US20140338254A1

US20140338254A1 - Process For Reducing Fouling In The Processing Of Liquid Hydrocarbons

Info

Publication number: US20140338254A1
Application number: US14/375,077
Authority: US
Inventors: Michael Feustel; Dominko Andrin; Matthias Krull
Original assignee: Clariant Finance BVI Ltd
Current assignee: Clariant International Ltd
Priority date: 2012-01-31
Filing date: 2013-01-29
Publication date: 2014-11-20
Also published as: EA201400774A1; JP2015508829A; CN104066820A; EP2809750A1; WO2013113491A1; EA025207B9; CN104066820B; KR20140128289A; BR112014011956A2; CA2863267A1; EA025207B1; JP6095131B2

Abstract

The present invention relates to the use of a polyester which bears hydroxyl groups and is preparable by polycondensation of a polyol containing two primary OH groups and at least one secondary OH group with a dicarboxylic acid or anhydride thereof or ester thereof bearing a C₁₆- to C₄₀₀-alkyl radical or a C₁₆- to C₄₀₀-alkenyl radical as an antifoulant in the thermal treatment of liquid hydrocarbon media in the temperature range from 100 to 550 DEG C.

Description

The present invention relates to a process for reducing fouling by liquid hydrocarbons during the processing thereof at relatively high temperatures, for example in refinery operations.
In the course of processing, hydrocarbons such as crude oil and intermediates in mineral oil processing, for example, but also petrochemicals and petrochemical intermediates, are generally heated to temperatures between 100° C. and 550° C., frequently between 200° C. and 550° C. In heating and heat exchange systems too, hydrocarbons used as heat carriers are exposed to such temperatures. In virtually all these cases, hydrocarbons used form unwanted breakdown products or by-products at elevated temperatures, which can separate out and accumulate at the hot surfaces of the heat transferers. The formation of these deposits is generally attributed to the presence of comparatively unstable compounds, for example oxidized and/or oxidizable hydrocarbons and olefinically unsaturated compounds, but this is also blamed on high molecular weight organic compounds and inorganic impurities. In specific cases, the extraneous substances which separate out and accumulate may even already be present in the raw material or precursor to be processed. In the specific case of mineral oil distillation, the crude oils used for that purpose generally comprise constituents which lead to deposits, for example alkali metal and alkaline earth metal salts, compounds or complexes containing transition metals, for example iron sulfide or porphyrins, sulfur compounds, for example mercaptans, nitrogen compounds, for example pyrroles, compounds containing carbonyl groups or carboxyl groups, and polycyclic aromatics, for example asphaltenes and/or coke particles. In addition, the hydrocarbons used for processing virtually always contain small amounts of dissolved oxygen.
The deposits which form in the course of processing of the hydrocarbons at elevated temperatures and settle out on the surfaces in contact with the liquid are referred to as fouling. They form particularly on the hot insides of pipes, machines or heat exchangers.
These deposits in the processes mentioned gradually reduce the bore of pipelines and vessels, which impairs both the process throughput and heat transfer. Often, the deposits even block filter screens, valves and traps, and as a result cause plant shutdowns for cleaning and maintenance. In all cases, these deposits are additionally unwanted by-products which reduce the yield of target product and hence lower the economic viability of the plant. In the case of heat exchange systems, the deposits form an insulating layer on the surfaces present, which restricts heat transfer. Consequently, the deposits necessitate frequent shutdowns of the plants for cleaning and in some cases even replacement thereof. Accordingly, these deposits are highly undesirable in industry.
The above-described deposits are usually higher molecular weight materials, the consistency of which may range from tar through rubber and “popcorn” to coke. The composition thereof may differ in nature and in many cases defies any detailed analysis. They often contain a combination of carbonaceous phases which are coke-like in nature, polymers and/or condensates which are formed from the hydrocarbons or impurities present therein by various mechanisms. Further deposit constituents are frequently salts composed primarily of magnesium chloride, calcium chloride and sodium chloride. The formation of polymers and/or condensates is attributed to catalysis by metal compounds, for example compounds of copper or iron, which are present as impurities in the hydrocarbons to be processed. Metal compounds of this kind can, for example, accelerate the hydrocarbon oxidation rate by promoting degenerative chain branching. The free radicals formed can in turn trigger oxidation and polymerization reactions, which leads to the formation of resins and sediments. Often, relatively inert carbonaceous deposits are enclosed by more adhesive condensates or polymers.
Fouling deposits are equally encountered in the petrochemical field, where petrochemicals are either produced or purified. The deposits in this environment are primarily polymeric in nature and have a severe effect on the economic viability of the petrochemical operation. The petrochemical operations include, for example, the preparation of ethylene or propylene, or else the purification of chlorinated hydrocarbons. Fouling is also observed in the processing of biogenic raw materials, for example in the processing of fatty acids and derivatives thereof, for example fatty acid esters.
To prevent the formation of deposits, oil-soluble, polar nitrogen compounds are used in many cases. These are predominantly reaction products of alkyl- or alkenylsuccinic acids or anhydrides thereof with polyamines, which are optionally derivatized further.
For instance, U.S. Pat. No. 3,271,295 discloses reaction products of alk(en)ylsuccinic anhydrides with polyamines for prevention of deposits on metal surfaces in heat transferers in mineral oil refining.
WO-2011/014215 discloses the use of mono- and bisimides formed from polyamines and C₁₀- to C₈₀₀-alkyl- or -alkenylsuccinic anhydrides for prevention of deposits in plants for mineral oil refining.
U.S. Pat. No. 5,342,505 discloses the use of reaction products formed from poly(alkenyl)succinimides with epoxyalkanols as antifoulants in liquid hydrocarbons during the processing thereof at elevated temperatures
U.S. Pat. No. 5,171,420 discloses reaction products formed from alkenylsuccinic anhydrides, polyols, amines bearing hydroxyl groups, polyalkylenesuccinimides and polyoxyalkyleneamines for prevention of deposits in the course of heating of liquid hydrocarbons. In the preferred embodiments, which are demonstrated by examples, polyfunctional reagents which lead to highly branched structures are used.
