Systems and methods for enhanced recovery of styrene

Abstract

The present application provides a process for enhanced recovery of styrene, the process comprising feeding a hot liquid residue stream from a styrene monomer purification and purification system to a stripping vessel. The hot liquid residue stream comprises styrene and a compound that is less volatile than styrene. The method comprises: introducing a gas into the stripping vessel to strip a portion of the styrene to a vapor, thereby generating an evaporated styrene fraction; returning the evaporated styrene part and the gas to the styrene monomer purification refining system; recovering at least a portion of the vaporized styrene fraction to the styrene monomer product; and generating a final liquid residue stream from the bottom of the stripping vessel, the final liquid residue stream having a lower concentration of styrene than the hot liquid residue stream fed to the stripping vessel.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] The subject application claims the benefit of priority to U.S. patent application No. 17 / 870,821, filed on July 22, 2022, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to improvements to methods and systems for recovering styrene products. Specifically, the systems and methods of the present disclosure reduce utility costs, improve efficiency, and enable savings compared to current methods practiced in the industry. Background Art

[0004] It is well known in the field of styrene monomer manufacturing that the crude styrene from the dehydrogenation section is stripped of compounds lighter than styrene in one or more distillation towers (hereinafter collectively referred to as styrene pre-fractionation systems). The remaining stream containing styrene monomer and less volatile impurities is fed to a styrene monomer purification and refining system, which usually starts with a distillation refining tower operated under vacuum and recovers the final styrene monomer product as a distillate. The heavier impurities in the stream include C9 aromatics (such as propylbenzene and methylstyrene), C10 aromatics (such as diethylbenzene and divinylbenzene isomers), polycyclic aromatics (mainly condensation products of styrene, including diphenylethylene) and styrene polymers that are less volatile than styrene.

[0005] The bottom stream from the distillation refining tower is usually partially evaporated in a steam-heated exchanger and fed to a residue flash tank, in which steam is separated from the liquid and fed back to the distillation refining tower. The residue flash tank liquid contains most of the low volatility (heavy) by-products of the styrene process and residual styrene, and its concentration is usually about 5 weight %. A portion of this liquid from the residue flash tank can be optionally recycled to the beginning of the styrene pre-fractionation system upstream of the distillation refining tower to reuse the residual active polymerization inhibitor. The rest of the liquid is the net weight by-product stream (residue) of the styrene process, and the styrene content in this stream is a yield loss for this process.

[0006] U.S. Patent 3,515,647 describes a system similar to that described above; however, the residue flash tank and the steam heated exchanger upstream thereof are replaced by a thin film evaporator with a steam heated jacket that provides the same function. This prior art alternative uses a hot gaseous stream fed to the thin film evaporator to assist in stripping styrene from the net residue. However, the evaporator evaporates relatively large amounts of styrene and other components, so that the hot gaseous stream must be relatively large to have a great effect on the separation. Using large amounts of nitrogen or steam, as recommended, would be expensive and would require changes to the overhead system of the distillation refining column to separate the added components from the styrene monomer product.

[0007] At least for economic reasons, it is desirable to have a process with lower styrene monomer yield losses. Thus, the inventive process disclosed herein effectively minimizes styrene product losses to a heavy byproduct residue stream while also reducing energy costs. Summary of the invention

[0008] The present application provides a method for enhancing the recovery of styrene, the method comprising: feeding a hot liquid residue stream from a styrene monomer purification and refining system to a stripping vessel. The hot liquid residue stream contains styrene and compounds less volatile than styrene. The method comprises: introducing a gas into the stripping vessel to strip a portion of the styrene into vapor, thereby generating an evaporated styrene portion. The method comprises: returning the evaporated styrene portion and the gas to the styrene monomer purification and refining system. The method comprises: recycling at least a portion of the evaporated styrene portion into a styrene monomer product. The method comprises: producing a final liquid residue stream from the bottom of the stripping vessel, the final liquid residue stream having a lower concentration of styrene than the hot liquid residue stream fed to the stripping vessel.

