Process for purification of a hydrocarbon gas stream

Abstract

The invention is directed to a process for purification of a hydrocarbon gas stream comprising alkanes, olefines, hydrogen, at least one acid gas and at least one polymer precursor, in particular an aldehyde, a ketone and / or an aromatic hydrocarbon, wherein the hydrocarbon gas stream is contacted in a wash unit with an aqueous solution stream comprising an alkali hydroxide, and wherein a mass ratio between the alkali hydroxide fed to the wash unit and the hydrocarbon gas stream fed to the wash unit is less than 0.00095. The invention is further related to a process for control of a wash unit.

Description

[0001] 240816

[0002] 1

[0003] Process for purification of a hydrocarbon gas stream

[0004] Description

[0005] The present invention relates to a process for purification of a hydrocarbon gas stream comprising alkanes, olefines, hydrogen and at least one polymer precursor, wherein the hydrocarbon gas stream is contacted in a wash unit with an aqueous solution stream comprising an alkali hydroxide. The invention further relates to a process for control of a wash unit.

[0006] Hydrocarbon gas streams are typical product streams originating from crackers and / or alternative sources, which represent starting points for numerous chemical value chains.

[0007] Crackers are applied to produce building blocks such as ethylene and propene from a hydrocarbon feedstock for the chemical industry in large scale. Typically, a hydrocarbon feedstock is thermally cracked into smaller molecules by high temperature pyrolysis, in particular in presence of steam. The resulting cracked gas is submitted to refinery and separated into desired products by a sequence of separation and chemical-treatment steps. The mentioned hydrocarbon feedstock can be of fossil, bio, recycled or other origin. Liquified petroleum gas comprising propane and butane, natural gas liquids comprising ethane, propane and butane, gas oils and naphtha can be used for example as hydrocarbon feedstock. Naphtha contains more than 100 individual components with different boiling points and properties, for example paraffins, isoalkanes, olefins, naphthenes and aromatics.

[0008] In addition to the target products, by-products are formed and comprised in the hydrocarbon gas stream. Among the by-products and residuals from the hydrocarbon feedstock, the presence of polymer precursor compounds in the hydrocarbon gas stream is challenging in refinery and separation steps, which are performed in particular downstream of the cracking step. Polymerization and therefore blockage of apparatus can occur. In particular in wash units, where the gas is contacted with aqueous liquids to separate for example carbon dioxide from the hydrocarbon gas stream, polymerization is a prevalent problem, which has to be considered and overcome by the process design and carefully selected process parameters.

[0009] Mentioned polymer precursor compounds present in the hydrocarbon gas stream typically polymerize under the process conditions mainly via two polymerization mechanisms: via aldol condensation of carbonyl compounds, especially vinyl acetates and acetaldehyde, and depending on specific conditions such as temperature elevation and presence of peroxides, hydro peroxides and / or free oxygen, radical polymerization occurs. Resulting high molecular weight compounds are also known as red oil or fouling. Red oil formation is an undesired side reaction, and it causes technical challenges in the caustic wash unit and further downstream steps. Reduction of red oil production contributes significantly to the process efficiency and stability of operation.

[0010] During steam cracking of naphtha, a plurality of different materials is produced. Besides the target products such as ethylene and / or propylene also acidic gases are present in the product gas stream. The acidic gases such as carbon dioxide have to be removed from the product gas stream as for example carbon dioxide tends to cause the build up of dry ice and gas hydrates in the downstream separation sections. Typically, the raw product gas stream is treated with a caustic solution in a wash unit also referred to as caustic wash column or tower. The efficiency of the caustic wash can be determined for example by monitoring of the residual carbon dioxide concentration at the top of the caustic wash column. Typically, to ensure a reliable performance of the wash unit, the amount of caustic solution fed to the caustic wash tower is increased, in case carbon dioxide is detected at the top of caustic tower. Therefore, in general an excess amount of caustic solution is used that can be partially recycled within the system.

[0011] As a side reaction within the wash unit, unsaturated hydrocarbons such as aldehydes, ketones and / or aromatic hydrocarbons for example styrene and / or indene, polymerize, which is in turn promoted by the caustic solution. As a result, polymeric deposits appear in the wash unit and in plant sections downstream of the wash unit.

[0012] US 2023 / 0089058 A1 is directed to the use of spent caustic solution from a pygas treatment to neutralize halogens from liquified waste plastic. Caustic scrubbers for effluent streams from cracker furnaces are mentioned, but fouling and specific proportions or amounts concerning the caustic solution are not addressed.

[0013] WO 2011 / 138305 A2 aims at reducing the presence of polymeric deposits in caustic scrubbers by addition of a solvent such as benzene, toluene or xylene. After the treatment, the solvent is separated from the alkaline solution. 240816

[0014] 2

[0015] WO 2014 / 102289 A1 discloses a process for preparation of an olefinic product, wherein a non-aqueous liquid stream comprising aromatic C?+ hydrocarbons is fed to the caustic solution in a caustic tower to counteract the formation of viscous oily polymer referred to as red oil.

[0016] US 6,235,961 B1 proposes an inline, high-shear mixer to be applied in a cracked gas stream upstream of a caustic tower in an ethylene production unit in order to avoid polymer deposition in the caustic tower.

[0017] It is an object of the present invention to provide a process for caustic washing of a gas stream, wherein polymer formation and therefore fouling in the caustic wash unit is reduced or prevented and the addition of a further organic compound or use of additional apparatuses are avoided.

[0018] This object is achieved by a process for purification of a hydrocarbon gas stream comprising alkanes, olefines, hydrogen, at least one acid gas such as carbon dioxide and at least one polymer precursor, in particular an aldehyde, a ketone and / or an aromatic hydrocarbon, wherein the hydrocarbon gas stream is contacted in a wash unit with an aqueous solution stream comprising an alkali hydroxide, and wherein a mass ratio between the alkali hydroxide fed to the wash unit and the hydrocarbon gas stream fed to the wash unit is less than 0.00095, preferably less than 0.00090, more preferably less than 0.00060, even more preferably less than 0.00050.

