Method and plant for processing a plastics pyrolysis oil

Abstract

A method (100) of processing plastics pyrolysis oil (1) is proposed, comprising steamcracking (10) the plastics pyrolysis oil (1) to obtain a cracking gas (2), wherein at least some of the cracking gas (2) is subjected successively to oil fractionation (20), a water wash (30), and a compression (50) via one or more intermediate stages, and further comprising eliminating hydrogen cyanide in laden wash water (4) from the water wash (30) and / or the cracking gas (2) at the intermediate stage or one of the plurality of intermediate stages of the compression (50). A corresponding plant is likewise proposed.

Description

[0001] Description

[0002] Process and plant for processing a plastic pyrolysis oil

[0003] Area

[0004] The present disclosure relates to a process for processing a plastic pyrolysis oil and a corresponding plant.

[0005] background

[0006] Processes and plants for the pyrolysis of plastics, especially so-called solid plastic waste (SPW), are known. Among many others, reference can be made, for example, to an article by BA Perez et al., “Characterization of SPW pyrolysis oils: Products spectra and opportunities”, in: D. Moscatelli and M. Pelucchi (eds.), “Towards Circular Economy: Closing the Loop with Chemical Recycling of Solid Plastic Waste”, Adv. Chem. Eng. 60(1), 169-214, 2022.

[0007] The present disclosure relates to the processing and use of liquid pyrolysis products from plastic pyrolysis, which, as is common in the field, are also referred to here as plastic pyrolysis oil.

[0008] Plastic pyrolysis oil can be further processed by steam cracking with other feedstocks. The most widespread approach to steam cracking plastic pyrolysis oil for commercial applications to date is dilution with conventional feedstocks such as naphtha, atmospheric gas oil (AGO), unhydrogenated or hydrogenated vacuum gas oil ((H)VGO), and other fractions, particularly from refinery processes. However, due to various limitations, often only small amounts of plastic pyrolysis oil can be added to the conventional feedstock, which has so far severely restricted the application possibilities and quantities.

[0009] To improve its integration capacity, plastic pyrolysis oil can undergo pretreatment, for example by fractionation through distillation, filtration, extraction, adsorption, or catalytic treatment with and without hydrogen. These measures are intended to enable an increase in the proportion of plastic pyrolysis oil used in a steam cracker, up to and including undiluted processing.

[0010] US Patent 2022 / 380681 A1 discloses a process for processing plastic pyrolysis oil by steam cracking, in which a fraction of compounds with five or more carbon atoms and a fraction of compounds with four carbon atoms are separated. Processes for removing hydrogen cyanide in other environments are described in AR Yeddou et al., “Removal of cyanide in aqueous solution by oxidation with hydrogen peroxide in the presence of copper-impregnated activated carbon”, Minerals Eng. 24(8), 2011, 788-793, as well as in US Patents 5,463,168 A, 6,063,349 A, 4,271,133 A, and 2022 / 402840 A1.

[0011] There is still a need for improvements in processes where plastic pyrolysis oil is converted by steam cracking.

[0012] Overview

[0013] Against this background, a method for processing a plastic pyrolysis oil with the features of the independent claims is proposed. Embodiments are the subject of the dependent claims and the following description.

[0014] The proposed process for processing plastic pyrolysis oil comprises steam cracking the plastic pyrolysis oil to obtain a cracking gas, wherein at least a part of the cracking gas is successively subjected to oil fractionation, water washing and compression via one or more intermediate stages, and the elimination of hydrogen cyanide in the cracking gas at the intermediate stage or one of the several intermediate stages of compression and / or loaded wash water from the water washing.

[0015] The term "fission gas" is used here for a mixture of compounds that, initially in gaseous form, is taken from a cracking furnace used in steam cracking, or for a mixture formed from it upstream of a cryogenic separation unit. Elsewhere, such a mixture is initially also referred to as raw gas or cracking raw gas. The terminology used here therefore also applies to the raw gas or cracking raw gas and any mixture formed from it. This mixture is processed using known process steps.

[0016] Processes and equipment for steam cracking hydrocarbons are described, for example, in the article "Ethylene" in Ullmann's Encyclopedia of Industrial Chemistry, online edition, April 15, 2009, DOI: 10.1002 / 14356007.a10_045.pub2, which also describes suitable processing steps. Typically, these processing steps include, in a first section (front-end section) of a corresponding processing sequence, the removal of heavy compounds, if present, followed by raw gas compression, sour gas removal, and drying. The oil fractionation, water scrubbing, and compression used in the proposed process and its embodiments are part of this first section. After processing in the first section, fractionation takes place, in which thermal separation processes using ethylene or C2 refrigerant and propylene or...Certain fractions of C3 refrigerant are formed and can be further separated if necessary. For technical details, please refer to the aforementioned article, in particular sections 5.3.2.1, "Front-End Section," and 5.3.2.2, "Hydrocarbon Fractionation Section."

[0017] The proposed process and its configurations address a problem that was only recently recognized as relevant. Depending on the proportion and pretreatment, impurities continue to enter the steam cracker via the plastic pyrolysis oil, even after the aforementioned processing steps. These impurities can then be transformed into novel and unexpected compounds. The degree of formation and the nature of such unexpected compounds also depend on the pretreatment. Little is currently known about this. However, detailed investigations by the applicant revealed that the cracking gas from a steam cracker using plastic pyrolysis oil can contain hydrogen cyanide (HCN). Without further measures, this can spread into multiple streams within the plant and cause problems as a highly toxic and corrosive compound.

