Abstract: A hydrocarbon reforming apparatus with a controller which manipulates the addition of halogen to the reforming catalyst in response to an isopentane signal representative of the isopentane content in the reformate.

Abstract: A process and associated apparatus for the cooling of hot fluidized solid particles. The particles flow from a first dense phase fluidized bed into the shell side of a vertically oriented shell and tube heat exchanger where cooling occurs via indirect heat exchange with a cooling medium circulating in the tubes. The extent of cooling is controlled by the varying of the heat transfer coefficient between the tubes and particles in the heat exchanger which are maintained as a second dense phase fluidized bed. The coefficient is varied by varying the quantity of fluidizing gas to the fluidized bed in the heat exchanger. The particles flow freely to and from the first and second dense phase fluidized beds through which the particles recirculate and are backmixed. The process has particular applicability to a combustive regeneration process and most particular applicability to the FCC process.

Abstract: In a catalytic cracking unit, the amount of the products removed from the fractionator associated with the catalytic cracking unit which has a boiling point above the highest boiling point of the desired product is calculated based on internal reflux to vapor ratios in various portions of the fractionator. The conversion of the catalytic cracking unit (the amount of the feed to the reactor having a boiling point above the highest boiling point of the desired product which is converted to a product having a boiling point below the highest boiling point of the desired product) is then determined and the reactor is controlled so as to maintain the actual conversion substantially equal to a desired conversion.

Abstract: An apparatus for regenerating coke-contaminated fluid catalyst is disclosed which comprises a vertical combustion chamber, an inlet to said chamber for spent catalyst and regeneration gas, a heat removal chamber located superadjacent to the combustion chamber and in communication therewith, heat removal means disposed within the heat removal chamber, a catalyst withdrawal conduit connected at one end to the heat removal chamber for withdrawing regenerated fluid catalyst from the heat removal chamber, and a catalyst recycle conduit connecting the withdrawal conduit with the lower portion of the combustion chamber, such that regenerated fluid catalyst can pass from the heat removal chamber to the combustion chamber. There may also be a means and control system for manipulating the extent of immersion of the heat removal means.

Abstract: A catalyst regeneration process and apparatus for the oxidative removal of coke from a coke contaminated fluid catalyst. The process comprises a high temperature coke combustion zone, a catalyst disengagement zone and an external heat removal zone. A mixture of coke contaminated catalyst, oxygen containing gas, hot regenerated catalyst and cool regenerated catalyst from the heat removal zone are contacted in the high temperature combustion zone, the temperature of which is controlled by adjusting the rate at which catalyst is recycled from the heat removal zone. The temperature of the catalyst-gas mixture is controlled by adjusting the rate at which hot catalyst is recycled from the disengagement zone.

Abstract: Controlling gas to slurry volume ratio to achieve a gas holdup of about 0.4 when heating a flowing coal-oil slurry and a hydrogen containing gas stream allows operation with virtually any coal to solvent ratio and permits operation with efficient heat transfer and satisfactory pressure drops. The critical minimum gas flow rate for any given coal-oil slurry will depend on numerous factors such as coal concentration, coal particle size distribution, composition of the solvent (including recycle slurries), and type of coal. Further system efficiency can be achieved by operating with multiple heating zones to provide a high heat flux when the apparent viscosity of the gas saturated slurry is highest.

Abstract: A catalyst regeneration process and apparatus for the oxidative removal of coke from a coke contaminated fluid catalyst. The process comprises a high temperature coke combustion zone, a catalyst disengagement zone and an external heat removal zone comprising a shell and tube heat exchanger. Catalyst is cooled by passing it through the shell side of the heat exchanger with a cooling medium through the tube side. A mixture of coke contaminated catalyst, oxygen containing gas, and cool regenerated catalyst from the heat removal zone are contacted in the high temperature combustion zone, the temperature of which is controlled by adjusting the rate at which fluidizing gas is passed to the bottom portion of the shell of the heat exchanger which varies the degree of turbulence of the fluidized bed in the shell side and thus the heat transfer coefficient at the outside surface of the tubes which in turn varies the quantity of heat transferred to the cooling medium in the tubes.

