EP0436151B2 - Coldstable petroleum middle distillate containing copolymers as paraffindispersants - Google Patents

Abstract

Cold-stable petroleum middle distillates contain small quantities of polymers or copolymers which contain substantial fractions of units of formula I <IMAGE> or consist of these, in which R<1> and R<2> independently of one another are hydrogen or low-molecular alkyl or together form a tetramethylene radical and in which R<3> and R<4> are unbranched alkyl radicals having 14 to 24 C atoms. The additives effect very good dispersing and flow improvement.

Description

The invention relates to low-temperature petroleum middle distillates containing low molecular weight copolymers with polymerized monoethylenically unsaturated dicarboxylic acids which are reacted with unbranched, secondary fatty amines and have improved cold flowability and better dispersion of the separated wax crystals.

Middle distillates such as gas oils, diesel oils or heating oils, which are obtained from petroleum by distillation, have different paraffin contents depending on the origin of the crude oil. At lower temperatures, solid paraffins are excreted (cloud point or cloud point, CP). Upon further cooling, the platelet-shaped n-paraffin crystals form a "house of cards" structure and the middle distillate stops, although the majority of the middle distillate is still liquid. The fluidity of the petroleum distillate fuels is considerably impaired by the unusual n-paraffins in the temperature region between the cloud point and pour point or pour point. The paraffins clog filters and cause uneven or completely interrupted fuel supply to the combustion units. Similar problems occur with heating oils.

It has long been known that suitable additives can be used to modify the crystal growth of the paraffins in the petroleum middle distillate fuels. Good-acting additives on the one hand prevent middle distillates from forming such house of cards structures and, at temperatures a few degrees Celsius below the temperature at which the first wax crystals crystallize, already become solid and, on the other hand, they form fine, well-crystallized, separate wax crystals that pass through filters in motor vehicles and heating systems or at least form a filter cake which is permeable to the liquid part of the middle distillates, so that trouble-free operation is ensured

A disadvantage of the additives mentioned is that the failed paraffin crystals tend to settle more and more on the bottom of the container due to their higher density than the liquid part. This creates a homogeneous low-paraffin phase in the upper part of the container and a two-phase paraffin-rich layer on the bottom. Since both in vehicle tanks and in storage or delivery tanks of mineral oil dealers, the middle distillate is mostly removed slightly above the bottom of the container, there is a risk that the high concentration of solid paraffins will lead to blockages of filters and metering devices. This danger becomes greater the further the storage temperature falls below the paraffin separation temperature, since the amount of paraffin excreted is a function of the temperature and increases with falling temperature.

The paraffin crystal modifiers, the so-called flow improvers, are polymers which change the crystal growth of the n-paraffins through co-crystallization (interaction) and improve the flow properties of the middle distillate at low temperatures. The effectiveness of the flow improver is expressed indirectly according to DIN 51428 by measuring the "Cold Filters Plugging Point" (CFPP).

Known ethylene copolymers, especially copolymers of ethylene and unsaturated esters, are used as cold flow improvers. DE-A-11 47 799 and 19 14 756, for example, describe copolymers of ethylene with vinyl acetate, with a content of 25 to 45% by weight of vinyl acetate and a molecular weight of 500 to 5000.

It is also known from GB-A-2 095 698 to add middle distillates a combination of the copolymers mentioned with amides of long-chain amines and aromatic or cycloaliphatic carboxylic acids.

DE-A-2 531 234 the addition of dialkyl diamides or monoalkylimides of e.g. Styrene / maleic acid amide copolymers recommended as stabilizers in mineral oils, i.e. the carboxyl groups of the maleic anhydride radical are completely reacted with amines, so that there are no free carboxyl groups.

According to US Pat. No. 3,506,625, reaction products of monoamines with maleic anhydride polymers to give the corresponding imides are also described, wherein when using less than one mole of amine per mole of maleic anhydride unit, any remaining carboxyl groups are neutralized. Although dialkylamines are also mentioned as reactants in the general description in the cited patent specification, there is no concrete information about a reaction with a secondary amine. Rather, there is always talk of imide to be formed, the formation of which is only possible with a primary amine. Finally, the reaction product should not contain any free acid groups; these are converted into metal salts by neutralization.