[0018] The proposed process and its embodiments, based on a combination of suitable purification methods such as catalytic and oxidative processes and scrubbing, enable a particularly advantageous removal of hydrogen cyanide from especially relevant material streams, i.e., at particularly advantageous positions. This applies to both effluent and product streams, i.e., both unseparated streams from the steam cracker and fractions formed therefrom, and especially wastewater streams. The use of the proposed process and its embodiments allows for the clean and specification-compliant discharge of aqueous material streams. Particularly in aqueous environments, this results in a reduction of corrosion risks. If selective hydrogenation is carried out in conjunction with the proposed process and its embodiments, no catalyst impairment is to be expected.Ultimately, this results in a reduced risk of contamination of product streams or loss of product value.

[0019] In embodiments of the proposed process, the elimination of hydrogen cyanide can be carried out by catalytic conversion using one or more catalysts. The catalytic conversion can be used, in particular, for the cracked gas at the aforementioned intermediate stage of compression, whereas the wash water from the water scrubbing can be treated, especially oxidatively or otherwise, for example, by treatment with hydrogen peroxide. The catalytic conversion for the cracked gas is particularly effective and results in exceptionally low residual concentrations.

[0020] In embodiments of the proposed process, hydrogen cyanide is eliminated (in each case) to a residual content of less than 100 ppm by weight, 10 ppm by weight, 5 ppm by weight, or 1 ppm by weight, or a content below the detection limit, i.e., practically zero. In this way, the proposed process enables compliance with the typically stringent specifications for wastewater and cracking products.

[0021] In embodiments of the proposed process, hydrogen cyanide is eliminated in addition to the caustic scrubbing for carbon dioxide removal. In such cases, this elimination step occurs upstream of the caustic scrubbing and, in particular, in a (partially) compressed gas stream leading to the caustic scrubbing in the form of cracking gas at the aforementioned intermediate compression stage. While eliminating hydrogen cyanide from gas within the caustic scrubbing itself would also be possible in principle, this can lead to a variety of reactions and side effects such as foaming or deposit formation. Hydrogen cyanide can also be released in gaseous form during the neutralization of the caustic solution in the caustic scrubbing process. Therefore, in the proposed process and its embodiments, the elimination of hydrogen cyanide takes place upstream of the caustic scrubbing.This avoids the undesirable reactions and side effects in the caustic scrubbing process, as well as the outgassing of the caustic solution. Therefore, the caustic scrubbing process itself, or a correspondingly loaded caustic solution, does not represent a position or medium considered advantageous in the present process.

[0022] In various embodiments of the proposed process, the plastic pyrolysis oil can contain 30 to 13,000 ppm (parts per million by weight) or 50 to 3,500 ppm by weight of nitrogenous compounds. Simultaneously, the plastic pyrolysis oil can contain less than 100 ppm or less than 10 ppm by weight of hydrogen cyanide, which is either introduced into the plastic pyrolysis process via appropriate plastic waste or generated during the pyrolysis process itself. The plastic pyrolysis oil itself thus contributes little or no hydrogen cyanide. Instead, this is generated from corresponding precursors during steam cracking.

[0023] The nitrogen-containing compounds can be, in particular, organic compounds with nitrile groups. These organic compounds with nitrile groups originate especially from polyurethanes, certain types of polyamides, and polyacrylonitriles such as ABS (an acrylonitrile-butadiene-styrene copolymer) used in plastics for plastic pyrolysis.

[0024] In certain embodiments of the proposed process, the plastic pyrolysis oil is therefore not, or only to a limited extent, freed from such nitrogen-containing compounds. As explained below, the removal of such compounds from the plastic pyrolysis oil is complex, and the proposed removal of hydrogen cyanide at specific downstream positions represents a particularly advantageous alternative to removing the precursor compounds. Thus, in certain embodiments, the proposed process incorporates the seemingly counterintuitive approach of first allowing the formation of hydrogen cyanide and then removing it.In other words, the elimination of hydrogen cyanide is particularly advantageous compared to the removal of a wide range of possible precursor compounds, especially since identifying the corresponding precursor compounds may not be readily possible and therefore a very wide range of compounds needs to be removed.

[0025] Hydrogen cyanide elimination is fundamentally possible at various stages, three of which are compared below. These are (a) cracked gas between oil fractionation and water scrubbing, (b) cracked gas at an intermediate compression stage, and (c) loaded scrubbing water from water scrubbing. The proposed process and its embodiments utilize alternatives (b) and (c), either individually or in combination. As mentioned, the term cracked gas refers to the (partially) processed component mixture (also called raw gas, cracked raw gas, etc.) obtained during steam cracking, as described in the first section.

[0026] Depending on the overall plant design and the proportion of impurities in use, the respective alternatives (a) to (c) or corresponding combinations thereof may be considered advantageous. For example, alternative (a) may initially appear particularly desirable for the removal of hydrogen cyanide, as it largely prevents the distribution of hydrogen cyanide into downstream parts of the plant. However, this position places the highest demands on catalyst and reactor selection (with regard to flow control, reactor size, and catalyst deactivation), so it does not represent a solution within the scope of the proposed process. Instead, alternatives (b) and (c), or combinations thereof, were identified as particularly advantageous as a result of an overall assessment.

[0027] In embodiments of the proposed process, it may be provided, in particular, that the cracked gas is subjected to carbon dioxide capture at the intermediate compression stage and then returned to the compression stage for further compression. The elimination of hydrogen cyanide according to alternative (b) is thereby carried out, in particular, in the (partially) compressed cracked gas upstream of the carbon dioxide capture at the intermediate stage.

