Method for removing fouling downstream of an ODH reactor

Abstract

A method for preventing or removing water-soluble foulants (7) located downstream of an oxidative dehydrogenation (ODH) reactor (3) is described. The method employs the continuous or intermittent introduction of water upstream of the fouling site, which acts to solubilize and carry away the fouling material. This method has the advantage that it can be used while the ODH process is running, avoiding the need for costly shutdowns.

Description

Technical Field

[0001] This disclosure relates to the use of a reactor for the oxidative dehydrogenation (ODH) of lower (C2-4) paraffins to their corresponding olefins. More specifically, the disclosure relates to methods for preventing and / or removing fouling (contamination, deposits, fouling) in the output line of an ODH reactor.

Background Art

[0002] The catalytic oxidative dehydrogenation (ODH) of ethane to ethylene has been known previously as an alternative to thermal cracking, but concerns about the safety of mixing hydrocarbons and oxygen, and the capital injection required to replace familiar equipment that is costly to operate but highly reliable and exhibits very high conversion rates have prevented its commercial acceptance. In recent years, a number of ODH-related patents have been published or granted in connection with improvements in safety or efficiency using mixed metal oxide catalysts with higher conversion and selectivity. In contrast, little discussion has taken place regarding the operation and maintenance of ODH reactors, which have a major impact on cost efficiency if ODH were to be commercially accepted. ODH is inherently exothermic and, in contrast to thermal cracking, does not require a furnace for coke formation, although large amounts of greenhouse gases are of course produced. The advantage of ODH is that it is not associated with the inevitable, time-consuming, and costly delays that are characteristic of coke removal.

[0003] As with many oxidation processes, ODH requires monitoring and control of the reactor's state, and its variations can affect the yield and the extent of the formation of undesirable by-products. For example, the ODH reaction conditions can affect the types and amounts of oxygen-containing compounds such as acetic acid and maleic acid present in the off-stream from the reactor. Separation downstream of the reaction products is a given, and while the operator changes conditions to promote the formation of some products, it avoids other things that require more costly efforts for separation. Cooling is required for downstream separation, but unfortunately, when water-soluble components such as maleic acid condense and freeze and eventually adhere to the reactor infrastructure, it can have an undesirable effect of fouling. If this occurs, the advantages that ODH holds against thermal decomposition in relation to shutdown of operation can be nullified.

[0004] Generally in the petrochemical industry, fouling and clogging of pipes and equipment are common problems, along with solutions to address issues including chemical and mechanical treatments. For example, oil and gas pipelines accumulate solid deposits that require periodic shutdowns for cleaning, and various methods for this exist in the prior art. As an example, WIPO application 2013169679 filed on May 7, 2013 in the names of Fisher et al. (applicant M-1 L.L.C.) teaches a method for cleaning natural gas pipelines by introducing a cleaning liquid containing a diluent selected from glycol, glycerin, or a mixture thereof. This patent teaches the use of a selective diluent because the use of water in natural gas pipelines is not recommended as it can lead to the formation of mud, rust, and hydrate plugs.

[0005] The prior art also includes various methods for removing hydrate plugs from oil and gas pipelines. The plugs can be removed by depressurization, chemical injection, or heating. Mechanical removal using a pig is also known, as taught in U.S. Patent No. 7,279,052, issued to Kinari et al. on October 9, 2007, and assigned to Statoil Asia and Crawford Technical Services, Inc. In this case, as the pig advances with the aid of a thrust fluid and a return flow line, the deposits are removed continuously or intermittently.

[0006] Solutions for removing blockages in oil and gas pipelines are beneficial, but not necessarily practical when applied to the problem of blockages immediately downstream of the ODH reactor. Oil and gas pipelines are much larger in diameter, much longer in distance, and dislike using water for cleaning. In the case of an oxidation process, the equipment including pipes and related components is much smaller in scale and is not affected by damage caused by moisture of the types generally avoided in the oil and gas industry.

