Stripping columns typically feed a top tray and take main product from the bottom.

As used herein, the term "a component-rich stream" means that the rich stream coming out of a vessel has a greater concentration of the component than the feed to the vessel.

As used herein, the term "a component-lean stream" means that the lean stream coming out of a vessel has a smaller concentration of the component than the feed to the vessel.

As used herein, the term "boiling point temperature" means atmospheric equivalent boiling point (AEBP) as calculated from the observed boiling temperature and the distillation pressure, as calculated using the equations furnished in ASTM D1160 appendix A7 entitled "Practice for Converting Observed Vapor Temperatures to Atmospheric Equivalent Temperatures".

As used herein, the term "True Boiling Point" (TBP) means a test method for determining the boiling point of a material which corresponds to ASTM D-<NUM> for the production of a liquefied gas, distillate fractions, and residuum of standardized quality on which analytical data can be obtained, and the determination of yields of the above fractions by both mass and volume from which a graph of temperature versus mass % distilled is produced using fifteen theoretical plates in a column with a <NUM>:<NUM> reflux ratio.

As used herein, the term "T5" or "T95" means the temperature at which <NUM> mass percent or <NUM> mass percent, as the case may be, respectively, of the sample boils using ASTM D-<NUM> or TBP.

As used herein, the term "initial boiling point" (IBP) means the temperature at which the sample begins to boil using ASTM D-<NUM>, ASTM D-<NUM> or TBP, as the case may be.

As used herein, the term "end point" (EP) means the temperature at which the sample has all boiled off using ASTM D-<NUM>, ASTM D-<NUM> or TBP, as the case may be.

As used herein, the term "separator" means a vessel which has an inlet and at least an overhead vapor outlet and a bottoms liquid outlet and may also have an aqueous stream outlet from a boot. A flash drum is a type of separator which may be in downstream communication with a separator that may be operated at higher pressure.

As used herein, the term "predominant" or "predominate" means greater than <NUM>%, suitably greater than <NUM>% and preferably greater than <NUM>%.

The term "Cx" is to be understood to refer to molecules having the number of carbon atoms represented by the subscript "x". Similarly, the term "Cx-" refers to molecules that contain less than or equal to x and preferably x and less carbon atoms. The term "Cx+" refers to molecules with more than or equal to x and preferably x and more carbon atoms.

The present disclosure endeavors to convert aromatics rich streams to make them suitable for steam cracking feed. For example, aromatics in pygas can be saturated into naphthenes that may be recycled to the steam cracking unit and then cracked into useful olefins. Steam cracking disadvantageously converts aromatics to fuel oil, but naphthenes can be steam cracked to valuable light olefins. Saturation of aromatics can generate large amounts of heat which must be managed. The process may also be suitable for other pure aromatics or aromatics-rich streams in addition to pygas or pygases other than steam cracked pygas. The disclosed process increases the concentration of naphthenes and decreases the concentration of aromatics while the concentration of normal and iso-paraffins remain essentially unchanged.

Turning to <FIG> of the present process, a pyrolysis stream in pyrolysis line <NUM> from a steam cracking unit <NUM> predominantly comprises C5+ hydrocarbons. The pyrolysis stream in line <NUM> may be subjected to selective hydrogenation before it is ready for saturation. The pyrolysis stream in line <NUM> may be subjected to selective hydrogenation to convert diolefins and conjugated-diolefins in the pyrolysis line <NUM> to monoolefins. A recycle stream in line <NUM> and hydrogen from a hydrogen line <NUM> is added to the pyrolysis stream in line <NUM> and a resulting combined pyrolysis stream in line <NUM> is charged to a selective hydrogenation reactor <NUM>. The selective hydrogenation reactor <NUM> is normally operated at relatively mild hydrogenation conditions. The reactants will normally be maintained under the minimum pressure sufficient to maintain the reactants as liquid phase hydrocarbons. A broad range of suitable operating pressures therefore extends from <NUM> kPa(g) (<NUM> psig) to <NUM> kPa(g) (<NUM> psig), or <NUM> kPa(g) (<NUM> psig) to <NUM> kPa(g) (<NUM> psig). A relatively moderate temperature between <NUM> (<NUM>°F) and <NUM> (<NUM>°F), or <NUM> (<NUM>°F) and <NUM> (<NUM>°F) is typically employed. The liquid hourly space velocity of the reactants through the selective hydrogenation catalyst should be <NUM> hr-<NUM> and <NUM> hr-<NUM>. To avoid the undesired saturation of a significant amount monoolefinic hydrocarbons, the mole ratio of hydrogen to diolefinic hydrocarbons in the material entering the bed of selective hydrogenation catalyst is maintained between <NUM>:<NUM> and <NUM>:<NUM>.