[0019] The invention is further directed to a process for control of a wash unit, wherein a hydrocarbon gas stream comprising alkanes, olefines, hydrogen, at least one polymer precursor and at least one acid gas, and an aqueous solution stream comprising an alkali hydroxide are fed to the wash unit and a purified gas stream comprising optionally carbon dioxide, and a used aqueous solution stream are withdrawn from the wash unit, and wherein the carbon dioxide concentration is measured in the purified gas stream and a mass flow of the aqueous solution stream fed to the wash unit is adjusted based on the carbon dioxide measurement in the purified gas stream.

[0020] The tendency to polymerization within the wash unit increases with increasing application of caustic solution. The specific ratio of amounts of applied alkali hydroxide and the raw gas fed to the wash unit enables a removal of the acid gases comprised in the hydrocarbon gas stream, wherein the formation of polymers is significantly reduced. The addition of further additives such as inhibitors is dispensable.

[0021] By tracking the carbon dioxide concentration at the outlet of the caustic wash column the mass flow of the aqueous solution stream fed to the wash unit and thereby the ratio between aqueous solution and the hydrocarbon gas stream can be adjusted to ensure a sufficient or even complete removal of acidic gases but at the same time to run on minimal possible concentration of alkali hydroxide.

[0022] With the suggested processes polymerizing side reactions are minimized and longer run times of the production plant, in particular of the wash unit section, are possible. Pretreatments, in particular absorptive pretreatments, of the hydrocarbon gas stream upstream of the wash unit can be avoided.

[0023] The hydrocarbon gas stream can be produced for example from wet natural gases, natural gasolines and / or refinery waste gases, in particular by pyrolysis or steam cracking. Often, the hydrocarbon gas stream originates from a device for cracking a hydrocarbon feedstock, in particular a cracker. Further, the hydrocarbon gas stream can be produced from ethanol, in particular bioethanol based on bio feedstocks, by catalytic dehydration, from fluid catalytic cracking (FCC) offgas, from methanol by dehydration and catalytic conversion, from syngas, in particular produced by gasification, by Fischer-Tropsch processes, from methane by oxidative coupling, from ethane by dehydrogenation and / or from propene by metathesis. More details on different routes to gain the hydrocarbon gas stream are discussed in Ullmann’s Encyclopedia of Industrial Chemistry, Ethylene, 2012, Vol. 13, pp. 465, Wiley-VCH, DOI: 10.1002 / 14356007. a10_045.pub3.

[0024] The hydrocarbon gas stream comprises or is in particular at least part of a product gas stream, also referred to as raw gas stream, cracking product gas stream or cracked gas stream, which is at least partly withdrawn from the device for cracking a hydrocarbon feedstock, in particular a cracker. The wash unit is preferably arranged downstream, referring to the flow direction of the hydrocarbon gas stream, of the device for cracking a hydrocarbon feedstock.

[0025] Preferably, the hydrocarbon gas stream is formed at least partly by cracking a hydrocarbon feedstock in presence of steam at a cracking temperature of at least 750°C. 240816

[0026] 3

[0027] In a preferred embodiment, the process further comprises at least one of the following steps: a. providing a device for cracking a hydrocarbon feedstock, which comprises one or more furnaces, one or more heat exchangers, in particular transfer line exchangers (TLE), b. cracking a hydrocarbon feedstock, wherein the hydrocarbon gas stream is formed, withdrawn from the device for cracking a hydrocarbon feedstock and fed into the wash unit.

[0028] The device for cracking a hydrocarbon feedstock can comprise for example 1 to 15 furnaces. More preferably, the hydrocarbon gas stream is submitted to one or more processing steps, such as cooling, in particular direct cooling by oil and / or water washing, also referred to as quench, and / or compression, before being fed into the wash unit. Additional compression steps can be arranged downstream of the wash unit. In a preferred embodiment, the hydrocarbon gas stream is cooled to temperatures of less than 100°C, more preferably less than 80°C, even more preferably less than 50°C, then fed to one or more, for example 2 to 4, compressing steps, and then fed to the wash unit.

[0029] The device for cracking a hydrocarbon feedstock preferably comprises or is a steam cracker. The device for cracking a hydrocarbon feedstock is preferably used for thermal cracking of the hydrocarbon feedstock, wherein the hydrocarbon gas stream is obtained. The hydrocarbon gas stream comprises in particular unsaturated hydrocarbons. The hydrocarbon gas stream preferably comprises olefines, more preferably ethylene and optionally propylene, butadiene, benzene, toluene and / or xylene.

[0030] The cracking operation of the cracker can also be described as a cracking mode. During the cracking mode the hydrocarbon feedstock is fed to at least one of the one or more furnaces and the hydrocarbon gas stream is obtained. The cracking of the hydrocarbon feedstock is in particular characterized in that a split reaction of hydrocarbons is performed in the one or more furnaces of the device for cracking a hydrocarbon feedstock. In the split reaction typically chains of hydrocarbons are broken off into at least two parts. In the cracking mode, the hydrocarbon feedstock is fed to the one or more furnaces, wherein the hydrocarbon feedstock preferably comprises more than 25 wt.-%, more preferably more than 50 wt.-%, even more preferably more than 75 wt.-%, of hydrocarbons, referring to the total hydrocarbon feedstock. An outlet of the one or more furnaces is in particular connected to a refinery unit, in particular to the wash unit.

[0031] The mentioned hydrocarbon feedstock can be for example of fossil, bio, recycled, in particular chemcyded, or other origin. The hydrocarbon feedstock can comprise at least parts of pyrolysis oils, for example manufactured from pyrolysis of plastic waste.