The reaction products of dicarboxylic acids with polyamines typically have a relatively low molecular weight, since dicarboxylic acids, when condensed with primary amines, react preferentially to give imides and form only minor proportions, if any, of diamides. Typically, the condensation is restricted to the reaction of the primary amino groups of the polyamine with one dicarboxylic acid each, such that the result is typically molecular weights of not more than 3000 g/mol. Higher molecular weight compounds, which are desirable for the efficient reduction of fouling, are thus not obtainable in this way.
In addition, it is desirable from an economic point of view to use additives having a minimum nitrogen content. As a result, any increase in the nitrogen content of the products obtained in the thermal treatment of liquid hydrocarbons and any occurrence of by-products and residues can be avoided. Both in the thermal treatment of liquid hydrocarbons themselves and in the subsequent further use of the products, by-products and residues obtained, an elevated content of nitrogen compounds can lead to unwanted by-products and conversion products. For example, the combustion thereof forms nitrogen oxides.
There have been no descriptions to date of higher molecular weight oligomeric or even polymeric compounds and more particularly of higher molecular weight oligomeric or even polymeric nitrogen-free compounds for reduction of fouling by liquid hydrocarbons during the processing thereof at relatively high temperatures.
Higher molecular weight and additionally nitrogen-free condensates of alkenylsuccinic acids are obtainable only by condensation with polyols, but these have been used to date only in entirely different applications.
For instance, EP-0809623 discloses oligomeric and polymeric bisesters of alkyl- or alkenyldicarboxylic acid derivatives and polyalcohols, and the use thereof as solubilizers, emulsifiers and/or wash-active substances. Preferred polyalcohols are glycerol and oligomeric glycerols.
WO-2008/059234 discloses oligo- and polyesters based on alk(en)ylsuccinic anhydrides and polyols having at least 3 hydroxyl groups and the use thereof as emulsifiers. These polymers are additionally useful in the oilfield as foaming agents in foam drilling fluids, as kinetic gas hydrate inhibitors and as lubricants in aqueous drilling fluids.
U.S. Pat. No. 4,216,114 discloses condensation products of C_9-18-alkyl- or -alkenylsuccinic anhydrides with water-soluble polyalkylene glycols and polyols having at least 3 OH groups and the use thereof for splitting water-in-oil emulsions.
U.S. Pat. No. 3,447,916 discloses condensation polymers of alkenylsuccinic anhydrides, polyols and fatty acids for lowering the pour point of hydrocarbon oils. In these polymers, the hydroxyl groups of the polyol are very substantially esterified.
DE-A-1920849 discloses condensation polymers of alkenylsuccinic anhydrides, polyols having at least 4 OH groups and fatty acids for lowering the pour point of hydrocarbon oils. Preferably, the stoichiometry of the reactants used for the condensation is selected such that the number of moles of OH groups and carboxylic groups is the same, meaning that there is substantially complete esterification.
WO-2011/076338 discloses low-temperature additives for middle distillates comprising polycondensates of a polyol containing two primary OH groups and at least one secondary OH group with a dicarboxylic acid or anhydride thereof or ester thereof bearing a C₁₆- to C₄₀-alkyl radical or a C₁₆- to C₄₀-alkenyl radical.
The additives used according to the prior art for suppression or at least for reduction of fouling often show deficits in the efficacy thereof.
Consequently, there is a need for additives for more efficient suppression or at least for reduction of the formation of sparingly soluble deposits on the apparatus walls in the thermal treatment of hydrocarbons, for example in processing and purifying plants, and also in heat exchange systems. These should preferably be nitrogen-free. Specifically, this need exists in the distillation of crude oils and in the further processing of the mineral oil distillation fractions which remain in distillation processes.
It has been found that, surprisingly, specific polycondensates of dicarboxylic acids or dicarboxylic anhydrides bearing C₁₆-C₄₀₀-alkyl radicals or C₁₆-C₄₀₀-alkenyl radicals and polyols having two primary and at least one secondary OH group achieve the stated objects. It has been found that higher molecular weight condensates having an essentially linear polymer backbone are particularly useful.
The invention accordingly provides for the use of a polyester which bears hydroxyl groups and is preparable by polycondensation of a polyol containing two primary OH groups and at least one secondary OH group with a dicarboxylic acid or anhydride thereof or ester thereof which bears a C₁₆- to C₄₀₀-alkyl radical or a C₁₆- to C₄₀₀-alkenyl radical as an antifoulant in the thermal treatment of liquid hydrocarbon media within the temperature range from 100 to 550° C.
The present invention further provides a method for reducing fouling in a liquid hydrocarbon medium during the thermal treatment of the medium at temperatures between 100 and 550° C., in which a polyester which bears hydroxyl groups and is preparable by polycondensation of a polyol containing two primary OH groups and at least one secondary OH group with a dicarboxylic acid or anhydride thereof or ester thereof which bears a C₁₆- to C₄₀₀-alkyl radical or a C₁₆- to C₄₀₀-alkenyl radical is added to the liquid hydrocarbon before and/or during the thermal treatment.
The invention further provides a method for increasing the service life of plants for thermal treatment of liquid hydrocarbon media within the temperature range from 100 to 550° C., in which a polyester which bears hydroxyl groups and is preparable by polycondensation of a polyol containing two primary OH groups and at least one secondary OH group with a dicarboxylic acid or anhydride thereof or ester thereof which bears a C₁₆- to C₄₀₀-alkyl radical or a C₁₆- to C₄₀₀-alkenyl radical is added to a liquid hydrocarbon medium to be processed in the plant before and/or during the thermal treatment.
The polyester bearing hydroxyl groups is generally obtained by the polycondensation of a dicarboxylic acid bearing a C₁₆- to C₄₀₀-alkyl radical or -alkenyl radical, also referred to collectively hereinafter as C₁₆-C₄₀₀-alk(en)yl radical, with the primary hydroxyl groups of the polyol. It is preferable that the secondary OH groups remain essentially unesterified. The preferred structure of the polyester bearing hydroxyl groups can thus be represented, for example, by formula (A):
in which