Abstract: A desired cloud point temperature for a side draw product stream from a fractional distillation column is maintained at a desired level by comparing a calculated actual cloud point temperature, which is updated periodically by a cloud point measurement, to a desired cloud point temperature with results of the comparison being utilized to control the flow rate of the side draw product stream so as to maintain the actual cloud point temperature of the side draw product stream substantially equal to the desired cloud point temperature. Use of the calculated cloud point temperature provides a means by which continuous control of the actual cloud point temperature of the side draw product stream can be maintained.

Abstract: A fractional distillation process, in which the flow rate of the external reflux is utilized to control the bottoms product composition, is controlled so as to maintain a desired bottoms product composition to the extent that maintaining the desired bottoms product composition does not cause a process constraint to be violated. A plurality of actual process conditions are compared to process constraints for those process conditions to determine if the flow rate of the external reflux should remain the same, be increased by an incremental amount or be decreased by an incremental amount. The order of the comparison of process constraints to actual process conditions is such that the most important comparison will control the external reflux flow rate if that comparison indicates that a process constraint has been violated.

Abstract: Measurements of process variables associated with a cracking furnace are utilized to calculate the actual maximum tube skin temperature for the cracking furnace. The thus calculated actual maximum tube skin temperature is then utilized to derive a control signal which may be utilized to manipulate the rate at which heat is supplied to the cracking furnace. The accurate prediction of the actual tube skin temperature based on actual process measurements enables the conversion rate for the cracking furnace to be substantially maximized while preventing damage to the cracking tubes.

Abstract: Antimony compounds are employed to reduce catalyst attrition and improve catalyst performance in catalytic cracking units, especially in conjunction with strontium compounds.

Abstract: A method for decreasing the external energy input to a fractionation system is disclosed. The method utilizes a compression zone communicating with either the overhead fraction or the bottoms fraction from a fractionation zone. The method is directed at regulating the rate of energy addition and/or removal to the fractionation zone by at least one of the fractions returned to the fractionation zone to maintain the separated fractions within the desired limits.

Abstract: Sulfur trioxide concentration in the flue gas of catalytic cracking regenerators is maintained at a predetermined level by controlling the flow rate of oxygen-containing regeneration gas into the regenerator, and, optionally, the amount of carbon monoxide combustion promoter in the regenerator.

Abstract: In fractionating tower apparatus including a plurality of modules including multi-stage segments, the improvement comprising the combination of a plurality of vaporizers for heating feed liquid to different temperatures to vaporize fractions of correspondingly different volatilities, means for feeding the vaporized fractions to different multi-stage segments of the tower, means for withdrawing liquid sidestreams from a plurality of the segments, temperature control means for limiting the level of heavy components in at least one selected liquid sidestream withdrawn from a segment below a module in which vapor is flashed, and means for separately feeding reflux liquid from at least one segment to a segment below it.

Abstract: In a fluid catalytic cracking process in the case of a catalyst load dump the catalyst-laden gas is prevented from passing through the power recover expander. This is achieved by establishing a tangential, low pressure bypass flow path in the supply line to the expander which causes the catalyst to be bypassed. An optical sensor senses the catalyst load dump and causes the actuation of the valve necessary to establish the low pressure bypass flow path.

Abstract: A catalyst regeneration process and apparatus for the oxidative removal of coke from a coke contaminated fluid catalyst. The process comprises a high temperature coke combustion zone, a catalyst disengagement zone and an external heat removal zone. A mixture of coke contaminated catalyst, oxygen containing gas, hot regenerated catalyst and cool regenerated catalyst from the heat removal zone are contacted in the high temperature combustion zone, the temperature of which is controlled by adjusting the rate at which catalyst is recycled from the heat removal zone. The temperature of the catalyst-gas mixture is controlled by adjusting the rate at which hot catalyst is recycled from the disengagement zone.

Abstract: A control system for a fluid catalytic cracking unit is provided in which either the wet gas compressor or the air blower is selected for base loading. After the device selected for base loading has been base loaded, load is shifted to the device which was not base loaded. The feed flow rate is forced to continue to increase until both the wet gas compressor and the air blower are loaded so as to substantially maximize the production of the desired product.

Abstract: The heat supplied to an ethylene process cracking furnace is manipulated so as to maintain a desired ethylene production rate while the flow of feed to the ethylene process cracking furnace is manipulated so as to substantially maximize the selectivity of the ethylene process cracking furnace to the production of ethylene. In this manner a desired ethylene production rate is maintained while the process economics are improved in the case where the cost of the feed is a more important economic consideration than the cost of the energy required for the cracking furnace by substantially minimizing the conversion of the feed to undesired byproducts.

