LU85337A1 - HIGH-PRESSURE RECIPROCATING COMPRESSOR AND ITS APPLICATION TO A PROCESS FOR THE POLYMERIZATION OF ETHYLENE AND GAMMA-OLEFINS - Google Patents

Description

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The present invention relates to a reciprocating compressor intended for very high pressures and to its application to a process for the polymerization of ethylene and tf-olefins.

The present invention aims to solve the technical problems posed by the compression, under a discharge pressure greater than or equal to 150 bars, of a fluid meeting at least one of the following three conditions: - the fluid contains traces of abrasive particles .

- The fluid is not self-lubricating and it intervenes in a process not accepting external lubricant.

* - the fluid is not self-lubricating and it is involved in a process accepting external lubricants but it is a solvent for possible lubricants.

The fluid concerned by the problem of the present invention may be in the liquid or gaseous state, its temperature may be in a range from -30 ° to + 200 ° C on any stage of the compression machine, and finally it can be of very diverse chemical nature, for example helium, ammonia, ethylene, mixture of ethylene and at least one compound chosen from o-olefins, hydrogen, saturated hydrocarbons such as propane or butane.

The compression of a fluid under a discharge pressure greater than or equal to 150 bars is generally carried out in a compression machine with one or more stages, with reciprocating movement, each cylinder of which comprises on the one hand a plunger or a jacket of cylinder 25 and on the other hand a sealing element which can be either (in the case of a plunger) at least one static seal in several parts or (in the case of a cylinder liner) at least one segment movable together with the piston. In the case of packings, the dynamic seal takes place between the pack (s) and the plunger; in the case of 30 segments, dynamic sealing takes place between the cylinder liner and the segment (s).

When the fluid intended to be compressed does not meet any of the three conditions listed above, in particular when said fluid is self-lubricating and / or when it does not contain traces of abrasive particles, and / or when the surfaces in contact constituting dynamic seals are lubricated, the compression machine operates satisfactorily for a period of a few thousand hours without disturbances, up to 7000 hours, without the need to. choose with particular attention the materials constituting the k; 2 different parts of said machine. For example when Ton compresses ethylene or a mixture of ethylene and polar comonomers up to a discharge pressure often between 150 and 3000 bars, in the context of a high temperature polymerization process in the presence of ini-5 free radical initiators as described in particular in French patents no 1,584,529, 2,025,887, 2,099,267 and 2,313,399, it is common to use a reciprocating compressor including the plunger or the jacket cylinder are made of treated steel or cast iron or a material of the family of metal carbides such as tungsten and whose 10 sealing elements are made of bronze such as bronze EU 9 P containing * 9% d tin, 0.3 to 1% phosphorus and the rest in copper. This is due to the fact that the flow of ethylene to be compressed does not contain traces of abrasive particles.

On the other hand, it has been found that when the fluid to be compressed meets at least one of the three conditions listed above, a reciprocating compressor whose parts are made of commonly used materials can no longer function satisfactorily, ie that is to say without interruption, that for a number of hours limited to a few hundred or even a few tens, this number being function-20 tion of the lack of lubrication and / or the content of abrasive particles in the fluid. The analysis of the causes of this unsatisfactory operation has shown, in particular by examination with an optical and electronic microscope of the surfaces of the sealing elements, an excessively rapid wear of these, inevitably leading to seizure of the sealing elements of the compressor. . The present invention aims to provide solutions to the technical problem thus exposed.

A first object of the present invention consists of a reciprocating compressor intended for very high pressures comprising on the one hand at least one plunger or a cylinder liner made of a material chosen from cast irons, steels and tungs carbides tene and on the other hand at least one sealing element, characterized in that said sealing element consists of a composite material associating on the one hand at least one metal alloy chosen from bronzes and alloys based on titanium and on the other hand, at least one polymer chosen from polytetrafluoroethylenes, polyimides and polyimide-amides, or a graphite-silver sintered compound.

Among the bronzes that can be used for the sealing element of the compressor according to the invention is notably the UE 9 P bronze described above! -above. Among the titanium-based alloys which can be used for the sealing element 3 ″ of the compressor according to the invention include in particular titanium and silver alloys such as an alloy comprising 94% by weight of titanium and 6% by weight silver. The polymers which can be used for the sealing element of the compressor according to the invention can optionally be loaded with graphite and / or fiberglass; they can also be reinforced by a bronze weaving.

The two parts of the composite material used for the sealing element of the compressor according to the invention can be combined according to the two methods described below. According to a first method, V as-10 sociation is obtained by a bonding system of the bonding type using an adhesive. The adhesive should preferably be able to reach a peel strength of 3 KN / m or more at 100 ° C for a 10 mm wide sample. According to a second method, the association is obtained by a connection system of the vacuum brazing type using, for example, a gold-germanium or silver alloy. The association can be favored, according to the method used by a particular geometry of the composite material consisting in inserting the polymer or the sintered graphite - silver compound in a metal alloy ring.

The compressor according to the invention operates satisfactorily, that is to say without disturbances, for a duration greater than or equal to 1000 hours: - when the fluid to be compressed, liquid or gaseous, is on any stage of said compressor at a temperature between -30 ° and + 200 ° C, - when the discharge pressure of said fluid is greater than or equal to 25 150 bars, - when the reciprocating movement of the compressor is such that the relative average sliding speed d 'one part compared to the other reaches up to 5 m / s, depending on the discharge pressure, - when the fluid to be compressed contains traces of abrasive particles 30 in such a quantity that the coefficient of friction can reach 0.5 , - when the fluid to be compressed is not self-lubricating and it is not possible to obtain assistance from an external lubricant.