However, these mixtures are not yet satisfactory with regard to the handling and dispersing properties of the paraffins which have been eliminated. Furthermore, the diamides mentioned have the disadvantage that a 50% solution in a conventional solvent, e.g. Solvesso® is only homogeneous in the heat and the diamide is precipitated at room temperature. Finally, the preparation of the diamides is only possible with great effort because higher temperatures, longer residence times and removal of the water formed are required to introduce the second amide group.

From Japanese patent specification 56-54038 copolymers of C ₁₀ -C ₃₂ -α-olefins and structural units of the following formula I are known as flow improvers for petroleum middle distillate fuels.

The object was therefore to propose additives to middle distillates which have improved handling with simpler production and at least equally good paraffin dispersing action with good flow improvement.

bestehen, wobei
R³ und R⁴ unverzweigte Alkylreste mit 14 bis 24 C-Atomen bedeuten, diese Forderung erfüllen.Surprisingly, it has now been found that low-temperature petroleum middle distillates containing, in amounts effective as flow improvers, copolymers consisting of propylene, n-butene, isobutene or diisobutene and units of the formula I

consist of
R ³ and R ^{4 are} unbranched alkyl radicals with 14 to 24 carbon atoms, meet this requirement.

In particular, the radicals of the formula I are derived from z. B. maleic acid, fumaric acid, preferably maleic anhydride. Isobutene is preferred as the comonomer.

The following may be mentioned as examples of copolymers: an alternating copolymer of maleic acid and i-butene. The molar masses of the copolymers are generally 500 g / mol to 20,000 g / mol, preferably 700 to 2000 g / mol.

in Betracht, in der
R³, R⁴ einen geradkettigen Alkylrest mit 14 bis 24 Kohlenstoffatomen, bedeutet. Im einzelnen seien Dioleylamin, Dipalmitinamin und Dibehenylamin und vorzugsweise Ditalgfettamin genannt.Dialkylamines of the formula:

considered in the
R ³ , R ^{4 is} a straight-chain alkyl radical having 14 to 24 carbon atoms. Dioleylamine, dipalmitinamine and dibehenylamine and preferably ditallow fatty amine may be mentioned in particular.

As a rule, it is advantageous to use the dicarboxylic acids in the form of the anhydrides, if available, in the copolymerization, e.g. Maleic anhydride, since the anhydrides usually copolymerize better with the olefins. The anhydride groups of the copolymers can then be reacted directly with the amines.

The copolymers are reacted with the secondary fatty amines at from 50 to 200 ° C. in the course of 0.3 to 30 hours. The secondary fatty amine is in amounts of about one mole per mole of polymerized dicarboxylic acid anhydride, i.e. 0.9 to 1.1 mol / mol applied. The use of larger or smaller amounts is possible, but has no advantage. If larger amounts than one mole are used, ammonium salts are obtained in part because the formation of a second amide group requires higher temperatures, longer residence times and water removal. If less than one mole is used, there is no complete conversion to the monoamide and a correspondingly reduced effect is obtained.

Instead of the subsequent reaction of the carboxyl groups in the form of the dicarboxylic anhydride with amines to give the corresponding amides, it can sometimes be advantageous to prepare the monoamides of the monomers and then to copolymerize them directly during the polymerization. In most cases, however, this is technically much more complex because the amines can attach to the double bond of the monomeric mono- and dicarboxylic acid and then copolymerization is no longer possible.

The copolymers are prepared by known discontinuous or continuous polymerization processes such as bulk, suspension, precipitation or solution polymerization and initiation with customary radical donors such as acetylcyclohexanesulfonyl peroxide, diacetyl peroxidicarbonate, dicyclohexyl peroxidicarbonate, di-2-ethylhexyl peroxidyl carbonate, tert. -Azobis (4-methoxy-2,4-dimethylvaleronitrile), tert-butyl perpivalate, tert-butyl per-2-ethyl hexanoate, tert-butyl permaleinate, 2,2'-azobis (isobutyronitrile), bis (tert- butyl peroxide) cyclohexane, tert-butyl peroxy isopropyl carbonate, tert-butyl peracetate, dicumyl peroxide, Di-tert-amyl peroxide, di-tert-butyl peroxide, p-methane hydroperoxide, cumene hydroperoxide and tert-butyl hydroperoxide and mixtures with one another. In general, these initiators are used in amounts of 0.1 to 20% by weight, preferably 0.2 to 15% by weight, based on the monomers.