[0007] The use of water for removing dirt from components of industrial plants is described in the art. U.S. Patent No. 4,773,357, issued to Scharton et al. on September 27, 1998, and assigned to Anco Engineers, Inc., teaches a method of cleaning a tube sheet of a heat exchanger using a water cannon. A reactor is equipped with a number of heat exchangers that experience sludge accumulation at the bottom. The sludge includes copper oxide, magnetite, corrosion products, oxidation products, and other deposits. This patent teaches that the sludge found on the tube sheet can be removed by explosively discharging water at the sludge using a directional nozzle as needed. Unfortunately, this method requires the heat exchanger to be shut down.

[0008] U.S. Patent No. 3,531,541, issued to Woerner on September 29, 1970 and assigned to Petro-Tex Chemical Corporation, teaches reducing fouling of compressor pistons and cylinders used in compressing gaseous compositions, preferably fouling produced in oxidative dehydrogenation processes. This process is designed to sweep away fouling believed to include oxygenated and / or unsaturated organic compounds that adhere to the contact surfaces after compression. This patent teaches spraying water on the suction side of the compressor into which the gaseous composition, preferably excluding water, is introduced.

[0009] U.S. Patent No. 3,728,413, issued to Woerner on April 17, 1973 and assigned to Petro-Tex Chemical Corporation, teaches reducing compressor fouling downstream of an oxidative dehydrogenation process. The risk of fouling by polymeric compounds formed from precursors during compression is reduced by removing the precursors with water. This process teaches removing water from the effluent from the oxidation process during a cooling step, then reintroducing the water before or during compression, followed by a scrubbing step to remove impurities. This patent does not teach removing water-soluble fouling immediately downstream of the reactor and before quenching and compression. SUMMARY OF THE INVENTION

[0010] Provided herein is a method for removing or preventing the accumulation of water-soluble fouling that accumulates downstream of an ODH reactor used for the oxidative dehydrogenation of lower paraffins to the corresponding olefins. In one embodiment, the solvent is introduced from an upstream ODH reactor where fouling may occur into an outlet pipe in a manner that promotes a cyclic, laminar, or turbulent flow of the solvent mainly in a liquid state along the inner surface of the pipe immediately downstream of the introduction location. In one embodiment, the solvent introduced into the outlet pipe is water.

[0011] In another embodiment, the solvent is introduced via a pipe-in-pipe arrangement, and the outlet pipe from the reactor is mounted within a downstream pipe having a larger diameter. In this case, the solvent is introduced through the gap between the outlet pipe and the downstream pipe.

[0012] In another embodiment, the solvent can be sprayed onto the inner surface of the outlet pipe by an in-stream atomizer or, alternatively, strategically placed on the inner surface of the outlet pipe and by a series of jets continuous with the inner surface.

[0013] In another embodiment, the solvent is introduced through a plurality of holes having outlets continuous with the inner surface of the outlet pipe.

[0014] In a further embodiment, water is introduced via a pipe-in-pipe arrangement combined with an in-stream atomizer, inner surface jets, a plurality of holes, or combinations thereof.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4-A

Figure 4-B

Figure 5-A

Figure 5-B

Best Mode for Carrying Out the Invention

[0016] Unless otherwise indicated by an operating example or otherwise, all numbers or expressions referring to amounts of components, reaction conditions, etc. used in the specification and claims should be understood to be modified in all cases by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and the appended claims are approximations that may vary depending on the properties desired to be obtained by the present invention. At a minimum, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the reported number of significant digits and by applying ordinary rounding techniques.

[0017] Disclosed herein is a method for removing substantially water-soluble fouling and / or preventing accumulation in the outlet pipe of an oxidative dehydrogenation (ODH) reactor used to convert lower paraffins (C2-C4) to the corresponding olefins. The term "fouling", including "plugging" in the worst case, refers to water-soluble by-products of the ODH process that solidify after exiting the reactor and may adhere to and accumulate on the inner surface of the outlet pipe of the ODH reactor. If left unchecked, fouling can even restrict or block the flow through the outlet pipe, affecting the efficiency of the ODH process. Plugging refers to a complete blockage of the pipe. The water-soluble by-products of the ODH process include oxygen-containing compounds such as maleic acid. The modifier "substantially water-soluble" refers to the possibility that the fouling may also include small amounts of miscible products such as acetaldehyde and ethanol, or other insoluble fine particles entrapped within the solid fouling. Lower paraffins refer to paraffins having 2 to 4 carbon atoms. In one embodiment, the lower paraffin is ethane and its corresponding olefin is ethylene. For simplicity, the method will be described for use in ethane ODH, but it is also applicable to the use of propane and butane ODH.

