KR100619123B1 - A method for removing polymer from an acid gas treating system - Google Patents

Abstract

The present invention comprises the steps of contacting an acidic gas containing stream with an aqueous alkanolamine solution and a hydrocarbon solvent to form a mixture of an alkanolamine rich solution containing an acidic gas or a by-product thereof with a hydrocarbon solvent. And a hydrocarbon solvent having an aromatic content of about 60-100% by volume and a binary ring aromatic content of about 50-85% by volume, based on the total volume of the solvent.

Description

A METHOD FOR REMOVING POLYMER FROM AN ACID GAS TREATING SYSTEM

The present invention relates to a method for removing polymer from an acidic gas treatment system.

In a typical petrochemical plant having a steam cracker for pyrolysis of ethane, propane, naphtha, gas oil and other suitable steam cracking feedstocks, the effluent from the steam cracker is the desired olefin product, In addition to ethylene and propylene, for example, they contain acid gases such as carbon dioxide, hydrogen sulfide and trace amounts of carbonyl sulfide. To recover the desired product, it is necessary to purify the steam cracker effluent to remove acid gases, for example by contacting the effluent with an appropriate solvent such as an aqueous solution of alkanolamine in the absorption tower. In the absorption tower, the acidic gas is absorbed by the alkanolamine aqueous solution to produce an alkanolamine rich solution, which is discharged from the absorption tower. The alkanolamine rich solution is then transferred to a regenerator where the alkanolamine rich solution is heated to blow away most of the acidic gas. The alkanolamine lean solution from the regenerator is recycled to the absorption tower and brought into contact with further steam cracker effluents. On the other hand, acidic gases can further improve the quality grade in the sulfur recovery unit to obtain sulfur products that can be sold.

The problem of polymer formation in acidic gas systems, namely the formation of acidic gas byproducts and contamination of units, is well known in the art. For example, US Pat. No. 3,696,162 describes polymerization problems encountered in alkanolamine regenerators and heat exchange systems caused by the presence of dienes and complexes formed from dienes and acid gas anions in the steam cracker effluent entering the absorption tower. have. This reference solves the polymerization problem by introducing a hydrocarbon solvent with an aqueous alkanolamine solution into the absorption tower to absorb the diene and act as a solvent for any complex of the diene. In this method, an alkanol rich solution phase and a hydrocarbon solvent phase containing the absorbed diene and complex are formed at the bottom of the absorption tower upon sedimentation. The reference indicates that the alkanolamine rich solution phase and the hydrocarbon solvent phase containing the absorbed diene and complex can be discharged separately or together. When discharged together, the portion on the hydrocarbon solvent must eventually be separated to provide a purge stream to prevent accumulation of dienes and complexes in the system. The alkanolamine rich aqueous solution phase and the hydrocarbon solvent phase are then passed together through a heat exchanger and a regenerator. Hydrocarbon solvents are described as preferred because aromatic solvents generally have a starting boiling point of about 80 ° C.

In the system described in USP 3,696,162, the alkanolamine solution and the hydrocarbon solvent are circulated together through an absorption tower, a heat exchanger and a regenerator. Since hydrocarbon solvents of USP 3,696,162 generally have a starting boiling point of about 80 ° C., contamination of the regenerator overhead occurs and the quality of sulfur recovered from acid gases in the sulfur recovery unit is poor. US Pat. No. 3,696,162 solves the polymerization problem that occurs when removing acid gases from steam cracker effluents, but this reference describes the contamination of regenerator overhead with hydrocarbon solvents and the sulfur recovered in sulfur recovery units when such contamination occurs. The associated deterioration of the product is not recognized.

Typically, the acid gases in the purifier gas are removed as part of the purifier operation before improving the quality grade in the sulfur recovery unit. In some cases, it may be desirable to utilize an absorption tower used to remove acid gas from the steam cracker effluent for further work of absorbing acid gas from the purifier gas. This can be achieved, for example, by combining the steam cracker effluent with the purifier gas prior to contacting the aqueous alkanolamine solution in the absorption tower. The acid gas and purifier gas recovered from the steam cracker effluent may then improve the quality grade in the sulfur recovery unit.

