Abstract: A method for an underground gas storage reservoir management may include obtaining feed from a domestic master gas system. The method further includes simultaneously injecting and reproducing in an underground gas storage via a plurality of dual producer and injector wells. The method further includes enhancing the underground gas storage reservoir management to maximize gas and condensate recovery. The method further includes improving a switching cycle between dual operation modes in the underground gas storage for faster reaction times through a completely segregated injection and reproduction lines.

Abstract: A method and system for liquid sulfur degassing is disclosed. The method and system generally involve degassing liquid sulfur in a degassing vessel, and the level of the liquid sulfur in the degassing vessel is controlled in the degassing vessel by determining the level of liquid sulfur in the degassing vessel.

Abstract: Contemplated systems and methods for removing polysulfides and hydrogen sulfide from liquid sulfur of a Claus plant include (a) physically separated steps of catalytic decomposition of polysulfides and gas stripping, or (b) use of the stripping gas as the continuous phase in a packed column with decomposition catalyst to so avoid catalyst attrition.

Abstract: The present invention is intended for effectively removing copper, iron, sulfur, which are impurities, from activated carbon on which gold is adsorbed before gold eluting in the point of view of gold recovery, and is related to a method for eluting gold from an activated carbon on which at least sulfur (S) and gold (Au) are adsorbed, whereas the activated carbon is washed with an alkali solution before eluting the gold, and then the gold is eluted from the activated carbon.

Abstract: Contemplated systems and methods for removing polysulfides and hydrogen sulfide from liquid sulfur of a Claus plant include (a) physically separated steps of catalytic decomposition of polysulfides and gas stripping, or (b) use of the stripping gas as the continuous phase in a packed column with decomposition catalyst to so avoid catalyst attrition.

Abstract: A process of producing degassed liquid sulfur using process gas containing H2S to agitate the liquid sulfur being degassed while in contact with a degassing catalyst. Process gas is less costly and less complicated and quickly accomplishes substantial degassing rendering the liquid sulfur much safer in storage and transportation.

Abstract: A process is provided for liquid sulfur degasification in an underground container, comprising: collecting liquid sulfur which contains polysulfides and hydrogen sulfide in a first compartment of the underground container; agitating and creating turbulence in the liquid sulfur in the first compartment of the underground container; transferring the liquid sulfur into a second compartment of the underground container; injecting gas into the liquid sulfur into the second compartment of the underground container via gas spargers, and also injecting morpholine catalyst into the liquid sulfur in the second compartment of the underground container to produce a degassed liquid sulfur; and transferring the degassed liquid sulfur into a third compartment of the underground container for storage and subsequent removal.

Abstract: A process of producing degassed liquid sulfur using agitation gas to agitate the liquid sulfur being degassed while in contact with a degassing catalyst. Process gas may be blended with the agitation gas wherein the process gas contains H2S to accomplish substantial degassing rendering the liquid sulfur much safer in storage and transportation.

Abstract: The invention relates to a process and apparatus for recovering sulfur (9). In a sour gas scrubbing apparatus (S) comprises a scrubbing part (SP) and a regeneration part (RP), wherein sulfur components and carbon dioxide are selectively removed from a crude synthesis gas (2) with the aid of a circulating scrubbing agent (3). A sulfur-containing gas fraction (8) produced during the regeneration of loaded scrubbing agent is supplied to a sulfur recovery system (SR) in which an off-gas (10) comprising carbon dioxide and also sulfur components is formed. The off-gas is hydrogenated (H) and subsequently subjected to a gas scrubbing operation (Z). The hydrogenated off-gas (12) is scrubbed, independently of the crude synthesis gas (2), and scrubbing agent (13) removed from the scrubbing agent circuit of the sour gas scrubbing apparatus (S) is used to scrub out sulfur components from the hydrogenated off-gas (12).

Abstract: The invention relates to a process and apparatus for recovering sulfur (9). In a sour gas scrubbing apparatus (S) comprises a scrubbing part (SP) and a regeneration part (RP), wherein sulfur components and carbon dioxide are selectively removed from a crude synthesis gas (2) with the aid of a circulating scrubbing agent (3). A sulfur-containing gas fraction (8) produced during the regeneration of loaded scrubbing agent is supplied to a sulfur recovery system (SR) in which an off-gas (10) comprising carbon dioxide and also sulfur components is formed. The off-gas is hydrogenated (H) and subsequently subjected to a gas scrubbing operation (Z). The hydrogenated off-gas (12) is scrubbed, independently of the crude synthesis gas (2), and scrubbing agent (13) removed from the scrubbing agent circuit of the sour gas scrubbing apparatus (S) is used to scrub out sulfur components from the hydrogenated off-gas (12).