[0018] The advantage of the disclosed method is that it can prevent fouling from being removed or formed while the ODH reactor is operating. Operating means being in operation, as opposed to a stop period where the reactor is idle and the flow of reactants and product gas has stopped. Shutting down the operation can be time-consuming and costly. The disclosed method reduces the risk of the need for shutdown, which has a significant economic impact. However, this method can also be used during reactor shutdown. In this case, the volume and flow rate of the solvent introduced need to be changed accordingly.

[0019] The method described is intended for use in connection with an ODH process in which a gas mixture 1 containing at least ethane and oxygen is introduced via one or more inlets 2 into an ODH reactor 3 containing an ODH catalyst (Figure 1). The conditions within the ODH reactor 3 promote the conversion of ethane to ethylene. The oxidation reaction catalyzed by the ODH catalyst can produce various by-products, such as water-soluble oxygen-containing compounds like carbon dioxide, acetic acid, and maleic acid. An outlet stream 4 containing ethylene and by-products, and unreacted ethane and oxygen if present, is discharged from the ODH reactor 3 via an outlet pipe 5. The temperature of the outlet stream 4 can vary and is controlled by the operator according to the desired reaction conditions, which are adjusted to match a specific target product profile. Upon exiting the ODH reactor 3, the outlet stream 4, in the absence of exothermic reaction conditions, naturally begins to cool using a cooling mechanism, such as a heat exchanger 6, starting with a quench tower for removing, for example, water and acetic acid, before being subjected to a series of separation steps as required.

[0020] Although not intended to be bound by theory, as the temperature of the outlet stream 4 decreases, the water-soluble oxygen-containing compound present in gaseous form within the ODH reactor 3 condenses as droplets within the gaseous outlet stream 4 and then adheres to the inner surface of the outlet pipe 5 and ultimately solidifies as fouling 7 at another location upstream of where the separation step begins, downstream of the ODH reactor 3. The proportion of the outlet stream 4 consisting of the water-soluble oxygen-containing compound is minimal, but the fouling 7 can grow over time without intervention as particles of the solid water-soluble oxygen-containing compound adhere to form a mass. Growth of the fouling 7 can be detected as an abnormal pressure drop between the reactor and a location downstream of the outlet pipe 5. Introduction of a solvent at one or more locations (arrow 8) upstream of the location of the fouling promotes dissolution of the fouling 7. This can then be carried to the separation step by the gas and liquid flows and removed along with the acetic acid.

[0021] ODH reactor The disclosed method contemplates the use of any known reactor type applicable to the ODH of hydrocarbons. In some embodiments, the method disclosed herein uses one or more conventional fixed-bed reactors. In a typical fixed-bed reactor, reactants are introduced into the reactor at one end and flow through or over the immobilized catalyst, products are formed, and exit at the other end of the reactor. The reactor may include separate inlets for each reactant or may include a single inlet (similar to FIG. 1) through which the reactants are premixed and introduced into the reactor as a homogeneous mixture. Those skilled in the art will know which features are required regarding the shape and dimensions, control and monitoring of reactant input, product output, temperature and pressure, and means for securing the catalyst.

[0022] In another embodiment, the use of one or more fluidized bed reactors is contemplated. These types of reactors are well known. Typically, the catalyst is supported by a porous structure or a distribution plate located near the lower end of the reactor, and the reactants are pushed against it at a rate sufficient to balance the weight of the catalyst, as a result of which the catalyst rises and begins to swirl around in a fluidized manner. The reactants are converted to products upon contact with the fluidized catalyst and are then removed from the upper end of the reactor. Design considerations include the shape of the reactor and the distribution plate, the inputs and outputs, and the control and monitoring of temperature and pressure, all of which fall within the knowledge of those skilled in the art.

[0023] Another embodiment contemplates the use of multiple ODH reactors, either in series or in parallel. The use of multiple reactors, including ODH reactors, either in a parallel configuration or in a series configuration is well known in the art. When parallel ODH reactors are used, the methods disclosed herein can be used downstream of each ODH reactor, after the streams from each ODH reactor are combined, or both downstream of each ODH reactor and after the streams from each ODH reactor are combined.