The inventors have found that the polymerization problem when using absorption towers for the further operation of absorbing acid gases from purifier gases is that the solvents described in USP 3,696,162 generally have a starting boiling point of about 80 ° C. It has been found that they have different properties than those described in US Pat. No. 3,696,162 to the extent that they do not effectively reduce contamination. In particular, the polymers formed when absorbing acidic gases from steam cracker effluent and purifier gas have a higher molecular weight, ranging from viscous syrup form liquids to solid stone solids. Such polymers may be amine degradation products, carbonyl polymers, diene polymers, free radical polymers and corrosion products and may be present in the molecular weight range of 150 to 10,000.

Thus, when an absorption tower is used to remove acidic gases from a mixed feedstock of steam cracker effluent and purifier gas, contamination of the regenerator overhead with hydrocarbon solvents can be used to improve the quality grade of sulfur recovered in the sulfur recovery unit. It may be desirable to reduce contamination of the regenerator and heat exchanger while eliminating it. It is also desirable to easily separate the hydrocarbon solvent phase from the alkanolamine rich solution.

Summary of the Invention

In order to solve the polymerization problems arising from the use of absorption towers to absorb acid gases from steam cracker effluent and / or purifier gas and at the same time minimize the contamination of regenerator overhead, the inventors have found that the use of heavy aromatic solvents It has been found to achieve that goal, ie to dissolve larger molecular weight polymers and to minimize contamination of the regenerator overhead. However, the use of heavy aromatic solvents, such as hydrocarbon solvents, prevents sufficient settling of the hydrocarbon solvent phase in the absorption tower to cause separation of the alkanolamine rich solution phase and the hydrocarbon solvent phase. In other words, the density of the hydrocarbon solvent phase approaches the density of the alkanolamine rich solution such that the separation of the alkanolamine rich solution by sedimentation or gravity from the hydrocarbon solvent phase is inefficient and very difficult to achieve.

In FIG. 1, fresh hydrocarbon solvent 1 is added to the circulating alkanolamine solution 2. This blended hydrocarbon solvent and alkanolamine rich solution are then separated via hydrocyclone (3). The alkanolamine rich solution contains residual hydrocarbon solvent 4 and is then sent to regenerator 5. This residual hydrocarbon solvent dissolves the polymer accumulated in the system. At the initial high rate of hydrocarbon solvent addition, the amount of hydrocarbon solvent accumulates to equilibrium levels in the system. The reject stream 6 from the hydrocyclone thus becomes a mixture of the hydrocarbon solvent and the dissolved polymer, ie the polymer rich hydrocarbon solvent stream. In the regenerator 5, the alkanolamine rich solution containing the residual hydrocarbon solvent 4 is heated to blow most acidic gas 10 away. The alkanolamine deficient solution 11 exiting the regenerator 5 is recycled to the absorption tower 13 and brought into contact with the further steam cracker effluent 14.

In FIG. 2, such reject stream 6 is sent to a separation drum 7 where the polymer rich hydrocarbon solvent and any residual alkanolamine rich solution each form a separate phase. The heavier alkanolamine rich solution phase 8 is returned to the circulating alkanolamine system, while the lighter polymer rich hydrocarbon solvent phase 9 is sent to the purifier for further processing. After the hydrocarbon solvent has accumulated to the equilibrium level, the addition of fresh hydrocarbon solvent is slowly reduced.

 Detailed description of the invention

In the present specification, the alkanolamine rich solution refers to an alkanolamine aqueous solution containing acidic gas, while the alkanolamine deficient solution refers to an alkanolamine aqueous solution in which most acidic gases are removed. Polymer rich hydrocarbon solvent means a hydrocarbon solvent in which a polymer is contained.

In the process according to the invention, the steam cracker effluent and / or purifier gas, each containing acidic gas, is sent to an absorption tower, where the two streams are brought into contact with the alkanolamine deficient solution. Initially, the alkanolamine deficient solution is a fresh alkanolamine solution. However, once a steady state is achieved, the alkanolamine deficient solution may be recycled from the regenerator and may contain a polymer rich hydrocarbon solvent, which is a novel alkanolamine solution for any construction purpose. Added at that point. In either case, the alkanolamine deficient solution absorbs acid gases from the steam cracker effluent and / or purifier gas to form an alkanolamine enriched solution, which is recovered from the absorption tower. The absorption tower is typically operated at an inert temperature of 20 to 60 ° C., preferably 40 to 50 ° C., and a pressure of 3 to 50 atm (300 to 5100 kPa), preferably 5 atm (510 kPa) or more.