Any suitable catalyst which is capable of selectively hydrogenating diolefins in a naphtha stream may be used. Suitable catalysts include, but are not limited to, a catalyst comprising platinum, palladium, copper, titanium, vanadium, chrome, manganese, cobalt, nickel, zinc, molybdenum, and cadmium or mixtures thereof. The metals are preferably supported on inorganic oxide supports such as silica and alumina, for example.

The selectively hydrogenated pyrolysis stream in line <NUM> is separated in a separator <NUM> to provide the recycle stream in a bottoms line <NUM> and a vaporous pyrolysis stream in an overhead line <NUM> which is fed to a pyrolysis fractionation column <NUM>.

The vaporous pyrolysis stream in line <NUM> may be preliminarily fractionated in a pyrolysis fractionation column <NUM> to remove C9+ hydrocarbons. Light olefins preferably are removed from the pyrolysis stream in line <NUM> in a steam cracking light olefin recovery section prior to selective hydrogenation and pyrolysis fractionation. The pyrolysis fractionation column <NUM> is operated to separate an off gas stream in line <NUM> comprising C4- hydrocarbons wet gas, a net overhead stream comprising C5-C8 hydrocarbons pyrolysis product in a net overhead line <NUM>, rich in benzenes, toluene and xylenes, and a pyrolysis bottoms stream rich in C9+ hydrocarbons in line <NUM>. The pyrolysis overhead stream is withdrawn from the pyrolysis fractionation column <NUM> in an overhead line <NUM>, condensed in a cooler and fed to a separator <NUM>. A portion of the condensed pyrolysis overhead stream is recycled to the pyrolysis fractionation column <NUM> as reflux through a reflux line and the remaining portion of the condensed net pyrolysis overhead stream is withdrawn through a net pyrolysis overhead line <NUM>. Wet gases are withdrawn in the off gas line <NUM> while the C5-C8 hydrocarbon pyrolysis product is withdrawn in the net overhead line <NUM>.

The pyrolysis bottoms stream is withdrawn from pyrolysis fractionation column <NUM> through a bottoms line <NUM> where a portion of the pyrolysis bottoms stream flows through a reboiler line <NUM> to a reboiler heater and returns heated to the pyrolysis fractionation column <NUM>. A net pyrolysis bottoms stream flows through line <NUM> rich in C9+ hydrocarbons which may be recovered or further processed. The pyrolysis fractionation column <NUM> operates in bottoms temperature range of <NUM> to <NUM> and an overhead pressure of <NUM> to <NUM> kPa (gauge).

The combined pyrolysis product stream in line <NUM> may be heated and charged to the hydrotreating reactor <NUM>. The hydrotreating reactor <NUM> may have one or more beds of hydrotreating catalyst to hydrodemetallate, hydrodenitrogenate and hydrodesulfurize the combined selectively hydrogenated pyrolysis stream. The combined pyrolysis product stream may be charged to the hydrotreating reactor <NUM> at a hydrotreating inlet temperature that may range from <NUM> (<NUM>°F) to <NUM> (<NUM>°F). The hydrotreating reactor <NUM> may employ interbed hydrogen quench streams from the hydrogen manifold <NUM>.

