Abstract: The present invention relates to a catalytic composition which comprises an ERS-10 zeolite, a metal of group VIII, a metal of group VI and optionally one or more oxides as carrier. According to a preferred aspect, the catalytic composition also contains a metal of group II B and/or III A. The catalytic system of the present invention can be used in the upgrading of hydrocarbon mixtures having boiling ranges within the range of C4 to 250° C., preferably mixtures of hydrocarbons which boil within the naphtha range, containing impurities of sulfur, i.e. in hydrodesulfuration with the contemporaneous skeleton isomerization of olefins contained in these hydrocarbons, the whole process being carried out in a single step.

Abstract: A process where the need to circulate hydrogen through the catalyst is eliminated. This is accomplished by mixing and/or flashing the hydrogen and the oil to be treated in the presence of a solvent or diluent in which the hydrogen solubility is “high” relative to the oil feed. The type and amount of diluent added, as well as the reactor conditions, can be set so that all of the hydrogen required in the hydroprocessing reactions is available in solution. The oil/diluent/hydrogen solution can then be fed to a plug flow reactor packed with catalyst where the oil and hydrogen react. No additional hydrogen is required, therefore, hydrogen recirculation is avoided and trickle bed operation of the reactors is avoided. Therefore, the large trickle bed reactors can be replaced by much smaller tubular reactor.

Abstract: A process for using a hydrogen sensor in a liquid metal heat exchange loop in a hydrocarbon conversion process with high hydrogen permeation. The hydrogen sensor of the present invention consists essentially of a hollow nickel membrane probe in intimate contact with liquid metal. A vacuum chamber in fluid communication with the hollow nickel membrane probe through which hydrogen permeates, wherein the vacuum chamber is initially evacuated to a vacuum pressure and is in equilibrium with the vacuum chamber. The hydrogen sensor is useful for measuring the partial pressure of the hydrogen in the liquid metal to provide advisory control for the removal of hydrogen from the liquid metal exchange loop to avoid the problem of metal hydride formation and associated plugging problems.

Abstract: A process for the hydrodesulfurization of a diesel boiling range petroleum fraction wherein the hydrodesulfurization is carried out concurrently with a fractional distillation in a distillation column reactor containing a catalyst bed. The diesel is fed above the catalyst bed and hydrogen is fed below the bed. The bottoms from the distillation column reactor is then separated by fractional distillation to remove a bottoms containing most of the unconverted sulfur.

Abstract: During the hydrotreating process, hydrogen sulfide and short chain hydrocarbons such as methane, ethane, propane, butane and pentane are formed. The separation of gas from hydrotreated liquid hydrocarbons is achieved using a stripper and a flash drum. High pressure steam or nitrogen is contacted with the hydrotreated liquid hydrocarbon material. This high pressure steam strips the volatiles, i.e., hydrogen, the volatile hydrocarbons, hydrogen sulfide, and the like, from the oil. The gaseous stream is then separated and cooled to remove condensables, including primarily water, short chain hydrocarbons, and hydrogen sulfide in the water. The condensables are advantageously sent to the gasifier, where the hydrocarbons are gasified, the water moderates the gasifier temperature and increases the yield of hydrogen, and where hydrogen sulfide is routed with the produced synthesis gas to the acid gas removal process.

Abstract: A catalyst composition comprising a cobalt compound, a molybdenum compound, and a SAPO molecular sieve is used to hydrodesulfurize a hydrocarbon feed containing organic sulfur compounds.

Abstract: Process for the catalytic hydrotreating of a hydrocarbon feed stock containing silicon compounds by contacting the feed stock in presence of hydrogen with a hydrotreating catalyst at conditions to be effective in the hydrotreating of the feed stock, the improvement of which comprises the step of moisturising the hydrotreating catalyst with an amount of water added to the feed stock between 0.01 and 10 vol %.

Abstract: The present invention pertains to a process for sulfiding a catalyst composition comprising at least one hydrogenation metal component of Group VI and/or Group VIII of the Periodic Table, and an organic additive wherein the catalyst composition is first contacted with an organic liquid, followed by the catalyst being contacted with hydrogen and a sulfur-containing compound in the gaseous phase, wherein less than about 40%, preferably less than about 35%, more preferably less than about 25%, most preferably less than about 15%, of the sulfur present in the sulfided catalyst is added with the organic liquid. The process of the present invention makes it possible to use additive-containing catalysts without loss of activity in units which cannot effect gas phase start-up or are required to carry out a pressure test. The organic liquid may be a hydrocarbon with a boiling range of about 150-500° C., preferably gasoline, white spirit, diesel, gas oil, mineral lube oil, or white oil.