Fresh hydrocarbon solvents or constituent hydrocarbon solvents can be added to the alkanolamine deficient solution prior to introduction into the absorption tower and introduced into the absorption tower separately from the alkanolamine deficient solution, or after exiting the absorption tower. It can be added to the amine rich solution. The hydrocarbon solvent is initially added in an amount ranging from about 1 to 5% by volume of the solvent, based on the total volume of the circulating alkanolamine solution, and gradually, about 0.05 to 1% by volume, depending on the severity of contamination in the heat exchanger and regenerator. It can be added in amounts in the range. Hydrocarbon solvents are believed to dissolve polymers formed due to the high temperature operation associated with these two units in heat exchangers and regenerators. However, polymer rich hydrocarbon solvents and alkanolamine rich solutions have relatively similar densities, so sedimentation or separation by gravity is inefficient and difficult to achieve. According to the invention, the mixture of the polymer rich hydrocarbon solvent and the alkanolamine rich solution is transferred to a hydrocyclone, wherein the polymer rich hydrocarbon solvent is removed from the alkanolamine rich solution containing acidic gas. The polymer rich hydrocarbon solvent can then be separated from the hydrocyclone and sent to the purifier for further processing, for example in a catalytic cracked fractionator from which a polymer tar stream or fuel can be obtained.

Hydrocyclones are typically pressure vessels containing a flow of fluid evenly distributed between each liner and a plurality of liners that work in equilibrium inside the vessel. The feedstock stream is typically a fluid and is pumped into each liner, which is designed to force fluid into the sailing spiral path through the liner outlet end. This spiral path generates a denser fluid, a centrifugal force that pushes the water phase to the wall of the liner. The less dense fluid, ie the polymer rich hydrocarbon solvent, is disposed towards the center of the liner. By maintaining the pressure at the upstream end of the hydrocyclone lower than the pressure at the outlet, the central core of the fluid, i.e. the polymer rich hydrocarbon solvent, flows in the opposite direction of the water phase, i.e., the alkanolamine rich solution, and rejects at the upstream end of the hydrocyclone. The orifice is discharged through the orifice, while the aqueous phase exits the outlet of the hydrocyclone. The aqueous phase typically contains from 0.1 to 2.0% by volume, more preferably from 0.1 to 0.5% by volume of the residual hydrocarbon solvent, based on the total volume of the circulating alkanolamine solution.

From the hydrocyclones, the alkanolamine rich solution is sent to a regenerator, which has a temperature in the range of 100 to 160 ° C., preferably 110 to 140 ° C. and 0.5 to 10 atm (51 to 1000 kPa), preferably 0.5 to It operates at pressures ranging from 1.5 atm (51 to 150 kPa). The hydrocarbon solvent trapped in the alkanolamine rich solution dissolves the polymer formed in the heat exchanger and regenerator. Since the regenerator is operated at high temperatures, it is desirable to have a heavy hydrocarbon solvent that does not pass over the regenerator overhead and does not contaminate the regenerator overhead. The alkanolamine deficient solution and the polymer rich hydrocarbon solvent are withdrawn from the regenerator and recycled to the absorption tower.

Acidic gas components typically found in steam cracker effluents are H ₂ S in the range of 10 to 100,000 wppm, preferably in the range of 20 to 20,000 wppm, CO _{2 in the} range of 10 to 100,000 wppm, preferably 10 to 2,000 wppm, and It consists of COS in the range of 1 to 100 wppm, preferably 1 to 10 wppm.

Purifier gases that may be used as feedstock in the process according to the invention include flue gases generated from the purifier conversion unit, such as cokers, fluid catalytic cracking units, and saturated lightweight ends of hydrocracking or hydrogenation units. Typically, the purifier gas is 10 to 100,000 wppm, preferably 100 to 60,000 wppm H ₂ S, 10 to 50,000 wppm, preferably 10 to 20,000 wppm CO ₂ , 0 to 50,000 wppm, preferably 0 to 2,000 It may contain wppm mercaptan, 0-1,000 wppm, preferably 0-200 wppm COS and 0-1,000 wppm, preferably 0-10 wppm, where the level of acid gas component is It depends on whether it is pretreated in the purifier.