Suitable hydrotreating catalysts are any known conventional hydrotreating catalysts and include those which are comprised of at least one Group VIII metal, preferably iron, cobalt and nickel, more preferably cobalt and/or nickel and at least one Group VI metal, preferably molybdenum and tungsten, on a high surface area support material, preferably alumina. Other suitable hydrotreating catalysts include zeolitic catalysts, as well as noble metal catalysts where the noble metal is selected from palladium and platinum. It is within the scope of the present description that more than one type of hydrotreating catalyst be used in the same hydrotreating reactor <NUM>. The Group VIII metal is typically present in an amount ranging from <NUM> to <NUM> wt%, preferably from <NUM> to <NUM> wt%. The Group VI metal will typically be present in an amount ranging from <NUM> to <NUM> wt%, preferably from <NUM> to <NUM> wt%. Generally, hydrotreating conditions include a pressure of <NUM> kPa (<NUM> psig) to <NUM> MPa (<NUM> psig). The hydrotreating outlet temperature may range between <NUM> (<NUM>°F) and <NUM> (<NUM>°F).

The hydrotreated effluent stream may exit the hydrotreating reactor in line <NUM> and enter a hydrotreating separator <NUM> to provide an overhead stream rich in hydrogen in line <NUM> that may be scrubbed (not shown) to remove hydrogen sulfide and ammonia or other compounds and compressed and returned back to the hydrogen manifold <NUM> and the hydrogen line <NUM> after perhaps supplementation with a make-up hydrogen stream. A hydrotreated pyrolysis stream is provided from a bottoms line <NUM> from the hydrotreater separator <NUM> and stripped in a stripper column <NUM> to remove C4- off gases in a stripper off-gas line <NUM> and a stripped pyrolysis stream in a stripper bottoms line <NUM>.

The stripped pyrolysis stream may in line <NUM> is rich in C6-C8 aromatics. In an embodiment, the preliminary overhead feed comprises at least <NUM> to <NUM> wt% C6-C8 aromatics. The stripped pyrolysis stream may have the composition shown in Table <NUM>.

The pyrolysis stream may comprise <NUM> to <NUM> wt% pentanes, <NUM> to <NUM> wt% hexane, <NUM> to <NUM> wt% benzene, <NUM> to <NUM> wt% heptanes, <NUM> to <NUM> wt% toluene, <NUM> to <NUM> wt% ethylbenzene, <NUM> to <NUM> wt% xylenes and <NUM> to <NUM> wt% total aromatics.

The hydrotreated pyrolysis stream in line <NUM> may have a sulfur concentration of below <NUM> wppm, but the saturation catalyst may be very sensitive to sulfur. Hence the hydrotreated pyrolysis stream is further desulfurized by passing it through sulfur guard beds <NUM> and <NUM> set up in a lead-lag arrangement and operated at hot conditions of <NUM> (<NUM>°F) to <NUM> (<NUM>°F). Low temperature sulfur guard beds may be used as well depending on the species of sulfur. The hydrotreated pyrolysis stream in line <NUM> is heated by heat exchange with a first saturated effluent stream in line <NUM> in a fresh feed reactor effluent exchanger <NUM> and then may be further heated in a guard bed charge heater <NUM> before it is fed to the sulfur guard beds <NUM> and <NUM> to have sulfur removed down to less than <NUM> wppm and preferably, <NUM> to <NUM> wppm sulfur. The hydrotreated pyrolysis stream in line <NUM> may also be heated by heat exchange with a second saturated effluent stream in line <NUM> in the fresh feed reactor effluent exchanger <NUM>, but this embodiment is not shown.

