Process for producing gaseous hydrocarbons

Abstract

The invention relates to a process for producing gaseous hydrocarbons with at least one double bond, comprising: (a) cracking of starting hydrocarbons in a cracking reactor, thereby obtaining a gaseous intermediate product stream; (b) cooling of the gaseous intermediate product stream by adding a quenching liquid in a quenching apparatus, thereby obtaining a crude product stream containing the gaseous hydrocarbons with at least one double bond and a liquid stream containing the quenching liquid and condensed by-products; (c) separating the quenching liquid from the condensed by-products in a phase separator; (d) separating a BTX stream from the condensed by-products; wherein at least a part of the BTX stream is fed into the quenching apparatus, into an apparatus for caustic wash of the crude product stream and / or into at least one compressor at the suction side of the at least one compressor and / or directly into the at least one compressor.

Description

[0001] Process for producing gaseous hydrocarbons

[0002] Description

[0003] The invention relates to a process for producing gaseous hydrocarbons with at least one double bond, comprising:

[0004] (a) cracking of starting hydrocarbons in a cracking reactor, thereby obtaining a gaseous intermediate product stream;

[0005] (b) cooling of the gaseous intermediate product stream by adding a quenching liquid in a quenching apparatus, thereby obtaining a crude product stream containing the gaseous hydrocarbons with at least one double bond and a liquid stream containing the quenching liquid and condensed by-products;

[0006] (c) separating the quenching liquid from the condensed by-products in a phase separator; and

[0007] (d) separating a BTX stream from the condensed by-products.

[0008] Hydrocarbons with at least one double bond, for example ethylene, propylene or a mixture of butylene and aromatics are produced for example by steam cracking of raw materials like liquefied petroleum gas, natural gas liquids or naphtha. The steam cracking process is described for example in Ullmann’s Encyclopedia of Industrial Chemistry, “Ethylene”, Volume 13, 2012, pages 469 to 522. For cracking, the raw materials are fed into a furnace, in which the raw material are cracked by steam cracking forming a raw gas containing the desired product. The raw gas obtained in the furnace then is fed into a working up section for cooling and separating the components in the raw gas.

[0009] It is a disadvantage of the known process that during a continuous operation fouling occurs in different parts of the production plant. For removing fouling after a defined production period plant parts are cleaned with washing oils.

[0010] Washing oils used for removing fouling are for example oils containing 9 or more carbon atoms and are described for example in KR-A 2001-0002601 . Further, also CN-A 103087759 mentions that hydrocarbons with more than 9 carbon atoms may be used as compressor washing oil. KR- A 2001-0092131 describes the use of heavy gas oil, light gas oil and kerosene for cleaning devices in refineries and petrochemical plants. The cleaning process is carried out during a shutdown of the plant.

[0011] A process for cleaning compressors and / or intercoolers, particularly in a methanol to olefin separation system in described in EP-B 1 562 881 . Here, a washing liquid, particularly an aromatic compound is injected into the compressor and / or the intercooler. Alternatively, the washing liquid may comprise water or may be a heavy hydrocarbon stream from the separation system or a byproduct from the methanol to olefin reaction system. For removing contaminants, the washing liquid is injected into the compressor and / or the intercooler from 1 to 24 hours.

[0012] EP-A 3 026 101 describes a wash oil as an antifouling agent in gas compressors. The wash oil preferably is prepared out of mono-, di- and tri-substituted benzene. Depending on the use, the wash oil may comprise further components, e.g. other antifouling agents, metal scavenger and / or pH control additives.

[0013] Discontinuous cleaning of a coking tower using crude benzene is described in CN-A 101157865. For cleaning, the crude benzene is cooled down to a temperature in a range from 25 to 40 °C and then sprayed into the coking tower at the top. After cleaning, the crude benzene with the dirt is collected and the dirt is removed from the crude benzene to reuse the crude benzene for a following washing step when required.

[0014] The separation of water, liquid and gas phases in general is known from several processes. DE- A 102018 103 552 describes a process for producing a synthetic gasoline by catalytic reaction of alcohols to form a reaction mixture comprising water and a mixture of hydrocarbons, separating the reaction mixture into a liquid phase containing hydrocarbons, an aqueous phase containing not reacted alcohols and a gas phase containing Ci- to Cs-hydrocarbons and subsequently separating the not reacted alcohols from the aqueous phase. The alcohols then may be recycled into the reaction.

[0015] Phase separation of pyrolysis gasoline from water obtained from a steam cracking step using a coalescer is described in WO-A 2012 / 104769. The coalescer is made of metal, fiber glass or a combination of metal and fiber glass.

[0016] It is a disadvantage of all processes that for removing fouling from devices used in a steam cracking process either the process must be shut down or that for cleaning additional washing liquids are used which are not part of the components used in the process and, thus, must be removed after the washing and undergo further treatment or be disposed causing additional waste.

[0017] Therefore, it is an object of the present invention to provide a process for producing gaseous hydrocarbons with at least one double bond which allows removal of fouling without shutting down the process and where no additional components are used for washing.

