EP0726306A1 - Process for the simultaneous hydrogenation of hydrocarbon containing gases and condensates - Google Patents

Abstract

Catalytically hydrogenating a gas and a condensate contg. (un)satd. aromatic hydrocarbons with hydrogen comprises: using a hydrogen partial pressure of 1-50 bar to catalytically hydrogenate the condensate and a part of the gas in a 1st step at 160-250 degrees C; adding the remaining part of the gas to the hydrogenated prod.; and catalytically hydrogenating the mixt. obtd. in a 2nd step at 250-420 degrees C.

Description

The present invention relates to a new process for the joint hydrogenation of hydrocarbon-containing gases and condensates.

The thermal processing of plastic waste by depolymerization produces gaseous and liquid product flows. These product streams essentially contain saturated hydrocarbons, but also some significant proportions of olefinically or acetylenically unsaturated compounds. The unsaturated hydrocarbons tend to polymerize, which forms solids which interfere in subsequent processing processes, for example by deactivating catalysts or causing coking on the reactor, tube and heat exchanger surfaces.

DE-A 43 11 034 proposes that the gas, condensate and the bottom phase containing the viscous depolymerization products obtained by depolymerizing plastic wastes be drawn off in separate substreams and that the condensate and the bottom phase be worked up separately. This document also teaches a one-step hydrogenation of the condensate over a fixed bed catalyst.

This method of processing is technically complex because it requires separate technical systems for the individual streams. Furthermore, fluctuating compositions of the condensate and gas as a result of different compositions of the plastic waste can lead to large temperature fluctuations in the hydrogenation catalyst and shorten its service life.

It was an object of the present invention to provide a process which, in a technically simpler manner than in the prior art, leads to a reduction in the formation of polymers in gases and condensates containing essentially saturated and unsaturated hydrocarbons. Another aspect of the task was to find a method that enables constant operation despite fluctuations in the composition of the feed streams.

Accordingly, a process for the catalytic hydrogenation of a gas and a condensate, each containing essentially saturated and unsaturated and, in the case of the condensate, aromatic hydrocarbons, was found with hydrogen, which is characterized in that the condensate is used at a hydrogen partial pressure of 1 to 50 bar and also catalytically hydrogenated part of the gas in a first stage at 160 to 250 ° C., added the remaining part of the gas to the hydrogenation product and catalytically hydrogenating the mixture thus obtained in a second stage at 250 to 420 ° C.

The gases and condensates to be hydrogenated according to the invention are gaseous or liquid under normal pressure.

The main components of the gases contain saturated hydrocarbons such as methane, ethane, propane, n-butane, isobutane and n-pentane. As unsaturated hydrocarbons they contain e.g. Ethylene, propylene, 1-butene and 1-pentene.

In addition, the gases to be processed can contain other components such as hydrogen, carbon monoxide, carbon dioxide, ammonia, hydrogen sulfide and nitrogen, but also chlorinated carbons such as methyl chloride.

As a rule, the content of unsaturated hydrocarbons in the gas is 0.5 to 35% by weight, but in special cases it can also be higher. The gas preferably contains at least 50% by weight of saturated hydrocarbons, but only about 30% by weight can also occur in gases which contain particularly high proportions of unsaturated hydrocarbons. The total chlorine content is generally 0.01 to 1% by weight.

The condensates generally have a boiling point at approximately 60 ° C. and a boiling point at approximately 180 ° C. (ASTM D-86). As saturated hydrocarbons, they generally contain isomeric pentanes, hexanes, heptanes, octanes and nonanes, but also longer-chain alkanes. Aromatic hydrocarbons such as benzene, toluene and ethylbenzene can occur as main components, as can xylenes. The polymerizable unsaturated compounds are generally essentially styrene, methyl styrene and indene. Condensates with a styrene content of 1 to 4% by weight are preferred. The diene number of the condensate is preferably 5 to 50 g / 100 g (UOP method 326-82), the bromine absorption is 20 to 60 g / 100 g (DIN 51 774), particularly preferably 40 to 50 g / 100 g. The condensate can also contain chlorine-containing compounds (usually less than 500 ppm chlorine content, based on the mass) and sulfur compounds (usually less than 100 ppm sulfur by mass).