The hot desulfurized pyrolysis stream from the guard beds <NUM> and <NUM> is routed to a feed surge drum <NUM>, which may be blanketed by makeup gas and/or separator off gas. Further, this hot desulfurized pyrolysis stream in line <NUM> is pumped to the saturation reactor pressure of <NUM> MPa (<NUM> psig) to <NUM> MPa (<NUM> psig) by a charge pump.

The saturation of aromatics is extremely exothermic. Managing the exothermicity is conducted by splitting the pyrolysis stream in line <NUM> into multiple streams each for a dedicated saturation reactor. This arrangement can maintain the exotherm for each saturation reactor below <NUM> (<NUM>°F), preferably below <NUM> (<NUM>°F). The desulfurized pyrolysis stream in line <NUM> is split into at least a first pyrolysis stream in line <NUM> and a second pyrolysis stream in line <NUM>.

Hydrogen partial pressure should be minimized so as to avoid side reactions such as hydrocracking reactions. To reduce the tendency for side reactions, a hydrogen stream in line <NUM> is split into a number of streams, perhaps like the number of multiple pyrolysis streams, each dedicated to a single saturation reactor. A hydrogen stream in line <NUM> entering the process is split into a first hydrogen stream in line <NUM> and a second hydrogen stream in line <NUM>. The first hydrogen stream in line <NUM> is added to the first pyrolysis stream in line <NUM>. The second hydrogen stream in line <NUM> is added to the second pyrolysis stream in line <NUM>. For systems using a platinum-based catalyst, the hydrogen should be very pure make-up hydrogen, preferably from a pressure-swing adsorption unit, with no more than <NUM> mole-ppm carbon monoxide and no more than <NUM> mole-ppm hydrogen sulfide, preferably no more than <NUM> mole-ppm hydrogen sulfide. The saturation process can endure a higher concentration of impurities in the hydrogen if the operating temperature and/or the outlet hydrogen-to-hydrocarbon ratio is increased. Hydrogen with some concentration of light paraffinic gases can also be used provided the applicable contaminant limit is met for the catalyst.

The first pyrolysis stream in line <NUM> may comprise <NUM> to <NUM> vol-% and typically <NUM> to <NUM> vol-% of the hot desulfurized pyrolysis stream in line <NUM>. The second pyrolysis stream in line <NUM> may comprise the balance. A third pyrolysis stream to a third saturation reactor is also contemplated but not shown. A third hydrogen stream to a third saturation reactor is also contemplated but not shown. The first hydrogen stream in line <NUM> is added to the first pyrolysis stream in line <NUM>. Moreover, to further manage the saturation exotherm a recycle stream in line <NUM> is added to the first pyrolysis stream in line <NUM> to provide a first combined charge stream in line <NUM>. The recycle stream is taken from a second saturated effluent stream in line <NUM>. The first combined charge stream in line <NUM> may be heated to <NUM> (<NUM>°F) to <NUM> (<NUM>°F) and charged to the first saturation reactor <NUM>.

In the first saturation reactor <NUM>, aromatics are saturated over a bed of saturation catalyst to naphthenes to provide a first saturated effluent stream. The saturation catalyst may be the same in both saturation reactors <NUM> and <NUM>. The saturation catalyst may comprise a noble metal, platinum or palladium, a platinum-lithium or nickel on a porous carrier material or any know commercial hydrogenation catalyst.

The porous carrier material may have a surface area of <NUM> to <NUM> square meters per gram, preferably <NUM> to <NUM> square meters per gram, and may comprise non-acidic, amorphous alumina. Gamma alumina may be preferred. In addition, a preferred alumina will have an apparent bulk density of <NUM> to <NUM> gm/cc and surface area characteristics such that the average pore diameter is <NUM> to <NUM> Angstroms and the pore volume is <NUM> to <NUM> milliliter per gram. The alumina carrier may be prepared by adding a suitable alkaline reagent, such as ammonium hydroxide, to a salt of aluminum, such as aluminum chloride, or aluminum nitrate, in an amount to form an aluminum hydroxide gel which, upon drying and calcination, is converted to alumina. The carrier material may be formed in any desired shape such as spheres, pills, cakes, extrudates, powders, granules, etc., and may further be utilized in any desired size.