[0028] The caustic scrubbing process can include the elimination of hydrogen cyanide, particularly in the loaded caustic solution, as explained below, but only in addition to the solutions proposed in the proposed process. It may be possible to carry out the caustic scrubbing with a caustic utilization of 50 to 80%, and / or to neutralize the scrubbing solution using an acid to which hydrogen peroxide is added in a predetermined ratio before being added back to the scrubbing solution. This results in a particularly effective reduction of the hydrogen cyanide content and, as explained in detail below, prevents the release of hydrogen cyanide during the neutralization of the scrubbing solution.

[0029] A hydrogen cyanide removal process provided in the cracking gas at the intermediate compression stage, i.e., alternative (b) as previously explained, is particularly advantageous in one aspect of the invention. Here, the cracking gas can be selected, in particular, after the fourth stage of a cracking gas compressor that, for example, has four or five stages overall (without limiting the invention to this), is particularly advantageous. At this point, the hydrogen cyanide removal can be implemented in a particularly compact manner. Furthermore, a considerable portion of the water and potential catalyst poisons (tar, polycyclic aromatic hydrocarbons, acids, and other oxygenates, etc.) is already removed or reduced at this stage. The reaction temperatures can therefore be selected to be lower than in alternative (a).

[0030] Alternatives (b) and (c) can be used in combination and comprise a partial transfer of hydrogen cyanide from the cracked gas into the process water in the scrubbing column and, in particular, the oxidative elimination of hydrogen cyanide from this (alternative (c)), followed by a catalytic conversion of the hydrogen cyanide remaining in the cracked gas according to alternative (b), especially upstream of an alkaline scrubber. In this way, a substantial elimination of hydrogen cyanide can be achieved in the relevant material streams. For alternative (b), only the residual amount of hydrogen cyanide that was not already removed with the scrubbing water in alternative (c) remains. The majority of the hydrogen cyanide remains in the gas phase during water scrubbing; approximately 10 to 30% is expected to be in the water. If the water is recycled to the steam cracker, the hydrogen cyanide can be thermally converted.The remaining residue can be treated accordingly, and the resulting water quality can be checked via a blowdown.

[0031] In embodiments of the proposed process, the elimination of hydrogen cyanide, as mentioned, can be carried out by catalytic conversion using one or more catalysts, particularly with regard to the cracking gas at the intermediate compression stage. In principle, all suitable catalysts can be used, especially those comprising one or more metals and a support system.

[0032] The one or more metals may be selected, in particular, from one or more alkaline earth metals and / or one or more transition metals, especially sodium, potassium, barium, nickel, cobalt, molybdenum, iron and / or zinc, and / or the support system may comprise one or more oxides of titanium, aluminum, zinc and / or zirconium. Further advantageous embodiments are explained in detail below.

[0033] In embodiments of the proposed process, the catalytic reaction can be carried out at a reaction temperature between 80 and 300 °C and / or a space velocity between 5,000 and 20,000 standard cubic meters of gas per cubic meter of catalyst per hour. Such reaction conditions have proven particularly advantageous. Since it has been found that the inhibitory effect of water is particularly low in a temperature range between 150 and 220 °C, the reaction temperature can be set especially within this range.

[0034] In embodiments of the proposed process, the reaction temperature can be adjusted, at least in part, using heat of compression, sensible heat from a heavy oil circuit, and / or heat supplied by steam. As explained in detail below, different heat sources are particularly suitable for specific temperature ranges and are therefore used in corresponding embodiments. In embodiments of the proposed process, a gas mixture removed from the catalytic reaction can be cooled against a feedstock supplied to the catalytic reaction. Such a feed-effluent heat exchange can, in particular, reduce the heating requirement.

[0035] As an alternative to, or in combination with, catalytic conversion, the elimination of hydrogen cyanide in embodiments of the proposed process can be carried out using an oxidative, complexing, and / or adsorptive process. In particular, the loaded wash water according to alternative (c) can be treated oxidatively, especially using hydrogen peroxide. Depending on the overall concept and the alternatives implemented, specific advantages arise in each case. Details of the corresponding processes are explained below.

[0036] The elimination of hydrogen cyanide can be carried out, in particular, using a reactor equipped with a honeycomb structure, especially a radial flow reactor and / or a lateral flow reactor. These are reactor alternatives with particularly low differential pressures, which can therefore be especially advantageous. In a radial flow reactor, the honeycomb structure equipped with the appropriate catalyst can be flowed through radially, while in a lateral flow reactor, the flow occurs laterally.

[0037] The proposed plant for processing plastic pyrolysis oil includes means that are set up for steam cracking the plastic pyrolysis oil to obtain a cracking gas, wherein at least a part of the cracking gas is successively subjected to oil fractionation, water washing and compression, and which is further set up for eliminating hydrogen cyanide in the cracking gas at the intermediate stage or one of the several intermediate stages of compression and / or loaded wash water from the water washing.

[0038] For further features and advantages of a corresponding system and its embodiments, reference is expressly made to the above explanations concerning the method proposed according to the invention and its embodiments, as these apply equally. The same applies to a system which, according to an embodiment of the invention, is configured to carry out a method according to any embodiment of the present invention.

[0039] The proposed process for processing plastic pyrolysis oil can be integrated into a process for the production of hydrocarbons, which may include the pyrolysis of plastic to obtain plastic pyrolysis oil and the processing of the plastic pyrolysis oil using any previously described embodiment. This process thus also benefits from the advantages of the described process and its embodiments. This also applies accordingly to a plant equipped for the pyrolysis of plastic to obtain plastic pyrolysis oil and its processing in the described manner.