[0009] The stripping vessel may be operated at a lower temperature than the hot liquid residue stream. The gas may be non-condensable at ambient temperature and pressure, such as natural gas, nitrogen or air. The gas may also be steam.

[0010] In some embodiments, the stripping vessel is operated under vacuum. The method may include: recycling a portion of the liquid residue stream from the bottom of the stripping vessel back to the start of the styrene pre-fractionation system upstream of the styrene monomer purification and refining system. The liquid residue leaving the stripping vessel may be at a temperature of about 250°F to 340°F. Introducing the gas into the stripping vessel may include at least one of the following items: (i) feeding the gas to the stripping vessel independently of the liquid residue stream, or (ii) feeding the gas to a hot liquid residue stream and subsequently feeding the stripping vessel. In some embodiments, feeding a hot liquid residue stream includes: feeding a hot liquid residue stream to the top of the stripping vessel, and wherein introducing the gas includes: introducing the gas to the bottom of the stripping vessel.

[0011] A system for enhancing the recovery of styrene from a residue stream is also disclosed. The system includes a stripping vessel constructed and adapted to receive a hot liquid residue feed from a styrene monomer purification finishing system.

[0012] In some embodiments, the system includes a return conduit connected to a stripping vessel, which is constructed and adapted to feed a portion of the styrene contained in the hot liquid residue feed to the front of the styrene pre-fractionation system. The stripping vessel can be a tank, which is constructed and adapted to receive a stripping gas, which goes directly to the tank, or to a hot liquid residue feed pipe and then to the tank. The stripping vessel can also be a stripping tower with filler and / or tray. The top of the stripping tower is constructed and adapted to receive the hot liquid residue feed, and the bottom of the stripping tower is constructed and adapted to receive the stripping gas feed.

[0013] The stripping vessel can be a tank without internal elements for enhancing gas-liquid contact. The stripping vessel can include internal elements for enhancing gas-liquid contact. The internal elements inside the stripping vessel can be distillation trays. The internal elements inside the stripping vessel can be distillation fillers. The gas can be steam, natural gas, nitrogen or air. The stripping vessel can be constructed and adapted to promote contact between the hot liquid residue feed from the styrene monomer purification and refining system and the gas for recovering a part of the styrene contained in the liquid feed. The stripping vessel can include an outlet, which is constructed and adapted to return a part of the styrene contained in the hot liquid residue feed to the front of the styrene pre-fractionation system.

[0014] These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] So that those skilled in the art to which the subject invention pertains will readily understand how to make and use the apparatus and methods of the subject invention without undue experimentation, preferred embodiments of the present invention will be described in detail below with reference to certain drawings, in which:

[0016] Figure 1 is a schematic diagram of a prior art styrene monomer purification and refining system; and

[0017] Figure 2 is a schematic diagram of a styrene monomer purification finishing and gas stripping system constructed according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0018] Referring now to the drawings, in which the same reference numerals represent similar structures or features of the subject invention, Figure 2 A novel and useful styrene monomer purification finishing and stripping system constructed in accordance with a preferred embodiment of the subject disclosure and generally designated by reference numeral 200 is illustrated in the accompanying drawings. Styrene monomer purification finishing and stripping system 200 includes a novel stripping vessel that results in lower heat input into styrene monomer purification finishing and stripping system 200 and lower styrene monomer yield loss than conventional systems.