[0032] Bio-based hydrocarbon feedstock preparation for cracking processes typically starts 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 often not directly suitable to be used in cracking processes to obtain olefins, aromatics, and other cracking products, but is preferably 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.4-hydrocarbons (bio-Ci-4-HCs) which can be used as feed streams for cracking processes, e.g. steam cracking.

[0033] 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 Ci.4hydrocarbons, in particular Ci-4alkanes) like bio-based liquefied petroleum gas (LPG; e.g. bio-based butane, propane, and ethane). 240816

[0034] 4

[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] Optionally feedstocks can originate from the recycling, in particular chemcycling, of waste products to pyrolysis oils and pyrolysis gases suitable for cracking processes, e.g. steam cracking. The term pyrolysis relates to a thermal decomposition or degradation of a feedstock such as plastic waste under inert conditions and 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-4-alkanes, C2-4-alkenes, ethyne, propyne, 1 -butyne, pyrolysis oil 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 oil is then separated by condensation from the pyrolysis gas. In addition, water is formed during the pyrolysis which may be partially dispersed in the pyrolysis oil and may be partially contacted with the pyrolysis oil 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 oil is understood to mean any oil originating from the pyrolysis of e.g. plastic waste. The term plastic waste includes for example rubber waste such as end-of-life tires and feedstocks comprising plastic waste. 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. The term plastic waste further includes industrial and domestic plastic waste and including used tires and agricultural and horticultural plastic material.

[0037] 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., nitrogencontaining 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.

[0038] Pyrolysis processes as such are known. They are described, e.g., in EP 0713906 A1 and WO 95 / 03375 A1 . Optionally, the pyrolysis oil or mixture of pyrolysis oils is subjected to one or more methods selected from filtration, centrifugation, adsorption, washing, extraction before used as e.g., a feedstock for a steam cracking process. Such optional pre-treatment methods are for example described in WO 2021 / 224287 A1, WO 2023 / 061834 A1 , EP 0713906 A1 and WO 95 / 03375 A1 .

[0039] The hydrocarbon feedstock preferably comprises or consists of preferably ethane, propane, butane, liquefied petroleum gas, gasoline fractions, such as light naphtha, for example having a boiling point at 0.1 MPa in a range from 30°C to 150°C, full-range naphtha, for example having a boiling point at 0.1 MPa in a range from 30°C to 180°C, heavy naphtha, for example having a boiling point at 0.1 MPa in a range from 150°C to 220°C, kerosene, for example having a boiling point at 0.1 MPa in a range from 200°C to 260°C, gas oils, such a light gas oil, for example having a boiling point at 0.1 MPa in a range from 200°C to 360°C, and heavy gas oil, for example having a boiling point at 0.1 MPa in a range from 310°C to 430°C, vacuum distillates, for example having a boiling point at 0.1 MPa in a range from 400°C to 560°C, and / or chemically recycled fractions having a boiling point at 0.1 MPa of up to 400°C. The hydrocarbon feedstock can comprise or consists of chemically recycled fractions having a boiling point at 0.1 MPa in a range from 160°C to 400°C. The hydrocarbon feedstock can comprise or consist of gasoline fractions, kerosene and / or gas oils, wherein gas oil typically has a boiling point at 0.1 MPa in a range from 180°C to 350°C. Further, the hydrocarbon feedstock can comprise sulfur, for example in a concentration of more than 0.02 wt.-%, referring to the total hydrocarbon feedstock. The presence of sulfur in the hydrocarbon feedstock enhances the 240816

[0040] 5 tendency to coking and thus to solid deposition in the device for cracking a hydrocarbon feedstock, in particular in the one or more furnaces and heat exchangers.

[0041] The, in particular thermal, cracking is preferably performed in presence of steam, also referred to as steam cracking. Steam is understood to be water in gaseous form. In a preferred embodiment, the hydrocarbon feedstock is fed to the device for cracking a hydrocarbon feedstock together with steam for cracking. In particular, a feed mixture, comprising the hydrocarbon feedstock and steam, is fed to the device for cracking a hydrocarbon feedstock. A weight ratio of steam to the hydrocarbon feedstock, comprised in the feed mixture respectively, is preferably in a range from 0.1 to 1 .0, more preferably from 0.2 to 0.8, especially from 0.3 to 0.7, for example from 0.35 to 0.50.

[0042] The one or more furnaces comprised in the device for cracking a hydrocarbon feedstock are preferably indirectly heated tube furnaces, more preferably fired tubular reactors, even more preferably radiant coils. Tubes of the one or more furnaces are preferably arranged vertically. Further, tubes of the one or more furnaces can comprise fins to enlarge the inner surface of the tubes. The tubes of the one or more furnaces are preferably made of a steel alloy material comprising nickel, chrome and optionally silicon, manganese, niobium, molybdenum, tungsten and / or titanium. The steel alloy material comprises preferably 40 wt.-% or more of nickel and 30 wt.-% or more of chrome, referring to the total steel alloy material. The one or more furnaces can comprise each a firebox with tubes, in particular vertical radiant coils, arranged therein, more preferably centrally arranged therein. A temperature in the firebox outside of the tubes is preferably in a range from 700°C to 1200°C.

[0043] Further, the one or more furnaces comprised in the device for cracking a hydrocarbon feedstock can be at least in part directly or indirectly heated by electricity. The one or more furnaces can comprise an electrical heater, wherein the electrical heater in particular comprises electrical resistors in order to generate heat.