[0018] This object is achieved with a process for producing gaseous hydrocarbons with at least one double bond, comprising:

[0019] (a) cracking of starting hydrocarbons in a cracking reactor, thereby obtaining a gaseous intermediate product stream; (b) cooling of the gaseous intermediate product stream by adding a quenching liquid in a quenching apparatus, thereby obtaining a crude product stream containing the gaseous hydrocarbons with at least one double bond and a liquid stream containing the quenching liquid and condensed by-products;

[0020] (c) separating the quenching liquid from the condensed by-products in a phase separator; and

[0021] (d) separating a BTX stream from the condensed by-products, wherein at least a part of the BTX stream is fed into at least one of: the quenching apparatus, an apparatus for caustic wash of the crude product stream at least one compressor for conveying and / or compressing gas streams containing the gaseous hydrocarbons with at least one double bond at the suction side of the at least one compressor and / or directly into the at least one compressor.

[0022] Surprisingly it has shown that the BTX stream which is fed into the quenching apparatus, and / or into the apparatus for caustic wash of the crude product and / or at the suction side of at least one compressor acts as an efficient processing aid. Feeding the at least part of the BTX stream into the quenching apparatus has the positive effect that it improves the phase separation of the quenching liquid and the condensed by-products and feeding a part of the BTX stream into the suction side of a compressor has the advantage that deposits are removed and fouling is reduced or even prevented.

[0023] It is a further advantage of using the part of the BTX stream as processing aid, that the BTX must not be separated from the process streams in a specific, additional device but can follow the standard process and be removed at that stage at which aromatic by-products usually are removed. Particularly it is not necessary to remove the BTX stream directly after passing the respective device in which it is used as processing aid.

[0024] In the context of the present invention, the BTX stream comprises benzene, toluene, xylene and essentially no styrene. Additionally, the BTX stream may comprise ethyl benzene and further components. “Essentially no styrene” in the context of the present invention means that the amount of styrene in the BTX stream is below 800 wt-ppm, preferably below 600 wt-ppm, more preferred below 400 wt-ppm and particularly such that it is below the detection limit.

[0025] Preferably, the BTX stream consists of 30 to 50 wt-% benzene, 10 to 25 wt-% toluene, 2 to 8 wt- % xylene, 4 to 9 wt-% ethyl benzene and less than 45 wt-% other components, particularly, the BTX stream consists of 35 to 45 wt-% benzene, 10 to 20 wt-% toluene, 4 to 6 wt-% xylene, 5 to 8 wt-% ethyl benzene and less than 40 wt-% other components. Other components which may be contained in the BTX stream for example are cyclopentane, cyclopentene, n-hexane, methyl cyclopentane and methyl cyclopentene. In a process for producing hydrocarbons with at least one double bond, generally cracking processes are used. A typical cracking process is steam cracking as described for example in Ullmann’s Encyclopedia of Industrial Chemistry, “Ethylene”, Volume 13, 2012, pages 469 to 522.

[0026] In a first stage of the process, starting hydrocarbons are fed into a cracking reactor in which they are cracked into the hydrocarbons with at least one double bond, thereby obtaining a gaseous intermediate product stream.

[0027] Typical hydrocarbons used for producing the hydrocarbons with at least one double bond are saturated aliphatic hydrocarbons. The starting hydrocarbons may be of fossil origin, or biobased hydrocarbons or may be obtained by chemical recycling, for example by ChemCycling® as described for example on “https: / / www.basf.com / global / de / who-we-are / sustainability / we- drive-sustainable-solutions / circular-economy / mass-balance-approach / chemcycling.html”, retrieved on March 22, 2024, or may be obtained by any other chemical process or be a byproduct thereof, or may be a combination of hydrocarbons obtained by any of those or all mentioned processes. Preferably, the starting hydrocarbons used in the process for producing hydrocarbons with at least one double bond are at least one of liquefied petroleum gas, natural gas liquids, like ethane, propane, butane, isobutane, and pentane, particularly ethane or propane, and naphtha. Hydrocarbons with at least one double bond produced in the cracking reactor and contained in the gaseous intermediate product stream preferably are ethylene, propylene and / or butene, particularly butene with one double bond. Particularly preferably the hydrocarbons produced with one double bond are ethylene and / or propylene.