Abstract: A level-controlled water tank in an extractive distillation system is used to maintain the water concentration in the solvent mixture constant.

Abstract: In a fractional distillation column, a desired ratio between the flow rate of a side draw product stream and the internal reflux flow rate at the particular tray from which a side draw stream is withdrawn from the fractional distillation column is maintained by manipulating the flow rate of the center reflux stream so as to maintain the desired ratio between the internal reflux flow rate and a side draw product stream flow rate at the particular tray from which the side draw stream is withdrawn from the fractional distillation column. Control of the ratio between the flow rate of a side draw product stream and the internal reflux flow rate results in an improved side draw product stream purity.

Abstract: In a process in which a feedstock is provided through a desalting unit prior to introduction into a fractional distillation column, the feed stream flowing to the desalting unit is split and passed through multiple heat exchangers. Method and apparatus is provided for maintaining a desired effluent flow rate out of the desalting unit and for maintaining the temperatures of the split feed streams substantially equal. After passing through the desalting unit the feed stream may again be split and passed through multiple heat exchangers prior to introduction of the feed stream into the fractional distillation column. Method and apparatus is also provided for maintaining the temperature of the split feed streams provided from the desalting unit substantially equal.

Abstract: In a fluid catalytic cracking (FCC) process and apparatus, the heat balance between the reactor and the regenerator of the FCC operation is partially uncoupled by transferring at least a portion of thermal energy from the reactor vessel riser to the regenerator vessel. The transfer of thermal energy results in a higher regenerating temperature. The thermal energy is recirculated to the bottom section of the reaction riser through a regenerated catalyst having higher temperature. Consequently, the outlet of the reaction vessel is maintained at a substantially constant temperature (e.g., 100.degree. F.) and the rate of conversion of the oil feed and the octane number of gasoline produced in the process are increased.

Abstract: For hot hydrocarbon liquids and slurries containing a vapor portion derived from a hydrogenation process, the vapor portion is separated from the liquid portion within a separation zone by providing a liquid vortex flow pattern having a gas core. The vapor is withdrawn from the vortex core through an inwardly-extending conduit, and the remaining rotating liquid portion is passed to below the vortex. If catalyst particles are also contained in the hot hydrocarbon liquid, such as in a coal or oil hydrogenation reaction effluent liquid at elevated temperature and pressure conditions, such catalysts can be conveniently separated from a product liquid stream and returned to the reaction zone along with the recycled liquid. A clean liquid stream is withdrawn from the recycled liquid for further processing. If desired, the phase separation zone utilizing a liquid vortex can be provided within the catalytic reaction zone.

Abstract: A regenerator control system is disclosed which is particularly useful in connection with combustion promoted cracking catalysts. The air flow is controlled responsive to the temperature in the dilute phase, first in a direct mode when the amount of combustibles on the catalyst is at or above a set point and second in accordance with a reverse mode when the amount of combustibles is below such a set point. This versatile control system allows the safe operation of a regenerator under a variety of conditions.

Abstract: In a fractionation distillation process in which a pump around stream and an external reflux are utilized to control heat removal from an upper portion of the fractional distillation column and in which the pump around stream is utilized to preheat the feed stream flowing to the fractional distillation, the flow rate of the pump around stream is manipulated so as to substantially maximize the flow rate of the pump around stream while maintaining a desired heat removal from the fractional distillation column. Priority control of the pump around stream and external reflux result in a maximum preheating of the feed stream flowing to the fractional distillation column which improves the energy efficiency of the fractional distillation process.

Abstract: A control system for distillation column with a sidedraw line is provided wherein a prediction on the influence by changes in the feed stream with respect to the sidedraw composition is made.

Abstract: A fluid catalytic cracking unit (FCCU) is controlled in such a manner that load is automatically shifted from the wet gas compressor to the air blower for the catalyst regenerator if the loading on the wet gas compressor becomes a limiting factor on the yield of a desired product from the FCCU. Also, the preheat temperature for the feed flowing to the reactor is automatically increased if the desired temperature in the reactor cannot be maintained by increasing catalyst flow to the reactor without exceeding an air blower limitation. This automatic load shifting provides for a substantially maximum production of a desired product without exceeding a process limitation.