The gases and condensates mentioned occur in thermal and catalytic cracking plants of hydrocarbon mixtures. In particular, coke ovens, coke ovens, catalytic crackers or visbreakers are to be mentioned, the product range of which includes the gases and condensates mentioned, which can be provided, for example, by distillation if they are worked up accordingly.

However, preference is given to gases and condensates which arise after thermal depolymerization of plastic waste, such as those e.g. are described in DE-A 43 11 034 or German patent application P 43 24 112.3. The starting product in these processes is plastic waste, which is usually a mixture of polymers such as polyolefins, e.g. Polyethylene and polypropylene, polystyrene, polyvinyl chloride, blends such as ABS, but also of polyurethanes, polyesters and polyamides. If necessary, this waste is crushed and melted. The melt, e.g. at 400 to 550 ° C, converted into products, which are then usually separated by distillation. The resulting distillation residues can be returned to the reactor, solids and any acids being separated beforehand.

Particularly preferred for the preparation of the starting materials to be hydrogenated according to the invention is a procedure according to which a top product obtained at 200 to 280 ° C. is separated from the depolymerization product in a first column directly downstream of the reactor, and after partial condensation of a second column at 70 to 150 ° C. is fed and separated into a gas mixture emerging at the top of the column and a condensate obtained at the bottom of the column. The gas and condensate obtained in this way can then be hydrogenated according to the invention.

The hydrogenation is carried out in two stages. The temperature in the first stage is 160 to 250 ° C., that in the second stage 250 to 420 ° C., the temperatures of the individual stages preferably being different. The partial pressure of the hydrogen used for the hydrogenation is 1 to 50 bar, the pressure range from 5 to 20 bar being preferred.

The hydrogenation is carried out on catalysts as are known per se to the person skilled in the art for the hydrogenation of hydrocarbon streams containing unsaturated compounds (for example pyrolysis gasoline hydrogenation, Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, VCH, vol. A3, p. 489, and benzene pressure refining, Ullmann's Encyclopedia of Technical Chemistry, 4th edition, VCH, Vol. 8, p. 389). There are in particular catalysts with the catalytically active metals nickel, cobalt, chromium, molybdenum, tungsten, palladium and platinum and mixtures thereof, which are applied to supports such as aluminum oxide, silicon dioxide, titanium oxide and zirconium oxide. Commercial catalysts which contain cobalt and molybdenum or nickel and molybdenum as active catalysts, so-called Co-Mo and Ni-Mo catalysts, as described, for example, in DE-A 23 00 038, DE-A 29 03 193, are particularly preferred and EP-A 8 424.

The catalysts in both hydrogenation stages are arranged as a fixed bed. A vertical first hydrogenation reactor is preferably flowed through from bottom to top in order to allow any liquid which may occur in the reactor feed or on the catalyst to drip down and to be removed in the bottom of the reactor.

According to the invention, the gas to be hydrogenated is divided between the two hydrogenation stages. A part is hydrogenated with the condensate to be hydrogenated in the first stage. The remaining part of the gas is then added to the hydrogenation product thus obtained before the mixture thus obtained is fed to the hydrogenation in the second stage. The result is that the heat of reaction released is distributed over both stages. The distribution of the gas quantities to the first and second stages is advantageously regulated so that the temperature of the hydrogenation product emerging from the first stage is measured. If this temperature rises, less gas is metered into the first stage and vice versa. This procedure allows a relatively constant reaction temperature at the catalyst to be guaranteed even with fluctuating concentrations of individual components in the mixture to be hydrogenated.

The amount of gas added to the first stage depends on the amount of heat released during the hydrogenation of the gas, which depends on its composition, and on the reaction temperature in the first reactor. It can easily be determined by the expert by means of corresponding preliminary tests.