[0028] For obtaining starting hydrocarbons by ChemCycling® or recycling, waste products are converted by pyrolysis, i.e. by thermal decomposition or degradation of a feedstock such as plastic waste under inert conditions that results in a gas, a liquid, and a solid char fraction. During the pyrolysis, the feedstock is converted in a pyrolysis unit into a great variety of chemicals including gases such as H2, Ci- to C4-alkanes, C2- to C4-alkenes, ethyne, propyne, 1 -butyne, pyrolysis liquid having a boiling temperature of 25 °C to 500 °C or more and char. The direct products from such a pyrolysis are “pyrolysis gas” and solid products. The liquid product “pyrolysis liquid” is then separated by condensation from the “pyrolysis gas”. In addition, water is formed during the pyrolysis. The water may be partially dispersed in the pyrolysis liquid and may be partially contacted with the pyrolysis liquid in a separate phase. The water formed during pyrolysis comprises various organic compounds and / or salts thereof, which were also formed during the pyrolysis. The term “pyrolysis” includes slow pyrolysis, fast pyrolysis, flash pyrolysis and catalytic pyrolysis. These pyrolysis types differ regarding process temperature, heating rate, residence time, feed particle size, etc. resulting in different product quality. The pyrolysis unit may be operated adiabatically, isothermally, nonadiabatically, non-isothermally, or combinations thereof. The pyrolysis reactions of this disclosure may be carried out in a single stage or in multiple stages. For example, the pyrolysis unit can comprise two reactor vessels fluidly connected in series. The term “pyrolysis liquid” is understood to mean any oil originating from the pyrolysis of plastic waste. The term “plastic waste” includes rubber waste such as end-of-life tires and feedstocks comprising plastic waste. The pyrolysis liquid is obtained and / or obtainable from pyrolysis of such plastic waste.

[0029] The term “plastic waste” refers to any plastic material discarded after use, i.e., the plastic material has reached the end of its useful life and is considered post-consumer waste. The plastic waste can be pure polymeric plastic waste, mixed plastic waste or film waste, including soiling, adhesive materials, fillers, residues etc. The plastic waste may have an oxygen content, a nitrogen content, sulfur content, halogen content and optionally also a heavy metal content. The plastic waste can originate from any plastic material containing source.

[0030] Accordingly, the term “plastic waste” includes industrial and domestic plastic waste and including used tires and agricultural and horticultural plastic material.

[0031] Typically, plastic waste is a mixture of different plastic materials, including hydrocarbon plastics, e.g., polyolefins such as polyethylene (HDPE, LDPE) and polypropylene, polystyrene, and copolymers thereof, etc., and polymers composed of carbon, hydrogen, and other elements such as chlorine, fluorine, oxygen, nitrogen, sulfur, silicone, etc., for example chlorinated plastics, such as polyvinylchloride (PVC), polyvinylidene chloride (PVDC), etc., nitrogen-containing plastics, such as polyamides (PA), polyurethanes (PU), acrylonitrile butadiene styrene (ABS), etc., oxygen-containing plastics such as polyesters, e.g., polyethylene terephthalate (PET), polycarbonate (PC), etc., silicones and / or sulfur bridges crosslinked rubbers.

[0032] Typically, the plastic material comprises additives, such as processing aids, plasticizers, flame retardants, pigments, light stabilizers, lubricants, impact modifiers, antistatic agents, antioxidants, etc. These additives may comprise elements other than carbon and hydrogen. For example, bromine is mainly found in connection to flame retardants. Heavy metal compounds may be used as lightfast pigments and / or stabilizers in plastics. Cadmium, zinc, and lead may be present in heat stabilizers and slip agents used in plastics manufacturing. The plastic waste can also contain residues. Residues in the sense of the invention are contaminants adhering to the plastic waste. The additives and residues are usually present in an amount of less than 50 wt.- %, preferably less than 30 wt.-%, more preferably less than 20 wt.-%, even more preferably less than 10 wt.-%, based on the total weight of the dry weight plastic.

[0033] Examples of rubber waste (which is also considered “plastic waste” in the sense of the present invention) include end-of-life tires, rubber waste produced during manufacturing processes and discarded rubber containing products such as latex examining gloves and gaskets. End-of-life tires comprise further ingredients such as textiles and organic and inorganic additives which may be separated from the rubber portion of end-of-life tires prior to pyrolysis. Pyrolysis liquids obtained by pyrolysis of (predominantly) end-of-life tires are also known as tire pyrolysis oils (TPO). Processes to obtain bio-based starting hydrocarbons typically start with the conversion of biomass to bio-oil, e.g., via mechanical operations and chemical processes. Due to its chemical composition, especially due to its high oxygen content, said bio-oil is typically not directly suitable to be used in cracking processes to obtain olefins, aromatics, and other cracking products, but needs to be further refined and / or upgraded, especially catalytically hydrotreated. This hydrotreatment yields hydrocarbons that may be separated into different fractions like renewable fuels (HVO, SAF), bio-naphtha, and bio-based Ci ^-hydrocarbons (bio-Ci-4-HCs) which can be used as feed streams for cracking processes, e.g. steam cracking.