Abstract: A batch distillation operation is automatically controlled from a start-up mode to a shut-down mode.

Abstract: The heat provided to a cracking furnace is manipulated so as to substantially maximize the value of the product stream flowing from the cracking furnace. An analysis of the product stream may be utilized to measure the ratio of a first constituent in the product stream to a second constituent in the product stream (severity equivalent). The severity equivalent required to substantially maximize the value of the product stream may be calculated in response to process operating conditions. The heat supplied to the cracking furnace is manipulated in response to a comparison of the actual severity equivalent to the desired severity equivalent to thereby substantially maximize the value of the product stream.

Abstract: Whole crude and residual fractions from distillation of petroleum and like feed stocks are subjected to selective vaporization to prepare heavy fractions of reduced Conradson Carbon and/or metals content by short-term, high temperature riser contact with a substantially inert solid contact material of low surface area in a selective vaporization zone. High boiling point components of the charge which are of high Conradson Carbon number and/or high metal content remain on the contact material as a combustible deposit which is then burned off in a combustion zone whereby the contact material is heated to a high temperature for return to the selective vaporization zone to supply the heat required therein. The system is dynamically controlled for fuel supply in the combustion zone by the lower hydrogen content, least valuable components of the feed.

Abstract: An improved method for controlling the fluidized dense catalyst phase temperature in the regeneration zone of a fluid catalytic cracking unit. In this method, the level of the fluidized catalyst bed above the riser discharge in the reaction vessel is adjusted in response to a change detected in the temperature of the fluidized dense catalyst phase of the regeneration zone. Adjustment of the catalyst bed level in the reaction zone affects the coke laydown on the catalyst in the reactor and, consequently, the amount of heat liberated in the regeneration zone upon combustion of the coke contained in the partially deactivated catalyst. A reactor bed level is attained where the resulting coke laydown on the catalyst is sufficient to maintain the desired temperature in the fluidized dense catalyst phase.

Abstract: A control system for a fractional distillation column, where the feed to the fractional distillation column is supplied from at least one separator, is disclosed in which overloading of the fractional distillation column is prevented by monitoring a control parameter associated with the fractional distillation column which indicates that the fractional distillation column is being overloaded. The flow of the feed from the at least one separator is manipulated to maintain the control parameter which indicates that the fractional distillation column is being overloaded within desired limits. The feed to the at least one separator is manipulated to maintain the liquid level in the at least one separator within desired limits.

Abstract: In a fractionation process in which at least two fractional distillation columns are cascaded in series, the vapor from the overhead accumulator associated with the first fractional distillation column in the series is provided to a feed tank for the second fractional distillation column in the series. The flow of the vapor from the overhead accumulator associated with the first fractional distillation column in the series is controlled by means of a control valve. An interactive control system is utilized to maintain the pressure in the overhead accumulator associated with the first fractional distillation column within safe operating limits, maintain the control valve at a substantially fully open position so as to minimize the pressure drop across the control valve controlling the flow of vapor from the overhead accumulator associated with the first fractional distillation column, and maintain the pressure in the feed tank to the second fractional distillation column at a desired value.

Abstract: A control system for a fractional distillation column in which a pumparound stream is used to remove heat from the fractional distillation column is disclosed. A combination of feedforward and feedback control is utilized to control the heat removal by the pumparound stream in such a manner that smoother operation of the fractional distillation column will be accomplished and a closer approach to required product specifications will be approached.

Abstract: An improved catalytic process wherein trace amounts of a metal of period 5 and 6 of Group VIII or rhenium is added to the total catalyst inventory for cracking of gas oil is disclosed. With this improved process, the yield of gasoline, fuel oil, total cracked products, or predetermined combinations thereof, is maximized for the fluid catalytic cracking of any predetermined feed, usually a gas oil. The process employs a kinetic predictive or a correlative model of the cracking process to identify proper operating conditions, including CO.sub.2 /CO ratio in the effluent from the regenerator, and also requires control of the metal activity to achieve optimal conversion of CO to CO.sub.2 in the regenerator.Also disclosed is an improved method for control of a fluid catalytic process wherein the cracking catalyst comprises trace amounts of a metal selected from the group Pt, Pd, Rh, Ru, Ir, Os and Re.