[0044] A residence time of the hydrocarbon feedstock in the one or more furnaces is preferably in a range from 0.05 seconds to 1 .00 second, more preferably from 0.10 seconds to 0.60 seconds, even more preferably from 0.10 seconds to 0.50 seconds. A cracking temperature, in particular a mean average cracking temperature, particularly over the length of the tubes, during the cracking mode in the furnace is preferably 750°C or more, more preferably in a range from 780°C to 900°C, even more preferably in a range from 800°C to 900°C. The cracking temperature can be measured for example by thermowell or thermocouple or via skin thermocouple. In a preferred embodiment, the cracking temperature increases from an inlet of the furnace to the outlet of the furnace. An inlet temperature can be for example in a range from 500°C to 680°C and an outlet temperature in a range from 775°C to 875°C. An energy supply to the tubes of the one or more furnaces during cracking is preferably in a range from 40 000 kcal / (m2h) (167.36 MJ / (m2h)) to 80 000 kcal / (m2h) (334.72 MJ / (m2h)), more preferably from 50 000 kcal / (m2h) (209.2 MJ / (m2h)) to 70 000 kcal / (m2h) (292.88 MJ / (m2h)). A capacity of each furnace is typically in a range from 130 000 t / a to 300 000 t / a of the hydrocarbon feedstock.

[0045] The one or more furnaces are preferably heated by firing with burners, in particular radiant burners. The burners can consume gaseous and / or liquid fuels for combustion with air. The burners are preferably run with a fuel gas. The fuel gas is fed to the burners, whereas the hydrocarbon feedstock is fed into the interior of the furnace, in particular to a reaction zone of the furnace, preferably into the tubes of the furnace, which is heated by the burners. Flue gas from the burners can be used to preheat the hydrocarbon feedstock. The preheating can be carried out by direct or indirect heat exchange. The optionally preheated hydrocarbon feedstock is then preferably mixed with steam.

[0046] The flue gas of the burners is preferably submitted to a NOx separation step. The term NOx is understood to describe nitrogen oxides comprising or consisting of nitric oxide (NO) and nitrogen dioxide (NO2). The device for cracking a hydrocarbon feedstock can further comprise a NOx removal unit, wherein NOx is preferably removed from the flue gas by selective catalytic reduction. In a preferred embodiment, NOx present in the flue gas of the burners reacts with ammonia in the NOx removal unit.

[0047] After leaving the furnace, the hydrocarbon gas stream is preferably cooled to temperatures of 700°C or less, more preferably to 650°C or less, even more preferably to 500°C or less, for example to temperatures in a range from 300°C to 700°C, preferably from 300°C to 500°C, to prevent undesired side-reactions. The hydrocarbon gas stream can be cooled by direct or indirect cooling methods. For direct cooling methods liquid hydrocarbons, such as an oil, and / or water can be injected into the hydrocarbon gas stream. Preferably, the hydrocarbon gas stream is further cooled by oil washing to a temperature of 120°C or less. In a preferred embodiment, the hydrocarbon gas stream is submitted to an oil washing step and then to a water washing step. 240816

[0048] 6

[0049] In a preferred embodiment, the hydrocarbon gas stream is cooled in the one or more heat exchangers, in particular with water. The one or more heat exchangers can be indirect heat exchangers or direct heat exchangers. Preferably, the one or more heat exchangers are indirect heat exchangers. The one or more heat exchangers are preferably tube bundle heat exchangers, more preferably each of the one or more heat exchangers comprises at least 50 exchanger tubes. The one or more heat exchangers are more preferably transfer line exchangers (TLE), even more preferably linear transfer line exchangers. A heat exchanger is preferably mounted on top of a furnace, more preferably one heat exchanger is mounted on top of each furnace. Alternatively, the heat exchanger can be mounted along a side of the furnace or underneath the furnace. The one or more heat exchangers can be orientated vertically or horizontally. Preferably each furnace is connected to at least one heat exchanger. Each furnace can be connected to a series of at least two heat exchangers. Accordingly, the hydrocarbon gas stream can be cooled stepwise.

[0050] The one or more heat exchangers serve preferably for cooling of the hydrocarbon gas stream and for heat recovery in form of high-pressure steam. In particular, steam is generated in the one or more heat exchangers under high pressure in a range from 5 MPa to 15 MPa, more particularly from 8 MPa to 13 MPa. The pressurized stream can be used as energy source in the process.

[0051] The hydrocarbon gas stream, in particular the cooled hydrocarbon gas stream, is preferably led to the refinery unit, in particular the wash unit, for separation of desired components or fractions comprised in the hydrocarbon gas stream. The hydrocarbon gas stream, in particular the cooled hydrocarbon gas stream, is preferably submitted to at least one refinery step comprising a wash step in the wash unit. In a preferred embodiment, the hydrocarbon gas streams from all furnaces are combined. The at least one refinery step can comprise, in addition to the wash step, removal of heat, condensation of water and heavy hydrocarbons, compression, drying, thermal separation such as condensation, extraction and / or distillation, and / or hydrogenation.

[0052] During cracking, acetylenic, olefinic, diolefinic and aromatic compounds are typically formed. The hydrocarbon gas stream can comprise for example ethylene (ethene), acetylene, aromatics such as benzene, toluene and / or xylene; butadiene, butene, isobutene, 2-methyl-1 ,3-butadiene, 1 ,3-pentadiene, cyclopentadiene, ethane, fuel oil, hydrogen, methane, naphthalene, propane, propene, tar and / or sulfur containing species.

[0053] Preferably, the hydrocarbon gas stream is led from an outlet of the one or more furnaces to a heat exchanger of the one or more heat exchangers, in particular a transfer line exchanger of the one or more transfer line exchangers, and then to the wash unit. More preferably, the hydrocarbon gas stream is led from the outlet of the one or more furnaces to a heat exchanger of the one or more heat exchangers, in particular a transfer line exchanger of the one or more transfer line exchangers, to an oil wash unit, to a water wash unit, to a compression unit and then to the wash unit.

[0054] More preferably, the device for cracking a hydrocarbon feedstock comprises a device for solid separation and the hydrocarbon gas stream is led from the heat exchanger, in particular the transfer line exchanger (TLE), through the device for solid separation and then to the wash unit.