[0040] Drawings

[0041] Exemplary embodiments of the solutions proposed here are described below with reference to the attached drawing, wherein

[0042] Figure 1 illustrates a method according to one embodiment, and

[0043] Figures 2A to 2C illustrate reactors for catalytic conversion.

[0044] Designs

[0045] The embodiments and configurations described below are provided solely to assist the reader in understanding the claimed and previously explained features. They represent only representative examples and are not intended to be considered exhaustive or limiting with regard to the features of the proposed methods and devices.

[0046] It is understood that the advantages, embodiments, examples, functions, features, structures and / or other aspects described above and below are not to be considered as limitations of the scope of the claims or as limitations of equivalents thereto, and that other embodiments may be used and modifications made without deviating from the scope of the claims.

[0047] Different embodiments may include, feature, consist of, or essentially consist of further advantageous combinations of the described elements, components, features, parts, steps, means, etc., even if such combinations are not specifically described herein. Furthermore, other embodiments may be included that are not currently claimed but could be claimed in the future, particularly if they are within the scope of the independent claims.

[0048] Explanations relating to devices, apparatus, arrangements, systems, etc., according to proposed embodiments may also apply to procedures, processes, methods, etc., according to other embodiments, and vice versa. Identical, functionally equivalent, structurally identical, or comparable elements, process steps, etc., may be indicated with identical reference numerals.

[0049] The following explanations and definitions relating to some fundamental aspects of the invention may apply to all or part of the embodiments presented here, and the explanation of certain aspects relating to only one part or one of the embodiments should not be understood to mean that these aspects cannot also be realized with other or all embodiments, insofar as technically possible and sensible.

[0050] Liquid and gaseous flows, gas mixtures or the like may, in the terminology used herein, be “rich” or “poor” in one or more components, where “rich” may refer to a content of at least 50%, 75%, 90%, 95%, 99%, 99.5%, 99.9% or 99.99% and “poor” to a content of at most 50%, 25%, 10%, 5%, 1%, 0.1% or 0.01% on a molar, weight or olm basis.

[0051] Liquid and gaseous material streams, gas mixtures, or the like may, in the terminology used herein, be enriched or depleted of one or more components, these terms referring to a concentration in another material stream from which the material stream was formed. A material stream under consideration is "enriched" if it has at least 2, 5, 10, 100, or 1,000 times the concentration of the designated component(s), and "depleted" if it has at most 0.5, 0.1, 0.01, or 0.001 times the concentration of the designated component(s), in each case with respect to the material stream from which the material stream under consideration was formed.

[0052] Terms such as "essentially containing" and the like are understood here to mean, in particular, that a composition, material flow, etc., described in this way may contain other components in addition to those specified as mandatory or those implied by the name of the gas mixture (e.g., "hydrogen"), provided that the essential characteristics of the described composition are not significantly altered by these other components. The same applies to terms such as "essentially free of" and the like. A gas or gas mixture that "essentially" contains or consists of one or more components may, in particular, contain these components in amounts exceeding 95%, 99%, 99.9%, or 99.99% in total or as individual values. Conversely, a gas or gas mixture is "essentially free" of one or more components if it contains less than 5%, 1%, 0.1%, or 0.01% of these components in total or as individual values.

[0053] All percentages used here may refer to molar, quantity, or volume fractions. Unless otherwise stated, pressure values ​​in bar are to be understood as absolute pressures.

[0054] The conjunction "and / or," when used before the last item in a list, should be understood to mean that all items mentioned before and after it can be combined in any way. In other words, "A, B and / or C" means "A and / or B and / or C" or "at least one of the elements A, B, and C in any combination."

[0055] When referring to a "part" of a material stream, this can mean a fraction with the same composition that has simply been diverted from an original stream, but also a fraction with a different composition and possibly only a component of the original stream that is formed by a process such as condensation, evaporation, boiling, distillation, rectification, absorption, adsorption, flash filtration, membrane separation, deposition, or the like, or that remains as a residue after a corresponding step. A "part" can also exist after a combination of any of the aforementioned steps, for example, after separation processing of a diverted fraction.

[0056] The processing of plastic waste for use in pyrolysis, i.e.

[0057] Plastic pyrolysis can include, in particular, a pretreatment process involving sorting, washing, and pre-reduction of impurities. The subsequent pyrolysis, which can be thermal and / or catalytic, possibly with the addition of additives, yields plastic pyrolysis gas and the plastic pyrolysis oil under consideration. The gas can be subjected to any desired pretreatment. The oil can, in particular, first undergo purification, which may include, for example, filtration, desalting, distillation, or processing by adsorptive and / or absorptive methods. In addition to direct feeding of the purified plastic pyrolysis oil into a steam cracker, hydrogenation, for example, in the form of hydrodemetallization (HDM), a known hydrotreating process, or a known hydrocracking process, can also be carried out.

[0058] The term "plastic pyrolysis oil" is used here to refer to a pyrolysis oil produced using a pyrolysis feed that consists at least partially of plastic, particularly plastic waste. This may include polyethylene and polypropylene. Typically, other types of plastic may also be present in smaller proportions, especially polystyrene, polyethylene terephthalate, polyamide, and the like. However, a pyrolysis oil produced only partially using plastic, but otherwise using conventional feeds such as naphtha, also constitutes a "plastic pyrolysis oil" in the sense used here, as it may contain the components identified as critical.