Abstract: The present invention provides a process for reducing sulfur emission of a sulfur plant, wherein the sulfur plant includes a thermal reaction unit, a catalytic reaction unit and a tail-gas purification unit, the process is characterized in that the waste-gas from the degassing of the liquid sulfur in the liquid sulfur tank is introduced into the catalytic reaction unit, and/or the waste-gas from the degassing of the liquid sulfur in the liquid sulfur tank is introduced into the tail-gas purification unit. In present invention, the H2S in purified tail-gas can be reduced to no more than 10 ppm(v) and the SO2 emission concentration of the sulfur plant can be reduced to no more than 100 mg/m3.

Abstract: A method for removing elemental sulfur from a hot gas stream, such as an autoclave vent gas, while simultaneously cooling the gas stream. The method results in conversion of sulfur in the hot gas stream to the form of solid, non-sticky sulfur allotropes such as rhombic sulfur while avoiding formation of sticky sulfur allotropes such as monoclinic sulfur, thereby avoiding scaling and fouling of plant equipment. According the method, the hot gas stream is contacted with an aqueous medium containing a particulate material inside a quench vessel having a first inlet for the hot gas stream, a second inlet for the aqueous medium, and an outlet for removing a sulfur-containing liquid fraction from the vessel. At least a portion of the sulfur contained in the hot gas stream, along with other condensable materials, becomes incorporated into the aqueous medium and is subsequently drained from the vessel.

Abstract: Sulfur emissions from liquid sulfur are reduced, or even entirely avoided by degassing the liquid sulfur at pressure in an out-of-pit vessel and by sweeping the rundown pit (or vessel) with a sweep gas that is non-poisonous for a hydrogenation catalyst. Acid gases from degassing are fed at pressure to the Claus unit, while sweep gases are fed to the tail gas treatment unit to substantially recycle the acid gases to extinction. In preferred plants and methods, motive fluids and booster eductors or compressors are not needed, and incineration of the acid gases can be avoided.

Abstract: A novel system and method for degassing H2S and H2Sx from liquid sulfur (sulphur) is disclosed. The system includes a degassing vessel with a plurality of cells. The cells include a sparging gas mat with a perforated surface at the bottom of the cell to allow the release of air bubbles (or sparging gas) into the cells. A catalyst may be used during the process. As a result, hydrogen sulfide and hydrogen polysulfide are efficiently and effectively removed from the liquid sulfur.

Abstract: Disclosed is a reactor, a structured packing, and a method for increasing the rate of decomposition of polysulfides and oxidation of polysulfides and hydrogen sulfide in liquid sulfur. The reactor, the structured packing, and the method involve a structured packing for contacting a first stream and a second stream in a reactor including a catalyst. The catalyst increases the rate of decomposition of polysulfides and oxidation of polysulfides and hydrogen sulfide in the liquid sulfur of the first stream with the second stream. The first stream includes liquid sulfur containing polysulfides and dissolved hydrogen sulfide. The second stream includes an oxygen-containing gas.

Abstract: A system may include a sulfur recovery unit. The sulfur recovery unit may include an acid gas supply, which may supply acid gas, an oxygen supply, which may supply oxygen, a fuel supply, which may supply fuel. The fuel may have a higher heating value than the acid gas. Also, the sulfur recovery unit may include a thermal reaction zone, which may thermally recover sulfur from the acid gas by combustion of the fuel and the acid gas with the oxygen and through reaction of the acid gas with combustion products arising from the combustion.

Abstract: A system and method for removing contaminant from sulfur (or sulphur) to a level that allows re-processed sulfur to reenter a supply chain. The system and method melt contaminated sulfur, filters the molten sulfur, and then dispenses the less contaminated sulfur after filtering it. More specifically, solid contaminated sulfur enters a settling tank, while heated molten sulfur circulates through the system and the settling tank. The circulation of the molten sulfur assists in the melting process of the solid sulfur, allowing the solid sulfur to melt efficiently. In preferred embodiments of the system, the system uses steam jacked sulfur lines and steam tracing to maintain temperatures for melting the solid sulfur and maintaining the molten state of the sulfur circulating through the system. Furthermore, the system comprises a programmable logic controller, which controls motors, pumps, valves, and environment monitoring instruments.