The polymerization is generally carried out at from 40 to 400 ° C., preferably from 80 to 300 ° C., and when olefins or solvents are used, boiling temperatures below the polymerization temperature are advantageously carried out under pressure. The polymerization is expediently carried out in the absence of air, i.e. if it is not possible to work under boiling conditions, e.g. B. carried out under nitrogen, since oxygen delays the polymerization. The reaction can be accelerated by using redox coinitiators such as benzoin, dimethylaniline, ascorbic acid and organically soluble complexes of heavy metals such as copper, cobalt, manganese, iron, nickel and chromium. The amounts usually used are 0.1 to 2000 ppm by weight, preferably 0.1 to 1000 ppm by weight. When choosing the initiator or the initiator system, it is advisable to ensure at the chosen polymerization temperature that the half-life of the initiator or the initiator system is less than 3 hours

To achieve low molecular weight copolymers it is often advisable to work in the presence of regulators. Suitable regulators are, for example, allyl alcohols, such as buten-1-ol-3, organic mercapto compounds such as 2-mercaptoethanol, 2-mercaptopropanol, mercaptoacetic acid, mercaptopropionic acid, tert-butyl mercaptan, n-butyl mercaptan, n-octyl mercaptan, n-dodecyl mercaptan and tert.- Dodecyl mercaptan, which are generally used in amounts of 0.1% by weight to 10% by weight.

Equipment suitable for the polymerization is e.g. Conventional stirred kettles with, for example, anchor, blade, impeller or multi-stage impulse countercurrent stirrers and for the continuous production of stirred kettle cascades, tubular reactors and static mixers.

The simplest method of polymerization is bulk polymerization. The olefins and the acid group- or acid anhydride group-containing monomer are polymerized in the presence of an initiator and in the absence of solvents. This process is particularly suitable for those copolymers in which the olefin used is diisobutene. Advantageously, all monomers are mixed in the desired composition and a small part, e.g. about 5 to 10%, in the reactor before, heated to the desired polymerization temperature with stirring and metered in the remaining monomer mixture and the initiator and optionally coinitiator and regulator evenly within 1 to 10 hours, preferably 2 to 5 hours. It is expedient to meter in the initiator and the coinitiator separately in the form of solutions in a small amount of a suitable solvent. The copolymer can then be converted into the paraffin dispersant according to the invention directly in the melt or after dilution with a suitable solvent.

A continuous high-pressure process which permits space-time yields of 1 to 50 kg of polymer per liter of reactor and hour is also suitable for producing the desired copolymers. As a polymerization apparatus, e.g. a pressure vessel, a pressure vessel cascade, a pressure tube or a pressure vessel with a downstream reaction tube, which is provided with a static mixer, can be used. The monomers are preferably polymerized from olefins and monoethylenically unsaturated compounds containing acid anhydride groups or acid groups in at least 2 polymerization zones connected in series. One reaction zone can consist of a pressure-tight vessel, the other of a heatable static mixer. You get sales of more than 99%. The polymerization is expediently carried out at pressures of more than 1 bar, preferably between 1 and 200 bar. The copolymers obtained with solids contents above 99 ° C. can then be reacted further to give the corresponding amides.

Another method for the simple preparation of the copolymers is precipitation polymerization. In the case of precipitation polymerization, solvents are used in which the monomers are soluble and the copolymer formed is insoluble and fails. Examples of such solvents are ethers such as diethyl ether, dipropyl ether, dibutyl ether, methyl tert-butyl ether, diethylene glycol dimethyl ether, toluene, xylene, ethylbenzene, cumene, high-boiling aromatic mixtures such as Solvesso 100®, 150 and 200, aliphatic and cycloaliphatic hydrocarbons and mixtures with one another. When carrying out the precipitation polymerization, it is expedient, particularly when working at concentrations of more than 40% by weight, to use a protective colloid to prevent the formation of aggregates. Suitable protective colloids are polymeric substances which are readily soluble in the solvents and which do not react with the monomers. For example, copolymers of maleic anhydride with vinyl alkyl ethers and / or olefins with 8 to 20 C atoms and their monoesters with C ₁₀ to C ₂₀ alcohols or mono- and diamides with C ₁₀ to C ₂₀ alkyl amines are suitable as well as polyalkyl vinyl ethers whose alkyl group contains 1 to 20 carbon atoms, such as polymethyl, polyethylene, polyisobutyl and polyoctadecyl vinyl ether. The amounts of protective colloid added are usually 0.05 to 4% by weight (based on the monomers used), preferably 0.1 to 2% by weight, it often being advantageous to combine several protective colloids. In the case of the polymerization, it is expedient to put the solvent, the protective colloid and part of the monomer mixture in the reactor and to meter in the rest of the monomer mixture and the initiator and, if appropriate, the point and regulator at the selected polymerization temperature with vigorous stirring. The feed times for monomer and initiator are generally between 1 and 10 hours, preferably 2 and 5 hours. It is also possible to polymerize all of the starting materials together in one reactor, but problems with heat dissipation can occur, so that such an operation is less is appropriate. The concentrations of the monomers to be polymerized are between 20 and 80% by weight, preferably 30 to 70% by weight. The polymers can be isolated directly from the polymer suspensions in evaporators, for example belt dryers, paddle dryers, spray dryers and fluidized bed dryers.