The alkanolamine aqueous solution is any alkanolamine solution commonly used for acidic gas removal, such as monoethanolamine, diethanolamine, diisopropanolamine and methyldiethanolamine. Typically, the alkanolamine is in aqueous solution, where the amine concentration is 10-50% by weight, preferably 20-50% by weight, based on the total weight of the solution. For example, monoethanolamine is typically used at an amine concentration of 15 to 25% by weight, diethanolamine is used at an amine concentration of 20 to 30% by weight, and methyldiethanolamine is used at an amine concentration of 40 to 50% by weight. Used as

Solvents that can be used in the process of the present invention are based on the total volume of the solvent from 60 to 100% by volume, more preferably at least 80% by volume of aromatic components and from 50 to 85% by volume, preferably from 60 to 80% by volume. Heavy aromatic solvents having a binary ring aromatic component. The term "two-membered ring aromatic" as used herein is meant to include substituted and unsubstituted naphthalenes such as methyl naphthalene and dimethyl naphthalene.

Alternatively, the heavy aromatic solvent may have a starting and ending boiling point and a specific gravity in the range of 0.95 to 1.1, respectively, falling within the range of 150 to 450 ° C. Examples of solvents that meet the preferred criteria include catalytically heated gas oil, steam cracked gas oil and heavy aromatic solvents such as Solvesso 200, which are aromatic solvents commercially available from ExxonMobil Chemical Company. Such solvents have an aromatic content of at least 80% by volume, a bicyclic aromatic content of at least 20% by volume, a boiling point in the range of 190 to 400 ° C and a specific gravity of 0.95 to 1.1.

Example 1

Laboratory tests have shown that light aromatic solvents, such as toluene, are very effective at dissolving the polymer, but because of their high volatility, the amount of loss into the regenerator overhead is very high.

Example 2

1,500 liters of Solveso 200, a heavy aromatic solvent having an aromatic content of at least 99.8% by volume, a binary ring aromatic content of 80% by volume, a specific gravity of 1.0 and a boiling point of 232-277 ° C, was added to the 20% monomethanolamine aqueous solution system . The regenerator was operated at 110 ° C. The spent solvent was recovered via gravity separation.

The tendency of contamination of some heat exchangers was observed during operation, and the results showed that the tendency of contamination of the system was greatly reduced. In addition, the aromatic solvent had a sufficiently high boiling point to not contaminate the regenerator overhead at all. However, due to the small difference in specific gravity between the spent solvent and the aqueous alkanolamine solution, gravity separation was achieved with less than 50% recovery of the added solvent. Thus, in practice, very small amounts of polymer were removed from the system.

Example 3

As a result of practicing the present invention, several improvements in performance have been observed.

Conventionally, when no hydrocarbon solvent was used, the relative contaminant of clean exchangers typically increased exponentially from 1 to 60 over a period of 4 to 8 weeks. Cloning was invariably required after 8 months. The regenerator behaved in a similar fashion, but at a slower rate (cleaning was required after 8 months).

After practicing the present invention, the exchanger relative contamination factor was basically constant at 2. Based on current trends, exchange and regenerator operation periods are expected to exceed one and three years, respectively.

Claims (20)