[0019] Figure 1 A prior art styrene monomer purification and refining system represented by reference numeral 100 is shown. The prior art styrene monomer purification and refining system 100 is located downstream of a styrene pre-fractionation system, which includes one or more distillation towers that remove and recover lighter compounds from crude styrene from the dehydrogenation section of the styrene process. The styrene monomer purification and refining system 100 includes a distillation refining tower 1 that receives a feed 11 from the styrene pre-fractionation system. The feed 11 contains styrene monomer, methyl styrene, polystyrene and other heavy compounds. The overhead vapor stream 30 from the distillation refining tower is partially condensed in a condenser 3 to produce a vapor distillate stream 13, a liquid distillate stream 12 (styrene monomer product) and a tower reflux stream 14. Heat is provided to the distillation refining tower by a steam reboiler 2. The liquid bottom stream 15 from the distillation refining tower 1 flows to an exchanger (residue reboiler) 4 heated by steam. The partially vaporized bottoms stream from the residue reboiler 4 is fed to the residue flash tank 5 where the liquid and vapor streams are separated. The vapor stream 16 is returned to the distillation finishing column 1 and the liquid stream 17 leaves the residue flash tank 5. A portion 20 of the liquid stream 17 is optionally recycled to the beginning of the styrene prefractionation system and the remainder of the liquid is a net heavy byproduct residue stream 19 from the styrene monomer process.

[0020] The distillation and refining tower 1 and the residue flash tank 5 are operated under deep vacuum, which causes low operating temperature, and this low operating temperature minimizes the loss of the styrene of the polymer formed by polymerization and the consumption of polymerization inhibitor. The tower 1 internals can include all trays, the distillation filler above the feed and the tray below the feed, or all fillers. Compared with the tray, the filler has the advantage of lower pressure drop, which causes lower temperature, less liquid retention (both of which reduce the loss of styrene to polymer) and smaller tower diameter. In contrast, the tray is a less expensive internal component, is unlikely to have poor performance due to uneven flow distribution, is less prone to fouling due to insoluble polymers, and is easier to clean up polymers. Having the filler above the feed and the tray below the feed is a preferred design for the distillation and refining tower 1, because the structure provides the lower pressure and the partial beneficial effect of lower liquid retention when there is filler, and provides a tray at the place where polymer fouling is most likely to occur.

[0021] In order to use Figure 1 To further reduce the styrene concentration in the residue using the current system shown, the heat input to the residue reboiler 4 would typically be increased. However, doing so may result in excessively high residue temperatures, which may degrade the polymerization inhibitors in the residue. Figure 2 As shown, stripping styrene from the residue with gas in an additional downstream stripping vessel 6 and feeding the gas with recovered styrene back to the styrene monomer purification finishing system is disclosed. The present disclosure further provides a cost-effective apparatus for reducing styrene losses to the residue in a styrene plant without excessive temperatures that can degrade polymerization inhibitors.

[0022] Figure 2 A styrene monomer purification finishing and stripping system 200 constructed in accordance with an embodiment of the present disclosure is shown. Figure 2 The styrene monomer purification and stripping system 200 includes Figure 1 The system 100 is provided with a hot liquid residue stream 21 having a net liquid stream 17 of residue from a flash tank 5, the hot liquid residue stream being fed to a stripping vessel 6 where it is contacted with a stripping gas 22 to remove a portion of the styrene from the liquid. Figure 2, the stripping gas 22 is shown as being fed independently of the hot liquid residue stream 21. When the stripping vessel 6 is a stripping column (i.e., a container with trays or packing (internal elements 18)), it is generally applicable to feed only the liquid 21 to the top and the gas 22 to the bottom, which will be described in more detail below. As an alternative embodiment, as shown by the dashed arrows, the stripping gas 22 can be fed into the hot liquid residue stream 21 and then fed to the stripping vessel 6. The gas mixture with styrene 23 recovered from the stripping vessel 6 is directed to the distillation refining column 1. A portion of the stripping vessel liquid residue bottoms stream (schematically depicted by pipe 20) is optionally recycled to the beginning of the styrene pre-fractionation system, and the remaining liquid is a net heavy by-product residue stream 19 from the styrene monomer process. The stripping vessel includes an outlet in fluid communication with the liquid residue bottoms stream 20.