one of the R¹to R⁴radicals is a C₁₆-C₄₀₀-alkyl or -alkenyl radical and
the other R¹to R⁴radicals are each independently hydrogen or an alkyl radical having 1 to 3 carbon atoms,
R⁵is a C—C bond or an alkylene radical having 1 to 6 carbon atoms,
R¹⁶is a hydrocarbyl group which bears at least one hydroxyl group and has 3 to 10 carbon atoms,
n is a number from 1 to 100,
m is a number from 3 to 250,
p is 0 or 1, and
q is 0 or 1.

Preferred dicarboxylic acids which bear C₁₆-C₄₀₀-alkyl- and/or -alkenyl radicals and are suitable for preparation of the polyesters A) bearing hydroxyl groups correspond to the formula (1)
in which

one of the R¹to R⁴radicals is a C₁₆-C₄₀₀-alkyl or -alkenyl radical and
the other R¹to R⁴radicals are each independently hydrogen or an alkyl radical having 1 to 3 carbon atoms, and
R⁵is a C—C bond or an alkylene radical having 1 to 6 carbon atoms.

More preferably, one of the R¹to R⁴radicals is a C₁₆-C₄₀₀-alkyl- or -alkenyl radical, one is a methyl group and the rest are hydrogen. In a specific embodiment, one of the R¹to R⁴radicals is a C₁₆-C₄₀₀-alkyl- or -alkenyl radical and the others are hydrogen. In a particularly preferred embodiment, R⁵is a C—C single bond. More particularly, one of the R¹to R⁴radicals is a C₁₆-C₄₀₀-alkyl- or -alkenyl radical, the other R¹to R⁴radicals are hydrogen and R⁵is a C—C single bond.
The dicarboxylic acids or anhydrides thereof bearing alkyl- and/or -alkenyl radicals can be prepared by known processes. For example, they can be prepared by heating ethylenically unsaturated dicarboxylic acids with olefins or with chloroalkanes. Preference is given to the thermal addition of olefins onto ethylenically unsaturated dicarboxylic acids or anhydrides thereof (“ene reaction”), which is typically conducted at temperatures between 100 and 250° C. The dicarboxylic acids and dicarboxylic anhydrides bearing alkenyl radicals which are formed can be hydrogenated to dicarboxylic acids and dicarboxylic anhydrides bearing alkyl radicals. Dicarboxylic acids and anhydrides thereof preferred for the reaction with olefins are maleic acid and more preferably maleic anhydride. Additionally suitable are itaconic acid, citraconic acid and anhydrides thereof, and the esters of the aforementioned acids, especially those with lower C₁-C₈-alcohols, for example methanol, ethanol, propanol and butanol.
In a first preferred embodiment, one of the R¹to R⁴radicals is a linear C₁₆-C₄₀-alkyl- or -alkenyl radical. For the preparation of such dicarboxylic acids or anhydrides thereof bearing alk(en)yl radicals, preference is given to using olefins having 16 to 40 carbon atoms and especially having 18 to 36 carbon atoms, for example having 19 to 32 carbon atoms. In a particularly preferred embodiment, mixtures of olefins having different chain lengths are used. Preference is given to using mixtures of olefins having 18 to 36 carbon atoms, for example mixtures of olefins in the C₂₀-C₂₂, C₂₀-C₂₄, C₂₄-C₂₈, C₂₆-C₂₈, C₃₀-C₃₆range. Olefin mixtures may also comprise minor proportions of shorter- and/or longer-chain olefins compared to the range specified, for example hexene, heptene, octene, nonene, decene, undecene, dodecene, tetradecene and/or olefins having more than 40 carbon atoms. Preferably, the proportion of the shorter- and longer-chain olefins in the olefin mixture is, however, not more than 10% by weight. More particularly, it is between 0.1 and 8% by weight, for example between 1 and 5% by weight.
Olefins particularly preferred for the preparation of the dicarboxylic acids or anhydrides thereof bearing C₁₆-C₄₀-alk(en)yl radicals have a linear or at least substantially linear alkyl chain. “Linear or substantially linear” is understood to mean that at least 50% by weight, preferably 70 to 99% by weight, especially 75 to 95% by weight, for example 80 to 90% by weight, of the olefins have a linear component having 16 to 40 carbon atoms and especially having 18 to 36 carbon atoms, for example having 19 to 32 carbon atoms. In a specific embodiment, α-olefins, wherein the C═C double bond is at the chain end, are used. Particularly useful olefins have been found to be technical grade alkene mixtures. These contain preferably at least 50% by weight, more preferably 60 to 99% by weight and especially 70 to 95% by weight, for example 75 to 90% by weight, of terminal double bonds (α-olefins). In addition, they may contain up to 50% by weight, preferably 1 to 40% by weight and especially 5 to 30% by weight, for example 10 to 25% by weight, of olefins having an internal double bond, for example having vinylidene double bonds having the structural element R¹⁷—CH═C(CH₃)₂where R¹⁷is an alkyl radical having 12 to 36 carbon atoms and especially having 14 to 32 carbon atoms, for example having 15 to 28 carbon atoms. In addition, minor amounts of secondary components present for technical reasons, for example paraffins, may be present, but preferably not more than 5% by weight. Particular preference is given to olefin mixtures containing at least 75% by weight of linear α-olefins having a carbon chain length in the range from C₂₀to C₂₄.
In a further preferred embodiment, one of the R¹to R⁴radicals is a C₄₁-C₄₀₀-alkyl- or -alkenyl radical and especially a C₅₀- to C₃₀₀-alkyl or -alkenyl radical, for example a C₅₅- to C₂₀₀-alkyl- or -alkenyl radical. Preferably, this alk(en)yl radical is branched. Additionally preferably, these C₄₁-C₄₀₀-alk(en)yl radicals derive from polyolefins preparable by polymerization of monoolefins having 3 to 6 and especially having 3, 4 or 5 carbon atoms. Particularly preferred monoolefins as base structures for the polyolefins are propylene and isobutene, which give rise to poly(propylene) and poly(isobutene) as polyolefins. Preferred polyolefins have an alkylvinylidene content of at least 50 mol %, particularly of at least 70 mol % and especially at least 80 mol %, for example at least 85 mol %. “Alkylvinylidene content” is understood to mean the content in the polyolefins of structural units which result from compounds of the formula (3):
in which R⁶or R⁷is methyl, ethyl or propyl and especially methyl and the other group is an oligomer of the C₃-C₆-olefin. The alkylvinylidene content can be determined, for example, by means of ¹H NMR spectroscopy. The number of carbon atoms in the polyolefin is between 41 and 400. In a preferred embodiment of the invention, the number of carbon atoms is between 50 and 3000 and especially between 55 and 200. The parent polyolefins of the C₄₁-C₄₀₀-alkyl- or -alkenyl radical are obtainable, for example, by ionic polymerization and are available as commercial products (e.g. Glissopal®, polyisobutenes from BASF with different alkylvinylidene content and molecular weight). Also suitable in accordance with the invention are mixtures of various polyolefins, in which case these may differ, for example, in terms of the parent monomers, the molecular weights and/or the alkylvinylidene content.