[0059] Depending on the proportion and pretreatment, impurities can still enter the steam cracker, as mentioned, and may be converted into new compounds. As also mentioned, hydrogen cyanide can be formed in this process, which, without additional measures, can lead to its distribution throughout the system and cause corresponding problems.

[0060] Hydrogen cyanide is produced during steam cracking from precursor compounds in the plastic pyrolysis oil, such as organic nitriles. Due to the relatively high stability of hydrogen cyanide under the conditions prevailing during steam cracking (high temperature, presence of hydrogen), its formation can only be partially controlled by the operating conditions; rather, it depends primarily on the composition of the plastic pyrolysis oil. Complete removal of the precursor components from the plastic pyrolysis oil is technically difficult and costly.

[0061] Processes otherwise widely used in the petrochemical industry for similar applications, operating purely on the principle of physical adsorption, are not efficient for treating larger flows of plastic pyrolysis oil due to the large number of other trace components that can be co-adsorbed. Hydrogenation processes promise high removal rates, but also require several pre- and post-treatment steps of the plastic pyrolysis oil and are therefore investment- and / or operating-cost-intensive.

[0062] Hydrogen cyanide is a gaseous compound that is somewhat soluble in water, but as a very weak acid, it is displaced by other components. As a result, untreated hydrogen cyanide is distributed downstream of the cracking furnace in the stream cracker system in different streams.

[0063] For the removal of hydrogen cyanide, a variety of methods are known in the scientific community, which are selected depending on the state of matter of the carrier stream (gaseous or liquid), concentration of the hydrogen cyanide, composition of the matrix medium as well as energetic and economic framework conditions.

[0064] A key method for separating hydrogen cyanide from gaseous streams is absorption. In this process, the gaseous hydrogen cyanide is transferred into a suitable liquid through contact, usually in a countercurrent flow. Both purely physical solvents and chemically reactive media can be used as absorbents.

[0065] In physical absorption, hydrogen cyanide dissolves due to its high solubility in solvents such as water or organic solvents like methanol. This form of absorption is reversible and particularly suitable for higher gas concentrations and low partial pressures. In contrast, chemical absorption is characterized by a reaction of hydrogen cyanide with components of the absorption solution. Alkaline solutions, such as sodium hydroxide or potassium hydroxide, are frequently used, in which the hydrogen cyanide is deprotonated to form cyanide ions. Amine solutions are also used for selective chemical absorption. The conversion to cyanide ions enables the targeted recovery or oxidative destruction of the toxic substance in subsequent steps. However, this approach, as previously described in connection with alkaline scrubbing, has certain disadvantages.

[0066] Another established method for removing hydrogen cyanide, particularly from low-concentration gas streams, is adsorption onto solid porous materials. In this process, the hydrogen cyanide is bound to the surface of an adsorbent either physically or chemically. Typical adsorbents include activated carbon, especially impregnated activated carbons with bases, metal salts, or oxidizing agents, which offer higher loading capacity and selectivity for hydrogen cyanide. Zeolites and metal-organic frameworks (MOFs) are also suitable for hydrogen cyanide adsorption due to their specifically tunable pore structures and chemical functionalization. Adsorption can be carried out continuously or discontinuously in fixed-bed, packed-bed, or moving-bed systems.The regeneration of the adsorbent can be carried out thermally or by purging with inert or reactive gases, whereby the recovery or destruction of the hydrogen cyanide must be ensured.

[0067] Oxidative processes are also used to directly destroy hydrogen cyanide, in which the hydrogen cyanide is converted into less toxic products by suitable oxidizing agents. Such processes can be carried out in both the gas and liquid phases. In the aqueous phase, the oxidation of hydrogen cyanide or cyanide ions is often achieved by adding hydrogen peroxide, ozone, potassium permanganate, or hypochlorite. The oxidation usually proceeds in several steps, forming intermediate products (e.g., cyanate), ideally resulting in carbon dioxide and non-toxic or less toxic nitrogen compounds as end products.

[0068] In the gas phase, catalytic oxidation can occur at elevated temperatures using noble metal or metal oxide catalysts. Such systems are particularly relevant in exhaust air purification systems, where catalytic afterburning is used as the final purification stage. Thermal treatment can also be used to completely destroy hydrogen cyanide, especially in exhaust gases with high concentrations. At temperatures above 1000 °C and with sufficient oxygen supply, hydrogen cyanide is completely oxidized. This high-temperature combustion is frequently used in combination with downstream denitrification processes. Furthermore, hydrogen cyanide can also be hydrolyzed over suitable catalysts (e.g., metal-doped titanium oxide) in the presence of water vapor in the gas phase. Combined processes are common to increase the separation efficiency and process flexibility.For example, pretreatment can be carried out by absorption, followed by adsorption for residual removal or subsequent catalytic or thermal destruction. The combination of chemical absorption with subsequent oxidation in the liquid phase (e.g., in an absorber circuit) is also frequently used to ensure complete removal while simultaneously avoiding the retention and disposal of cyanide ions. The selection of the appropriate method for removing hydrogen cyanide from liquid or gaseous streams depends on the specific process parameters. Combining several methods allows for flexible purification optimized for each application, always taking into account compliance with legal limits, the protection of human health and the environment, and the economic viability of the process.

[0069] Aspects of the proposed method also include a combination of removal methods at specific locations, which, however, are very specifically adapted to the processing process downstream of a steam cracker and offer particular advantages in this context.