[0023] Because the styrene monomer purification finishing and stripping system 200 operates under vacuum, the system design typically has an allowance for air leakage from the atmosphere into the equipment and piping, and the vacuum system is used to remove the air and maintain a constant vacuum pressure at the top of the distillation finishing column. The flow rate of air, natural gas, or nitrogen required in the stripping vessel 6 can be small relative to the typical allowance for air leakage into the design. Although the injection of gas reduces the air leakage capacity of the vacuum system, the reduction in capacity is typically minimal and is comparable to the air leakage capacity of the vacuum system. Figure 1 Compared with the prior art system in Figure 2 The styrene monomer purification and stripping systems usually do not require additional equipment.

[0024] The stripping gas 22 may also be steam. In this case, the styrene monomer product 12 will contain dissolved water. If the amount of injected steam is large enough, a separated liquid water phase may be formed in an overhead reflux vessel (not shown) and possibly in any container for storing or transporting styrene. Any water phase in the styrene product will generally need to be separated before using styrene.

[0025] By using the stripping vessel 6, the heat input into the styrene monomer purification and stripping system 200 is less than Figure 1 The prior art system, although the level of styrene recovery from the net residue is higher. The use of air in the stripping vessel 6 instead of nitrogen or natural gas has the additional advantage that it reduces the loss of styrene to polymer in the system because the presence of oxygen enhances the effect of the polymerization inhibitor 4-tert-butylcatechol (TBC) which is typically added to the distillation refining column reflux 14. The stripping vessel 6 and associated pumps are very small in size relative to the equipment in the prior art styrene monomer purification refining system.

[0026] Continue to refer Figure 2, the stripping vessel 6 can be a simple vapor-liquid separation tank or a stripping tower with contact internal elements 18 below the liquid feed. In terms of performance, the stripping tower is superior to a simple tank. In a stripping tower with a residue flash tank liquid introduced at the top and a stripping gas added at the bottom of the stripping tower, the contact internal elements 18 (such as trays or fillers) cause countercurrent stripping of styrene from the liquid, which can significantly reduce the amount of gas required to remove a certain amount of styrene from the residue. The gas strips a portion of styrene from the residue and returns it to the optimal entry point of the distillation and refining tower 1 through a return pipe 23. Because the gas dilutes the styrene in the gas phase from the stripping vessel 6, when the liquid from the residue flash tank 5 enters the stripping vessel 6 and contacts the stripping gas, the equilibrium temperature of the liquid decreases. This decrease in liquid temperature provides heat for evaporating the styrene in the liquid from the residue flash tank 5.

[0027] Three tables are given below, which have simulation results for a styrene monomer purification refining system in a baseline process (Table 1), a process with an embodiment of a stripping vessel as a tank (simple vapor-liquid separator) (Table 2), and an improved process with another embodiment of a stripping vessel as a stripping column (Table 3). All of these simulations are based on a production of 125,000 lb / hr of final styrene monomer product and a distillation refining column with packing above the feed and trays below the feed. For the results in Tables 2 and 3, air was used as the stripping gas. Using nitrogen instead of air will produce virtually the same results because air is primarily (79% by volume) composed of nitrogen and has very similar physical properties. Steam and natural gas have different physical properties than nitrogen or air, but because the gas flow is very small relative to the liquid feed to the stripping vessel, an equal molar flow of steam or natural gas has essentially the same effect on the balance as nitrogen or air.

[0028] In addition, the above process does not require heating of the stripping gas. Since the flow rate of the stripping gas is very small relative to the flow rate in US Pat. No. 3,515,647, the beneficial effect of heating the gas will be negligible.

[0029] Table 1 shows the changes in the styrene concentration in the net residue as it decreases. Figure 1Results of the prior art process shown. As shown, as the styrene content in the residue decreases and the load of the residue reboiler increases, the temperature of the residue in the residue flash tank increases significantly. In the case of 5.0 wt% styrene in the residue, the residue flash tank is operated at a temperature of about 324°F (162°C). Reducing the styrene concentration in the residue to 2.0 wt% requires increasing the temperature of the residue flash tank to 358°F (181°C). However, at least one commercially available inhibitor degrades at this temperature. Therefore, reducing the yield loss to 2.0 wt% in the residue flash tank causes excessively high temperatures that degrade the polymerization inhibitor in the residue.