[0034] Animal fats, vegetable oils (e.g., rapeseed, sunflower, soybean, palm, and camelina oil), waste oils and fats (e.g., used cooking oil, waste animal fats), microbial and algal oils, and fatty acids represent the most important biomass-derived raw materials for bio-based hydrocarbon production. Among the major pathways towards bio-based hydrocarbons is the catalytic hydrotreatment of these mono-, di-, and triglycerides and fatty acids, which includes hydrogenation, decarboxylation, decarbonylation, hydroisomerization, and cracking processes under high temperature and pressure conditions, frequently also including a catalytic isomerization step, resulting in a hydrocarbon mixture comprising n- and iso-paraffins, among others. These reaction products may be further separated into gaseous and liquid fractions, which constitute valuable transportation fuels and chemical feedstocks, e.g., as renewable diesel (hydrotreated vegetable oils: HVOs), renewable jet fuel (sustainable aviation fuel: SAF), bio-naphtha (a mixture of hydrocarbons mainly comprising paraffins, e.g. of up to 10 carbon atoms, that can be used - similar to naphtha of fossil origin - as a gasoline blending component or as a chemical feedstock, e.g., for crackers), and other low-boiling hydrocarbons (i.e. mainly C1-4 hydrocarbons, in particular C1-4 alkanes) like bio-based liquefied petroleum gas (LPG; e.g. bio-based butane, propane, and ethane).

[0035] The term biomass, as used herein, designates any material of vegetable or animal origin that is in principle suitable to be converted at least into bio-oils. In particular, the term biomass comprises plants or parts thereof like crops, wood, or residues thereof, marine organisms like algae, and bio waste such as organic food waste, e.g., animal fat from meat industry waste, fish fat from fish processing waste, or used cooking oil. For instance, biomass may comprise or be derived from algae, oil crops, oil palms, soybeans, rapeseed, mustard, flax, cottonseed, sunflower, corn, castor beans, hemp, field pennycress, pongamia, jatropha, macauba palm (kernel or pulp), mahua, camelina, salicornia, carinata, lignocellulose, wood, forestry residues, agricultural residues, crop residues, straw, residues from vegetable oil production, green waste, food waste, and used vegetable cooking oil. Of note, the biomass may be composed of biomass streams from various of the above-mentioned sources.

[0036] The processing of biomass into bio-oil may comprise both mechanical and physical operations, like harvesting and collecting as well as crushing, cracking, cutting, shredding, grinding, chipping, milling, extrusion, irradiation, squeezing, pressing, filtering, sieving, adsorption, and thermal treatments such as drying and torrefaction, and chemical processes, like extraction, distilla- tion, thermochemical conversions like pyrolysis or hydrothermal liquefaction, gasification followed by Fischer-Tropsch processes, hydrolysis, saponification, neutralization, ketonization, or hydrogenation. Also, the mechanical, physical, and / or chemical separation of the products and by-products of said operations and processes, in particular the separation of gaseous, liquid, and solid fractions like solid biomass residues and biomass waste, forms part of the processing of biomass into bio-oils. In essence, processing of biomass into bio-oil comprises purification steps, inter alia the removal of all by-products from the bio-oil that are not suitable or are detrimental for further use as a feedstock for subsequent hydrotreatment. The right choice of suitable process steps and operating conditions is mainly dependent on the biomass to be processed; but the one skilled in the art will be familiar with such considerations. In particular, processing biomass into bio-oil suitable may include the removal of solids, ash particles, and / or metal residues, e.g., via filtration and adsorption steps. Further, said processing may include extraction, distillation, neutralization, esterification, and ketonization steps, e.g., to remove water, oxygen-rich species, and / or high-boiling components. Said process steps may also be used to increase the stability and / or the heating value of the bio-oil or to reduce its viscosity and / or its corrosivity.

[0037] It is to be understood that processing the biomass into bio-oil may also comprise purification steps, e.g., to remove contaminants or impurities that may be detrimental for the further process steps or for further use of the end-products of the process.

[0038] In common and generally used processes, the starting hydrocarbons are thermally cracked with steam in the cracking reactor. As this reaction is highly endothermic, the reaction is carried out at high temperatures, generally in the range from 750 to 875 °C. For achieving the required temperature, the starting hydrocarbons may be heated for example by heat exchange against flue gas in a convection section of the cracking reactor, mixed with steam and further heated to incipient cracking temperature, which generally is in a range from 500 to 680 °C. Subsequently, the reaction stream containing the steam and the starting hydrocarbons may be fed for example into a fired tubular reactor, in which the starting hydrocarbons are cracked into smaller molecules within a short reaction time which generally is in a range between 0.1 and 0.5 s. However, besides this process, the cracking may be carried out in any other reactor known to the skilled person in which the starting hydrocarbons can be cracked into the desired hydrocarbons having at least one double bond.

[0039] To stop the reaction and achieve a sufficient yield of the desired hydrocarbons with at least one double bond, the gaseous intermediate product stream is rapidly cooled to a temperature at which the reaction stops and also the formation of undesired by-products is minimized. This rapid cooling generally is carried out in a transfer line exchanger (TLE). Subsequently, the gaseous intermediate product stream is cooled by adding a quenching liquid in a quenching apparatus, thereby obtaining the crude product stream containing the gaseous hydrocarbons with at least one double bond and the liquid stream containing the quenching liquid and condensed byproducts. Typical by-products obtained in the cracking process are acetylenic, diolefinic and aromatic compounds, for example benzene, ethyl benzene, toluene, xylene, styrene, cyclopentane, cyclopentene, n-hexane, methyl cyclopentane, and methyl cyclopentene. As these by-products usually have a boiling temperature above the boiling temperature of the hydrocarbons with at least one double bond which are obtained in the cracking reaction as desired products, they are condensed in the cooling step (b) and then referred to as the “condensed by-products”.