Preferred polyesters bearing hydroxyl groups are preparable by reaction of alkyl- or alkenylsuccinic acids and/or anhydrides thereof bearing a C₁₆-C₄₀₀-alkyl- or -alkenyl radical with polyols bearing two primary and at least one secondary hydroxyl group.
Preferred polyols may be monomeric, oligomeric or polymeric in terms of structure. Polymers and oligomers are referred to collectively as polymers. R¹⁶in formula A) is preferably a radical of the formula (2)
—(CH₂)_r—(CH(OH))_t—(CH₂)_s— (2)
in which
t is a number from 1 to 6,
r and s are each independently a number from 1 to 9 and
t+r+s is a number from 3 to 10.
In monomeric polyols, n in formula A) is 1. Preferred monomeric polyols have three to 10 and especially four to six carbon atoms. They additionally have at least one and preferably 1 to 6, for example 2 to 4, secondary OH groups, but not more than one OH group per carbon atom. Suitable monomeric polyols are, for example, glycerol, 1,2,4-butanetriol, 1,2,6-trihydroxyhexane, and also reduced carbohydrates and mixtures thereof. Reduced carbohydrates are understood here to mean polyols which derive from carbohydrates and bear two primary and two or more secondary OH groups. Particularly preferred reduced carbohydrates have 4 to 6 carbon atoms. Examples of reduced carbohydrates are erythritol, threitol, adonitol, arabitol, xylitol, dulcitol, mannitol and sorbitol. A particularly preferred monomeric polyol is glycerol.
In polymeric polyols, n in formula A) is a number from 2 to 100, preferably a number from 2 to 50, more preferably a number from 3 to 25 and especially a number from 4 to 20. Preferred polymeric polyols have six to 150, especially eight to 100 and particularly nine to 50 carbon atoms. They bear at least one, preferably two to 50 and especially three to 15 secondary OH groups, but not more than one OH group per carbon atom. Polymeric polyols suitable in accordance with the invention are preparable, for example, by polycondensation of polyols having two primary and at least one secondary OH group. A preferred polymeric polyol is poly(glycerol). “Poly(glycerol)” is especially understood to mean structures derivable by polycondensation from glycerol. The condensation level of poly(glycerols) preferred in accordance with the invention is between 2 and 50, more preferably between 3 and 25 and especially between 4 and 20, for example between 5 and 15.
The preparation of poly(glycerol) is known in the prior art. It can be prepared, for example, via addition of 2,3-epoxy-1-propanol (glycide) onto glycerol. In addition, poly(glycerol) can be prepared by polycondensation, as known per se, of glycerol. The reaction temperature in the polycondensation is generally between 150 and 300° C., preferably between 200 and 250° C. The polycondensation of glycerol is normally conducted at atmospheric pressure. Catalyzing acids include, for example, HCl, H₂SO₄, organic sulfonic acids or H₃PO₄; catalyzing bases include, for example, NaOH or KOH. The catalysts are added to the reaction mixture preferably in amounts of 0.01 to 10% by weight, more preferably 0.1 to 5% by weight, based on the weight of the reaction mixture. The polycondensation of glycerol can be conducted without solvent, or else in the presence of solvent. If the polycondensation is effected in the presence of solvent, the proportion thereof in the reaction mixture is preferably 0.1 to 70% by weight, for example 10 to 60% by weight. Preferred organic solvents here are the solvents also used and preferred for the condensation of the dicarboxylic acid, anhydride thereof or ester thereof bearing alk(en)yl radicals with the polyol. The polycondensation of glycerol generally takes 3 to 10 hours. This process is also applicable mutatis mutandis to the polycondensation of other polyols.
The dicarboxylic acid, anhydride thereof or ester thereof bearing alk(en)yl radicals are converted to the polyester bearing hydroxyl groups preferably in a molar ratio of 1:2 to 2:1, more preferably in a molar ratio of 1:1.5 to 1.5:1 and especially in a molar ratio of 1:1.2 to 1.2:1, for example in a equimolar ratio. More preferably, the conversion is effected with an excess of polyol. In this context, molar excesses of 1 to 10 mol % and especially 1.5 to 5 mol % based on the amount of dicarboxylic acid used have been found to be particularly useful.
The polycondensation of the dicarboxylic acid, anhydride thereof or ester thereof bearing alkyl radicals with the polyol is effected preferably by heating C₁₆-C₄₀₀-alkyl- or -alkenyl-substituted dicarboxylic acid or the anhydride or ester thereof together with the polyol to temperatures above 100° C. and preferably to temperatures between 120 and 320° C., for example to temperatures between 150 and 290° C. For adjustment of the molecular weight, which is important for the efficacy of the polyester bearing hydroxyl groups, it is typically necessary to remove the water of reaction or the alcohol of reaction, which can be effected, for example, by distillative removal. Azeotropic removal by means of suitable organic solvents is also suitable for this purpose. Preferred solvents for the polycondensation of the dicarboxylic acid, anhydride thereof or ester thereof bearing alk(en)yl radicals with the polyol are relatively high-boiling, low-viscosity solvents. Particularly preferred solvents are aliphatic and aromatic hydrocarbons and mixtures thereof. Aliphatic hydrocarbons preferred as solvents have 9 to 20 carbon atoms and especially 10 to 16 carbon atoms. They may be linear, branched and/or cyclic. They are preferably saturated or at least substantially saturated. Aromatic hydrocarbons preferred as solvents have 7 to 20 carbon atoms and especially 8 to 16, for example 9 to 13, carbon atoms. Preferred aromatic hydrocarbons are mono-, di-, tri- and polycyclic aromatics. In a preferred embodiment, these bear one or more, for example two, three, four, five or more, substituents. In the case of a plurality of substituents, these may be the same or different. Preferred substituents are alkyl radicals having 1 to 20 and especially having 1 to 5 carbon atoms, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, n-pentyl, isopentyl, tert-pentyl and neopentyl radical. Examples of suitable aromatics are alkylbenzenes and alkylnaphthalenes. For example, aliphatic and/or aromatic hydrocarbons or hydrocarbon mixtures, e.g. petroleum fractions, kerosene, decane, pentadecane, toluene, xylene, ethylbenzene or commercial solvent mixtures such as Solvent Naphtha, Shellsol® AB, Solvesso® 150, Solvesso® 200, Exxsol® products, ISOPAR® products and Shellsol® D products, are particularly suitable. As well as the solvents based on mineral oils, solvents based on renewable raw materials and synthetic hydrocarbons obtainable, for example, from the Fischer-Tropsch process, are suitable as solvents. Also suitable are mixtures of the solvents mentioned. If the polycondensation is effected in the presence of solvent, the proportion thereof in the reaction mixture is preferably 1 to 75% by weight and especially 10 to 70% by weight, for example 20 to 60% by weight. The condensation is preferably conducted without solvent.