[0024] When series ODH reactors are used, fouling between the ODH reactors is not expected because the temperature drops sufficiently so that condensation and freezing of the oxygen-containing compounds do not occur. The introduction of a solvent for the prevention or removal of fouling is preferably used only downstream of the last ODH reactor in a series. However, if fouling is seen between the ODH reactors, the methods of the present invention may be used between the series ODH reactors, provided that the user ensures that water introduced between the ODH reactors is removed before entering the next series of ODH reactors, or that the temperature of the stream is high enough to convert the water to steam before entering the next ODH reactor. Liquid water within the ODH reactor can potentially damage the catalyst, the reactor, and associated components.

[0025] ODH process The use of an ODH reactor as applicable to the present disclosure is within the knowledge of those skilled in the art. The oxidative dehydrogenation of ethane can be carried out at a temperature of 300 °C to 550 °C, or 300 °C to 500 °C, or 350 °C to 450 °C, at a pressure of 0.5 to 100 psi (3.447 to 689.47 kPa), or 15 to 50 psi (103.4 to 344.73 kPa), and the residence time of the lower alkane in the reactor is usually 0.002 to 30 seconds, or 1 to 10 seconds.

[0026] Any ODH catalyst known in the art is suitable for use in the methods disclosed herein. When selecting a catalyst, a skilled user will understand that catalysts can vary with respect to selectivity and activity. In one embodiment, mixed metal oxides are used because they can provide high selectivity to ethylene without significantly sacrificing activity. Examples of catalysts are those of the following formula:

[0027] [Chemical formula] Here, Me is a metal selected from the group consisting of Ta, Ti, W, Hf, Zr, Sb, and mixtures thereof. a is from 0.1 to 3. b is from 0.5 to 1.5. c is from 0.001 to 3. d is from 0.001 to 5. e is from 0 to 2. And f is a number that satisfies the valence state of the catalyst.

[0028] In the ODH process, various combinations of the ratios of ethane, oxygen, and, optionally, an inert diluent can be used. Those skilled in the art will understand that, for safety reasons, it is preferable to select a composition in which the ratio of oxygen to ethane is outside the flammable envelope, in the presence or absence of an inert or substantially inert component. This includes ratios that exceed the upper flammability limit or are below the lower flammability limit. Those skilled in the art will know how to determine the flammability limits and whether a particular composition, including those containing an inert diluent, is outside the flammable envelope.

[0029] Oxygen can be supplied as pure oxygen or as a component of a gas mixture such as air. Air contains nitrogen and functions as an inert diluent. When using pure oxygen and an inert diluent, the inert diluent needs to be in a gaseous state under the conditions in the reactor and must not increase the flammability of the hydrocarbons added to the reactor. This is a characteristic that a person skilled in the art would understand when determining which insoluble diluent to use. The inert diluent can be added to either ethane or oxygen, or to an oxygen-containing gas if used, before entering the ODH reactor, or can be added directly into the ODH reactor.

[0030] The reaction conditions and the composition of the reactants, such as the ratio of ethane to oxygen and the presence, absence, or nature of an inert diluent, can affect the product profile, together with the catalyst used, such as the selectivity to ethylene, the conversion rate, and the extent to which oxygen-containing compounds such as maleic acid are produced in the ODH reaction. A person skilled in the art would be well aware of how adjustments to the conditions and components affect the product profile. To detect the presence of fouling that needs to be removed, monitor the pressure drop when the outlet stream 4 exits the reactor. Under normal operating conditions, the pressure decreases with temperature. When fouling begins to occur, the pipes downstream of the ODH reactor start to become blocked, and the pressure profile may change. The pressure upstream of the fouling is usually higher, and the pressure downstream of the fouling is lower than the pressure that occurs under normal circumstances. A complete blockage can result in a very large pressure increase upstream of the fouling. When the pressure is higher than normal upstream of the fouling and lower than normal downstream of the fouling, a larger pressure drop than usual is expected.