[0040] To rapidly cool the gaseous intermediate product stream, the TLE preferably is directly connected to the cracking reactor to avoid additional reaction time due to the time the gaseous intermediate product stream needs for flowing from the cracking reactor to the TLE.

[0041] The quenching liquid used for cooling the intermediate product stream in the quenching apparatus following the TLE may be any liquid which is inert to the components in the intermediate product stream and which is inflammable at the temperature the intermediate product stream comes into contact with the quenching liquid.

[0042] Quenching of the intermediate product stream may be carried out in one or more quenching steps. Independently of whether cooling is carried out in only one quenching step or in more than one quenching step, it is preferred that the quenching liquid used in the quenching apparatus in step (b) is water.

[0043] However, preferably, quenching is carried out in more than one quenching step, more preferred in at least two quenching steps and particularly in two quenching steps. In these cases, it is preferred that the intermediate product stream is cooled in an oil quench before being fed into the cooling in step (b). The oil being used for cooling the gaseous intermediate reaction product in the oil quench may be for example C to Ci6 hydrocarbons with a boiling point below 250 °C, preferably with a boiling point below 200 °C.

[0044] If cooling is carried out in more than two quenching steps, at least the last quenching step is the cooling step (b) in which water is used as the quenching liquid. In this case, in the first quenching step an oil as previously described for quenching in two steps is used and in the at least one further quenching step between the first quenching step and the last quenching step in which water is used as quenching liquid, either an oil, particularly the same oil as in the first quenching step, or water may be used as quenching liquid. In this case it is further preferred, that in quenching steps following the first quenching step in which water is used as a quenching liquid also water is used as the quenching liquid, so that independently of the number of quenching stages, first an oil is used as quenching liquid and then water.

[0045] In the case of two or more quenching steps, the first quenching apparatus is connected downstream to the TLE and the following quenching apparatuses may be connected directly to the outlet of the first quenching apparatus or may be connected to the first quenching apparatus by suitable connecting pipes. By quenching, a crude product stream containing the gaseous hydrocarbons with at least one double bond and a liquid stream containing the quenching liquid and condensed by-products are obtained.

[0046] The crude product stream containing the gaseous hydrocarbons with at least one double bond may be transferred into a working-up treatment for removing undesired gaseous products and separating the different hydrocarbons with at least one double bond and obtain the desired products in desired purity. For removing acid gases from the crude product stream, the crude product stream is treated by a caustic wash. Subsequently, the crude reaction product may be further worked up to obtain the pure products. Working-up the crude product stream after the caustic wash may be carried out by any processes known to a skilled person, for example condensation, extraction, distillation, rectification and / or hydrogenation. Preferably, the crude product stream is worked-up by rectification and subsequent hydrogenation.

[0047] As the condensed by-products in the quenching liquid also may be valuable products and as it is further preferred to reuse the quenching liquid, the liquid phase containing the quenching liquid and the condensed by-products is further worked up to separate the condensed by-products from the quenching liquid. Separating the condensed by-products from the quenching liquid may be carried out in any suitable separation apparatus. Particularly if the quenching liquid is water, the separation apparatus preferably is a phase separator. Suitable phase separators for example are gravity separators or coalescers.

[0048] After being separated, the quenching liquid can be recycled into the quenching apparatus. As residues of the condensed by-products which are not separated in the separation step result from the reaction process, it is not necessary to further clean the quenching liquid before recycling into the quenching apparatus. However, to avoid accumulation of components which are not separated off in the phase separation, it either may be necessary to further clean the quenching liquid for removing these components or to withdraw a part of the quenching liquid and add a respective amount of fresh quenching liquid.

[0049] The valuable compounds contained in the condensed by-products preferably are separated and stored or delivered to processes which use these components. The products obtained from the condensed by-products may be either pure components or mixtures of at least two components of the by-products. Such mixtures for example are pyrolysis oil and BTX. Pyrolysis oil generally contains aliphatic and aromatic hydrocarbons compounds, with aliphatic hydrocarbons, particularly saturated aliphatic hydrocarbons being the majority of the components contained in the pyrolysis oil. BTX is a mixture of particularly aromatic hydrocarbons from which styrene has been removed and which has a composition as described above.

[0050] The BTX stream may be separated from the condensed by-products for example by rectification, the BTX stream being condensed over the top of the column, by distillation or by extraction. Further, be BTX stream may be separated from the condensed by-products by distillation or extraction processes. If the BTX stream still contains reactive components, for example styrene, a selective hydrogenation of the reactive components, particularly the styrene, may be carried out.

[0051] According to the invention, a part of the BTX stream obtained by separation from the condensed by-products is fed into the quenching apparatus, and / or into an apparatus for caustic wash of the crude product stream and / or the suction side of at least one compressor. The part of the BTX stream not being fed into the quenching apparatus, the apparatus for caustic wash and / or the at least one compressor may be hydrogenated.