[0030] Table 1

[0031] Styrene in residues weight% 5.0 4.0 3.0 2.0 1.0 Styrene in the bottom of the refining tower weight% 56.9 50.9 43.2 32.7 18.9 Styrene in Residue Flash Tank Liquid weight% 5.0 4.0 3.0 2.0 1.0 Refining tower bottom temperature ℉ 214.9 218.1 223.0 230.7 245.3 Residue flash tank temperature ℉ 324.3 333.7 345.0 358.0 374.7 Refining tower reboiler duty Million BTU / hr 18.01 18.01 18.01 18.01 18.01 Residue reboiler duty Million BTU / hr 6.72 6.73 6.70 6.80 6.83 Tower condenser load Million BTU / hr 31.23 31.23 31.20 31.29 31.31 Styrene loss to residue lb / hr 28.3 22.4 16.6 11.0 5.4

[0032] Table 2 shows the results of the improved process of the present disclosure for an embodiment with a single stage stripping tank utilizing air as the stripping medium. Compared to Table 1, for all styrene concentrations in the net residue in Table 2, the temperature of the residue flash tank is lower and the stripping tank temperature is lower than the residue flash tank temperature. Although the separation of the system is improved, the residue reboiler duty is actually slightly reduced at lower styrene concentrations in the net residue. As Figure 2 The gas may be fed directly to the stripping tank as shown, but it is preferred to feed the gas into a residue pipe which feeds the tank, such as Figure 2 For the same molar flow rate of stripping gas, the results of using nitrogen, natural gas or steam as stripping gas will be almost the same.

[0033] Table 2

[0034] Styrene in residues weight% 5.0 4.0 3.0 2.0 1.0 Styrene in the bottom of the refining tower weight% 58.4 56.3 53.2 48.2 37.4 Styrene in Residue Flash Tank Liquid weight% 5.4 5.3 5.0 4.6 3.6 Refining tower bottom temperature ℉ 214.0 214.9 216.1 218.1 222.8 Residue flash tank temperature ℉ 320.0 320.0 320.0 320.0 320.0 Air stripping tank temperature ℉ 317.5 311.7 304.5 294.3 275.7 Refining tower reboiler duty Million BTU / hr 18.01 18.01 18.01 18.01 18.01 Residue reboiler duty Million BTU / hr 6.80 6.74 6.68 6.56 6.56 Tower condenser load Million BTU / hr 31.32 31.27 31.21 31.06 30.95 Styrene loss to residue lb / hr 28.4 22.5 16.7 10.9 5.4 Air flow rate lb / hr 0.6 7.5 28.6 88.9 306.7 Styrene in stripping tank vapor lb / hr 25.5 87.9 173.0 315.1 658.2

[0035] Table 3 shows the results for another embodiment of the improved process of the present disclosure, wherein a stripper acts as a stripping vessel, which utilizes air as the stripping gas. The stripper has multiple stages, wherein the liquid feed from the residue flash tank enters the top of the stripper and the air enters the bottom of the stripper. Comparing Tables 2 and 3, for the case where there is 2.0 wt% styrene in the net residue, it is shown that going from a single stripper (the fourth data column in Table 2) to a two-stage stripper (the third data column in Table 3) reduces the required stripping air by 64% (from 88.9 lb / hr to 32.3 lb / hr). The last three columns in Table 3 illustrate that adding additional stripper stages reduces the stripping gas consumption or reduces the amount of styrene lost in the final residue at the same stripping gas flow rate.

[0036] Table 3

[0037]

[0038]

[0039] By stripping styrene from the residue with gas in a stripping vessel and feeding the gas with the stripped styrene back to the distillation refining column, styrene losses to the residue are significantly reduced without the need to operate at higher temperatures. With the addition of a stripping vessel, not only is styrene recovery higher, but the heat input into the styrene monomer purification refining system is actually lower, which is an unexpected result in a distillation system because the components are separated more completely.