[0055] Preferably, the hydrocarbon gas stream is submitted to a solid separation step. The solid separation can imply dry and / or wet separation, in particular a water quench and / or application of a cyclone. Preferably, the device for cracking a hydrocarbon feedstock comprises the device for solid separation. The device for solid separation is preferably a cyclone. The device for solid separation is more preferably arranged upstream of wash unit.

[0056] At least part of the hydrocarbon gas stream is typically led to further purification and separation steps downstream of the wash unit.

[0057] Preferably, the hydrocarbon gas stream has a temperature in a range from 5°C to 100°C, more preferably from 10°C to 40°C, even more preferably from 15°C to 35°C, when entering the wash unit. Preferably, the hydrocarbon gas stream has an absolute pressure in a range from 1.0 MPa to 3.0 MPa, more preferably from 1.3 MPa to 2.9 MPa when being contacted with the aqueous solution stream.

[0058] Preferably, a mass ratio between the aqueous solution stream fed to the wash unit and the hydrocarbon gas stream fed to the wash unit is less than 0.010, more preferably less than 0.009, even more preferably less than 0.006, even more preferably equal to or less than 0.005. 240816

[0059] 7

[0060] The at least one polymer precursor comprises for example aldehydes such as acetaldehyde, ketones and / or aromatic hydrocarbons such as styrene and / or indene. The at least one polymer precursor is typically an aldehyde, a ketone and / or an aromatic hydrocarbon such as styrene and / or indene, in particular an aldehyde such as acetaldehyde. The at least one polymer precursor is preferably an aldehyde, more preferably acetaldehyde.

[0061] Preferably, a mass ratio between the alkali hydroxide fed to the wash unit and the at least one polymer precursor fed to the wash unit is less than 1 .4, more preferably less than 1.3. The at least one polymer precursor is fed to the wash unit in particular as part of the hydrocarbon gas stream.

[0062] Preferably, the temperature of a used aqueous solution stream withdrawn from the wash unit is 20°C or higher. The temperature of the used aqueous solution stream at an outlet of the wash unit, in particular at the bottom of the caustic wash column, is preferably in a range from 15°C to 80°C, more preferably from 20°C to 60°C. The pH value in the used aqueous solution stream withdrawn from the wash unit is preferably in a range from 10 to 14, more preferably from 11 to 13. Preferably, the concentration of the alkali hydroxide in the used aqueous solution stream is less than 0.5 wt.-%, based on the total used aqueous solution stream, in particular at the outlet of the wash unit. The used aqueous solution stream can be fed into a stripping unit.

[0063] Preferably, the wash unit comprises an, in particular vertically arranged, caustic wash column. The wash unit can further comprise a phase separator, in which an organic phase and an aqueous phase are separated, the aqueous phase containing the used aqueous solution. More preferably the purified gas stream is withdrawn from the wash unit at the top of the caustic wash column. A liquid phase containing the used aqueous solution is typically obtained at the bottom of the caustic wash column. The liquid phase is preferably fed into the phase separator. The caustic wash column preferably comprises separating internals such as trays and / or packings.

[0064] Preferably, the hydrocarbon gas stream and the aqueous solution stream are contacted in the wash unit in a countercurrent mode. Typically, the hydrocarbon gas stream ascends in the caustic wash column and the aqueous solution stream descends in the caustic wash column.

[0065] Preferably, the hydrocarbon gas stream and the aqueous solution stream are contacted in at least two stages, wherein the hydrocarbon gas stream is led from a first circuit to a second circuit and the aqueous solution stream fed to the wash unit is added into the second circuit. The second circuit is in particular arranged above the first circuit. Preferably, all stages are carried out in the same column.

[0066] Preferably, at least one liquid recycle stream, for example two liquid recycle streams, are established within the wash unit. In particular, one or more of the at least one liquid recycle stream is established in each stage. For example, a first liquid recycle stream is established in the first circuit and a second liquid recycle stream is established in the second circuit. In a preferred embodiment, the aqueous solution stream is added to the second liquid recycle stream in the second circuit.

[0067] More preferably, a mass ratio of the at least one liquid recycle stream, specifically all of the at least one liquid recycle stream, to the aqueous solution stream fed to the wash unit is at least 50, even more preferably at least 100, for example in a range from 200 to 350.

[0068] Particularly, the aqueous solution stream fed to the wash unit is the alkali hydroxide in aqueous solution, which is freshly added to the wash unit. Preferably, the aqueous solution stream fed to the wash unit is diluted, in particular in the wash unit, before being contacted with the hydrocarbon gas stream. In a preferred embodiment, the aqueous solution stream is diluted with at least part of the at least one liquid recycle stream, in particular before being contacted with the hydrocarbon gas stream.

[0069] In a preferred embodiment, the hydrocarbon gas stream is contacted with a diluted aqueous solution stream. The diluted aqueous solution stream is in particular formed by mixing the aqueous solution stream freshly added to the wash unit with at least part of the at least one liquid recycle stream. Preferably, the concentration of the alkali hydroxide in the diluted aqueous solution stream is less than 2 wt.-%, more preferably less than 1 wt.-%, even more preferably less than 0.6 wt.-%, based on the total diluted aqueous solution stream. 240816

[0070] 8

[0071] Further, the process can comprise a water wash step, wherein at least part, preferably part, of the hydrocarbon gas stream is contacted with a water stream. The part of the hydrocarbon gas stream contacted with the water stream corresponds preferably to the part of the hydrocarbon gas stream, which remains after the last wash step applying the aqueous solution, more preferably to the part of the hydrocarbon gas stream which is withdrawn from the second circuit. The water stream has preferably a lower content in the alkali hydroxide than the aqueous solution stream and more preferably lower than the at least one liquid recycle stream, in particular lower than all liquid recycle streams, considered separately, respectively. For example, the water stream comprises less than 0.01 wt.-%, preferably less than 0.001 wt.-%, more preferably less than 0.0005 wt.-%, of the alkali hydroxide, referring to the total water stream. In particular, the water stream is free of the alkali hydroxide. The water stream can also be referred to as fresh water. The water wash step is preferably performed in a water wash section. The water wash step is preferably performed in a counter-current mode. The water wash section can be part of the wash unit or arranged separately from the wash unit. The water wash step is preferably arranged downstream of the last wash step, where the hydrocarbon gas stream is contacted with the aqueous solution stream. Thus, preferably at least part of the hydrocarbon gas stream is led from the wash step to the water wash step. The water wash section is preferably arranged above the second circuit.