Abstract: There are disclosed a process for hydrogenating treating crude oil in the presence of a catalyst which comprises subjecting crude oil to consecutive hydrogenation demetalling treatment, hydrocracking treatment and hydrodesulfurization treatment to carry out hydrogenation treatment, subsequently carrying out gas-liquid separation in a gas-liquid separation step, and hydroreforming the resultant gas-phase fluid; and reformed crude oil or reformed topped crude oil which is produced by mixing the liquid-phase fluid produced from the gas-liquid separation step and the hydroreformed gas-phase fluid. The process markedly improves the qualities of kerosene and gas oil fractions in the product oil, which are expected to clear the prospectively intensified worldwide regulations on sulfur contents, aromatic components and the like.

Abstract: A process for the hydrodesulfurization of a diesel boiling range petroleum fraction wherein the hydrodesulfurization is carried out concurrently with a fractional distillation in a distillation column reactor containing a catalyst bed. The diesel is fed above the catalyst bed and hydrogen is fed below the bed. The heat for the distillation is provided by the heat of reaction of the hydrodesulfurization.

Abstract: The present invention pertains to a process for sulfiding a catalyst composition comprising at least one hydrogenation metal component of Group VI and/or Group VIII of the Periodic Table, and an S-containing organic additive wherein the catalyst composition is first contacted with an organic liquid, followed by the catalyst being contacted with hydrogen and a sulfur-containing compound in the gaseous phase. The invention also pertains to the sulfided catalyst composition and its use in hydrotreating. Catalysts sulfided by the process according to the invention show a higher activity than the same catalysts which have not been sulfided. It is preferred to carry out the process according to the invention ex situ, where it solves the problem of difficult start-up and the formation of undesirable side products in the hydrotreating unit. The organic liquid may be a hydrocarbon with a boiling range of about 150-500° C., preferably white oil, gasoline, diesel, gas oil, or mineral lube oil.

Abstract: The present invention pertains to a process for sulfiding a catalyst composition comprising at least one hydrogenation metal component of Group VI and/or Group VIII of the Periodic Table, and an S-containing organic additive wherein the catalyst composition is contacted with hydrogen and a sulfur-containing compound in the gaseous phase, characterized in that the process is carried out ex situ. By carrying out the process ex situ, the formation of undesirable side compounds in the hydrotreating unit is prevented. Additionally, the reactor suffers less downtime, and the start-up of the reactor may be simplified. The S-containing organic additive preferably is a mercapto-compound, more preferably a mercaptocarboxylic acid represented by the general formula HS—R1-COOR, wherein R1 stands for a divalent hydrocarbon group with 1-about 10 carbon atoms and R stands for a hydrogen atom, an alkali metal, an alkaline earth metal, ammonium, or a linear or branched alkyl group having 1 to about 10 carbon atoms.

Abstract: The invention relates to a process for the sulphurization of catalysts for the hydrotreating of hydrocarbon feedstocks. According to the invention, a small amount of at least one orthophthalic acid ester is added to the sulphurization agent used to sulphurize the catalyst. The catalysts thus sulphurized prove to be more active than conventionally sulphurized catalysts.

Abstract: The present invention pertains to a process for sulfiding a catalyst composition comprising at least one hydrogenation metal component of Group VI and/or Group VIII of the Periodic Table, and an S-containing organic additive wherein the catalyst composition is contacted in two steps with hydrogen and a sulfur-containing compound in the gaseous phase, with the first step being performed at a temperature which is lower than that of the second step. Catalysts sulfided by the process according to the invention show a higher activity than the same catalysts which have been sulfided in a one-step process. It is preferred to carry out the process according to the invention ex situ, where it solves the problem of difficult start-up and the formation of undesirable side products in the hydrotreating unit.

