System and method for improving the recovery of styrene

Abstract

A method for improving the recovery of styrene includes supplying a high-temperature liquid residue stream from a styrene monomer purification finishing system to a stripping tank. The high-temperature liquid residue stream includes styrene and a compound less volatile than styrene. The method includes introducing a gas into the stripping tank to strip a portion of the styrene as vapor, generating a vaporized styrene portion, returning the vaporized styrene portion and the gas to the styrene monomer purification finishing system, recovering at least a portion of the vaporized styrene portion into the styrene monomer product, and generating a final liquid residue stream having a styrene concentration lower than the high-temperature liquid residue stream supplied to the stripping tank from the bottom of the stripping tank.

Description

Technical Field

[0001] Cross - Reference to Related Applications This application claims the benefit of priority to U.S. Patent Application No. 17 / 870,821, filed on July 22, 2022, the entire disclosure of which is incorporated herein by reference.

[0002] This disclosure relates to improvements in methods for recovering styrene products and systems for recovering styrene products. Specifically, the systems and methods of this disclosure reduce utility costs, improve efficiency, and provide savings as compared to current methods practiced in the industry.

Background Art

[0003] In the technical field of styrene monomer production, it is well known to strip compounds lighter than styrene from crude styrene from the dehydrogenation section in one or more distillation towers (hereinafter collectively referred to as the styrene pre - fractionation system). The remaining stream, which contains styrene monomer and less volatile impurities, is fed to a styrene monomer purification finishing system, which typically starts with a distillation finishing tower that operates under vacuum and recovers the final styrene monomer product as a distillate. Heavier impurities in this stream include C9 aromatics (such as propylbenzene and methylstyrene), C10 aromatics (such as diethylbenzene isomers and divinylbenzene isomers), polycyclic aromatics less volatile than styrene (mainly condensation products of styrene containing stilbene), and styrene polymers.

[0004] The bottom stream from the distillation finishing column is typically partially vaporized in a steam-heated exchanger and fed to a residue flash drum where the vapor is separated from the liquid and returned to the distillation finishing column. The liquid in this residue flash drum contains most of the low volatility (heavy) by-products of the styrene process and typically about 5 wt% of residual styrene. A portion of this liquid from the residue flash drum can optionally be recycled to the starting point of the styrene prefractionation system upstream of the distillation finishing column to reuse the residual active polymerization inhibitor. The remainder of this liquid is the net heavy by-product stream (residue) of the styrene process, and the styrene content in this stream represents a yield loss to the process.

[0005] U.S. Patent No. 3,515,647 describes a system similar to the above. However, the residue flash drum and the steam-heated exchanger upstream thereof are replaced by a thin film evaporator having a steam-heated jacket that provides the same function. This prior art has the option of using a hot gas stream fed to the thin film evaporator to assist in stripping styrene from the net residue. However, since the evaporator vaporizes a relatively large amount of styrene and other components, the hot gas stream would have to be relatively large to have a significant impact on the separation. Using a large amount of nitrogen or steam as recommended is costly and requires modifications to the overhead system of the distillation finishing column to separate the added components from the styrene monomer product.

[0006] It is desirable to have a process with lower styrene monomer yield loss, at least for economic reasons. Thus, the method of the present invention disclosed herein efficiently minimizes styrene product loss to the heavy by-product residue stream while also reducing energy costs. SUMMARY OF THE INVENTION

[0007] A method for improving the recovery of styrene includes supplying a high-temperature liquid residue stream from a styrene monomer purification finishing system to a stripping tank. The high-temperature liquid residue stream includes styrene and a compound less volatile than styrene. The method includes introducing a gas into the stripping tank to strip a portion of the styrene as vapor to produce a vaporized styrene portion. The method includes returning the vaporized styrene portion and the gas to the styrene monomer purification finishing system. The method includes recovering at least a portion of the vaporized styrene portion into the styrene monomer product. The method includes producing a final liquid residue stream having a styrene concentration lower than that of the high-temperature liquid residue stream supplied to the stripping tank from the bottom of the stripping tank.

[0008] The stripping tank can operate at a temperature lower than that of the high-temperature liquid residue stream. The gas may be non-condensable at ambient temperature and pressure, such as natural gas, nitrogen, or air. The gas may be steam.