[0072] Preferably, the hydrocarbon gas stream comprises at least 0.1 wt.-ppm of sulfur, based on the total hydrocarbon gas stream.

[0073] As the formation of polymers can be attributed to oxygenates such as aldehydes, in particular acetaldehyde, and their precursors such as methanol present in the hydrocarbon gas stream, the invention can have an even higher benefit for chemcycled (chemically recycled) or bio-based feed streams to the cracker, which have naturally elevated contents of these substances compared to fossil based naphtha. More preferably, the hydrocarbon gas stream originates from a chemcycled or bio-based feed stream, which is optionally processed in the device for cracking a hydrocarbon feedstock.

[0074] The hydrocarbon gas stream is preferably continuously fed to the wash unit.

[0075] Preferably, the hydrocarbon gas stream comprises, based on the total hydrocarbon gas stream, 10.0000 to 60.0000 vol.-%, more preferably 25.0000 to 50.0000 vol.-%, even more preferably 25.0000 to 40.0000 vol.-%, of Ci to C4 alkanes, 30.0000 to 70.0000 vol.-%, more preferably 30.0000 to 50.0000 vol.-%, even more preferably 35.0000 to 50.0000 vol.-%, of Ci to C4 olefines, in particular ethylene and optionally propylene, butadiene; and aromatic hydrocarbons, in particular benzene, toluene and / or xylene, 1 .0000 to 20.0000 vol.-%, more preferably 5.0000 to 20.0000 vol.-%, even more preferably 8.0000 to 18.0000 vol.-%, of hydrogen and 0.0001 to 5.0000 vol.-%, more preferably 0.0010 to 1.0000 vol.-%, even more preferably 0.0010 to 0.5000 vol.-%, even more preferably 0.0010 to 0.0500 vol.-%, even more preferably 0.0010 to 0.0400 vol.-%, even more preferably 0.0010 to 0.0300 vol.-%, even more preferably 0.0010 to 0.0200 vol.-%, of the at least one polymer precursor, in particular an aldehyde, a ketone and / or an aromatic hydrocarbon.

[0076] More preferably, the hydrocarbon gas stream comprises, based on the total hydrocarbon gas stream, 0.0001 to 5.0000 vol.-%, more preferably 0.0010 to 1.0000 vol.-%, even more preferably 0.0010 to 0. 5000 vol.-%, even more preferably 0.0010 to 0.0500 vol.-%, even more preferably 0.0010 to 0.0400 vol.-%, even more preferably 0.0010 to 0.0300 vol.-%, even more preferably 0.0010 to 0.0200 vol.-%, of the at least one polymer precursor, in particular an aldehyde, a ketone and / or an aromatic hydrocarbon, for example of acetaldehyde.

[0077] The term “Ci to Cn" denominates a group of linear or branched hydrocarbon radicals or molecules having from 1 to n carbon atoms.

[0078] Preferably, the alkali hydroxide is sodium hydroxide and / or potassium hydroxide. The aqueous solution stream is preferably continuously fed to the wash unit.

[0079] Preferably, the concentration of the alkali hydroxide in the aqueous solution stream fed to the wash unit is at least 5 wt.-%, more preferably at least 15 wt.-%, even more preferably in a range from 15 wt.-% to 40 wt.-%, even more preferably in a range from 20 wt.-% to 30 wt.-%, even more preferably in a range from 22 wt.-% to 28 wt.-%, for example 25 wt.-%, based on the total aqueous solution stream fed to the wash unit. The aqueous solution stream is 240816

[0080] 9 in particular a solution of the alkali hydroxide in water. Particularly, the aqueous solution stream fed to the wash unit comprises less than 1 wt.-%, more particularly less than 0.1 wt.-%, of hydrocarbons, based on the total aqueous solution stream fed to the wash unit, is for example free of hydrocarbons or other additives. This concentration particularly applies before the aqueous solution stream is mixed with any other stream present in the wash unit, for example before being mixed with one of the at least one liquid recycle streams, for example the second liquid recycle stream in the second circuit.

[0081] Preferably, the hydrocarbon gas stream comprises alkanes, olefines, hydrogen, the at least one polymer precursor, alkynes and aromatic hydrocarbons. The hydrocarbon gas stream can comprise further organic and / or inorganic components.

[0082] Preferably, the purified gas stream comprises less than 10 vol.-ppm of carbon dioxide, based on the total purified gas stream. For the control of the wash unit and in addition to measuring the carbon dioxide concentration in the purified gas stream, preferably the concentration of the alkali hydroxide is measured within the wash unit, more preferably in the at least one liquid recycle stream, in particular in the first circuit and / or the second circuit.

[0083] The carbon dioxide concentration in the purified gas stream is preferably measured at the top of the wash unit, in particular at the top of the caustic wash column. The measurement is preferably performed by means of an infrared (IR) sensor, for example according to DIN CEN / TS 17405:2020.

[0084] In a preferred embodiment, the purified gas stream is withdrawn from the wash unit via a gas outlet line, which is equipped with a measurement unit. The carbon dioxide concentration is preferably measured at the measurement unit. The aqueous solution is preferably fed to the wash unit via a feed line, which is equipped with a valve, in particular a control valve. The mass flow of the aqueous solution stream is preferably adjusted at the valve of the feed line in dependency on the carbon dioxide measurement in the purified gas stream.