[0031] Introduction of solvent The solvent that can be used according to the described method is a solvent that can dissolve substantially water-soluble dirt, exists in liquid form at the temperature and pressure in the outlet pipe 5 downstream of the ODH reactor, and does not adversely affect the downstream separation step. A preferred solvent is water. The water used in this method need not be distilled or deionized and can include impurities if the impurities are non-reactive with respect to the olefins and are unlikely to adversely affect downstream processing. The collected water from the bottom of the acetic acid scrubber, which is likely to be the first separation unit downstream of the ODH reactor, can also be used as a solvent. The collected water from the bottom of the acetic acid scrubber may be described as dilute acetic acid, and the concentration of acetic acid ranges from 0 to 50 wt.%, preferably 1 to 10 wt.%. Using dilute acetic acid has the advantage that the water used in the acetic acid scrubber can be reused, and additional water sources may not be required. Embodiments of the disclosed method are described using water as the solvent. In each case, the solvent need not be water, but can be any other substance that can dissolve substantially water-soluble dirt, exists mainly in the liquid state under the main conditions, and does not adversely affect downstream processing.

[0032] The introduction of water downstream of the ODH reactor and upstream of the dirt can be done at multiple locations before the separation step. The water introduced must mainly be in the liquid state at the temperature and pressure at which the water is introduced. In liquid form, water has the ability to dissolve substantially water-soluble dirt, but this is not possible as steam. The introduction of water is such that the circulating flow of water indicated by the dashed curve arrows in FIGS. 2-5 substantially contacts the inner surface of the pipe, begins to dissolve the dirt 7, and carries the dirt particles 9 away from the side of the pipe (FIG. 2).

[0033] In one embodiment, as shown in FIG. 2, a pipe-in-pipe arrangement 10 can be used to deliver water in a desired manner. In this arrangement, the pipe 11 emerging from the outlet pipe 5 or the outlet pipe (FIGS. 1 and 2) downstream of the cooling mechanism (e.g., heat exchanger 6) and continuous with it becomes the inner pipe 12 and ends inside the outer pipe 13 with a larger diameter and fits with it. As long as there is a gap 15, whether continuous or discontinuous, through which water can be introduced between the inner surface of the outer pipe 13 and the end of the inner pipe 12, the end of the inner pipe 12 may or may not have a flare 14. The gap is selected so that water preferentially forms a film on the inner surface of the outer pipe 13, often generating a flow called an annular flow. The inner pipe 12 may also include rifling to generate vortices of water upon introduction, facilitating complete coverage of the inner surface of the outer pipe 13. The end of the inner pipe 12 should be placed at an upstream location where fouling may occur. The plurality of pipes of the pipe arrangement 10 can be used at multiple locations. The outer pipe 13 of the upstream pipe of the pipe array 10 functions like the inner pipe 12 of the subsequent pipe of the pipe array 10. The user may taper the diameter of the outer pipe 12 as it approaches the next arrangement, or select an inner pipe 12 with a consistent diameter. In this case, the diameter of each subsequent outer pipe 13 increases.

[0034] In another embodiment, water may be introduced via an in-stream atomizer head 16 (FIG. 3), and the atomizer sprays onto the inner wall from a position within the flow of the reactants exiting the ODH reactor. In another embodiment, a plurality of jets 17 can be used to introduce water into the wall of the outlet pipe 11 (FIGS. 4A and 4B). The jets 17 can spray an area of the inner wall of the pipe on the opposite side of the jet (indicated by the straight dashed arrow), and the size of that area depends on the configuration of the nozzle at the end of the jet 17. A plurality of jets can be used to ensure coverage of the inner wall of the outlet pipe 11, and the required number varies depending on the spray area of the nozzle and the jet (see the cross-section of the pip with jets in FIG. 4B). The jets may be within concentric circles or may be offset from some of the jets further upstream of the subsequent jets.

[0035] In another embodiment, water (shown by the curved dashed arrow) can be introduced through a plurality of holes or perforations 18 on the inner surface of the pipe into which the water is introduced (Figs. 5A and 5B). The holes are designed so that water can leak or penetrate into the pipe. This includes options to close the holes and options to control the flow of water through the holes to promote annular flow within the pipe. This type of design is within the knowledge of those skilled in the art. The perforations or holes 18 can be spaced in a uniform pattern or can be dispersed at various locations along the length of the outlet pipe 11.