Abstract: The present invention relates to trioctahedral phyllosilicates 2:1 of a stevensite or kerolite type containing fluorine, fluorinated in synthesis in an acid medium in the presence of hydrofluoric acid and/or another source of fluoride anions. The invention also relates to a method of preparation to obtain said phyllosilicate. The existence of gaps enables adjustable quantities of cations to be incorporated in the gaps and hence the exchange capacity of said solids to be modulated. These phyllosilicates, raw from synthesis or after post-synthesis modification, may be incorporated in the composition of catalysts used to convert hydrocarbons, in particular for hydrocracking.

Abstract: A feed stream is processed in a hydrotreating reaction zone and then the hydrotreating effluent is separated into two fractions in a first high pressure separator. The vapor fraction is passed into a second high pressure separator. A controlled portion of the liquid fraction from the first high pressure separator and the liquid from the second high pressure separator are passed into a hydrocracking zone. The remaining portion of the liquid from the first high pressure separator is passed into the product recovery zone.

Abstract: A process for the hydrodesulfurization of a cracked naphtha stream is disclosed where very little of the valuable olefins are saturated. The process is a two staged process wherein the H2S is removed between the stages to prevent recombinant mercaptans formation. Because the H2S is removed between the stages milder conditions can be used in the second stage polishing reactor to achieve the same desulfurization levels with less olefin loss.

Abstract: Novel sorbent systems for the desulfurization of cracked-gasoline are provided which are comprised of a bimetallic promotor on a particulate support such as that formed of zinc oxide and an inorganic or organic carrier. Such bimetallic promoters are formed of at least two metals of the group consisting of nickel, cobalt, iron, manganese, copper, zinc, molybdenum, tungsten, silver, tin, antimony and vanadium with the valence of same being reduced, preferably to zero. Processes for the production of such sorbents are provided wherein the sorbent is prepared from impregnated particulate supports or admixed to the support composite prior to particulation, drying, and calcination. Further disclosed is the use of such novel sorbents in the desulfurization of cracked-gasoline whereby there is achieved not only removal of sulfur but also an increase in the olefin retention in the desulfurized product. Such sorbents can also be utilized for the treatment of other sulfur-containing streams such as diesel fuels.

Abstract: A two step sulfur removal for treatment of hydrocarbonaceous fuel intended for use in a fuel cell comprising a mild hydrotreating step followed by an extraction step reduces the sulfur content in fuel to 5 ppm total sulfur or less and a fuel processor suitable for carrying out the process.

Abstract: A process for treating a full boiling range naphtha is disclosed in which the mercaptans and diolefins are removed simultaneously in a debutanizer distillation column reactor. The mercaptans are reacted with the diolefins to form sulfides which are higher boiling than the C4 and lighter portion of the naphtha which is taken as overheads. The higher boiling sulfides are removed as bottoms along with any C5 and heavier materials. The bottoms are preferably taken to a splitter where a portion is taken as overheads and a heavier portion is recovered with the sulfides. This reduced volume of naphtha may be hydrogenated to convert the sulfides to H2S and diolefins, which may be hydrogenated to olefins and alkanes.

Abstract: A two step sulfur removal comprising a mild hydrotreating step followed by an extraction step reduces the sulfur content in gasoline to a very low level without significantly reducing the octane of the gasoline.

Abstract: Sulfur-containing petroleum feeds are desulfurized by contacting the feeds with staged addition of sodium metal at temperatures of at least about 250° C. in the presence of excess hydrogen to sodium metal. The formation of Na2S is substantially suppressed and the formation of NaSH is promoted in the desulfurization process.

Abstract: A process for the hydrodesulfurization (HDS) of multiple condensed ring heterocyclic organosulftir compounds found in petroleum and petrochemical streams. HDS is preferably conducted in a mixed bed containing: (a) a Ni-based catalyst on an inorganic refractory support, and (b) a hydrogen sulfide sorbent material. The desulfurized stream can then be passed to further processing, including aromatics saturation and/or ring opening.

Abstract: A process for hydroprocessing a fluid stream containing at least hydrogen and hydrocarbons. The process uses a hydrocarbon-selective membrane to reduce the concentration of hydrocarbons and contaminants in the hydrogen stream recycled to the hydroprocessing reactor. The membrane can operate in the presence of hydrogen sulfide. The process also provides the opportunity for increased NGL recovery from the hydrocarbon-enriched membrane permeate stream.