For acceleration of the polycondensation, it has often been found to be useful to conduct the polycondensation in the presence of homogeneous catalysts, heterogeneous catalysts or mixtures thereof. Preferred catalysts here are acidic inorganic, organometallic or organic catalysts and mixtures of two or more of these catalysts.
Acidic inorganic catalysts in the context of the present invention are, for example, sulfuric acid, phosphoric acid, phosphonic acid, hypophosphorous acid, aluminum sulfate hydrate, alum, acidic silica gel and acidic aluminum hydroxide. Additionally usable as acidic inorganic catalysts are, for example, aluminum compounds of the formula Al(OR¹⁵)₃and titanates of the formula Ti(OR¹⁵)₄, where the R¹⁵radicals may each be the same or different and are each independently selected from C₁-C₁₀-alkyl radicals, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, sec-pentyl, neopentyl, 1,2-dimethylpropyl, isoamyl, n-hexyl, sec-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl or n-decyl, C₃-C₁₂-cycloalkyl radicals, for example cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl, cycloundecyl and cyclododecyl; preference is given to cyclopentyl, cyclohexyl and cycloheptyl. Preferably, the R¹⁵radicals in Al(OR¹⁵)₃and Ti(OR¹⁵)₄are each the same and are selected from isopropyl, butyl and 2-ethylhexyl.
Preferred acidic organometallic catalysts are, for example, selected from dialkyltin oxides (R¹⁵)₂SnO where R¹⁵is as defined above. A particularly preferred representative of acidic organometallic catalysts is di-n-butyltin oxide, commercially available as “oxo-tin” or as the Fascat® brand.
Preferred acidic organic catalysts are acidic organic compounds having, for example, phosphate groups, sulfo groups, sulfate groups or phosphonic acid groups. Particularly preferred sulfonic acids contain at least one sulfo group and at least one saturated or unsaturated, linear, branched and/or cyclic hydrocarbyl radical having 1 to 40 carbon atoms and preferably having 3 to 24 carbon atoms. Especially preferred are aromatic sulfonic acids and specifically alkylaromatic monosulfonic acids having one or more C₁-C₂₈-alkyl radicals and especially those having C₃-C₂₂-alkyl radicals. Suitable examples are methanesulfonic acid, butanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, xylenesulfonic acid, 2-mesitylenesulfonic acid, 4-ethylbenzenesulfonic acid, isopropylbenzene-sulfonic acid, 4-butylbenzenesulfonic acid, 4-octylbenzenesulfonic acid, dodecyl-benzenesulfonic acid, didodecylbenzenesulfonic acid and naphthalenesulfonic acid. It is also possible to use acidic ion exchangers as acidic organic catalysts, for example poly(styrene) resins which bear sulfo groups and have been crosslinked with about 2 mol % of divinylbenzene.
For the performance of the process according to the invention, particular preference is given to boric acid, phosphoric acid, polyphosphoric acid and polystyrenesulfonic acids. Especially preferred are titanates of the formula Ti(OR¹⁵)₄and specifically titanium tetrabutoxide and titanium tetraisopropoxide.
If it is desirable to use acidic inorganic, organometallic or organic catalysts, according to the invention, 0.01 to 10% by weight, preferably 0.02 to 2% by weight, of catalyst is used. In a specific embodiment, the condensation is effected without addition of catalysts.
In a preferred embodiment, for adjustment of the molecular weight, minor amounts of the dicarboxylic acids, anhydrides thereof or esters thereof bearing alk(en)yl radicals are replaced in the reaction mixture by C₁- to C₁₈-monocarboxylic acids, preferably C₂- to C₁₆-monocarboxylic acids and especially C₃- to C₁₄-monocarboxylic acids, for example C₄- to C₁₂-monocarboxylic acids. At the same time, however, not more than 20 mol % and preferably 0.1 to 10 mol %, for example 0.5 to 5 mol %, of the dicarboxylic acids, anhydrides thereof or esters thereof bearing alk(en)yl radicals is replaced by one or more monocarboxylic acids. In addition, minor amounts, for example up to 10 mol % and especially 0.01 to 5 mol % of the alk(en)ylsuccinic acids or anhydrides thereof may also be replaced by further dicarboxylic acids, for example succinic acid, glutaric acid, maleic acid and/or fumaric acid. More preferably, the polyesters bearing hydroxyl groups are prepared in the absence of monocarboxylic acids.
In a further preferred embodiment, for adjustment of the molecular weight, minor amounts of the polyol are replaced in the reaction mixture by C₁- to C₃₀-monoalcohols, preferably C₂- to C₂₄-monoalcohols and especially C₃- to C₁₈-monoalcohols, for example C₄- to C₁₂-monoalcohols. At the same time, preferably not more than 20 mol % and more preferably 0.1 to 10 mol %, for example 0.5 to 5 mol %, of the polyol is replaced by one or more monoalcohols. More preferably, the polyesters bearing hydroxyl groups are prepared in the absence of monoalcohols. In addition, the polyol bearing two primary and at least one secondary hydroxyl groups may also be replaced by one or more diols in minor amounts of up to 10 mol %, for example 0.01 to 5 mol %. Preference is given here to diols, for example ethylene glycol, propylene glycol and/or neopentyl glycol. More preferably, the polyesters bearing hydroxyl groups are prepared in the absence of diols.
In a further preferred embodiment, to increase the molecular weight, minor amounts of the polyol bearing two primary and at least one secondary OH group are replaced in the reaction mixture by polyols having three or more primary OH groups, for example having four, five, six or more primary OH groups. At the same time, preferably not more than 10 mol % and more preferably 0.1 to 8 mol %, for example 0.5 to 4 mol %, of the polyol bearing two primary and at least one secondary OH group is replaced by a polyol having three or more primary OH groups. Suitable polyols having three or more primary OH groups are, for example, trimethylolethane, trimethylolpropane and pentaerythritol.
The mean condensation level of the polyesters bearing hydroxyl groups used in accordance with the invention is preferably between 4 and 200, more preferably between 5 and 150, especially between 7 and 100 and particularly between 10 and 70, for example between 15 and 50, repeat dicarboxylic acid and polyol units. The condensation level is understood here to mean the sum of m+p+q as per formula (A). The weight-average molecular weight Mw of the polyesters bearing hydroxyl groups, determined by means of GPC in THF against poly(ethylene glycol) standards, is preferably between 2000 g/mol and 600 000 g/mol. In the case of polyesters which derive from dicarboxylic acids bearing C₁₆-C₄₀-alk(en)yl radicals, it is more preferably between 2000 and 100 000 g/mol and especially between 3000 and 50 000 g/mol, for example between 4000 and 20 000 g/mol. In the case of polyesters which derive from dicarboxylic acids bearing C₄₁-C₄₀₀-alk(en)yl radicals, it is more preferably between 3000 and 500 000 g/mol, particularly between 5000 and 200 000 g/mol and especially between 8000 and 150 000 g/mol, for example between 10 000 and 100 000 g/mol.