When working in suitable solvents that can be added directly to fuels, the further conversion to the amide can be carried out directly in the suspension. This is the preferred form of preparation for the production of maleic anhydride copolymers with isobutene and diisobutene.

Another embodiment for the preparation of the copolymers is solution polymerization. It is carried out in solvents in which the monomers and the copolymers formed are soluble. All solvents that meet this requirement and that do not react with the monomers are suitable for this. For example, these are acetone, methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, ethyl acetate, butyl acetate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, tetrahydrofuran and dioxane. As with bulk and precipitation polymerization, it is also expedient here to introduce the solvent and part of the monomer mixture (for example about 5 to 20%) and to meter in the rest of the monomer mixture with the initiator and, if appropriate, co-initiator and regulator. Solvents and olefins can also be introduced into the polymerization reactor and then, after the polymerization temperature has been reached, the acid group-containing or acid anhydride group-containing monomers, optionally dissolved in the solvent, and the initiator and, if appropriate, coinitiator and regulator are metered in. The concentrations of the monomers to be polymerized are between 20 and 80% by weight, preferably 30 and 70% by weight. The solid copolymer can be isolated easily by evaporating the solvent. But here, too, it is expedient to choose a solvent in which the further reaction with amines can take place.

The additives according to the invention are added to the petroleum middle distillates in amounts of 50 to 1000 ppm, preferably 100 to 500 ppm. As a rule, such middle distillates already contain flow improvers such as ethylene-vinyl ester copolymers.

• a) 50 bis 1000 ppm, bevorzugt 100 bis 500 ppm der erfindungsgemäßen Umsetzungsprodukte niedermolekularer Co-Polymerer, die Einheiten monoethylenisch ungesättigter Dicarbonsäuren enthalten, mit unverzweigten, sekundären Fettaminen zu den Monoamiden,
• b) an sich bekannten Ethylencopolymerisat-Fließverbesserern, z. B. Ethylen-Vinylestercopolymerisaten, in Mengen von 50 bis 1000 ppm, bevorzugt 50 bis 500 ppm und
• c) Leitfähigkeitsverbesserer in Form von Salzen, insbesondere von Carbonsäuren und Sulfonsäuren bzw. deren Metall- und Ammoniumsalzen in Mengen von 0,1 bis 40 ppm, bevorzugt 0,25 bis 20 ppm.

According to a particularly preferred embodiment of the invention, the petroleum middle distillates contain small amounts of the following additive combination

• a) 50 to 1000 ppm, preferably 100 to 500 ppm, of the reaction products according to the invention of low molecular weight copolymers which contain units of monoethylenically unsaturated dicarboxylic acids, with unbranched, secondary fatty amines to the monoamides,
• b) known ethylene copolymer flow improvers, for. B. ethylene-vinyl ester copolymers, in amounts of 50 to 1000 ppm, preferably 50 to 500 ppm and
• c) conductivity improvers in the form of salts, in particular carboxylic acids and sulfonic acids or their metal and ammonium salts in amounts of 0.1 to 40 ppm, preferably 0.25 to 20 ppm.

The known flow improvers (b) are described in detail in the patent literature. Examples include German Patent 19 14 756, EP 214786 (α-olefin / MSA ester) and EP 155807 (alkyl fumarates / VAC copolymers), to which reference is hereby made. However, terpolymers which contain, in addition to ethylene and vinyl esters or acrylic esters, other comonomers in copolymerized form are also equally suitable.