[0036] The embodiments for water introduction are not intended to be used solely for isolation. In another embodiment, water introduction can be done at one or more locations, and each location employs one or more methods for introducing water into the pipe. For example, a pipe-in-pipe arrangement can be used in combination with an in-stream atomizer, or perforations, or a plurality of jets. When using multiple methods for introducing water into the pipe, the user should note that increasing the amount of water introduced may result in a more diluted flow than when introducing water by a single method.

[0037] The introduction of water into the pipe downstream of the reactor where there is a gas flow creates a multiphase flow arrangement consisting of the gas outlet stream 4 and the introduced liquid water. Those skilled in the art will be familiar with various mechanisms for introducing water at an appropriate flow rate so that annular flow of water occurs substantially on the inner surface of the pipe. Elements to consider when determining the flow rate of water to introduce include the flow rate, temperature, and pressure of the outlet stream 4 exiting the ODH reactor 3. The flow rate of the gaseous outlet stream 4 depends on the size of the ODH reactor and ranges from 2 L / min for bench-scale reactors to 80,000 L / min for commercial-scale reactors. The temperature and pressure depend on the aforementioned reactor configuration and reaction conditions. In the absence of fouling, both the temperature and pressure begin to decrease immediately after exiting the reactor.

[0038] In the selection of flow rate, it is necessary to consider the extent to which water begins to dilute the outlet stream. The user needs to determine an appropriate flow rate that can form an annular stream without adding excessive water that needs to be removed downstream. The removal of water is preferably carried out during the quenching process where the generated acetic acid is also separated. Introducing a large amount of water into the outlet stream adds the water used in the quenching process and further dilutes the acetic acid. Users who further concentrate acetic acid after separation may want to limit the amount of water added to prevent fouling in order to reduce the extent to which acetic acid needs to be concentrated.

[0039] Water is introduced into the pipe under the conditions of the gas flow through the pipe, resulting in an annular flow of the liquid. The formation of the annular flow of the liquid for a given gas flow rate is a function of the density, viscosity, surface tension, and flow rate of the gas and the liquid. For details of the factors related to multiphase flow, refer to Chapter 2, "Gas-Liquid Transport in Ducts" of the 2006 Multiphase Flow Handbook by Clayton T. Crowe.

[0040] In limited situations, short slug flow can be used. Short means a period short enough that it is not necessary to stop the reactor so as not to interrupt the operation of the reactor. In this case, the amount of water introduced into the pipe is sufficient to form a "slug" of liquid water, followed by pockets of gas flow before and after it. As the slug moves downstream, fouling is removed from the inner surface of the pipe by the passing slug of water and dissolves as the slug moves. If the introduction of a higher volume of water associated with slug flow is maintained, the pressure inside the reactor may increase to the point where the reaction conditions are not ideal. This should be avoided. Also, the downstream separation unit must be able to withstand the sudden pressure increase resulting from the interaction with a large slug of water.

[0041] To select the location for introducing water, it is necessary to consider the locations where fouling occurs or is likely to occur. These locations are generally expected to occur where the temperature and pressure contribute to the formation of water-soluble oxygen-containing compounds from solid particles that may adhere to the inner surface of the piping connecting the reactor to the downstream separation components. For example, the melting point of maleic acid is 135 °C at normal pressure. Using the temperature profile of the outlet stream when exiting the reactor, ideally a guide is provided for where water can be introduced at a location where the temperature exceeds the melting point of maleic acid. However, the user will understand that they need to select a location where the introduced water is unlikely to boil to a significant extent, or where the water dissolves the fouling faster than it boils. It is preferred that the introduced water remains substantially in a liquid state until it dissolves the fouling. The temperature of the water after introduction is likely to increase depending on the location of introduction and the temperature of the water at the location of introduction, peak as the flow goes further downstream, and then decrease. Therefore, if the peak temperature of the introduced water remains below the boiling point and a flow rate is introduced that can maintain the liquid state for the time required to dissolve the contaminants, the location of introduction can include locations above the boiling point of water.

[0042] Intermittent or continuous operation The methods disclosed herein can be used continuously or intermittently. That is, water may be continuously introduced at one or more locations during the operation of the ODH reactor. In this case, the user can select the lowest possible water flow rate to minimize the degree to which the product stream is diluted. Continuous operation is effective in preventing fouling formation from the start because the inner surface of the piping downstream of the reactor and before the separation components is always covered with a film of flowing water. Fouling particles are not exposed to conditions where adhesion can be maintained for a period sufficient for subsequent particles to adhere to the previous particles and grow into agglomerates.