Abstract: Disclosed is a reactor, a retained catalyst structure, and a method for increasing the rate of decomposition of polysulfides and removal of hydrogen sulfide in liquid sulfur. The reactor, the retained catalyst structure, and the method include a retained catalyst structure arranged and disposed for contacting a first stream and a second stream in a reactor including a catalyst. The catalyst increases the rate of decomposition of polysulfides and facilitates the removal of hydrogen sulfide in the liquid sulfur of the first stream with the second stream. The first stream includes liquid sulfur containing polysulfides and dissolved hydrogen sulfide. The second stream includes an inert gas or a low oxygen-containing gas.

Abstract: Disclosed is a reactor, a retained catalyst structure, and a method for increasing the rate of decomposition of polysulfides and removal of hydrogen sulfide in liquid sulfur. The reactor, the retained catalyst structure, and the method include a retained catalyst structure arranged and disposed for contacting a first stream and a second stream in a reactor including a catalyst. The catalyst increases the rate of decomposition of polysulfides and facilitates the removal of hydrogen sulfide in the liquid sulfur of the first stream with the second stream. The first stream includes liquid sulfur containing polysulfides and dissolved hydrogen sulfide. The second stream includes an inert gas or a low oxygen-containing gas.

Abstract: A method and apparatus for degasification of Claus-derived sulfur by the use of gas-liquid eductor using the liquid sulfur as the ejector motive force and ambient sweep air as the active degassing agent combined with a static mixer and packed bed for promoting the intimate contact of the air and the sulfur.

Abstract: A process is disclosed for removing hydrogen sulfide from liquid sulfur including: passing a liquid sulfur feed comprising liquid sulfur and hydrogen sulfide to a vaporizer; vaporizing at least a portion of the liquid sulfur feed in the vaporizer to thereby form a first vapor stream comprising gaseous sulfur and gaseous hydrogen sulfide; partially condensing the first vapor stream in a condenser to form a liquid product stream comprising liquid sulfur and a second vapor stream comprising hydrogen sulfide; wherein the liquid product stream has a lower concentration of hydrogen sulfide than the liquid sulfur feed.

Abstract: A process is disclosed for removing hydrogen sulfide from liquid sulfur including: passing a liquid sulfur feed comprising liquid sulfur and hydrogen sulfide to a vaporizer; vaporizing at least a portion of the liquid sulfur feed in the vaporizer to thereby form a first vapor stream comprising gaseous sulfur and gaseous hydrogen sulfide; partially condensing the first vapor stream in a condenser to form a liquid product stream comprising liquid sulfur and a second vapor stream comprising hydrogen sulfide; wherein the liquid product stream has a lower concentration of hydrogen sulfide than the liquid sulfur feed.

Abstract: Disclosed is a reactor, a structured packing, and a method for increasing the rate of decomposition of polysulfides and oxidation of polysulfides and hydrogen sulfide in liquid sulfur. The reactor, the structured packing, and the method involve a structured packing for contacting a first stream and a second stream in a reactor including a catalyst. The catalyst increases the rate of decomposition of polysulfides and oxidation of polysulfides and hydrogen sulfide in the liquid sulfur of the first stream with the second stream. The first stream includes liquid sulfur containing polysulfides and dissolved hydrogen sulfide. The second stream includes an oxygen-containing gas.

Abstract: Systems and methods for the collection of liquid sulfur with integrated degassing are described, wherein the system and methods include the use of one or more liquid jet pumps or eductors employing a pumped liquid sulfur recycle stream as motive fluid to boost sulfur rundown pressure. The new invention eliminates piping constraints inherent with conventional gravity flow, thus permitting location of the sulfur collection vessel above ground and remote from the sulfur recovery unit. In addition, the described methods provide entrainment and enough agitation in the liquid sulfur such that simultaneously degassing occurs within the sulfur collection piping and associated systems described herein. The instant systems and methods are integrated with the degassing system, meaning that the sulfur will be initially degassed during the collection process, and then further degassing occurs by the methods described herein.