The Group VIII noble metal component, for example platinum, may exist within the final catalytic composite as a compound such as an oxide, sulfide, halide, or in an elemental state. The Group VIII noble metal component generally comprises <NUM>% to <NUM>% by weight of the final composite, calculated on an elemental basis. The Group VIII noble metal component may be incorporated within the catalytic composite in any suitable manner including co-precipitation or cogellation with the carrier material, ion-exchange, or impregnation. Following impregnation, the composite may generally be dried at a temperature of <NUM> (<NUM>°F) to <NUM> (<NUM>°F), for a period of from <NUM> to <NUM> hours, or more, and finally calcined at a temperature of <NUM> (<NUM>°F) to <NUM> (<NUM>°F), in an atmosphere of air, for a period of <NUM> to <NUM> hours.

In order to avoid side reactions which results in the loss of naphthenes, an alkalinous metal component may be combined with the catalytic composite in an amount of from <NUM>% to <NUM>% by weight. This component is selected from the group of alkali metals, particularly lithium and/or potassium.

The saturation catalyst may be reduced in a water-free environment after calcination to reduce the noble metal component. Moreover, the saturation catalyst may be presulfided such as in the presence of hydrogen sulfide to activate the catalyst.

A first saturated effluent stream exits the first saturation reactor <NUM> in the first saturated effluent line <NUM> with a greater concentration of naphthenes and a lower concentration of aromatics than in the first combined charge stream in line <NUM>. The first saturated effluent stream in line <NUM> may be cooled in a first steam generator <NUM>, then by heat exchange with the hydrotreated pyrolysis stream in the hydrotreater separator bottoms line <NUM> in the fresh feed reactor effluent exchanger <NUM> and then in a first reactor effluent cooler <NUM>.

The second hydrogen stream in line <NUM> is added to the second pyrolysis stream in line <NUM>. Moreover, to manage the exotherm in the second saturation reactor <NUM>, the cooled first saturated effluent stream in line <NUM> is also added to the second pyrolysis stream in line <NUM> to provide a second combined charge stream in line <NUM>. The second combined charge stream in line <NUM> may be at a temperature of <NUM> (<NUM>°F) to <NUM> (<NUM>°F) and charged to the second saturation reactor <NUM>.

In the second saturation reactor <NUM> aromatics are saturated over a bed of saturation catalyst to naphthenes to provide a second saturated effluent stream. The saturation catalyst may be the same in both saturation reactors <NUM> and <NUM>. The saturation catalyst may comprise a porous carrier material having combined therewith a Group VIII noble metal component or any commercial hydrogenation catalyst as described for the first saturation reactor <NUM>.

Conditions in the saturation reactors <NUM> and <NUM> should include a hydrogen to hydrocarbon mole ratio of <NUM> to <NUM>, preferably <NUM> to <NUM> at the reactor outlet, an outlet reaction temperature of <NUM> (<NUM>°F) to <NUM> (<NUM>°F), preferably <NUM> (<NUM>°F) to <NUM> (<NUM>°F), a LHSV of <NUM> to <NUM> hr-<NUM>, preferably, <NUM> to <NUM> hr-<NUM>, and a reactor pressure at the last reactor outlet of <NUM> MPa (<NUM> psig) to <NUM> MPa (<NUM> psig), preferably <NUM> MPa (<NUM> psig) to <NUM> MPa (<NUM> psig). The saturation reactors <NUM> and <NUM> may be operated in a downflow mode although other reactor configurations and flow regimes may be suitable.