[0085] Preferably, the mass flow of the aqueous solution stream is decreased, when a decrease in the carbon dioxide concentration is detected. Further, the mass flow of the aqueous solution stream can be increased, when an increase in the carbon dioxide concentration is detected.

[0086] Further, the composition of the hydrocarbon gas stream entering the wash unit can be analyzed by means of gas chromatography. Preferably, the aldehyde concentration, in particular the acetaldehyde concentration, in the hydrocarbon gas stream entering the wash unit is determined. The mass flow of the aqueous solution stream can be adjusted in dependency the aldehyde concentration in the hydrocarbon gas stream.

[0087] Embodiments of the invention are illustrated in the figures and further described in the following.

[0088] The figures show:

[0089] Figure 1 an overview of a system comprising a device for cracking a hydrocarbon feedstock and

[0090] Figure 2 an illustration of a wash unit.

[0091] Figure 1 shows an overview of a system 2 comprising a device for cracking 3 a hydrocarbon feedstock 4 from which a hydrocarbon gas stream 1 is obtained. The device for cracking 3 the hydrocarbon feedstock 4 comprises a furnace 5, a transfer line exchanger (TLE) as heat exchanger 9, a unit for further cooling and compression 19 and a wash unit 7.

[0092] During cracking, the hydrocarbon feedstock 4 and water in form of steam 6 are fed to the furnace 5. The furnace 5 is heated by combustion of a fuel gas 8 and a flue gas is emitted.

[0093] In the wash unit 7, the hydrocarbon gas stream 1, which was withdrawn at an outlet 17 from the furnace 5 and passed through the heat exchanger 9 and the unit for further cooling and compression 19, is contacted with an aqueous solution stream 10 comprising an alkali hydroxide. At least one acid gas is at least partly removed from the hydrocarbon gas stream 1 and a purified gas stream 28 as well as a used aqueous solution stream 11 are withdrawn from the wash unit 7. 240816

[0094] 10

[0095] Figure 2 shows an illustration of a wash unit 7 with a caustic wash column 12 comprising trays 14 as separating internals. A hydrocarbon gas stream 1 comprising alkanes, olefines, hydrogen, at least one acid gas and at least one polymer precursor is contacted in the wash unit 7 in counter-current mode with an aqueous solution stream 10 comprising an alkali hydroxide. The hydrocarbon gas stream 1 is fed to the bottom of the caustic wash column 12 and a purified gas stream 28 is withdrawn from the top of the caustic wash column 12. A used aqueous solution stream 11 leaves the bottom of the caustic wash column 12.

[0096] The hydrocarbon gas stream 1 and the aqueous solution stream 10 are contacted in two stages 13, wherein the hydrocarbon gas stream 1 is led from a first circuit 15 to a second circuit 16. Two liquid recycle streams 18 are established within the wash unit 7. A first liquid recycle stream 20 is established in the first circuit 15 and a second liquid recycle stream 22 is established in the second circuit 16. In this illustrative embodiment, the aqueous solution stream 10 is added to the second liquid recycle stream 22 in the second circuit 16, resulting in a diluted aqueous solution stream 23.

[0097] A carbon dioxide concentration is measured in the purified gas stream 28 at a measurement unit 34 at a gas outlet line 35. A mass flow of the aqueous solution stream 10 is adjusted at a valve 30 of a feed line 32 to the wash unit 7 in dependency on the carbon dioxide measurement in the purified gas stream 28.

[0098] Further, a part of the hydrocarbon gas stream 1 , which leaves the second circuit 16, is contacted with a water stream 26 in a water wash section 24. This water wash step is performed in a counter-current mode. In this illustrative embodiment, the water wash section 24 is part of the wash unit 7. The water wash step is arranged downstream of the last wash step applying the aqueous solution stream 10, here downstream of the second circuit 16. The respective part of the hydrocarbon gas stream 1 is led from the wash step to the water wash step. The water wash section 24 is arranged above the second circuit 16.

[0099] Examples and comparative examples

[0100] A tube cracking furnace was employed to crack a hydrocarbon feedstock in the presence of steam. The applied hydrocarbon feedstock was a gasoline fraction having a boiling point at 0.1 MPa in a range from 40°C to 180°C, also referred to as naphtha. The temperature at the furnace outlet was 850°C and the resulting product gas was cooled in a heat exchanger to a temperature of less than 450°C.

[0101] Examples

[0102] A resulting hydrocarbon gas stream comprising 37 vol.-% of Ci to C4 alkanes, 42 vol.-% of Ci to C4 olefines, 14 vol. - % of hydrogen, 0.0075 vol.-% of acetaldehyde and further residuals was introduced at a temperature of 30°C and at a pressure of 2,5 MPa absolute into a wash unit with two stages, namely a first circuit and a second circuit, and an additional water wash step. A corresponding wash unit is illustrated in Figure 2. An aqueous solution stream is fed to the second circuit of the wash unit with a concentration of 25 wt.-% of sodium hydroxide, based on the total aqueous solution stream fed to the wash unit. A mass ratio between the liquid recycle streams within the wash unit and the aqueous solution stream fed to the wash unit accounted to 300.

[0103] I.

[0104] The mass ratio between the alkali hydroxide fed to the wash unit and the hydrocarbon gas stream fed to the wash unit accounted to 0.00070. 10 mg polymer per 100 mL used aqueous solution were detected by mixing a sample amount of the used aqueous solution with the same volume of toluene and separating the organic phase containing the polymer and evaporating the organic phase.

[0105] II.

[0106] At a mass ratio between the alkali hydroxide fed to the wash unit and the hydrocarbon gas stream fed to the wash unit of 0.00060, 8 mg polymer per 100 mL used aqueous solution were detected.

[0107] III.