[0070] The following section discusses results from pilot tests with a plastic pyrolysis oil containing approximately 479 ppm nitrogen by weight, which is cleaved to obtain a propylene-ethylene ratio of approximately 0.41 and approximately 0.68 mol / mol (“Case 1”) and a plastic pyrolysis oil containing approximately 2079 ppm nitrogen by weight, which is cleaved to obtain a propylene-ethylene ratio of approximately 0.61 (“Case 2”).

[0071] In case 1, after the separation of hydrocarbon condensate and aqueous condensate, approximately 14 to 28% of the total raw gas from the pilot cracking furnace remains as gas. The hydrocarbon condensate accounts for approximately 28 to 32% of the total raw gas from the pilot cracking furnace, and the aqueous condensate for approximately 44 to 55%.

[0072] The hydrocarbon condensate, the aqueous condensate, and the remaining

[0073] In case 1, the gas fraction typically exhibits the following concentrations of certain compounds (the ppm values ​​refer to weight fractions in the condensates and to volume fractions in the remaining gas fraction):

[0074] Hydrocarbon condensate:

[0075] - Hydrogen cyanide approx. 80 to 120 ppm

[0076] - Nitriles approx. 90 to 130 ppm

[0077] - Pyrroles and pyridine approx. 60 ppm

[0078] Aqueous condensate:

[0079] - Hydrogen cyanide approx. 12 to 17 ppm

[0080] - Ammonia approx. 30 ppm

[0081] Remaining gas content:

[0082] - Hydrogen cyanide approx. 80 to 120 ppm

[0083] - Ammonia below approximately 1 ppm

[0084] - Nitric oxide below approximately 0.01 ppm

[0085] In case 2, however, after the separation of hydrocarbon condensate and aqueous condensate, approximately 35% of the total raw gas from the pilot cracking furnace remains as gas. The hydrocarbon condensate constitutes approximately 35% of the total raw gas from the pilot cracking furnace, and the aqueous condensate approximately 30%.

[0086] In case 2, the hydrocarbon condensate, the aqueous condensate and the remaining gas fraction typically exhibit the following concentrations of certain compounds (the ppm values ​​refer to weight fractions in the condensates and to volume fractions in the remaining gas fraction):

[0087] Hydrocarbon condensate:

[0088] - Hydrogen cyanide approx. 1 ppm

[0089] - Nitriles approx. 250 ppm

[0090] - Pyrroles and pyridine approx. 50 ppm

[0091] Aqueous condensate:

[0092] - Hydrogen cyanide approx. 15 ppm - Ammonia approx. 40 ppm

[0093] Remaining gas content:

[0094] - Hydrogen cyanide approx. 140 ppm

[0095] - Ammonia below approximately 1 ppm

[0096] - Nitric oxide below 0.01 ppm

[0097] To overcome the disadvantages described above, measures are proposed that are particularly suitable for the targeted purification and removal of hydrogen cyanide from cracking gas from a steam cracker or from material streams derived therefrom, which result from the cracking of feedstocks that contain at least plastic pyrolysis oil or consist entirely of such oil.

[0098] As mentioned, possible alternatives include (a) hydrolysis of hydrogen cyanide in the raw product gas stream or cracking gas at elevated temperatures using a suitable catalyst (e.g., based on metal-doped titanium oxide) before condensation of the water contained in the raw cracking gas, (b) hydrolysis of existing hydrogen cyanide content in the gas phase before alkaline scrubbing using a suitable catalyst (e.g., based on metal-doped titanium oxide) or an adsorbent, or (c) treatment of the aqueous condensates or loaded scrubbing water before transfer to conventional wastewater treatment using oxidizing processes (e.g., using hydrogen peroxide and optionally iron, copper, and the like, or using ozone or chlorine bleach), complexation, or ion exchange, possibly also after stripping.Combinations of these alternatives are possible and the configurations proposed here relate in particular to alternatives (b) and (c), individually or in combination.

[0099] Alternatives or embodiments of a proposed process may further include an adapted operating mode of the caustic scrubbing process with reduced caustic utilization of, for example, less than 85% and oxidizing treatment of a scour stream, treatment by means of medium-pressure oxidation (MP-WAO), or trace removal of hydrogen cyanide traces by means of an adsorbent after caustic scrubbing (before hydrogenation). Figure 1 illustrates a process according to an embodiment proposed here and is designated in its entirety by 100. The process comprises a steam cracking step 10, to which a feedstock 1 containing plastic pyrolysis oil is added, and in which a cracking gas 2 is obtained. The cracking gas 2 is subjected to an oil fractionation 20, from which a cracking gas depleted of heavy components, further designated by 2, is extracted.This is fed to a water wash 30, wherein a catalytic removal of hydrogen cyanide, which is not part of the proposed process, is illustrated in Figure 1 by a according to the above alternative (a).

[0100] Process water 3 extracted from the water scrubber can be fed into a process steam system 40, which generates process steam 5 that can be supplied to the steam cracking step 10. A portion of the process water 2 can be subjected to hydrogen cyanide removal, as explained above in alternative (c) and illustrated in Figure 1 by c. This can be, in particular, a portion produced in process 100.

[0101] After water washing, a cracked gas remains, which is further designated 2 and is subjected to cracked gas compression 50. Partially compressed cracked gas can be extracted from this at an intermediate stage. This gas, optionally after removal of hydrogen cyanide as explained above for alternative (b) and illustrated in Figure 1 by b, is then fed to an alkaline scrubbing process 60. Correspondingly treated cracked gas, further designated 2 in Figure 1, is returned to the cracked gas compression 50, further compressed there, and subsequently subjected to pre-cooling and drying 70.