Preferably, the acid number of the polyesters bearing hydroxyl groups is less than 40 mg KOH/g and more preferably less than 30 mg KOH/g, for example less than 20 mg KOH/g. The acid number can be determined, for example, by titration of the polymer with alcoholic tetra-n-butylammonium hydroxide solution in xylene/isopropanol. Additionally preferably, the hydroxyl number of the polyesters is between 40 and 500 mg KOH/g, more preferably between 50 and 300 mg KOH/g and especially between 60 and 250 mg KOH/g. The hydroxyl number can, after reaction of the free OH groups with isocyanate, be ascertained by means of ¹H NMR spectroscopy, by quantitative determination of the urethane formed.
Preferably, the polyesters bearing hydroxyl groups used in accordance with the invention are nitrogen-free. “Nitrogen-free” is understood in accordance with the invention to mean that the nitrogen content thereof is below 1000 ppm by weight and more preferably below 100 ppm by weight and especially below 10% by weight, for example below 1 ppm by weight. The nitrogen content can be determined, for example, according to Kjeldahl.
The term “liquid hydrocarbon medium”, according to the invention, represents various different mineral oil hydrocarbons and petrochemicals. For example, mineral oil hydrocarbon feedstocks including crude oils and fractions obtainable therefrom, for example naphtha, gasifier fuel, kerosene, diesel, jet fuel, heating oil, gas oil, vacuum residues inter alia are covered by this definition. Examples of petrochemicals are olefinic or naphthenic process streams, aromatic hydrocarbons and derivatives thereof, ethylene dichloride and ethylene glycol. Likewise covered by the term “liquid hydrocarbon media” are hydrocarbons used as heat carriers, for example fused and/or substituted aromatics. Additionally covered by this definition are biogenic raw materials and products obtainable from biogenic raw materials by processing, for example animal and vegetable oils and fats and derivatives thereof, for example fatty acid alkyl esters. The liquid hydrocarbon media may also comprise constituents not consisting of hydrocarbons, for example salts, minerals and organometallic compounds.
The polyesters used in accordance with the invention are added to the liquid hydrocarbon media preferably in amounts of 0.5 to 5000 ppm by weight, more preferably of 1.0 to 1000 ppm by weight, for example of 2 to 500 ppm by weight. The polyesters may be dispersed or dissolved in the liquid hydrocarbon medium. They are preferably dissolved.
For easier handling, the polyesters used in accordance with the invention are preferably dissolved or dispersed in a polar or nonpolar organic solvent and added to the liquid hydrocarbon medium as a concentrate. Preferred solvents here are the solvents and solvent mixtures already mentioned as solvents for the condensation reaction between dicarboxylic acid and polyol. Particular preference is given to aromatic solvents. Preferably, the proportion of the polyester in the concentrate is 5 to 95% by weight, more preferably 10 to 80% by weight and especially 20 to 70% by weight, for example 25 to 60% by weight.
The polyester is preferably added to the liquid hydrocarbon medium prior to the thermal treatment thereof. The addition can be undertaken batchwise, for example into the storage vessel of the liquid hydrocarbon medium, or continuously into the feed line to the heat treatment plant. The addition is preferably effected at a site where the temperature of the liquid hydrocarbon medium is at least 10° C. and especially at least 20° C., for example at least 50° C., below the maximum heat treatment temperature. Especially in the case of hydrocarbon media of relatively high viscosity, it has often been found to be useful to promote the mixing of the polyester into the liquid hydrocarbon medium by means of static or dynamic mixing apparatus.
Particular advantages are shown by the inventive use of polyesters bearing hydroxyl groups and by the method that utilizes them in the processing or treatment of liquid hydrocarbon media above 100° C., especially between 150 and 500° C. and particularly between 200° C. and 480° C., for example between 250° C. and 450° C.
The polyesters used in accordance with the invention can be used together with one or more further additives. Preferred further additives are pour point depressants and demulsifiers, the latter preferably based on alkoxylated alkylphenol-aldehyde resins.
The inventive use of polyesters bearing hydroxyl groups in the thermal treatment of liquid hydrocarbon media leads to a reduction in fouling superior to the prior art additives and often also to the substantial and in some cases even complete suppression thereof. As a result, the energy requirement in the processing of liquid hydrocarbon is lowered and the throughput of the plant and the yield of target product are increased.
The method of the invention is generally suitable for reducing and often even for suppressing fouling in the processing of liquid hydrocarbon media at relatively high temperatures. This lowers the energy requirement of the process and increases the throughput of the plant and the yield of target product. The reduction in fouling reduces the frequency of maintenance shutdowns for removal of deposits and hence increases the plant availability.
For instance, the methods of the invention have been used successfully for reduction of fouling in crude oil distillation, in the processing of intermediates in mineral oil processing and in the processing of petrochemicals, and also of petrochemical intermediates, for example of gases, oils and reforming feedstocks, chlorinated hydrocarbons and liquid products from olefin plants, for example of bottoms phases from deethanization. The methods have likewise been used successfully for reduction and often for suppression of fouling by hydrocarbons used as heating media on the ‘hot side’ of heat exchange systems.
The suitability of the additives used in accordance with the invention for suppression or at least for reduction of fouling by liquid hydrocarbons in the course of thermal treatment thereof can be measured, for example, with commercially available HLPS (Hot Liquid Process Simulation) systems. In these systems, the oil to be treated thermally is pumped continuously through a capillary with a heating element present therein. As a result of fouling, deposits gradually form on the heating element, which impair heat transfer and lead to a pressure drop over the capillary. The extent of fouling can be assessed, for example, via the drop in the temperature at the outlet of the capillary. A significant drop in the temperature during the experiment indicates the occurrence of fouling. Measurements of this kind are generally regarded as a measure for assessment of the tendency of an oil to fouling in heat exchangers.