Preferred copolymers (b) are those which essentially contain ethylene and 25 to 45% by weight of vinyl acetate, vinyl propionate or ethylhexyl acrylate. Also to be mentioned are copolymers which contain, for example, fumaric acid esters. The molecular weight of the flow improvers is generally 500 to 5000, preferably 1000 to 3000.

In general, hydrocarbon-soluble carboxylic acids and / or sulfonic acids or their salts are suitable as conductivity improvers (c) for middle distillates.

The basic conductivity of middle distillates is approx. 5 to 10 ps / m, measured according to DIN 51412. Variations occur due to different contents of water, salts, naphthenic acids, phenols and others compounds containing sulfur and nitrogen.

An increase in the conductivity by a factor of 2 to 3, based on the basic conductivity, has proven to be advantageous for the dispersing behavior of the paraffins in some of the middle distillates investigated.

The addition of the conductivity improvers, e.g. are described in DE-OS 21 16 556, already in amounts of 0.3 to 1 ppm in middle distillate improve the response. Other, less effective conductivity improvers naturally require a higher concentration. The addition of significantly larger amounts than the specified 40 ppm is possible, but has no significant technical advantages.

In particular, metal salts of hydrocarbon-soluble carboxylic acids and sulfonic acids, as are commercially available under the name ASA3® / Shell, and other conventional conductivity improvers, such as the commercially available Stadis® 450 commercial product from DuPont, whose composition is not known, are also suitable.

The invention is illustrated by the following examples

Manufacture of paraffin dispersants

Example 1 describes the preparation of the co-polymer, which is then reacted with ditallow fatty amine according to Example 2 to give the paraffin dispersant according to the invention.

The molecular weights were determined by gel permeation chromatography, using tetrahydrofuran as eluent and narrowly distributed fractions of polystyrene for calibration.

Manufacturing examples example 1

980 g of maleic anhydride, 1440 g of toluene and 14 g of polyvinylethyl ether with a molecular weight of 50,000 were introduced into a pressure reactor which was provided with a stirrer, heating and feed devices, and the reactor was pressed 3 times with 4 bar of nitrogen and heated to 140 ° C. with stirring 600 g of isobutene within 5 hours and a solution of 23.1 g of tert within 6 hours. Butyl perethyl hexanoate and 15.4 g of ditertiary butyl peroxide in 100 g of toluene were added. The mixture was then reheated for 1 hour. A pressure of 7 bar is established during the polymerization. The mixture was then cooled and the thin, fine suspension of the molar copolymer of maleic anhydride and isobutene was further reacted. The molar mass of the polymer was 3500.

Example 2

30.8 g of co-polymer (produced by evaporating the suspension from Example 1) and 102.0 g of ditallow fatty amine were stirred at 150 ° C. for about 12 hours until an amine titer of <1 was reached.

Comparative examples: Example 3 (Production according to US 3506625, Example III)

A reaction mixture of 60 g of styrene / maleic anhydride copolymer and 60.2 g of stearylamine in 368 g of neutral oil IA is heated to 220 ° C. for 20 hours. The mixture is then allowed to cool to 125 ° C. and 10.9 g of calcium hydroxide (dissolved in water) are metered in . After 2 hours of stirring at this temperature, the reaction mixture was filtered and the water was separated off.

Example 4 (Production according to DE 2531234, Example 2)

A reaction mixture of 27 g stearylamine and 91.1 g 17% solution of an isobutene / maleic anhydride copolymer in xylene was heated at 100 to 150 ° C for 40 hours. The product, 37 g brown solid, showed clear imide bands in the IR spectrum.

Examples of use

A.

Umsetzungsprodukte niedermolekularen Co-Polymerer, die Einheiten monoethylenisch ungesättigter Dicarbonsäuren enthalten, mit Ditalgfettamin

FI

Fließverbesserer, im besonderen
Fl(A) Ethylen/Vinylpropionat (mit ca. 40 Gew.-% Vinylpropionat) mit einem mittleren Molekulargewicht von ca. 2500 (bestimmt durch Dampfdruckosmometrie)
Fl(B) Ethylen/Vinylacetat (mit ca. 30 Gew.-% Vinylacetat) mit einem mittleren Molekulargewicht von ca. 2500.
FI(C) Ethylen/Ethylhexylacrylat (mit 50 Gew.-% Ethylhexylacrylat) mit einem mittleren Molekulargewicht von ca. 2500.