A second saturated effluent stream exits the second saturation reactor <NUM> in the second saturated effluent line <NUM> with a greater concentration of naphthenes and a lower concentration of aromatics than in the second combined charge stream in line <NUM>. The second saturated effluent stream in line <NUM> may be cooled in a second steam generator <NUM>, then by heat exchange with the recycle stream in the recycle line <NUM> in a recycle oil reactor effluent exchanger <NUM>, then in an optional low-pressure steam generator <NUM> and then in an optional second reactor effluent cooler <NUM>. The second saturated effluent stream is cooled still further by heat exchange with the separator liquid stream in line <NUM> in a separator liquid reactor effluent exchanger <NUM> and then condensed in a product condenser <NUM> before it is fed to a separator <NUM>. It is envisioned that a third or additional saturation reactors can be employed. Additional reactors may be employed to allow the first saturation reactor <NUM> or the second saturation reactor <NUM> to be taken offline for catalyst regeneration or replacement while a pyrolysis feed stream and hydrogen is run to the third or additional reactor without reducing throughput.

The cooled second saturated effluent stream in line <NUM> separated into a vapor saturated stream in an overhead line <NUM> extending from an overhead of the separator <NUM> and a liquid saturated stream in bottoms line <NUM> extending from a bottom of the separator <NUM>. The vapor saturated stream in line <NUM> is rich in hydrogen and may be recycled to line <NUM> for recycle to the saturation reactors <NUM> and <NUM> perhaps with a purge or forwarded to a pressure swing adsorption unit or other unit for hydrogen recovery or to provide make-up gas for any unit. The liquid saturated stream is pumped in the bottoms line <NUM> and heated in the reactor effluent exchanger <NUM> by heat exchange with the second saturated effluent stream in line <NUM> and split into the recycle stream in line <NUM> and a product fractionator feed stream in line <NUM>. The separator <NUM> is operated at <NUM> (<NUM>°F) to <NUM> (<NUM>°F) and <NUM> MPa (<NUM> psig) to <NUM> MPa (<NUM> psig).

The recycle stream in line <NUM> is taken from the liquid saturated stream in line <NUM>, may be heated in the recycle oil reactor effluent exchanger <NUM> and is added to the first pyrolysis stream in line <NUM> and the first hydrogen stream in line <NUM> to provide the first combined charge stream in line <NUM> charged to the first saturation reactor <NUM>. It is also envisioned that the recycle stream in line <NUM> can be heated by heat exchange with the first saturation effluent in line <NUM>. It is also envisioned that the recycle stream in line <NUM> may be recycled to the first pyrolysis stream in line <NUM> and to the second pyrolysis stream in line <NUM>. The recycle-to-feed ratio can be <NUM> to <NUM> and suitably <NUM> to <NUM>.

The product fractionator feed stream in line <NUM> taken from the liquid saturated stream in line <NUM> is fed to the product fractionation column <NUM>. The product fractionation feed stream in line <NUM> is fractionated in the product fractionation column <NUM> to remove C6 naphthenes in the bottoms stream and n-hexane in the overhead liquid stream. The product fractionation column <NUM> is operated to separate two fractions, a product overhead stream rich in cyclopentane and normal hexane and particularly rich in paraffins and a product bottoms stream rich in cyclohexane and particularly rich in naphthenes. The product overhead stream rich in normal hexane is withdrawn from the product fractionation column <NUM> in an overhead line <NUM> extending from an overhead of the column. A stripper overhead stream in line <NUM> may be added to the product overhead stream in line <NUM> to provide a combined overhead stream in line <NUM>. The combined overhead stream in line <NUM> may be condensed in a cooler and fed to a product overhead separator <NUM>. A portion of the condensed product overhead stream is recycled to the product fractionation column <NUM> as reflux through a reflux line and the remaining portion of the condensed product overhead stream is withdrawn through a net product overhead line <NUM>. In one embodiment, the condensed product overhead stream may be forwarded to the steam cracking unit <NUM>. In another embodiment, the condensed product overhead product stream may be fed to the overhead liquid stripper <NUM> to strip out lights. Hydrogen and C3- hydrocarbons are withdrawn in a net product vapor line <NUM> from an overhead of the product overhead separator <NUM> and may be transported to a fuel gas header. If the product fractionation column <NUM> is operated at lower pressure to enable reboil by low pressure steam, the overhead stream from the product overhead separator <NUM> may be compressed, cooled and separated in a compressor drum. The compressor drum vapor stream in the overhead line could be transported to the fuel gas header and the compressor drum liquid stream in the bottoms line would be recycled back to the product overhead separator <NUM>.