[0102] The cooled and dried cracking gas, further referred to as 2 in Figure 1, can be subjected to fractionation, which is not illustrated.

[0103] As mentioned, the catalytic front-end oxidation or hydrolysis according to alternative (a) is not part of the proposed process, which involves treatment of the gas phase, i.e., the cracking gas, in particular with a catalyst, prior to the caustic scrubbing according to alternative (b) and / or treatment of the wash water from a water scrubbing process, in particular oxidatively, according to alternative (c). As mentioned, arrangements with a low differential pressure, as illustrated in examples in Figures 2A, 2B, and 2C, may be particularly advantageous for the catalytic removal of hydrogen cyanide.

[0104] Figure 2A illustrates a reactor 210 with a honeycomb structure 201. Figure 2B illustrates a radial flow reactor 220 equipped with a catalyst in a region 202 and through which the flow is radial. Figure 2C illustrates a lateral flow reactor 230 equipped with a catalyst in a region 203 and through which the flow is lateral. A reactor vessel is designated 200 in Figures 2A to 2C.

[0105] Particularly suitable catalysts include, for example, catalysts doped with alkali or alkaline earth metals such as sodium, potassium, barium, etc., and / or catalysts containing transition metals such as nickel, cobalt, molybdenum, iron, indium, etc. Support systems such as titanium dioxide, dialuminous trioxide, zinc oxide, or zirconium oxide, or combinations thereof, are also suitable.

[0106] Depending on the topological configuration, the reaction temperatures used range from 80 to 300 °C, with space velocities of 5,000 to 20,000 standard cubic meters of gas per cubic meter of catalyst per hour. The inhibitory effect of water can be minimized by carefully selecting the temperature range of 150 to 220 °C. Depending on the reactor's position, the heat of compression and / or sensible heat from a heavy oil circuit can be used to preheat the gas stream, up to a temperature of 200 °C. To achieve higher reactor temperatures (up to 300 °C), the feed gas or cracking gas can be heated against steam.

[0107] To minimize heating requirements, preheating of the cracking gas against the reactor effluent in an upstream heat exchanger can be provided. However, this is not strictly necessary.

[0108] The aforementioned alternative (a) would, without further consideration, be considered particularly advantageous, as it largely prevents the distribution of hydrogen cyanide to other parts of the plant. However, as mentioned, this position represents the highest requirement with regard to catalyst and reactor selection, and ultimately economic viability, and is not a solution considered within the framework of the proposed procedure. Reference is made to the above statements.

[0109] To set the required reactor temperature, sensible heat from the heavy oil circuit can be used to preheat the gas stream in a range up to 200 °C. To achieve higher reactor temperatures (up to 300 °C), the feed gas or cracking gas can be heated, in particular, against steam.

[0110] To minimize heating requirements, preheating of the cracking gas against the reactor effluent in an upstream heat exchanger can be provided. However, this is not strictly necessary.

[0111] Alternative (b) is particularly suitable for the outlet of the fourth stage of the cracking gas compressor, because here the reactor can be built more compactly and a large part of the water and potential catalyst poisons (tar, polycyclic aromatics, acids and other oxygenates, etc.) have already been removed or reduced.

[0112] Reaction temperatures can be chosen to be lower here than according to alternative (a).

[0113] Up to a required reactor inlet temperature of 100 to 105 °C, the compression heat is generally sufficient according to alternative (b). To set the inlet temperature in the range of 100 to 200 °C, the cracked gas can be preheated in a feed-effluent heat exchanger, and heat from the heavy oil circuit can also be used. The cracked gas, now free of hydrogen cyanide, can then be cooled first against the feed stream and then further against cooling water, and after condensate separation, fed into the caustic scrubber.

[0114] The absorption of hydrogen cyanide in process water depends on several factors (temperature, pH value, ion strength, etc.). Under normal process conditions, most of the dissolved hydrogen cyanide is expected to be stripped and, in the usual configuration, returned to the scrubbing water column. This leads to a concentration of hydrogen cyanide in the column. To minimize this, in the configurations proposed here, the stripping gas can be recirculated further downstream, particularly near the suction side of the first stage of the cracking gas compressor.

[0115] The hydrogen cyanide and other metal-cyanide complexes contained in the process water can be removed via blowdown. Due to their potential toxicity to organisms, the cyanide limits for industrial wastewater are very low and typically below 1 mg / L. Numerous methods for removing cyanides from wastewater are known (oxidation with oxygen, hydrogen peroxide, ozone, or chlorine bleach; adsorption / ion exchange; complexation / precipitation / flocculation, or combinations thereof, or the aforementioned methods after prior stripping) and are generally known from the prior art.

[0116] Within the framework of the proposed configurations, the principle of complexation by adding iron sulfate to the process water can be specifically employed to precipitate hydrogen cyanide in the form of ferrocyanides (and remove it by filtration). This step also makes it possible to minimize corrosion in the process water circuit, reduce the load on the downstream catalytic / adsorptive removal process, and thus optimize costs.

[0117] In a further embodiment of the process, the catalytic removal of the hydrogen cyanide remaining in the cracked gas after the water quench column can be omitted. This can then be addressed by appropriately adjusting the operation of the caustic column. To promote the absorption and retention of hydrogen cyanide in the circulating caustic solution, the remaining "free" content of caustic solution, for example, sodium hydroxide, can be increased. In such embodiments, the caustic solution utilization can be reduced from 85% to 50–80% to achieve particularly low hydrogen cyanide concentrations, typically less than 1 ppm by weight, in the resulting cracked gas.