Abstract: Process and system are described for control of sulfur oxide in emissions to the atmosphere from regenerators of Fluid Catalytic Cracking Units (FCC) in a manner which improves the operation of the regenerator and/or the reactor. Spent FCC cracking catalyst containing sulfur-bearing inactivating carbonaceous deposit is partially oxidized, preferably in the presence of steam, thereby producing a mixture of CO and CO.sub.2 and releasing sulfur in the resulting reducing atmosphere as vaporous hydrogen sulfide before the catalyst is regenerated, whereby the amount of sulfur convertible to noxious SOx in the regenerator flue gas is reduced. By the disclosed process and system, the quantity of oxygen introduced is predetermined and controlled in response to hydrogen content of the coke on the partially oxidized catalyst or an SOx analyzer associated with the regenerator can be used as a control tool.

Abstract: Control of the temperature profile across a reactor which contains a plurality of catalyst beds is accomplished by splitting the feed to the reactor in such a manner that a portion of the feed is introduced into the first catalyst bed in the reactor and the remainder of the feed is introduced into the second catalyst bed of the reactor. The temperature of the feed stream introduced into the first catalyst bed in the reactor is manipulated so as to maintain a desired temperature in the first catalyst bed. The feed stream introduced into the second catalyst bed of the reactor is utilized to cool the effluent flowing from the first catalyst bed to maintain a desired temperature in the second catalyst bed. A recycle stream is introduced into the remaining catalyst beds with the flow rate of the recycle stream being manipulated so as to maintain desired temperatures in the remaining catalyst beds.

Abstract: A process and apparatus for regenerating fluidizable particulate cracking catalysts wherein a mixture of spent catalyst particles contaminated with carbonaceous deposits from a catalytic cracker and sufficient hot regenerated catalyst particles to bring the temperature of the mixture to the minimum temperature at which rapid oxidation of the carbonaceous deposits will be induced, are entrained in a high velocity gas stream containing sufficient oxygen to oxidize substantially all of the carbonaceous deposits on the spent catalyst particles to carbon dioxide and carbon monoxide and the resulting dilute phase catalyst particle stream is passed upwardly through a primary regeneration zone wherein the carbonaceous deposits are oxidized and the catalyst particles regenerated to a carbon content of less than 0.

Abstract: A draw rate signal generator for an atmospheric crude distillation unit receiving crude oil and providing streams 1 through 5 and a reduced crude oil includes flow rate sensors sensing the flow rates of the crude oil and of streams 1 through 4 and providing corresponding signals. Analyzers sample the crude oil and provide signals corresponding to the 35% point temperature, the API gravity and the vapor pressure of the crude oil. End point analyzers sample streams Nos. 1 and 3 and provide signals representative of the end point temperatures of streams Nos. 1 and 3, respectively. 50% point analyzers sample streams 2, 3 and 4 and provide signals corresponding to the 50% point temperatures of streams Nos. 2, 3 and 4, respectively. A circuit connected to the sensors, to the crude analyzers, to the end point analyzers and to the 50% analyzers provide signals corresponding to the draw rates for streams Nos. 1 through 5 in accordance with signals from the sensors and analyzers.

Abstract: Apparatus for separating paraffin from oil in refineries wherein such separation is accomplished by a cooling process, in a heat exchanger, the apparatus controlling the cooling process by sensing either the pressure upstream of the heat exchanger and directly controlling the flow through a heat exchanger bypass, or sensing the temperature downstream of the heat exchanger and inversely controlling the flow through a heat exchanger bypass, the flow control in each case regulating the flow of cooling liquid through the heat exchanger.

Abstract: The flow rate of a diluent fluid, such as steam, hydrogen or methane, to a cracking furnace is controlled so as to maintain a desired outlet velocity for the effluent flowing from the cracking furnace or to maintain a desired residence time for the feed stream in the cracking furnace. Feed forward control for the flow rate of the diluent fluid is provided by using an empirical model of the cracking furnace to predict either the outlet velocity or the residence time based on measured system parameters. Feedback control for the flow rate of the diluent fluid is provided by calculating the actual outlet velocity or actual residence time based on measured system parameters. The measured outlet velocity or residence time is utilized to bias or correct the predicted outlet velocity or residence time to provide a corrected prediction of the outlet or residence time.