[0052] By feeding the at least part of the BTX stream into the quenching apparatus, the following phase separation of the liquid phase containing the quenching liquid and the water is improved. The additional BTX has the effect that the amount of styrene in the quenching liquid, particularly the water, is at least reduced. Preferably, by feeding the at least part of the BTX stream into the quenching apparatus, the styrene can be fully separated from the water used as quenching liquid. Here, the BTX acts as an extraction medium for the styrene. “Fully separated” in this context means that the amount of styrene in the water used as quenching liquid is below the detection limit.

[0053] For fully separating the styrene from the water used as quenching medium, it is preferred that the amount of the at least part of the BTX stream being fed into the quenching apparatus is in a range from 0.01 to 10 wt-%, preferably in a range from 0.1 to 7 wt-% and particularly in a range from 0.5 to 5 wt-%, each based on the amount of the gaseous intermediate product stream being fed into the quenching apparatus.

[0054] The part of the BTX stream may be fed into the quenching apparatus as a mixture with the quenching liquid, particularly the water, or, alternatively, as a separate stream. If the part of the BTX stream is fed into the quenching apparatus as a separate stream, it is preferred to feed the part of the BTX stream at the same position as the quenching liquid. However, preferably, the BTX stream is fed into the quenching apparatus as a mixture with the quenching liquid.

[0055] Removing the styrene from the water used as quenching liquid has the positive effect that fouling in following devices through which the water flows can be reduced or even prohibited as styrene tends to polymerize and form polystyrene deposits on walls of devices used in the process.

[0056] For cleaning the apparatus and also avoid fouling a part of the BTX stream may be fed into the caustic wash in which acidic gases are removed from the crude product stream.

[0057] The apparatus for caustic wash preferably comprises a caustic wash column and a phase separator. In the caustic wash column, the crude product stream is brought into contact with a caustic to remove acid gases from the crude product stream. A washed crude product stream is obtained as a gas stream at the top of the caustic wash column and a liquid phase containing the spent caustic is obtained at the bottom of the caustic wash column. The liquid phase is fed into the phase separator, in which the liquid phase is separated into an organic phase and an aqueous phase, the aqueous phase containing the spent caustic. The organic phase preferably is recycled into the process, particularly into the cracking reactor or into the quenching apparatus. The aqueous phase may be further worked up in a caustic stripper in which the caustic is cleaned, so that it can be used again in the caustic wash.

[0058] The spent caustic solution, which is obtained after caustic wash, particularly at the bottom of the caustic wash column, contains water, caustic, dissolved organic compounds and inorganic compounds. To improve the separation of organic compounds from water and caustic, the spent caustic solution preferably is mixed with organic phase before being fed into the phase separator. This organic phase may be pyrolysis oil, not hydrogenated pyrolysis oil or BTX, which all are produced in the cracking reactor and subsequently separated from the crude product. The not hydrogenated pyrolysis oil contains unsaturated hydrocarbons which may enhance fouling of the organic phase. Hydrogenated pyrolysis oil may be applied to enhance the separation of the organic phase from caustic and water, however, adding BTX to the spent caustic solution shows the best result in terms of extraction of organic compounds. Therefore, it is preferred to feed at least a part of the BTX stream into the phase separator downstream of the caustic wash column for improving the separation of organic compounds from the caustic and water.

[0059] The amount of the at least part of the BTX stream being fed into the apparatus for caustic wash of the crude product stream preferably is in a range from 0.01 to 1 wt-%, more preferred in a range from 0.05 to 0.9 wt-% and particularly in a range from 0.1 to 0.8 wt-%, each based on the amount of the crude product stream being fed into the apparatus for caustic wash.

[0060] The at least part of the BTX stream which is fed into the apparatus for caustic wash may be fed into the caustic wash column at any suitable position and / or into the phase separator of the apparatus for caustic wash. If the BTX stream is fed into the caustic wash column, it is preferred that the at least part of the BTX stream is fed into the column in a mixture with the caustic solution or, if the BTX stream is fed in a separate stream, preferably at the same position as the caustic solution.

[0061] However, preferably, the BTX stream is fed into the phase separator of the apparatus for caustic wash, or into the liquid phase obtained in the caustic wash column upstream the phase separator, to improve phase separation in the phase separator of the apparatus for caustic wash.

[0062] As unsaturated hydrocarbons contained in the gaseous intermediate product stream and also in following gaseous streams resulting from working up the gaseous intermediate product stream may deposit and oligomerize or polymerize on parts of the plant at higher temperatures, particularly at temperatures in a range from 50 to 90 °C, measures have to be taken to avoid the deposition of the unsaturated hydrocarbons. Such deposits particularly occur on surfaces of compressors, thereby forming surface layers reducing the efficiency of the compressor. To avoid such surface layers, a part of the BTX stream can be fed at the suction side of compressors by which gas streams containing the unsaturated hydrocarbons are conveyed and / or compressed and in which the gas stream still has a temperature above 50 °C.