[0118] Sodium cyanide and metal complexes dissolved in the black liquor can only be incompletely converted to cyanates under typical low-pressure black liquor oxidation conditions. Neutralization with acid, such as sulfuric acid, would cause hydrogen cyanide to escape, or the neutralized black liquor would fail to meet the discharge specifications. Therefore, the oxidation kinetics can be accelerated by increasing the temperature, for example, in medium-pressure black liquor oxidation (190 to 250 °C), resulting in lower residual cyanide concentrations.

[0119] Preferably, a modified neutralization process is used in accordance with this embodiment, in which hydrogen peroxide is added to the acid, for example sulfuric acid, in a ratio of approximately 3:1 (sulfuric acid to hydrogen peroxide) before it enters the neutralization tanks. The resulting peroxomonosulfuric acid (Caro's acid) oxidizes the cyanides and thiocyanates that are not reacted in the slurry oxidation at a pH of 9 to 10 in a temperature range of 20 to 60 °C.

[0120] Particular advantages of this process are the low process temperature, short residence time, and the oxidation of thiosulfates, sulfites, and hydrocarbons. The treatment of the two wastewater streams (black liquor and process water) can, of course, be carried out on a common / combined stream using a suitable process (oxidation (with oxygen, hydrogen peroxide, ozone, chlorine bleach, Caro's acid), adsorption / ion exchange, complexation / precipitation / flocculation, or a combination thereof).

[0121] Under certain conditions, caustic scrubbing processes in an ethylene plant tend to foam, which can lead to slippage of sour gases and also hydrogen cyanide. Hydrogen cyanide is not adsorbed in the cracking gas dryer. To protect the acetylene hydrogenation catalysts, alumina-zeolite hybrid adsorbents specifically developed for unsaturated streams can be used. The preferred operating range is -40 to 60 °C.

Claims

Patent claims 1. Method (100) for processing plastic pyrolysis oil (1), wherein the method (100) comprises the following steps: Steam cracking (10) of the plastic pyrolysis oil (1) to obtain a cracking gas (2), wherein at least a part of the cracking gas (2) is successively subjected to oil fractionation (20), water washing (30) and compression (50) via one or more intermediate stages; and Eliminating hydrogen cyanide in loaded wash water (4) from the water wash (30) and / or the cracking gas (2) at the intermediate stage or one of the several intermediate stages of compression (50).

2. Method (100) according to claim 1, wherein the elimination of hydrogen cyanide is carried out by catalytic reaction using one or more catalysts.

3. Method (100) according to claim 1 or 2, wherein the elimination of hydrogen cyanide is carried out to a residual content of less than 100 wt. ppm, 10 wt. ppm, 5 wt. ppm or 1 wt. ppm.

4. Method (100) according to one of the preceding claims, wherein the elimination of hydrogen cyanide is carried out in addition to an alkaline scrubbing for carbon dioxide separation (60).

5. Method (100) according to any of the preceding claims, wherein the plastic pyrolysis oil (1) contains 30 to 13,000 ppm by weight or 50 to 3,500 ppm by weight of nitrogenous compounds and / or less than 100 ppm by weight or less than 10 ppm by weight of hydrogen cyanide.

6. Method (100) according to claim 4, wherein the caustic washing is carried out with a caustic utilization of 50 to 80%, and / or wherein a lye solution from the lye washing process is neutralized using an acid, to which hydrogen peroxide is added in a predetermined ratio before being fed back into the lye solution.

7. Method (100) according to claim 2, wherein the one or more catalysts comprise one or more metals and a support system.

8. Method (100) according to claim 7, wherein the one or more metals are selected from one or more alkaline earth metals and / or one or more transition metals, in particular sodium, potassium, barium, nickel, cobalt, molybdenum, iron and / or zinc, and / or wherein the support system comprises one or more oxides of titanium, aluminum, zinc and / or zirconium.

9. Method (100) according to one of claims 7 or 8, wherein the catalytic reaction is carried out at a reaction temperature between 80 and 300 °C or between 150 and 220 °C and / or a space velocity between 5,000 and 20,000 standard cubic meters of gas per cubic meter of catalyst per hour.

10. Method (100) according to claim 9, wherein the reaction temperature is set at least partly using heat of compression, sensible heat from a heavy oil circuit and / or heat provided by means of steam.

11. Method (100) according to one of claims 7 to 10, wherein a gas mixture taken from the catalytic reaction is cooled against an input supplied to the catalytic reaction.

12. Method (100) according to any one of the preceding claims, wherein the elimination of hydrogen cyanide is carried out using an oxidative, complexing and / or adsorptive process.

13. Method (100) according to one of claims 2 or 7 to 11, wherein the elimination of hydrogen cyanide is carried out using a reactor (210) equipped with a honeycomb structure (201), which in particular consists of a The operation is carried out using a radial flow reactor (220) and a lateral flow reactor (230).

14. Plant for processing plastic pyrolysis oil (1) comprising means set up to carry out the following steps: Steam cracking (10) of the plastic pyrolysis oil (1) to obtain a cracking gas (2), wherein at least a part of the cracking gas (2) is successively subjected to oil fractionation (20), water washing (30) and compression (50) via one or more intermediate stages; and elimination of hydrogen cyanide in loaded wash water (4) from the Water washing (30) and / or the cracking gas (2) at the intermediate stage or one of the several intermediate stages of compression (50).

15. System according to claim 14, wherein the system comprises means which are set up to carry out a method (100) according to any one of claims 2 to 13.

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