Abstract: In an ethylene manufacturing process in which the effluent from the cracking furnaces is provided to an acetylene reactor to therein selectively hydrogenate acetylene to ethylene, method and apparatus is provided whereby methanol is injected into the cracking furnace to produce carbon monoxide in sufficient quantities to substantially optimize the selectivity of the conversion of acetylene to ethylene in the acetylene reactors. The concentration of the carbon monoxide in the effluent flowing from the cracking furnace is utilized to manipulate the flow of methanol to the cracking furnace so as to maintain a desired concentration of carbon monoxide in the effluent flowing from the cracking furnace.

Abstract: A control system for a fractional distillation column in which a pumparound stream is used to remove heat from the fractional distillation column is disclosed. A combination of feedforward and feedback control is utilized to control the heat removal by the pumparound stream in such a manner that smoother operation of the fractional distillation column will be accomplished and a closer approach to required product specifications will be approached.

Abstract: A minor portion of the regeneration gas used to regenerate spent fluid catalytic cracking catalyst is employed to combust volatile hydrocarbons in mixture with said spent catalyst prior to said mixture entering the regeneration zone. This serves to reduce and/or minimize transverse oxygen gradients in the dense phase catalyst bed and in the effluent gases therefrom such that excessive or undesirable afterburning in the dilute catalyst phase can be minimized or eliminated.

Abstract: A minor portion of the regeneration gas used to regenerate spent fluid catalytic cracking catalyst is introduced into the regeneration zone in the localized area of the terminus of the spent catalyst line so as to combust the volatile hydrocarbons in mixture with the spent catalyst being transferred from the reaction zone to the regeneration zone. This serves to reduce and/or minimize transverse oxygen gradients in both the dense phase catalyst bed and in the effluent gases therefrom such that excessive or undesirable afterburning in the dilute catalyst phase can be minimized or eliminated.

Abstract: A control system for a fractional distillation column, where the feed to the fractional distillation column is supplied from at least one separator, is disclosed in which overloading of the fractional distillation column is prevented by monitoring a control parameter associated with the fractional distillation column which indicates that the fractional distillation column is being overloaded. The flow of the feed from the at least one separator is manipulated to maintain the control parameter which indicates that the fractional distillation column is being overloaded within desired limits. The feed to the at least one separator is manipulated to maintain the liquid level in the at least one separator within desired limits.

Abstract: The heat supplied to a cracking furnace is controlled so as to maintain a desired conversion rate for the material being cracked in the cracking furnace by comparing a prediction of the conversion rate for the material being cracked in the cracking furnace to a desired conversion rate for the material being cracked in the cracking furnace to thereby determine the heat required to maintain a desired conversion rate for the material being cracked in the cracking furnace. A measured conversion rate for the material being cracked in the cracking furnace is utilized to correct the predicted conversion rate as required. A signal representative of the heat required to maintain a desired conversion rate for the materal being cracking in the cracking furnace is utilized to manipulate the flow of fuel to the cracking furnace to thereby control the heat supplied to the cracking furnace.

Abstract: In a selective hydrogenation process wherein at least two catalyst beds in series are utilized, the temperature of the feed stream to the first catalyst bed and the temperature of the feed stream from the first catalyst bed to the second catalyst bed are manipulated so as to maintain a desired reaction temperature in both catalyst beds. The desired reaction temperature of the first catalyst bed is manipulated so as to insure that a desired percent of a specific feed component is selectively hydrogenated in the first catalyst bed. The reaction temperature of the second catalyst bed is manipulated so as to maintain a desired concentration of the specific feed component in the product stream from the second catalyst bed.

Abstract: A method for controlling the oxides of nitrogen concentration in the exit flue gas from the regeneration zone of a catalytic cracking unit employing carbon monoxide combustion promoters which comprises monitoring the oxides of nitrogen concentration in the exit flue gas and adjusting the concentration of combustion promoter present in the regeneration zone to maintain the oxides of nitrogen concentration below a predetermined limit.

Abstract: In a mixed-phase hydrocarbon hydroprocessing catalytic reaction system, wherein a guard-bed of particulate matter is employed above one or more individual beds of catalyst particles, the adverse detrimental effects of excessive reaction zone pressure drop are overcome by sensing the differential pressure across the guard-bed. Vaporous stream components are withdrawn from the reaction chamber through a locus above the guard-bed and at a rate proportional to the differential pressure. These withdrawn vaporous components are reintroduced into a substantially particle-free void volume below the guard-bed and above the bed of catalyst particles.