[0040] The systems and methods of the present disclosure as described above and shown in the accompanying drawings provide an apparatus that economically recovers styrene from heavy byproduct streams in a styrene plant without exceeding the temperature degradation limit for polymerization inhibitors. Each of the above systems and methods is based on a single refining tower, followed by a residue flash tank, and then a stripping vessel with a stripping gas. Although the apparatus and methods of the subject disclosure have been shown and described with reference to the embodiments, those skilled in the art will readily appreciate that changes and / or modifications may be made to the matters upstream of the stripping vessel without departing from the spirit of the subject disclosure.

Claims

1. A method for enhancing the recovery of styrene, the method comprising: feeding a hot liquid residue stream from a styrene monomer purification finishing system to a stripping vessel, wherein the hot liquid residue stream comprises styrene and compounds less volatile than styrene; introducing a gas into the stripping vessel to strip a portion of the styrene as a vapor, thereby generating a vaporized styrene portion; Returning the evaporated styrene portion and the gas to the styrene monomer purification and refining system; recovering at least a portion of the evaporated styrene portion into a styrene monomer product; as well as A final liquid residue stream is produced from the bottom of the stripping vessel, the final liquid residue stream having a lower concentration of styrene than the hot liquid residue stream fed to the stripping vessel.

2. The process of claim 1 wherein the stripping vessel is operated at a lower temperature than the hot liquid residue stream.

3. The method of claim 1, wherein the gas is non-condensable at ambient temperature and pressure.

4. The method according to claim 1, wherein the gas is steam, natural gas, nitrogen or air.

5. The method of claim 1, wherein the stripping vessel is operated under vacuum.

6. The method according to claim 1, further comprising: A portion of the liquid residue stream from the bottom of the stripping vessel is recycled back to the start of a styrene pre-fractionation system upstream of the styrene monomer purification finishing system.

7. The process of claim 1 wherein the liquid residue stream exiting the stripping vessel is at a temperature of about 250°F to 340°F.

8. The method of claim 1, wherein introducing the gas into the stripping vessel comprises at least one of: (i) feeding the gas into the stripping vessel independently of the liquid residue stream, or (ii) feeding the gas into the hot liquid residue stream and subsequently into the stripping vessel.

9. The method of claim 1, wherein feeding the hot liquid residue stream comprises: The hot liquid residue stream is fed to the top of the stripping vessel and introduced therein The gas includes: introducing the gas into the bottom of the stripping vessel.

10. A system for enhancing the recovery of styrene from a residue stream, the system comprising: A stripping vessel constructed and adapted to receive a hot liquid residue feed from a styrene monomer purification finishing system.

11. The system of claim 10, further comprising a return conduit connected to the stripping vessel, the return conduit being constructed and adapted to feed a portion of the styrene recovered from the hot liquid residue stream back to the styrene monomer purification finishing system upstream of the stripping vessel.

12. The system of claim 10, wherein the stripping vessel is a tank constructed and adapted to receive a stripping gas either directly to the tank, or to a hot liquid residue feed pipe and subsequently to the tank.

13. The system of claim 10, wherein the stripping vessel is a stripping column having packing and / or trays, wherein a top portion of the stripping column is constructed and adapted to receive the hot liquid residue feed, wherein a bottom portion of the stripping column is constructed and adapted to receive a stripping gas feed.

14. The system of claim 10, wherein the stripping vessel is a tank without internal elements for enhancing gas-liquid contact.

15. The system of claim 10, wherein the stripping vessel comprises internal elements for enhancing gas-liquid contact.

16. The system of claim 15, wherein the internal element inside the stripping vessel is a distillation tray.

17. The system of claim 15, wherein the internal element inside the stripping vessel is distillation packing.

18. The system of claim 12, wherein the stripping gas is constructed and adapted to be steam, natural gas, nitrogen, or air.

19. The system of claim 10, wherein the stripping vessel is constructed and adapted to facilitate contact between a hot liquid residue feed from a styrene monomer purification finishing system and a gas for recovering a portion of the styrene contained in the liquid feed.

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