The product bottoms stream is withdrawn from the product fractionation column <NUM> in a bottoms line <NUM> extending from a bottom of the column. A reboil portion of the product bottoms stream flows through a reboiler line <NUM>, a reboiler heater which may include heat exchange with steam and returns heated to the product fractionation column <NUM>. A net product bottoms stream flows through line <NUM> rich in cyclohexane and other naphthenes which may be cooled in a product bottoms cooler <NUM> and charged back to the steam cracking unit <NUM> to produce more light olefins. The product fractionation column <NUM> operates in bottoms temperature range of <NUM> (<NUM>°F) to <NUM> (<NUM>°F) and an overhead pressure of <NUM> kPa to <NUM> kPa (gauge).

The condensed product overhead stream in the net product overhead line <NUM> is stripped in the overhead liquid stripper <NUM> to assure that light hydrocarbons and hydrogen are removed from the product overhead stream. The condensed product overhead stream is stripped into two streams: a stripper overhead stream rich in C3- hydrocarbons and hydrogen and a stripper bottoms stream rich in C4+ hydrocarbons. The stripper overhead stream is withdrawn in the stripper overhead line <NUM> extending from an overhead of the overhead liquid stripper and is combined with the product fractionator overhead stream in line <NUM> to have C3- hydrocarbons removed from both streams together in the combined overhead stream in line <NUM>.

A stripper bottoms stream is withdrawn from overhead liquid stripper column <NUM> in a bottoms line <NUM> extending from a bottom of the column. A reboil portion of the stripper bottoms stream flows through a reboiler line <NUM>, a reboiler heater which may include heat exchange with steam and returns heated to the overhead liquid stripper <NUM>. A net stripper bottoms stream flows through line <NUM> rich in C5 naphthenes and C6 paraffins. The overhead liquid stripper column <NUM> operates in bottoms temperature range of <NUM> (<NUM>°F) to <NUM> (<NUM>°F) and an overhead pressure of <NUM> kPa to <NUM> kPa (gauge).

The net stripper bottoms stream may be routed to an iso-normal separation unit <NUM> that separates iso-paraffins from normal paraffins in line <NUM> to provide a normal paraffin rich stream in line <NUM>. The normal paraffins stream in line <NUM> can be routed to the stream cracking unit <NUM> as excellent steam cracker feed. The iso-paraffins rich stream in line <NUM> can be further processed such as in an isomerization unit to convert iso-paraffins to normal paraffins to provide a superb steam cracker feed. Alternatively, the net stripper bottoms stream may be routed to the stream cracking unit <NUM>.

<FIG> shows an embodiment of the process in which a hot separator is used to produce the recycle stream <NUM>'. Elements in <FIG> with the same configuration as in <FIG> will have the same reference numeral as in <FIG>. Elements in <FIG> which have a different configuration as the corresponding element in <FIG> will have the same reference numeral but designated with a prime symbol ('). The configuration and operation of the embodiment of <FIG> is essentially the same as in <FIG>.