The reciprocating compressor according to the invention finds numerous applications in the chemical industry and in particular for the polymerization of ethylene or the copolymerization of ethylene and β-olefins under high pressure. Indeed, it is known from French Patent No. 2,021,952 to polymerize ethylene under a pressure greater than 500 bars and at a temperature between 140 ° and 300 ° C, in the presence of a catalytic system of the Ziegler type. Many catalytic systems /

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4 of this type appear to be suitable for the polymerization of ethylene, as well as for the copolymerization of ethylene with olefins, under high pressure and temperature. These systems have in common the presence, on the one hand, of at least one halogen transition metal compound and, on the other hand, of at least one activator chosen from hydrides and organometallic compounds of metals from groups I to III of the Periodic Table. Among the halogenated transition metal compounds which can be used, although they are very numerous, mention may be made of the * 7 / daily or benzyl complexes of chromium, zirconium and titanium, vanadium and titanium trichlorides 10 (the latter optionally syncrystallized with aluminum chloride in the form TiCl3, 1/3 A1C13) optionally fixed on a support comprising, for example, a halide, a monohalide or a magnesium hydrohalide.

All of these compounds can be used in the presence of complexing agents such as heavy alcohols, metal alcoholates, alkyl silicates, aryl or alkylsiloxanes, ethers or amines. Among the activators which can be used, mention may in particular be made of trialkylaluminiums, halodialkylaluminiums and alkylsiloxalanes.

In such a polymerization or copolymerization process, the mixture from the reactor is sent to a separator operating at a pressure of 50 to 500 bars, in which the polymer is separated from the unreacted monomers, then the monomers from the separator are compressed to the reaction pressure and returned to the reactor. It has been found that significant amounts of low polymers containing catalytic residues, in particular aluminum compounds or other reducing compounds, are entrained in the stream of unreacted monomers to be compressed. At the inlet of the compressor, a fraction of low polymers which can appear in the form of particles with a particle size of between 1 and 250 microns, is one of the causes of abrasion of the sealing elements of the compressor, * as soon as the residue content catalysts in the fluid to be recycled and compressed exceeds 0.5 ppm (parts per million).

A second object of the present invention consists in a process for the polymerization of ethylene in a reactor under a pressure greater than 200 bars and at a temperature greater than 170 ° C., in the presence of a catalytic system of the Ziegler type, the mixture coming from the reactor being sent to a separator under pressure between 50 and 500 bars in which the polymer is separated from the unreacted ethylene, the ethylene coming from the separator being compressed to the pressure of the reactor and returned therein, characterized in that said compression of the ethylene from the separator jj is carried out by means of a reciprocating compressor 5 according to the invention as described above. Due to the satisfactory operation of the compressor, such a method has improved reliability.

The process according to the invention can be carried out by polymerizing ethylene in the presence of 1 to 50% by weight of an inert hydrocarbon 5 such as propane or butane for example, and / or in the presence of up to 2% in moles of hydrogen. The process of the invention is carried out continuously and can be carried out by using either autoclave or tubular reactors or else by using a reactor of each type. Said reactors may each include one or more reaction zones. The invention is also applicable to the copolymerization of ethylene - with e * -olefins such as propylene and butene-1 for example, as well as to the terpolymerization of ethylene with a c * -olefin such as propylene and with an unconjugated diolefin.

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Claims (12)

1. Reciprocating compressor intended for very high pressures comprising on the one hand at least one plunger or one. cylinder liner made of a material chosen from cast irons, steels and tungsten carbides and on the other hand at least one sealing element, characterized in that said sealing element consists of a composite material combining on the one hand at least one metal alloy chosen from bronzes and titanium-based alloys and on the other hand either at least one polymer chosen from polytetrafluoroethylenes, polyimides and polyimide-amides, or a sintered graphite compound -silver. . 2. A reciprocating compressor according to claim 1, characterized in that the polymer is loaded with graphite and / or * glass fiber. 3. Reciprocating compressor according to one of claims 1 and 2, characterized in that the polymer is reinforced by a bronze fabric. 4. reciprocating compressor according to one of claims 1 to 3, characterized in that the metal alloy and the graphite-silver polymer or sintered compound of the composite material are combined by a bonding system chosen from bonding to the using adhesive and vacuum brazing. 5. Reciprocating compressor according to claim 4, characterized in that the combination of the metal alloy and the graphite-silver sintered polymer or compound of the composite material is reinforced by inserting the polymer into a metal alloy ring. 6. Application of a reciprocating compressor according to one of claims 1 to 5 to the compression of a fluid under a discharge pressure greater than or equal to 150 bars. 7. Application according to claim 6, characterized in that the temperature of said fluid is between -30e and + 200 ° C at any stage of said compressor. 8. Application according to one of claims 5 and 7, characterized in that said fluid contains traces of abrasive particles. 9. Application according to one of claims 6 to 8, characterized in that said fluid is not self-lubricating and intervenes in a process which does not accept external lubricant. 10. Application according to one of claims 6 to 9, characterized in that said fluid is not self-lubricating and intervenes in a process accepting external lubricants but is solvent for lubricants / 40 possible. "7" 11. Application according to one of claims 6 to 10, characterized in that said fluid is ethylene, optionally in admixture with at least one compound chosen from c <-olefins, hydrogen and saturated hydrocarbons. 12. Application according to claim 11 to a process for the polymerization of ethylene in a reactor under a pressure higher than 200 bars and at a temperature higher than 170 ° C, in the presence of a catalytic system of Ziegler type, the mixture from the reactor being sent to a separator under pressure between 50 and 500 bars in which the polymer is separated from the unreacted ethylene, the ethylene from the separator * being compressed to the reactor pressure and returned to - the latter, characterized in that the ethylene from the separator contains at least 0.5 ppm of residue from said catalytic system. VÀXliM