[0063] To avoid fouling in compressors, the amount of the at least part of the BTX stream being fed into at least one compressor preferably is in a range from 0.05 to 2 wt-%, more preferred in a range from 0.1 to 1 wt-% and particularly in a range from 0.2 to 0.5 wt-% based on the amount of gas being fed into the at least one compressor.

[0064] The part of the BTX stream may be fed into the production plant either into the quenching apparatus, or into the apparatus for caustic wash or into at least one compressor, into the quenching apparatus and the apparatus for caustic wash, or into the quenching apparatus and into at least one compressor, or into the apparatus for caustic wash and into at least one compressor, or into the quenching apparatus and into the apparatus for caustic wash and into at least one compressor. It is particularly preferred to feed a part of the BTX stream into the quenching apparatus, into the apparatus for caustic wash and into at least one compressor.

[0065] For improving the production process of the hydrocarbons with at least one double bond, it is preferred to feed the part of the BTX stream continuously into the quenching apparatus, into the apparatus for caustic wash and / or into at least one compressor. Further, it may be preferred to feed a BTX stream to at least one compressor and / or the apparatus for caustic wash discontinuously for cleaning the respective device. For cleaning, the BTX stream may be fed into the respective device either during regular operation of the plant or during a shutdown for cleaning. Preferably, the BTX stream for cleaning is fed into the compressor and / or into the apparatus for caustic wash during regular operation of the process for producing hydrocarbons with at least one double bond.

[0066] If a BTX stream is fed into at least one compressor or into the apparatus for caustic wash discontinuously for cleaning, it is preferred that the amount of the BTX stream being fed discontinuously to the at least one compressor at the suction side of the compressor and / or the caustic wash column and / or the caustic stripper is in a range from 0.1 to 1 wt-%, more preferred in a range from 0.3 to 0.8 wt-% and particularly in a range from 0.4 to 0.5 wt-%, each based on the gaseous feed stream fed to the respective apparatus.

[0067] The BTX stream fed discontinuously for cleaning into the at least one compressor and / or into the apparatus for caustic wash, particularly the caustic wash column and / or the caustic stripper, may be a separate BTX stream or a part of the BTX stream obtained in step (d) by separating the BTX stream from the condensed by-products. Preferably, the BTX stream used for cleaning the at least one compressor and / or the apparatus for caustic wash is a part of the BTX stream obtained in step (d).

[0068] Independently of whether being used as a processing aid or for cleaning, using the BTX stream has the advantage that it must not be removed from the gas streams or liquid streams (in the following “process stream”) leaving the respective apparatus in which it was added directly after passing the apparatus but can remain in the process stream, because the additional BTX may support following process steps and will be removed by the regular working-up processes of the crude product stream or the liquid streams obtained in the process.

[0069] Examples

[0070] 1 . BTX to improve phase separation after the water quench

[0071] Coming from the cracking furnaces and TLE, the 200 to 250°C cracked raw gas - containing all cracking products and water vapor -first enters an oil-quench column. Here, all high-boiling cracking by-products are condensed at about 100°C and separated from the cracked raw gas stream. The aromatic fraction BTX remains in the gas phase and leaves the column overhead with other light boilers. Within the following water-quench column, the 100°C hot cracked raw gas stream is cooled to about 30°C. The BTX components condense together with other cracking by-products and are finely dispersed in the quench-water. By phase separation, it is intended to separate all hydrocarbons from the quench-water. Styrene, in particular, must be removed from the water to hinder fouling of the downstream processes.

[0072] From the quench water prepared as explained above, a sample was taken and analyzed using gas chromatography. An average value of 17 mg styrene / kg quench-water was detected.

[0073] Into samples of quench-water before phase separation, different amounts of BTX were dosed and then mixed for 10 minutes using a stirrer. Afterwards, the time until phase separation was finalized was measured using a micro chronometer and the rest styrene content within the water fraction of the quench-water after phase separation was measured as described before for the styrene content. Starting with an amount of 5 wt-% BTX based on the amount of quench-water added to the quench-water before phase separation, no styrene could be detected within the water fraction of the quench-water after phase separation. Additionally, the time until phase separation was finished was reduced by about 40%.

[0074] 2. BTX to clean raw gas compressor during process

[0075] Parts from the overall amount of BTX and other cracking by-products that do not condense within the water-quench column leave it together with gaseous components in the raw gas stream and enter the raw gas compressors. Within the raw gas compressors, the raw gas is compressed at ambient temperature to a pressure of 0.5 to 30 bar(g) in five pressure stages. During this process step, hydrocarbons with double bonds such as styrene can oligomerize or polymerize and be deposited on the surfaces in the raw gas compressors. By injecting in total additional 0.3 wt-% BTX, based on the amount of raw gas, equally distributed into the wheel space of the raw gas compressors, these deposits / caking are removed. The additionally injected BTX as well as the dissolved deposits / caking leave the raw gas compressor together with the condensate. 3. BTX to improve phase separation after caustic wash

[0076] The compressed raw gas stream still contains CO2 and other acidic components, which are removed in a caustic wash. For this purpose, the compressed raw gas stream is brought into contact with a caustic. Afterwards, the used caustic also contains water and dissolved organic and inorganic molecules. With a phase separation, the dissolved molecules are separated from the caus- tic / water mixture.