(a) contacting an acidic gas containing stream with an aqueous alkanolamine solution and a hydrocarbon solvent to form a mixture of an alkanolamine rich solution containing an acidic gas or a by-product thereof with a hydrocarbon solvent, the hydrocarbon solvent being a solvent Having an aromatic content of 60 to 100% by volume and a binary ring aromatic content of 50 to 85% by volume, based on the total volume of (b) separating the mixture of step (a) in a hydrocyclone to separate the hydrocarbon solvent from an alkanolamine rich solution having an acidic gas or a by-product thereof, thereby providing an alkanolamine rich solution having an acidic gas or a by-product thereof and Producing a hydrocarbon solvent entrapped in this solution, and (c) an alkanolamine rich solution having an acid gas or a by-product thereof and a hydrocarbon solvent trapped in the solution by regeneration in the regenerator so as to be substantially free of the polymer rich hydrocarbon solvent and an acid gas or a by-product thereof. Wherein the alkanolamine rich solution with the acidic gas or by-product thereof is sufficient to dissolve the polymer formed in the regenerator And removing the polymer from the acidic gas treatment system. (a) contacting the acidic gas containing stream with an aqueous solution of alkanolamine to form an alkanolamine rich solution containing acidic gas or by-products of the acidic gas, (b) an alkanolamine rich solution having an acidic gas or a by-product thereof containing a hydrocarbon solvent having an aromatic content of 60 to 100% by volume and a binary ring aromatic content of 50 to 85% by volume, based on the total volume of the solvent; Mixing step, (c) separating the mixture of step (b) in a hydrocyclone to separate the hydrocarbon solvent from an alkanolamine rich solution having an acidic gas or a by-product thereof, thereby providing an alkanolamine rich solution having an acidic gas or a by-product thereof and Producing 0.1-2.0% by volume of hydrocarbon solvent trapped in this solution, and (d) passing the solution and hydrocarbon solvent having the acidic gas or by-product thereof in step (d) through a regenerator to dissolve the polymer accumulated in the regenerator, having a trapped polymer rich hydrocarbon solvent, the acidic gas or Obtaining an alkanolamine deficient solution substantially free of byproducts And removing the polymer from the acidic gas treatment system. The process of claim 1 further comprising recycling to the absorption tower an alkanolamine deficient solution substantially free of polymer rich hydrocarbon solvents and acid gases or by-products thereof for use as the aqueous alkanolamine solution of step (a). Way. The process of claim 2 further comprising recycling to the absorption tower an alkanolamine deficient solution substantially free of polymer rich hydrocarbon solvent and acid gases or by-products thereof for use as the aqueous alkanolamine solution of step (a). Way. The process of claim 1 wherein the acidic gas containing stream is a steam cracker effluent. The process of claim 2 wherein the acidic gas containing stream is a steam cracker effluent. The process of claim 1 wherein the acidic gas containing stream comprises a steam cracker effluent and a purifier gas. The method of claim 2, wherein the gas containing stream comprises a steam cracker effluent and a purifier gas. The method of claim 1 further comprising improving the quality grade of the acidic gas and its by-products in the sulfur recovery unit.   The method of claim 2 further comprising improving the quality grade of the acidic gas and its by-products in the sulfur recovery unit. The alkanolamine rich solution having the acidic gas or by-product thereof of step (c) and the hydrocarbon solvent entrapped in the solution are passed through a heat exchanger prior to step (d) and Dissolving the accumulated polymer. The process of claim 1, wherein 0.1 to 2.0% by volume of the hydrocarbon solvent is trapped in an alkanolamine rich solution having the acidic gas of step (c) or a by-product thereof. The method of claim 1 wherein the hydrocarbon solvent has an aromatic content of at least 80% by volume. The method of claim 2, wherein the hydrocarbon solvent has an aromatic content of at least 80% by volume. The process of claim 13, wherein the hydrocarbon solvent has a bicyclic aromatic aromatic content of 60-80% by volume. 15. The process of claim 14, wherein the hydrocarbon solvent has a 60-80% by volume binary ring aromatic content. The method of claim 1, wherein the starting and ending boiling points of the hydrocarbon solvent are each in the range of 150 to 450 ° C. 7. The method of claim 2, wherein the starting and ending boiling points of the hydrocarbon solvent are each in the range of 150 to 450 ° C. 4. (a) contacting an acidic gas containing stream with an aqueous alkanolamine solution and a hydrocarbon solvent to form a mixture of an alkanolamine rich solution containing an acidic gas or a by-product thereof with a hydrocarbon solvent, wherein the hydrocarbon solvent is 60 Having an aromatic content of ˜100% by volume and a specific gravity in the range of 0.95 to 1.1, (b) isolating the mixture of step (a) in a hydrocyclone to separate the hydrocarbon solvent from an alkanolamine rich solution having an acidic gas or a by-product thereof, thereby providing an alkanolamine solution having an acidic gas or a by-product thereof and Producing a hydrocarbon solvent entrapped in the solution, and (c) an alkanolamine rich solution having an acid gas or a by-product thereof and a hydrocarbon solvent trapped in the solution are regenerated in a regenerator such that the alkanolamine deficient solution substantially free of the polymer rich hydrocarbon solvent and an acid gas or a by-product thereof Wherein the hydrocarbon solvent trapped in the alkanolamine rich solution with acidic gas or by-product thereof is sufficient to dissolve the polymer formed in the regenerator. And removing the polymer from the acidic gas treatment system. The method of claim 19, wherein the hydrocarbon solvent has an aromatic content of at least 80 vol%.

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