Abstract: A system for removing contaminant from sulfur to a level that allows re-processed sulfur to reenter a supply chain. The system melts contaminated sulfur, filters the molten sulfur, and then dispenses the less contaminated sulfur after filtering it. More specifically, solid contaminated sulfur enters a settling tank, while heated molten sulfur circulates through the system and the settling tank. The circulation of the molten sulfur assists in the melting process of the solid sulfur, allowing the solid sulfur to melt efficiently. In preferred embodiments of the system, the system uses steam jacked sulfur lines and steam tracing to maintain temperatures for melting the solid sulfur and maintaining the molten state of the sulfur circulating through the system. Furthermore, the system comprises a programmable logic controller, which controls motors, pumps, valves, and environment monitoring instruments.

Abstract: A process comprising contacting a solvent with a sulfur stream comprising sulfur and a contaminant, separating the sulfur from at least some of the contaminant and at least some of the solvent, thereby producing a decontaminated sulfur stream, contacting the decontaminated sulfur with a washing agent, and separating the sulfur from at least some of the washing agent, thereby producing a purified sulfur stream comprising elemental sulfur.

Abstract: A process comprising contacting a solvent with a sulfur stream comprising sulfur and a contaminant, separating the sulfur from at least some of the contaminant and at least some of the solvent, thereby producing a decontaminated sulfur stream, contacting the decontaminated sulfur with a washing agent, and separating the sulfur from at least some of the washing agent, thereby producing a purified sulfur stream comprising elemental sulfur.

Abstract: Sulfur sought to be removed from deposits formed at a subterranean gas producing well is dissolved in a high boiling point relatively pure and low cost non-aqueous solvent. The dissolved sulfur is then removed from the solvent by lowering the temperature of the solution to precipitate the sulfur which is then separated and recovered as a relatively pure product. After additional processing the regenerated solvent is injected back down the well hole to dissolve additional sulfur and reutilized in successive cycles of the process. An amine or solubilizing agent may be added to the non-aqueous solvent, and an oxidizing agent may be provided during the process to oxidize polysulfides to sulfur.

Abstract: A relatively simple and energy efficient multiple stage cryogenic process for the purification of a hydrogen-rich stream by the removal of acid gases, mainly CO2 and H2S, by method of autorefrigeration and delivering or producing those acid gases, mainly CO2, at pressure sufficiently high for disposal by containment, commonly known as sequestration. Autorefrigeration is comprised of (a) condensing acid gases from the syngas stream by cooling the syngas, (b) separating the liquefied acid gases from the syngas, and (c) evaporating the liquefied acid gases at a pressure lower than that of the syngas to provide cooling. The process is composed of multiple autorefrigeration stages to generate multiple acid gas product streams with a pressure as high as practical in each stream so as to lessen the power needed to pressurize the acid gas streams for sequestration.

Abstract: A method for distillation of sulfur for preparing radioactive phosphorous nuclide includes the steps of: charging powdered sulfur into a target tube designed to have an upper and a bottom neck; degassing the target tube to form a vacuum therein, followed by heating the upper neck to seal the target tube; irradiating neutrons into the sealed target tube to produce radioactive phosphorous nuclide; heating the distillation zone to distill the remaining unreacted sulfur; and cleaving the target tube at the bottom neck to separate the distillation and the cooling zone from each other, the separated zones containing the radioactive phosphorous nuclide and the unreacted sulfur, respectively, whereby the radioactive phosphorous nuclide of high purity can be prepared while the sulfur can be recovered at high efficiency.

Abstract: A vessel is mounted external to a liquid sulfur storage pit for degassing liquid sulfur at atmospheric pressure. Liquid sulfur is re-circulated from the pit to a static mixing device extending from a head space to the liquid sulfur which provides intimate contact of the liquid sulfur as it flows downwardly and sweep air flowing through the head space above the pit. Further, the static mixing device prevents free fall of liquid sulfur and the hazards of static electricity associated therewith. Use of a heat traced gas outlet induces flow of sweep gas from the system, obviating the need for a steam eductor or blower.