LV

Leitfähigkeitsverbesserer, im besonderen
LV(E) gemäß Beispiel 1 DE 21 16 556
LV(F) (ASA 3/Shell) Kohlenwasserstofflösliches sulfocarbonsaures Salz
LV(G) (Stadis 450/DuPont) Leitfähigkeitsverbesserer unbekannter Zusammensetzung.

The following mean:

A.

Reaction products of low molecular weight copolymers, the units of monoethylenically unsaturated dicarboxylic acids included, with ditallow fatty amine

FI

Flow improver, in particular
Fl (A) ethylene / vinyl propionate (with approx. 40% by weight vinyl propionate) with an average molecular weight of approx. 2500 (determined by vapor pressure osmometry)
Fl (B) ethylene / vinyl acetate (with approx. 30% by weight vinyl acetate) with an average molecular weight of approx. 2500.
FI (C) ethylene / ethylhexyl acrylate (with 50% by weight ethylhexyl acrylate) with an average molecular weight of approx. 2500.

LV

Conductivity improvers, especially
LV (E) according to Example 1 DE 21 16 556
LV (F) (ASA 3 / Shell) Hydrocarbon soluble sulfocarboxylic acid salt
LV (G) (Stadis 450 / DuPont) conductivity improver of unknown composition.

As middle distillate I, diesel fuel in commercial West German refinery quality was used for the following tests. Middle distillate I. Cloud point (° C) -7 CFPP (° C) -10 Density b. 20 ° C (g / ml) 0.817 Initial boiling point (° C) 156 20% boiling point (° C) 204 90% boiling point (° C) 309 End of boiling point (° C) 350

Description of the test method

The middle distillate was tested with different amounts of flow improvers alone and / or together with paraffin dispersants in combination with / without conductivity improvers at temperatures below the cloud point. The cooling was carried out using a temperature program. The middle distillate I (Tables I, II) were cooled from room temperature to -12 ° C at a cooling rate of 1 ° C / h and stored at -12 ° C for 24 h. The experiments were carried out with 100 ml and 1000 ml middle distillate volumes. Tables I - II list; Volume of the sedimented paraffin phase (%) optically assessed, cloud point (CP) and cold filter plugging point (CFPP) of the lower area (lower 40 vol%), CP and CFPP of the upper area (upper 60 vol.%) As well as the CP and CFPP of the middle distillate containing the additives before the storage test

Tabelle I

Prüfung in Mitteldestillat I

Tabelle II

Prüfung in Mitteldestillat I

As can be seen from the following tables, the sedimentation of the paraffins is further reduced by adding conductivity-improving additives.

Table I

Examination in middle distillate I

Table II

Examination in middle distillate I

BS =

Bodensatz

K =

klar

T =

trüb

LT =

leicht trüb

LD =

leicht dispergiert

D =

dispergiert
FI =  Mitteldestillatfließverbesserer
PD =  Paraffindispergator
PD (D2) =  Paraffindispergator gem. Beispiel 2
LV =  Leitfähigkeitsverbesserer
PD (D3) =  Paraffindispergator gem. DE 25 11 234, Vergleichsbeispiel 4
PD (D4) =  Paraffindispergator gem. US 3,506,625, Vergleichsbeispiel 3

In these tables:

BS =

Sediment

K =

clear

T =

cloudy

LT =

slightly cloudy

LD =

slightly dispersed

D =

dispersed
FI = middle distillate flow improver
PD = paraffin dispersant
PD (D2) = paraffin dispersant acc. Example 2
LV = conductivity improver
PD (D3) = paraffin dispersant acc. DE 25 11 234, comparative example 4
PD (D4) = paraffin dispersant acc. US 3,506,625, comparative example 3

Claims (3)

1. A mineral oil middle distillate which is stable at low temperatures and contains, in amounts effective as a flow improver, a copolymer which consists of propylene, n-butene, isobutene or diisobutene and units of the formula I

   

   where R³ and R⁴ are each straight-chain alkyl of 14 to 24 carbon atoms.
2. A mineral oil middle distillate as claimed in claim 1, wherein the copolymer is present in an amount of from 50 to 1,000 ppm.
3. A mineral oil middle distillate as claimed in claim 1, which additionally contains

   A) from 50 to 1,000 ppm of flow improvers selected from the group consisting of ethylene/vinyl acetate, ethylene/vinyl propionate and ethylene/ethylhexyl acrylate copolymers having an average molecular weight of from 1,000 to 5,000 and

   B) from 0.1 to 40 ppm of conductivity improvers.