Abstract: Processes and apparatus for providing improved contaminant removal and hydrogen reuse in reactors, particularly in refineries and petrochemical plants. The improved contaminant removal is achieved by selective purging, by passing gases in the reactor recycle loop across membranes selective in favor of the contaminant over hydrogen.

Abstract: A method of hydrodesulfurizing a catalytic cracked gasoline which comprises providing a presulfided catalyst modified with a basic organic nitrogen compound and contacting the catalytic cracked gasoline with the catalyst.

Abstract: A method for producing a process oil in which a naphthenic rich distillate is processed through a single hydrotreating stage, the hydrotreated distillate is then solvent extracted to yield a process oil which passes IP-346 and AMES screening test.

Abstract: A gas conversion process comprising synthesis gas production, hydrocarbon synthesis and hydrogen production from the synthesis gas, also includes hydrodesulfurizing sulfur-containing hydrocarbon liquids separated from a mixture of natural gas and the liquids produced from a gas well. Hydrogen produced from the synthesis gas is used for the hydrodesulfurization, to remove at least a portion of the sulfur as H.sub.2 S and form desulfurized hydrocarbon liquid. The hydrogen is produced from the synthesis gas by one or more of (i) physical separation means such as pressure swing adsorption and (ii) chemical means such as a water gas shift reactor. The synthesized hydrocarbons are upgraded by hydroconversion. A portion of the hydrogen produced from the synthesis gas may also be used for at least one of (i) hydrocarbon synthesis catalyst rejuvenation and the hydroconversion.

Abstract: Described in a process for hydrotreating (HDT) a petroleum feedstock (1) that contains sulfur and nitrogen are the catalytic cracking of the ammonia, produced by the hydrotreating process, in a catalytic cracking furnace (F), the cooling (E2) and separating of the cracking effluent to produce an H.sub.2 S containing gas phase, the extraction of the hydrogen sulfide from said gas phase and from the hydrotreating purge gas in an amine washing unit (20), and the separation (SM) of the hydrogen from the resultant effluent. The recovered hydrogen is recycled to hydrotreating unit (HDT) via a pipe (17).

Abstract: A nitrided calcined modified zeolite composition having incorporated therein a molybdenum promoter and a co-promoter. A carburized nitrided calcined modified zeolite composition having incorporated therein a molybdenum promoter and a co-promoter. Producing the composition by incorporating a molybdenum promoter compound and a co-promoter compound into the zeolite followed by thermal treatment of the resulting zeolite with a nitriding agent and, optionally, further thermally treating the resulting zeolite with a hydrocarbon, preferably in the presence of hydrogen. A hydrotreating process that contacts a hydrocarbon-containing fluid with the nitrided, optionally carburized, calcined modified zeolite composition under a condition sufficient to effect the reduction of sulfur content in the hydrocarbon-containing fluid.

Abstract: A process for concurrently fractionating and hydrotreating of a full range naphtha stream. The full boiling range naphtha stream is subjected to simultaneous hydrodesulfurization and splitting into a light boiling range naphtha and a heavy boiling range naphtha. The two boiling range naphthas are treated separately according to the amount of sulfur in each cut and the end use of each fraction.

Abstract: A process for desulfurizing and enhancing the octane of cracked gasoline by first aromatizing the cracked gasoline and, second, hydrodesulfurizing the resulting intermediate product stream.

Abstract: A process for the alkylation of benzene contained in a mixed refinery stream is disclosed wherein the refinery stream is first subjected to hydrogenation of higher olefins prior to alkylation of the benzene with selected types and quantities of lower olefins. Streams containing sulfur compounds may be pretreated by hydrodesulfurization. All of the process steps are advantageously carried out in distillation column reactors to take advantage of that mode of operation.

Abstract: A catalyst system for selective hydrogenation of sulfur-containing and nitrogen-containing compounds of a heteroaromatic organic phase includes a mixture of a noble metal selected from Group VIII of the Periodic Table of Elements and a water-soluble ligand. A process for preparing the catalyst system and hydrogenation process using the catalyst system are also provided.

Abstract: Intensive hydrofining of petroleum fractions wherein an at least partly together with hydrogen circulate in a given direction, in at least one reactor containing at least one fixed bed of a hydrofining catalyst in solid form, is characterized in that the reactor is equipped with at least one inlet pipe for the mixture of the petroleum fraction and hydrogen and at least one outlet pipe for the resultant hydrofined petroleum fraction, and that at least one static mixer is positioned upstream from said outlet of the hydrofined petroleum fraction.