Abstract: The present invention provides a method for inhibiting the evolution of H2S from sulfhydryl compounds in molten sulfur by using scavenging agents such as anhydrides and polymers thereof, conjugated ketones, carbonates, epoxides, monoesters and diesters of unsaturated dicarboxylic acids and polymers of these esters, and the like and mixtures thereof. In one embodiment, it is preferred that the scavenging agent is in liquid form at contact temperature with the molten sulfur. In another embodiment, the scavenging agent may be atomized into the vapor space over the molten sulfur to contact the sulfur with the agent.

Abstract: Disclosed herein is a system and process for treating sulfur at an elevated pressure. Embodiments of the system comprise a vessel into which the sulfur is injected and a device for alleviating the pressure of the sulfur.

Abstract: A process for the purification of sulfur that contains impurities is described. In a contact zone (1), solid sulfur (2) is brought into contact with at least one organic solvent (16) that is selected from the group that is formed by monohydric alcohols with 8 to 40 carbon atoms, polyols that comprise 2 to 8 hydroxyl groups and that have 8 to 40 carbon atoms, polyalkylene glycols and polyalkylene glycol ethers under suitable conditions for carrying out the melting of sulfur. The supply of sulfur is halted, and the mixture that is obtained is allowed to decant so as to produce the segregation or liquid/liquid separation of the purified liquid sulfur that is obtained and organic solvent that contains impurities. At least a portion of the purified liquid sulfur is drawn off (8) from the lower portion of the contact zone. The solvent can be used until it is saturated with products that are dissolved and/or in suspension, no longer allowing the separation of the liquid sulfur and the solvent.

Abstract: A sulfurous compound such as a sulfur-rich “float” (concentrate) obtained by flotation of the residue from the leaching of a zinc concentrate is heated at a temperature not lower than the melting point of sulfur but below its boiling point, preferably not higher than 200° C., more preferably not higher than 140° C., and the evolving gas containing sulfur vapor is cooled at a temperature less than the melting point of sulfur, preferably at ordinary temperature, to condense sulfur.

Abstract: A method for removing hydrogen sulfide from liquid sulfur, comprising introducing liquid sulfur containing hydrogen sulfide in an upstream portion of a conduit. The conduit has a fluid outlet in a downstream portion thereof located within a lower portion of a first vessel. The method further includes causing the liquid sulfur to flow through the conduit outlet into the first vessel, up an annulus formed between the conduit and the first vessel and overflow through an outlet positioned at an upper portion of the first vessel into a second vessel which is connected to a liquid sulfur pit operating at atmospheric pressure. Air is introduced into the liquid sulfur located in the annulus at a point between the conduit outlet and said first vessel outlet. The flows concurrently to the direction of flow of the liquid sulfur in the annulus and out of the first vessel outlet into the second vessel thereby removing hydrogen sulfide from the liquid sulfur in the annulus.

Abstract: A method for removing hydrogen sulfide from liquid sulfur, comprising introducing liquid sulfur containing hydrogen sulfide in an upstream portion of a conduit. The conduit has a fluid outlet in a downstream portion thereof located within a lower portion of a first vessel. The method further includes causing the liquid sulfur to flow through the conduit outlet into the first vessel, up an annulus formed between the conduit and the first vessel and overflow through an outlet positioned at an upper portion of the first vessel into a second vessel which is connected to a liquid sulfur pit operating at atmospheric pressure. Air is introduced into the liquid sulfur located in the annulus at a point between the conduit outlet and said first vessel outlet. The flows concurrently to the direction of flow of the liquid sulfur in the annulus and out of the first vessel outlet into the second vessel thereby removing hydrogen sulfide from the liquid sulfur in the annulus.

Abstract: The present invention provides a method for inhibiting the evolution of H2S from sulfhydryl compounds in molten sulfur by using scavenging agents such as anhydrides and polymers thereof, conjugated ketones, carbonates, epoxides, monoesters and diesters of unsaturated dicarboxylic acids and polymers of these esters, and the like and mixtures thereof. In one embodiment, it is preferred that the scavenging agent is in liquid form at contact temperature with the molten sulfur. In another embodiment, the scavenging agent may be atomized into the vapor space over the molten sulfur to contact the sulfur with the agent.