[0043] Alternatively, the user can choose to adopt the method of the present invention only if they consider it to be wise. The operating conditions can be such that the accumulation of dirt takes a long time to develop and water-soluble by-products are generated at a low level. If the user determines that the pressure drop from the reactor is outside the normal range expected in the absence of dirt, this method can be used to introduce water at one or more locations selected by the user. When the pressure drop returns to normal, the introduction of water into the stream can be stopped. Pressure transducers placed at intervals downstream of the reactor can identify the locations where dirt is accumulating by highlighting the sections with abnormal pressure drops. Next, the user can choose to introduce water only at the location immediately upstream of the dirt, or at multiple locations. If the pressure drop is not corrected over time, the user can increase the amount of water added or the number of upstream locations where water is added. Alternatively, the user can choose to introduce water such that a slug flow is used. Finally, if the pressure drop does not improve even after sufficient time has been given, there may be problems related to dirt that is not water-soluble.

Example

[0044] Example Two fixed-bed reactors connected in series were used in the ODH process, and ethane, ethylene, carbon dioxide, and oxygen were premixed at ratios of 11 - 93 / 0 - 80 / 0 - 8 / 0 - 8 vol% respectively before being fed into the first reactor of the two reactors. The weight hourly space velocity was 0.65 - 2.70 h -1It was within the range. The output from the first reactor was sent directly to the second reactor without adding new reactants. For each reactor, the temperature was maintained in the range of 300 - 337 °C at ambient pressure. The process was continuously run over a period of 43 days, and the composition of the feed varied within the specified range. An abnormal pressure profile was detected immediately downstream of the second reactor, before the downstream condenser. The process was stopped due to the activation of the high-pressure alarm in the reactor, and the outlet line from the second reactor was disconnected. Contamination was detected during inspection, and the contamination blocked approximately 40% of the cross-section of the pipe. In the analysis of the contaminants using GC-MS, maleic acid / anhydride was identified as the main component (>90%) containing a small amount of acetic acid and trace amounts of 1,2-benzenedicarboxylic acid. When water was passed through the pipe, the contamination was removed almost immediately. The results show that the contaminants can be removed with water and that a person skilled in the art proficient in multiphase flow configurations can design an input for introducing water into the pipe downstream of the ODH reactor in the manner described by the method disclosed herein.

[0045] Industrial applicability The method described is applicable for use in an oxidative dehydrogenation (ODH) reactor to prevent or remove water-soluble fouling in the outlet line downstream of the ODH reactor.

Claims

1. A method for oxidative dehydrogenation of ethane to ethylene, comprising removing and / or preventing the accumulation of substantially water soluble foulants in a piping downstream of an operating ODH reactor, the method comprising: introducing a solvent into one or more locations in the piping; at least one of the one or more locations is a location upstream of a quench tower; the solvent is introduced primarily in a liquid state at the pressure and temperature within said pipe at the point of introduction; the operating ODH reactor is used for the oxidative dehydrogenation of ethane to ethylene; The process above, wherein the solvent is water or dilute acetic acid.

2. The method of claim 1 , wherein the solvent is introduced at a flow rate or pressure that promotes annular flow of the solvent in substantial contact with the interior surface of the tubing.

3. 10. The method of claim 1, wherein an ODH reactor is in operation and the solvent is introduced at a flow rate or pressure that promotes slug flow of the solvent and for a time short enough so as not to interfere with the operation of the ODH reactor.

4. The method of any of claims 1 to 3, wherein the solvent is introduced into at least one of said one or more locations via a pipe-in-pipe arrangement.

5. The method of any of claims 1 to 3, wherein the solvent is introduced into at least one of said one or more locations via an in-stream atomizer.

6. The method according to any one of claims 1 to 3, wherein the solvent is introduced by a plurality of holes around the circumference of the pipe.

7. The method of any of claims 1 to 3, wherein the solvent is introduced into at least one of said one or more locations continuously during operation of the ODH reactor.

8. 2. The method of claim 1, wherein the water is introduced to at least one of the one or more locations intermittently during operation of the ODH reactor.

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