EXAMPLES

The α-olefins used were commercially available mixtures of 1-alkenes or poly(isobutenes) having the compositions specified. The acid numbers were determined by titration of an aliquot of the reaction mixture with alcoholic tetra-n-butylammonium hydroxide solution in xylene/isopropanol. The hydroxyl numbers were determined, after reacting the free OH groups of the polymers with isocyanate, by means of ¹H NMR spectroscopy, by quantitative determination of the urethane formed. The values reported are based on the solvent-free polymers.
The molecular weights were determined by means of lipophilic gel permeation chromatography in THF against poly(ethylene glycol) standards and detection by means of an RI detector.
Polyesters used:

P1) Copolymer of equimolar proportions of C_20-24-alkenylsuccinic anhydride (prepared by thermal condensation of maleic anhydride with technical-grade C_20-24-olefin containing, as main constituents, 43% C₂₀-, 35% C₂₂- and 17% C₂₄-olefin, with 90% α-olefins and 7.5% linear internal olefins) and glycerol. The reactants, in the form of a 50% solution in Shellsol® AB (aromatic solvent mixture having a boiling range of about 185-215° C.), were heated to 150° C. while stirring until the acid number remained constant. The water which formed was distilled off. The acid number of the polymer thus prepared was 7.8 mg KOH/g, the hydroxyl number was 98 mg KOH/g and the weight-average molecular weight was 6100 g/mol.
P2) Copolymer prepared in analogy to example P1) from equimolar proportions of C_20/24-alkenylsuccinic anhydride (prepared by thermal condensation of maleic anhydride with technical-grade C_20/24-olefin containing, as main constituents, 43% C₂₀-, 35% C₂₂- and 17% C₂₄-olefin, with 90% α-olefins and 7.5% linear internal olefins) and poly(glycerol) having a mean condensation level of 3. The acid number of the polymer was 6.5 mg KOH/g, the hydroxyl number was 195 mg KOH/g and the weight-average molecular weight was 8700 g/mol.
P3) Copolymer prepared in analogy to example P1) from equimolar proportions of C_26/28-alkenylsuccinic anhydride (prepared by thermal condensation of maleic anhydride with technical-grade C_26-28-olefin containing, as main constituents, 57% C₂₆-, 39% C₂₈- and 2.5% C₃₀₊-olefin, with 85% α-olefins, 4% linear internal olefins and 9% branched olefins) and glycerol. The acid number of the polymer was 10.4 mg KOH/g, the hydroxyl number was 68 mg KOH/g and the weight-average molecular weight was 9100 g/mol.
P4) Copolymer of C_20/24-alkenylsuccinic anhydride as per example P1, 0.7 molar equivalent of glycerol and 0.3 molar equivalent of behenic acid. The acid number of the polymer was 15 mg KOH/g, the hydroxyl number was 32 mg KOH/g and the weight-average molecular weight was 1800 g/mol.
P5) Copolymer of equimolar proportions of C_20-24-alkenylsuccinic anhydride and ethylene glycol in analogy to example P1. The acid number of the polymer thus prepared was 8.2 mg KOH/g, the hydroxyl number was 2 mg KOH/g and the weight-average molecular weight was 5700 g/mol (comparative example).
P6) C_20-24-Alkenylsuccinic anhydride as per example P1, reacted with 2 molar equivalents of triethylenetetramine. The reactants, in the form of a 50% solution in Shellsol AB, were heated to 150° C. while stirring until the acid number remained constant. The water which formed was distilled off. The acid number of the polymer thus prepared was 10.2 mg KOH/g and the weight-average molecular weight was 1000 g/mol (comparative example).
P7) Copolymer prepared in analogy to example P1) from equimolar proportions of poly(isobutenyl)succinic anhydride (prepared by thermal condensation of maleic anhydride with poly(isobutene) having a mean molecular weight Mn of 1000 g/mol and an alkylvinylidene content of 87 mol %) and glycerol. The acid number of the polymer was 8.6 mg KOH/g, the hydroxyl number was 47 mg KOH/g and the weight-average molecular weight was 14 000 g/mol.
P8) Copolymer prepared in analogy to example P1) from equimolar proportions of poly(isobutenyl)succinic anhydride (prepared by thermal condensation of maleic anhydride with poly(isobutene) having a mean molecular weight Mn of 2300 g/mol and an alkylvinylidene content of 81 mol %) and poly(glycerol) having a mean condensation level of 5. The acid number of the polymer was 7.8 mg KOH/g, the hydroxyl number was 110 mg KOH/g and the weight-average molecular weight was 21 000 g/mol.

The efficacy of the additives in terms of their ability to prevent or reduce fouling by mineral oils on hot surfaces was tested with the aid of a modified Hot Liquid Process Simulation (HLPS) system from Alcor. In the HLPS system, the oil to be examined was pumped continuously from a stirred and heated reservoir vessel through an electrically heated heating element mounted in a stainless steel capillary (=hot capillary), before being returned to the reservoir vessel. During the experiment, the maximum oil temperature attained after switching on the heating (the surface temperature of the heating element was about 400° C.) was firstly registered at the output of the stainless steel capillary (T1). Secondly, the oil temperature was registered at the same point after an experimental duration of 5 hours (T2). Since the deposits formed on the heating element as a result of fouling have low thermal conductivity, the maximum temperature initially attained correlates indirectly (low initial temperature T1 implies immediate onset of fouling), and the difference in the temperatures T2 and T1 directly, with the extent of fouling.
For each experiment, about 500 ml of the oil sample to be examined were introduced into the reservoir vessel and heated to about 150° C. for better pumpability. The oil was then pumped at a volume flow rate of 3 ml/min through the stainless steel capillary which has been provided with a clean heating element with a bare surface. The heating element was then heated to a temperature of about 400° C. for test oil 1, about 375° C. for test oil 2 or about 390° C. for test oil 3, and the maximum oil temperature which was then established at the capillary outlet was noted (T1). After a run time of 5 hours, the oil temperature that was then present at the end of the stainless steel capillary (T2) was noted and the experiment was ended. A high maximum temperature T1 and a low ΔT (ΔT=T2−T1) indicate low coverage of the surface of the heating element with insulating deposits and hence effective suppression of fouling.
The following test oils were used for the assessment of the fouling-reducing effect of the additives:


Test oil	1	2	3

Origin	Brazil	Malaysia	Thailand
API gravity @ 15° C.	25.7	47.2	11.4
[°API]
Viscosity [mPas]	61	5	160
	(25° C.)	(25° C.)	(50° C.)
Density [g/cm³]	0.900	0.792	0.990
	(20° C.)	(20° C.)	(16° C.)
Pour point [° C.]	−27	+18	+33
Asphaltene content	7.9	3.2	10.3
[% by wt.]

The viscosity was determined to ASTM D-445, and the density to DIN EN ISO 12185. The pour point was determined to ASTM D-97. The asphaltene content was determined to IP 143.

Experimental Results in Test Oil 1


		Dosage	T1	T1		Fouling
Measure-	Addi-	rate	(t = 0)	(t = 5 h)	ΔT	reduction
ment	tive	[ppm]	[° C.]	[° C.]	[° C.]	[%]

1	none	—	278	256	22	0
2	P1	5	281	266	15	32
3	P2	5	282	268	14	36
4	P3	5	281	266	15	32
5	P4	5	280	262	18	18
6 (comp.)	P5	5	279	257	22	0
7 (comp.)	P6	5	280	261	19	14
8	P1	10	283	269	14	36
9	P2	10	285	274	11	50
10	P3	10	283	271	12	45
11	P4	10	281	265	16	27
12 (comp.)	P5	10	279	259	20	9
13 (comp.)	P6	10	282	264	18	18
14	P1	15	285	274	11	50
15	P2	15	286	278	8	64
16	P3	15	285	275	10	55
17	P4	15	284	270	14	36
18 (comp.)	P5	15	280	261	19	14
19 (comp.)	P6	15	283	266	17	23

Experimental Results in Test Oil 2

20	none	0	257	230	27	0
21	P1	10	261	238	23	15
22	P2	10	262	239	23	15
23	P3	10	261	238	23	15
24	P4	10	260	236	24	11
25 (comp.)	P5	10	258	231	27	0
26 (comp.)	P6	10	260	235	25	7
27	P1	25	263	244	19	30
28	P2	25	263	243	20	26
29	P3	25	262	243	19	30
30	P4	25	261	240	21	22
31 (comp.)	P5	25	258	234	24	11
32(comp.)	P6	25	262	239	23	15
33	P1	50	264	253	11	59
34	P2	50	265	256	9	67
35	P3	50	263	253	10	63
36	P4	50	261	247	14	48
37 (comp.)	P5	50	260	241	19	30
38 (comp.)	P6	50	261	245	16	41