Abstract: Disclosed is a method in which hydrogen sulfide-containing liquid sulfur is introduced into a containment vessel to partially fill the containment vessel and create a hydrogen sulfide-containing liquid sulfur phase and a hydrogen sulfide-containing vapor phase. A portion of the hydrogen sulfide-containing liquid sulfur phase is then treated to produce a liquid sulfur-containing phase and a gaseous hydrogen sulfide-containing phase, such that the gaseous hydrogen sulfide-containing phase has a pressure of at least about 60 psig. A portion of the hydrogen sulfide-containing vapor phase is then withdrawn from the containment vessel using at least one eductor driven by a motive fluid, where the motive fluid is the gaseous hydrogen sulfide-containing phase from the container vessel. The hydrogen sulfide-containing waste gas stream exiting the eductor is then treated to reduce the hydrogen-sulfide content of the waste gas.

Abstract: A method for distillation of sulfur for preparing radioactive phosphorous nuclide includes the steps of: charging powdered sulfur into a target tube designed to have an upper and a bottom neck; degassing the target tube to form a vacuum therein, followed by heating the upper neck to seal the target tube; irradiating neutrons into the sealed target tube to produce radioactive phosphorous nuclide; heating the distillation zone to distill the remaining unreacted sulfur; and cleaving the target tube at the bottom neck to separate the distillation and the cooling zone from each other, the separated zones containing the radioactive phosphorous nuclide and the unreacted sulfur, respectively, whereby the radioactive phosphorous nuclide of high purity can be prepared while the sulfur can be recovered at high efficiency.

Abstract: Elemental sulfur is recovered from the hydrogen sulfide present in natural gases and other process gases by treating the hydrogen sulfide-containing gas in a series arrangement of a liquid-phase reactor; a furnace and a sulfur dioxide absorber. The hydrogen sulfide-containing gas and a sulfur dioxide-containing gas are fed into the liquid-phase reactor where they are dissolved into a solvent, such as polyglycol monoethers, diethers of ethylene glycol, diethers of propylene glycol, etc., and react in the presence of a catalyst, such as tertiary amine, pyridine, isoquinoline, etc., to produce elemental sulfur. The feed rates of the hydrogen sulfide-containing gas and the sulfur dioxide-containing gas are selected so that there will be an excess of hydrogen sulfide in the solvent thereby ensuring that the reaction products will include, not only the elemental sulfur, but also residual, unreacted hydrogen sulfide.

Abstract: A process for the purification of sulfur that contains impurities is described. In a contact zone (1), solid sulfur (2) is brought into contact with at least one organic solvent (16) that is selected from the group that is formed by monohydric alcohols with 8 to 40 carbon atoms, polyols that comprise 2 to 8 hydroxyl groups and that have 8 to 40 carbon atoms, polyalkylene glycols and polyalkylene glycol ethers under suitable conditions for carrying out the melting of sulfur. The supply of sulfur is halted, and the mixture that is obtained is allowed to decant so as to produce the segregation or liquid/liquid separation of the purified liquid sulfur that is obtained and organic solvent that contains impurities. At least a portion of the purified liquid sulfur is drawn off (8) from the lower portion of the contact zone. The solvent can be used until it is saturated with products that are dissolved and/or in suspension, no longer allowing the separation of the liquid sulfur and the solvent.

Abstract: The present invention provides a method for inhibiting the evolution of H2S from sulfhydryl compounds in molten sulfur by using scavenging agents such as anhydrides and polymers thereof, conjugated ketones, carbonates, epoxides, monoesters and diesters of unsaturated dicarboxylic acids and polymers of these esters, and the like and mixtures thereof. In one embodiment, it is preferred that the scavenging agent is in liquid form at contact temperature with the molten sulfur. In another embodiment, the scavenging agent may be atomized into the vapor space over the molten sulfur to contact the sulfur with the agent.

Abstract: A sulfur purification process is provided. Ash-containing molten sulfur is fed to a centrifuge and subjected to centrifugation under controlled conditions at G forces at least about 4,000 times that of gravity. The centrifugation generates a purified sulfur product and a high-solids sulfur waste stream. A solid bowl disc centrifuge provided with conical discs is used to effect the centrifugal separation of solids from liquids at G forces of at least about 4,000. The centrifuge and related piping system are equipped with heating means so as to maintain the temperature of the sulfur above about 250° F. If large volumes of sulfur are to be treated, e.g., 40 or more long tons per hour, a pretreatment step, where the molten sulfur is first subjected to centrifugation at G forces at least about 4,000 times that of gravity in one or more nozzle bowl centrifuges, is added in order to obtain satisfactory results.