[0077] Before phase separation, samples were taken and analyzed. Simulating the phase separation as done within the large-scale process using a 1 I sample in small scale phase separator, about 45 wt-% of the organic molecules could be removed from the caustic / water mixture proved by the complete extraction of the present organic phase with dichloromethane (DCM). With the addition of about 36 wt% of BTX to the 1 I sample, which corresponds to 1 wt% BTX based on the amount of the gaseous intermediate product, the amount of removed organic molecules from the caustic / water phase could be increased to about 65 wt%. That helps to improve phase separation and reduce fouling within the caustic wash column as the caustic increases oligomerization and polymerization of the organic phase as well as during phase separation to remove the organic components.

[0078] 4. Comparative example

[0079] A phase separation test was performed as described in example 3 with using a pyrolysis oil fraction as an extraction medium instead of the BTX. Pyrolysis oil is one of the steam cracker products and can be partially recycled into the system. The pyrolysis oil contained 5 wt% ethylbenzene, 6 wt% xylene, 15 wt% C9+ aromatic hydrocarbons, the rest was other C9+ hydrocarbons.

[0080] With the addition of about 36 wt% of pyrolysis oil to the 1 I sample, which corresponds to 1 wt% pyrolysis oil based on the amount of the gaseous intermediate product, the amount of removed organic molecules from the caustic / water phase reached approximately 51 wt%.

Claims

Claims1 . A process for producing gaseous hydrocarbons with at least one double bond, comprising:(a) cracking of starting hydrocarbons in a cracking reactor, thereby obtaining a gaseous intermediate product stream;(b) cooling of the gaseous intermediate product stream by adding a quenching liquid in a quenching apparatus, thereby obtaining a crude product stream containing the gaseous hydrocarbons with at least one double bond and a liquid stream containing the quenching liquid and condensed by-products;(c) separating the quenching liquid from the condensed by-products in a phase separator;(d) separating a BTX stream from the condensed by-products; wherein at least a part of the BTX stream is fed into at least one of: the quenching apparatus, an apparatus for caustic wash of the crude product stream at least one compressor for conveying and / or compressing gas streams containing the gaseous hydrocarbons with at least one double bond at the suction side of the at least one compressor and / or directly into the at least one compressor.

2. The process according to claim 1 , wherein the starting hydrocarbons are at least one of liquefied petroleum gas, natural gas liquids and naphtha.

3. The process according to claim 1 or 2, wherein the hydrocarbons with at least one double bond are ethylene and / or propylene.

4. The process according to any of claims 1 to 3, wherein the BTX stream consists of 35 to 50 wt-% benzene, 10 to 20 wt-% toluene, 2 to 8 wt-% xylene, 4 to 8 wt-% ethyl benzene and less than 30 wt-% other components.

5. The process according to claim 4, wherein one of the other components is styrene and the amount of styrene in the BTX stream is less than 400 wt-ppm.

6. The process according to any of claims 1 to 5, wherein the amount of the at least part of the BTX stream being fed into the quenching apparatus is in a range from 0 to 10 wt-%based on the amount of the gaseous intermediate product stream being fed into the quenching apparatus.

7. The process according to any of claims 1 to 6, wherein the amount of the at least part of the BTX stream being fed into the apparatus for caustic wash of the crude product stream is in a range from 0 to 1 wt-% based on the amount of the crude product stream being fed into the apparatus for caustic wash.

8. The process according to any of claims 1 to 7, wherein the amount of the at least part of the BTX stream being fed into at least one compressor is in a range from 0 to 0.5 wt-% based on the amount of gas being fed into the at least one compressor.

9. The process according to any of claims 1 to 8, wherein the quenching liquid used in the quenching apparatus in step (b) is water.

10. The process according to any of claims 1 to 9, wherein the intermediate product stream is cooled in an oil quench before being fed into the cooling in step (b).11 . The process according to any of claims 1 to 10, wherein the cracking reactor is a steam cracker.

12. The process according to any of claims 1 to 11 , wherein a BTX stream is fed to at least one compressor and / or the apparatus for caustic wash discontinuously for cleaning.

13. The process according to claim 12, wherein the BTX stream being fed discontinuously for cleaning into the at least one compressor and / or the apparatus for caustic wash is fed during operating the process.

14. The process according to claim 12 or 13, wherein the amount of the BTX stream being fed discontinuously to the at least one compressor and / or the apparatus for caustic wash is in a range from 0 to 1 wt-% based on the gaseous feed stream fed to the respective apparatus.

15. The process according to any of claims 12 to 14, wherein the BTX stream being fed discontinuously to the at least one compressor and / or the apparatus for caustic wash is a part of the BTX stream obtained in step (d).