Experimental Results in Test Oil 3

39	none	0	266	244	22	0
40	P1	10	270	254	16	27
41	P2	10	271	256	15	32
42 (comp.)	P5	10	265	244	21	5
43 (comp.)	P6	10	268	250	18	18
44	P7	10	271	257	14	36
45	P8	10	273	261	12	45
46	P1	20	272	259	13	41
47	P2	20	272	260	12	45
48 (comp.)	P5	20	265	245	20	9
49 (comp.)	P6	20	270	253	17	23
50	P7	20	273	262	11	50
51	P8	20	274	264	10	55
52	P1	40	273	262	11	50
53	P2	40	274	264	10	55
54 (comp.)	P5	40	266	246	20	9
55 (comp.)	P6	40	271	258	13	41
56	P7	40	275	266	9	59
57	P8	40	275	268	7	68

The decreases in temperature after an experimental duration of 5 hours observed in the experiments using the method of the invention are much smaller than in comparative experiments using other methods or additives. Moreover, higher maximum temperatures are generally observed at first. Both indicate lower deposits on the heating element and hence more efficient suppression of fouling in the case of inventive use of the additives or of the method that utilizes them. Accordingly, the method of the invention entails less frequent maintenance of the plant for removal of the deposits and hence longer service lives of the plant. Since the target oil temperature is often preset in industrial plants, the method of the invention additionally leads to saving of energy.

Claims

1. The use of a polyester which bears hydroxyl groups and is preparable by polycondensation of a polyol containing two primary OH groups and at least one secondary OH group with a dicarboxylic acid or anhydride thereof or ester thereof which bears a C₁₆- to C₄₀₀-alkyl radical or a C₁₆- to C₄₀₀-alkenyl radical as an antifoulant in the thermal treatment of liquid hydrocarbon media within the temperature range from 100 to 550° C.

2. The use as claimed in claim 1, wherein the OH number of the polyester is at least 40 mg KOH/g.

3. The use as claimed in claim 1 and/or 2, wherein the dicarboxylic acid or anhydride thereof or ester thereof bears a C₁₆- to C₄₀-alkyl radical or a C₁₆- to C₄₀-alkenyl radical.

4. The use as claimed in claim 3, wherein the dicarboxylic acid is a C₁₆- to C₄₀-alkyl- or C₁₆- to C₄₀-alk(en)ylsuccinic acid or anhydride thereof.

5. The use as claimed in one or more of claims 1 to 4, wherein the alkyl or alkenyl radical of the dicarboxylic acid or anhydride thereof or ester thereof contains 18 to 36 carbon atoms.

6. The use as claimed in one or more of claims 3 to 5, wherein the alkyl or alkenyl radical derives from an α-olefin.

7. The use as claimed in claim 1 and/or 2, wherein the dicarboxylic acid or anhydride thereof or ester thereof bears a C₄₁- to C₄₀₀-alkyl radical or a C₄₁- to C₄₀₀-alkenyl radical.

8. The use as claimed in claim 7, wherein the alkyl or alkenyl radical of the dicarboxylic acid is branched.

9. The use as claimed in claim 7 and/or 8, wherein the alkyl or alkenyl radical derives from a polyolefin.

10. The use as claimed in claim 9, wherein the polyolefin derives from an olefin having 3 to 6 carbon atoms.

11. The use as claimed in claim 10, wherein the polyolefin is poly(isobutene).

12. The use as claimed in one or more of claims 1 to 11, wherein the polyol is of monomeric structure, and comprises three to 10 carbon atoms and 1 to 6 secondary OH groups, but not more than one OH group per carbon atom.

13. The use as claimed in one or more of claims 1 to 11, wherein the polyol is of polymeric structure and comprises six to 150 carbon atoms and two to 50 secondary OH groups, but not more than one OH group per carbon atom.

14. The use as claimed in one or more of claims 1 to 11, wherein the polyol is selected from glycerol and oligomers thereof having 2 to 10 monomer units.

15. The use as claimed in one or more of claims 1 to 14, wherein the polyester corresponds to the structural formula (A)

in which

one of the R¹to R⁴radicals is a C₁₆-C₄₀₀-alkyl or -alkenyl radical and

the other R¹to R⁴radicals are each independently hydrogen or an alkyl radical having 1 to 3 carbon atoms,

R⁵is a C—C single bond or an alkylene radical having 1 to 6 carbon atoms,

R¹⁶is a hydrocarbyl group which bears at least one hydroxyl group and has 3 to 10 carbon atoms,

n is a number from 1 to 100,

m is a number from 3 to 250,

p is 0 or 1, and

q is 0 or 1.

16. The use as claimed in claim 15, wherein one of the R¹to R⁴radicals is a linear C₁₆-C₄₀-alkyl or -alkenyl radical.

17. The use as claimed in claim 15, wherein one of the R¹to R⁴radicals is a C₄₁-C₄₀₀-alkyl or -alkenyl radical.

18. The use as claimed in claim 9, wherein R¹⁶is a radical of the formula (2)

—(CH₂)_r—(CH(OH))_t—(CH₂)_s— (2)

in which

t is a number from 1 to 6,

r, s are each independently a number from 1 to 9 and

t+r+s is a number from 3 to 10.

19. The use as claimed in one or more of claims 1 to 18, wherein the polyester is nitrogen-free.

20. The use as claimed in one or more of claims 1 to 19, wherein the molecular weight of the polyester is between 2000 g/mol and 100 000 g/mol.

21. The use as claimed in one or more of claims 1 to 20, wherein the liquid hydrocarbon medium is crude oil or a fraction obtainable from crude oil.

22. The use as claimed in one or more of, claims 1 to 21, wherein the liquid hydrocarbon medium is a petrochemical or a hydrocarbon used as a heat carrier.

23. The use as claimed in one or more of claims 1 to 22, wherein the liquid hydrocarbon medium is of biogenic origin.

24. The use as claimed in one or more of claims 1 to 23, wherein the use is effected at temperatures between 200 and 550° C.

25. A method for reducing fouling in a liquid hydrocarbon medium during the heat treatment of the medium at temperatures between 100 and 550° C., in which the polyester bearing hydroxyl groups as defined in one or more of claims 1 to 20 is added to the liquid hydrocarbon before and/or during the thermal treatment.

26. A method for increasing the service life of plants for thermal treatment of hydrocarbons, in which the polyester bearing hydroxyl groups as defined in one or more of claims 1 to 20 is added to the hydrocarbon media to be processed before and/or during the thermal treatment.

27. The method as claimed in claim 25 or 26, which proceeds at temperatures of 200 to 550° C.