The process according to the invention is preferably operated in a cycle gas mode, ie gas is circulated in the hydrogenation stages, from which only the amount of hydrogenation product corresponding to the freshly supplied gas and condensate is taken off. The product obtained after the second stage can be worked up in a manner known per se, for example by distillation. Prefers is an expansion of the mixture in a column and subsequent fractionation.

The process products can be used in many ways, for example as raw gas for a steam cracker or as gasoline. It is also possible to further process the hydrogenation product in an aromatics plant to obtain pure aromatics.

Two particularly preferred embodiments of the invention are described below.

According to FIG. 1, condensate 1 with part of the gas 2 to be hydrogenated and the circulating gas 3 and hydrogen 4 is fed to an evaporator 5. If necessary, high-boiling fractions in the form of sludge are withdrawn from the bottom of the evaporator. The overhead vapors are heated against the outlet stream 6 from the second hydrogenation reactor and fed to the first hydrogenation reactor 7, which is flowed through from bottom to top. Any liquid can be removed from the sump. The remaining part of the gas 2 to be hydrogenated is added to the hydrogenation product and fed to the second hydrogenation reactor 8. Before this, the feed stream 9 is heated against the outlet stream 6 from the second hydrogenation reactor; if appropriate, a further heater is interposed. The hydrogenated product is cooled and optionally washed, e.g. with water or with sodium hydroxide solution 10 for the separation of hydrogen chloride. In a separator 11, the hydrogenated condensate and optionally washing water are drawn off via a line 12. The gas phase is partly discharged via a line 13 in order to avoid undesired constituents in the circulating gas being leveled up. The remaining part is mixed with hydrogen 4 and returned to the process via a compressor 14.

FIG. 2 shows the described method with an optimized heat composite. Here, a pre-evaporator 15 is attached in front of the evaporator 5. The pre-evaporation takes place in countercurrent to the reactor outlet stream 6 after preheating the pre-evaporator inlet stream. The stream fed directly to the evaporator 5 is also preheated against the outlet stream of the pre-evaporator 16. These measures can result in significant savings in the heating capacity of the evaporator.

In the process according to the invention, gases and condensates which essentially contain hydrocarbons are freed of unsaturated compounds, so that no undesired polymerization products occur during further processing of the process products. The special distribution of the gas flow over both hydrogenation stages ensures a constant temperature on the catalyst, which leads to longer catalyst life. By avoiding excessive temperature increases on the catalyst, the process can be operated adiabatically, ie there are no costs for corresponding components for heat removal from the reactor. It was also found that the process dehalogenates chlorine-containing hydrocarbons, such as those present in feedstocks from the depolymerization of plastic waste. Hydrogen chloride formed here can easily be separated from the process product in a wash.

Claims (7)

Process for the catalytic hydrogenation of a gas and a condensate, each of which essentially contains saturated and unsaturated and, in the case of the condensate, also aromatic hydrocarbons, with hydrogen, characterized in that the condensate and part of the gas are at a hydrogen partial pressure of 1 to 50 bar catalytically hydrogenated in a first stage at 160 to 250 ° C, the remaining part of the gas is added to the hydrogenation product and the mixture thus obtained is catalytically hydrogenated in a second stage at 250 to 420 ° C. Process according to claim 1, characterized in that the first stage is carried out in a vertical reactor through which the gas and condensate to be hydrogenated flow from bottom to top. A method according to claim 1 or 2, characterized in that one works in a cycle gas mode. Process according to claims 1 to 3, characterized in that catalysts are used in the first stage, the second stage or in both stages which contain cobalt and molybdenum or nickel and molybdenum as catalytically active constituents. Process according to claims 1 to 4, characterized in that a gas and a condensate are used which are obtained in the thermal depolymerization of plastic waste. Process according to claims 1 to 5, characterized in that a condensate with a bromine uptake of 20 to 60 g / 100 g according to DIN 51 774 is used. Process according to Claims 1 to 6, characterized in that a condensate with a styrene content of 1 to 4% by weight is used.

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