The second saturated effluent stream in line <NUM>' be taken directly or indirectly to a hot separator <NUM>' with none or some heat exchange in <FIG>. A hot vapor saturated stream from an overhead line <NUM>' extending from an overhead of the hot separator <NUM>' can be cooled in the product condenser <NUM>' and fed to a cold separator <NUM>. The cold separator separates the hot vapor saturated stream into a cold vapor saturated stream in the overhead line <NUM> extending from an overhead of the cold separator <NUM> which can be routed to hydrogen recovery and a cold liquid saturated stream in a cold separator bottoms line <NUM> extending from a bottom of the cold separator <NUM>. A pump may be used to transport liquid in line <NUM>.

The recycle stream in line <NUM>' can be taken from the hot saturated liquid stream in the hot separator bottoms line <NUM>' with less or no need for reheating to the reaction temperature for the first saturation reactor <NUM>. A remaining net hot saturated liquid stream in a net line <NUM> taken from the hot saturated liquid stream in the hot separator bottoms line <NUM>' may be combined with the cold saturated liquid stream in line <NUM> from a bottom of the cold separator <NUM> to provide a product fractionator feed stream in line <NUM>'. The product fractionator feed stream in line <NUM>' may be cooled in the exchanger <NUM>' and fed to the product fractionation column <NUM>. The rest of <FIG> as is described for <FIG>.

<FIG>, which is not within the scope of the invention, shows a process in which the combined saturated liquid streams are charged to the steam cracking unit <NUM> instead of being fractionated. Elements in <FIG> with the same configuration as in <FIG> will have the same reference numeral as in <FIG>. Elements in <FIG> which have a different configuration as the corresponding element in <FIG> will have the same reference numeral but designated with a double prime symbol ("). The configuration and operation of the embodiment of <FIG> is essentially the same as in <FIG>.

The net hot saturated liquid stream in line <NUM> taken from the hot saturated liquid stream in bottoms line <NUM>' and the cold saturate liquid stream in the cold separator bottoms line <NUM> combined in line <NUM>" can be taken directly to the stream cracking unit <NUM> instead of undergoing fractionation. The rest of <FIG> as is described for <FIG>.

The foregoing description provides a process for saturating aromatics in a pyrolysis stream which may make it suitable for steam cracking feed.

A simulation of the disclosed process was run with <NUM>,<NUM>/hr (<NUM> lb/hr) of make-up hydrogen with <NUM>% of the pyrolysis stream split to the first reactor and the balance to the second reactor. Reactor inlet temperatures were <NUM> (<NUM>°F) and reactor outlet temperatures were <NUM> (<NUM>°F) and <NUM> (<NUM>°F), respectively. Reactor pressure was <NUM> MPa (<NUM> psig), the hydrogen-to-hydrocarbon ratio was <NUM> at each reactor outlet, LHSV was <NUM> hr-<NUM> and the recycle-to-feed ratio was <NUM> on a volume basis. Conversion across both reactors was <NUM>%. The component mass flow rates are provided in Table <NUM>.

Table <NUM> reveals that total aromatics was reduced by <NUM>% while naphthenes increased by over three-fold. Paraffins were insubstantially converted.

Claim 1:
A process for saturating a pyrolysis gas stream comprising:
splitting the pyrolysis gas into at least a first pyrolysis gas stream and a second pyrolysis gas stream;
adding a first hydrogen stream to the first pyrolysis gas stream;
saturating aromatics in said first pyrolysis gas stream over a saturation catalyst to provide a first saturated effluent stream;
adding a second hydrogen stream to the second pyrolysis gas stream; and
saturating aromatics in said second pyrolysis gas stream over a saturation catalyst to provide a second saturated effluent stream;
wherein the process further comprises:
mixing a recycle stream taken from the second saturated effluent stream with the first pyrolysis gas stream and the first hydrogen stream; wherein the second saturated effluent stream is separated into a vapor saturated stream and a liquid saturated stream and the recycle stream is taken from the liquid saturated stream; and
fractionating a fractionator feed stream taken from the liquid saturated stream to produce a product bottoms stream rich in cyclohexane and a product overhead stream rich in n-hexane.