[0108] At a mass ratio between the alkali hydroxide fed to the wash unit and the hydrocarbon gas stream fed to the wash unit of 0.00054, less than 2 mg polymer per 100 mL used aqueous solution were detected. 240816

[0109] 11

[0110] Comparative example

[0111] The hydrocarbon gas stream according to the examples was contacted in the same wash unit with an aqueous solution stream fed to the wash unit with a concentration of 25 wt.-% of sodium hydroxide as described above. The mass flow of the aqueous solution fed to the wash unit was increased compared to the examples. Here, the mass ratio between the alkali hydroxide fed to the wash unit and the hydrocarbon gas stream fed to the wash unit accounted to 0.00101. More than 16 mg polymer per 100 mL used aqueous solution were detected.

[0112] 240816

[0113] 12

[0114] List of reference numerals

[0115] 1 hydrocarbon gas stream

[0116] 2 system

[0117] 3 device for cracking

[0118] 4 hydrocarbon feedstock

[0119] 5 furnace

[0120] 6 steam

[0121] 7 wash unit

[0122] 8 fuel gas

[0123] 9 heat exchanger

[0124] 10 aqueous solution stream

[0125] 11 used aqueous solution stream

[0126] 12 caustic wash column

[0127] 13 stage

[0128] 14 tray

[0129] 15 first circuit

[0130] 16 second circuit

[0131] 17 outlet

[0132] 18 liquid recycle stream

[0133] 19 unit for further cooling and compression

[0134] 20 first liquid recycle stream

[0135] 22 second liquid recycle stream

[0136] 23 diluted aqueous solution stream

[0137] 24 water wash section

[0138] 26 water stream

[0139] 28 purified gas stream

[0140] 30 valve

[0141] 32 feed line

[0142] 34 measurement unit

[0143] 35 gas outlet line

Claims

24081613Claims1 . A process for purification of a hydrocarbon gas stream (1) comprising alkanes, olefines, hydrogen, at least one acid gas such as carbon dioxide and at least one polymer precursor, in particular an aldehyde, a ketone and / or an aromatic hydrocarbon, wherein the hydrocarbon gas stream (1) is contacted in a wash unit (7) with an aqueous solution stream (10) comprising an alkali hydroxide, and wherein a mass ratio between the alkali hydroxide fed to the wash unit (7) and the hydrocarbon gas stream (1) fed to the wash unit (7) is less than 0.00095.

2. Process according to claim 1 , wherein a mass ratio between the aqueous solution stream (10) fed to the wash unit (7) and the hydrocarbon gas stream (1) fed to the wash unit (7) is less than 0.010, preferably less than 0.006.

3. Process according to claim 1 or 2, wherein a mass ratio between the alkali hydroxide fed to the wash unit (7) and the at least one polymer precursor fed to the wash unit (7) is less than 1.4.

4. Process according to any of claims 1 to 3, wherein the temperature of a used aqueous solution stream (11) withdrawn from the wash unit (7) is 20°C or higher.

5. Process according to any of claims 1 to 4, wherein the hydrocarbon gas stream (1) and the aqueous solution stream (10) are contacted in the wash unit (7) in a counter-current mode.

6. Process according to any of claims 1 to 5, wherein the hydrocarbon gas stream (1) and the aqueous solution stream (10) are contacted in at least two stages (13), wherein the hydrocarbon gas stream (1) is led from a first circuit (15) to a second circuit (16) and the aqueous solution stream (10) fed to the wash unit (7) is added into the second circuit (16).

7. Process according to any of claims 1 to 6, wherein at least one liquid recycle stream (18) is established within the wash unit (7) and a mass ratio of the at least one liquid recycle stream (18) to the aqueous solution stream (10) fed to the wash unit (7) is at least 50, preferably at least 100.

8. Process according to any of claims 1 to 7, wherein the hydrocarbon gas stream (1) comprises at least 0.1 wt.- ppm of sulfur, based on the total hydrocarbon gas stream (1).

9. Process according to any of claims 1 to 8, wherein the hydrocarbon gas stream (1) is formed by cracking a hydrocarbon feedstock (4) in presence of steam (6) at a cracking temperature of at least 750°C.

10. Process according to any of claims 1 to 9, wherein the hydrocarbon gas stream (1) comprises, based on the total hydrocarbon gas stream (1),10.0000 to 60.0000 vol.-% of Ci to C4 alkanes,30.0000 to 70.0000 vol.-% of Ci to C4 olefines, in particular ethylene and optionally propylene, butadiene; and aromatic hydrocarbons, in particular benzene, toluene and / or xylene, 1.0000 to 20.0000 vol.-% of hydrogen and0.0001 to 5.0000 vol.-% of the at least one polymer precursor, in particular the aldehyde, a ketone and / or an aromatic hydrocarbon.

11. Process according to any of claims 1 to 10, wherein the alkali hydroxide is sodium hydroxide and / or potassium hydroxide.

12. Process according to any of claims 1 to 11 , wherein the concentration of the alkali hydroxide in the aqueous solution stream (10) fed to the wash unit (7) is at least 15 wt.-%, based on the total aqueous solution stream (10) fed to the wash unit (7).

13. Process for control of a wash unit (7), wherein a hydrocarbon gas stream (1) comprising alkanes, olefines, hydrogen, at least one polymer precursor and at least one acid gas, and an aqueous solution stream (10)comprising an alkali hydroxide are fed to the wash unit (7) and a purified gas stream (28) comprising optionally carbon dioxide, and a used aqueous solution stream (11) are withdrawn from the wash unit (7), and wherein the carbon dioxide concentration is measured in the purified gas stream (28) and a mass flow of the aqueous solution stream (10) fed to the wash unit (7) is adjusted based on the carbon dioxide measurement in the purified gas stream (28).

14. Process according to claim 13, wherein the purified gas stream (28) comprises less than 10 vol. -ppm of carbon dioxide, based on the total purified gas stream (28).

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