Abstract: A layered hydrocracking catalyst system has high middle distillate selectivity when used for hydrocracking a high sulfur and high nitrogen containing feedstock. The layered system comprises a first layer catalyst with contains a zeolite having a unit cell size of greater than about 24.35 Angstroms, and a second layer catalyst which contains a zeolite having a unit cell size of less than about 24.30 Angstroms. The layered system is particularly beneficial in terms of catalyst life and product selectivity for reactors operated under conditions of a high temperature profile.

Abstract: A catalyst system for treating sulfur and nitrogen contaminated hydrocarbon feedstock includes a matrix, at least one support medium substantially uniformly distributed through said matrix and comprising a silica alumina molecular sieve material having a composition xSiO.sub.2 :Al.sub.2 O.sub.3 :yP.sub.2 O.sub.5, wherein x is at least about 0.1, a first catalytically active metal phase supported on said support medium, said first catalytically active metal phase comprising a first metal and a second metal each selected from group VIII of the Periodic Table of Elements, said first metal being different from said second metal, a second catalytically active metal phase supported on said matrix, said second catalytically active metal phase comprising a third metal and a fourth metal each selected from group VIII of the Periodic Table of Elements and a fifth metal selected from group VIb of the Periodic Table of Elements, said third metal being different from said fourth metal.

Abstract: Economies in capital cost and operation of a hydrocarbon refining unit for desulfurization of diesel fuel are provided by a heat exchange sequence which eliminates the need for a fired heater on the product fractionation zone. The feed to the fractionation zone is heat exchanged twice against both the effluent bottoms stream of the fractionation zone and the reaction zone effluent stream.

Abstract: An aromatic solvent together with methods for its preparation are described. A composition includes a paraffin fraction in an amount of from approximately 9 LV % to approximately 15 LV %; a naphthene fraction in an amount of from approximately 35 LV % to approximately 55 LV %; and an alkylbenzene fraction in an amount of from approximately 8 LV % to approximately 16 LV %. The solvent provides advantages in that the high solvency that is typical of an aromatic solvent is combined with a narrow distillation range, a high flash point and higher boiling range that is typical of an aliphatic solvent.

Abstract: A process for desulfurizing catalytically cracked gasoline containing sulfur compounds and olefin components, which comprises the steps of:1) first desulfurizing the catalytically cracked gasoline in the presence of a hydrodesulfurization catalyst at a desulfurization rate of 60 to 90%, a reaction temperature of 200 to 350.degree. C., a hydrogen partial pressure of 5 to 30 kg/cm.sup.2, a hydrogen/oil ratio of 500 to 3,000 scf/bbl, and a liquid hourly space velocity of 2 to 10 1/hr, said first desulfuriing step comprising supplying a feed having a hydrogen sulfide vapor concentration of not more than 0.1% by volume, and2) next desulfurizing the treated oil obtained in the first step in the presence of a hydrodesulfurization catalyst at a desulfurization rate of 60 to 90%, a reaction temperature of 200 to 300.degree. C., a hydrogen partial pressure of 5 to 15 kg/cm.sup.

Abstract: Hydrocarbon feeds are upgraded by contact of the stream under hydrodesulfurization (HDS) conditions with a catalyst system comprising a sulfided, transition metal promoted tungsten/molybdenum HDS catalyst, e.g., Ni/Co--Mo/Al.sub.2 O.sub.3 and a solid acid catalyst which is effective for the isomerization/disproportionation/transalkylation of alkyl substituted, condensed ring heterocyclic sulfur compounds present in the feedstream, e.g. zeolite or a heteropolyacid compound. Isomerization, disproportionation and transalkylation reactions convert refractory sulfur compounds such as 4- or 4,6-alkyl dibenzothiophenes into corresponding isomers or disproportionated isomers which can be more readily desulfurized by conventional HDS catalysts to H.sub.2 S and other products.

Abstract: A process for the alkylation of benzene contained in a mixed refinery stream is disclosed wherein the refinery stream is first subjected to hydrogenation of higher olefins prior to alkylation of the benzene with selected types and quantities of lower olefins. Streams containing sulfur compounds may be pretreated by hydrodesulfurization. All of the process steps are advantageously carried out in distillation column reactors to take advantage of that mode of operation.

Abstract: The present invention relates to a continuous in-situ process for reducing the viscosity, corrosivity and density of heavy oils comprising the steps of (a) contacting a heavy oil with an aqueous alkaline earth, Group IIA metal hydroxide at a temperature of about 380.degree. to about 450.degree. C. for a time sufficient to form the corresponding alkaline earth metal sulfide, recovering the reduced sulfur feed and regenerating the alkaline metal hydroxide for recycle to treat additional feed. Beneficially, the process removes heteroatoms (sulfur and nitrogen).

Abstract: A light hydrocarbon stream, such as a C.sub.3 -C.sub.5 stream recovered from an FCC unit, is catalytically treated for the selective hydrogenation of dienes and for the removal of mercaptans by thioetherification. The effluent of the reaction zone is fractionated to remove light ends and thioethers in a dual section fractionation zone, with the interconnection of the sections facilitating a reduction in capital and operating costs.

Abstract: A process for hydrotreating petroleum fractions early in the refining process by employing catalyst prepared as components of distillation structures or as contained beds of catalyst in atmospheric distillation columns and or side draw columns. For example, a crude petroleum is hydrotreated by taking side streams from an atmospheric distillation column and the vacuum gas oil from a vacuum distillation column which are individually fed to separate desulfurizations, preferably in distillation column reactors containing a hydrodesulfurization catalyst. The overheads from each of the distillation column reactors is returned to the atmospheric column and the bottoms from each distillation column reactor is withdrawn as hydrotreated product. The process may also be used for upgrading the effluent from a fluid catalytic cracking unit, preferably operated as a catalytic distillation reactor.

Abstract: A hydrocarbon feedstock is brought into contact with activated carbon at a temperature of about 300.degree. to about 650.degree. C. and a pressure of about 0-1000 psig in the absence of added hydrogen to reduce the content of aromatic, sulfur and nitrogen compounds contained therein and to simultaneously lower the specific gravity thereof.

Abstract: A process for the hydrodesulfurization of petroleum streams is disclosed wherein the sulfur containing petroleum stream is contacted along with hydrogen at a partial pressure of less than 70 psig in a distillation column reactor containing a hydrodesulfurization catalyst in the form of a catalytic distillation structure.

Abstract: A light hydrocarbon stream, such as a C.sub.3 -C.sub.5 stream recovered from an FCC unit, is catalytically treated for the selective hydrogenation of dienes and for the removal of mercaptans by thioetherification. The effluent of the reaction zone is fractionated to remove light ends and thioethers in a dual section fractionation zone, with the interconnection of the sections facilitating a reduction in capital and operating costs.

Abstract: This invention makes possible substantially continuous flow of uniformly distributed hydrogen and hydrocarbon liquid across a densely packed catalyst bed to fill substantially the entire volume of a reactor vessel by introducing the fluids as alternate annular rings of gas and liquid (i.e. a mixture of liquid hydrocarbon and a hydrogen-containing gas) at a rate insufficient to levitate or ebullate the catalyst bed. Catalyst are selected by density, shape and size at a design feed rate of liquids and gas to prevent ebullation of the packed bed at the design feed rates. Catalysts are selected by measuring bed expansion, such as in a large pilot plant run, with hydrocarbon, hydrogen, and catalyst at the design pressures and flow velocities. The liquid and gas components of the feed flow into the bed in alternate annular rings across the full area of the bed.

Abstract: The present invention relates to a continuous in-situ process for the removal of organically bound sulfur existing as mercaptans, sulfides and thiophenes comprising the steps of (a) contacting a heavy oil with aqueous sodium hydroxide at a temperature of about 380.degree. C. to about 450.degree. C. for a time sufficient to form sodium sulfide, and (b) steam stripping the sodium sulfide of step (a) at a temperature sufficient to convert said sodium sulfide to sodium hydroxide and recirculating the sodium hydroxide from step (b) back to step (a) and removing hydrogen sulfide and the metals from the organically bound metal complex of the sodium sulfide to convert it back to sodium hydroxide, in which case the sulfur may be recovered as H.sub.2 S rather than the metal sulfide. Optionally, molecular hydrogen may be added in the first step. The present invention is useful in removing organically bound sulfur that has been recognized to be difficult to remove, such as thiophenes.