[0009] In some embodiments, the stripping tank operates under vacuum. The method may include recirculating a portion of the liquid residue stream from the bottom of the stripping tank back to the starting point of a styrene pre-fractionation system upstream of the styrene monomer purification finishing system. The liquid residue exiting the stripping tank can be at a temperature of about 250°F to 340°F. Introducing the gas into the stripping tank can include at least one of (i) supplying the gas separately from the liquid residue stream to the stripping tank, or (ii) supplying the gas to the high-temperature liquid residue stream and then to the stripping tank. In some embodiments, supplying the high-temperature liquid residue stream includes supplying the high-temperature liquid residue stream to the top of the stripping tank, and introducing the gas includes introducing the gas to the bottom of the stripping tank.

[0010] A system for improving the recovery of styrene from a residue stream is also disclosed. This system includes a stripping tank configured and adapted to receive a high-temperature liquid residue feed stream from a styrene monomer purification finishing system.

[0011] In some embodiments, the system includes a return conduit connected to the stripping tank and configured and adapted to supply a portion of the styrene contained in the high-temperature liquid residue feed stream to the front of a styrene pre-fractionation system. The stripping tank can be a drum configured and adapted to directly receive stripping gas into the drum, or it can be a drum configured and adapted to receive the stripping gas into the high-temperature liquid residue feed line and then into the drum. The stripping tank can be a stripper tower having packing and / or trays. The top of the stripper tower is configured and adapted to receive the high-temperature liquid residue feed stream, and the bottom of the stripper tower is configured and adapted to receive the stripping gas feed stream.

[0012] The stripping tank can be a drum without internal elements for improving gas-liquid contact. The stripping tank can include internal elements for improving gas-liquid contact. The internal elements in the stripping tank can be distillation trays. The internal elements in the stripping tank can be distillation packing. The gas can be steam, natural gas, nitrogen, or air. The stripping tank can be configured and adapted to facilitate the contact between the high-temperature liquid residue feed stream from the styrene monomer purification finishing system and a gas for recovering a portion of the styrene contained in the liquid feed stream. The stripping tank can include an outlet configured and adapted to return a portion of the styrene contained in the high-temperature liquid residue feed stream to the front of the styrene pre-fractionation system.

[0013] These and other features of the systems and methods of the present 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 drawings.

Brief Description of the Drawings

[0014] Preferred embodiments of the present invention will be described in detail below in this specification with reference to specific figures so that those skilled in the art to which the present invention pertains can easily understand how to fabricate and use the devices and methods of the present invention without performing more experiments than necessary.

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0015] Referring now to the accompanying drawings, in which like reference numerals identify like structures or features of the present invention, FIG. 2 shows a novel and useful styrene monomer purification finishing and stripping system, generally designated by reference numeral 200, constructed in accordance with a preferred embodiment of the present disclosure. The styrene monomer purification finishing and stripping system 200 includes a novel stripping tank, which reduces the heat input to the styrene monomer purification finishing and stripping system 200 and reduces the yield loss of the styrene monomer as compared to conventional systems.

[0016] FIG. 1 shows a prior art styrene monomer finishing purification system designated by reference numeral 100. The prior art styrene monomer finishing purification system 100 is located downstream of a styrene prefractionation system that includes one or more distillation towers for removing and recovering light compounds from crude styrene from the dehydrogenation section of a styrene process. The styrene monomer finishing purification system 100 includes a distillation finishing column 1 that receives a feed stream 11 from the styrene prefractionation system. The feed stream 11 includes styrene monomer, methyl styrene, polystyrene, and other heavy compounds. The overhead vapor stream 30 from the distillation finishing column is partially condensed in a condenser 3 to produce a vapor distillate stream 13, a liquid distillate stream 12 (styrene monomer product), and a distillation column reflux stream 14. Heat to the distillation finishing column is provided by a steam reboiler 2. The liquid bottoms stream 15 from the distillation finishing column 1 flows to a steam heated exchanger (residue reboiler) 4. The partially vaporized bottoms stream from the residue reboiler 4 is fed to a residue flash drum 5 where a liquid stream and a vapor stream are separated. The vapor stream 16 is returned to the distillation finishing column 1 and the liquid stream 17 exits the residue flash drum 5. A portion 20 of the liquid stream 17 is optionally recycled to the start of the styrene prefractionation system and the remaining liquid is the net heavy byproduct residue stream 19 from the styrene monomer process.

[0017] The distillation finishing column 1 and the residue flash drum 5 operate at high vacuum, resulting in a low operating temperature that minimizes both the loss of styrene polymerizing to form polymers and the consumption of polymerization inhibitors. The internal structure of the distillation column 1 can include any of all trays, distillation packing above the feed stream and trays below the feed stream, or all packing. Compared to trays, packing has the advantage of a lower pressure drop, which results in a lower temperature, a lower liquid holdup (both of which reduce the loss of styrene to polymers), and a smaller distillation column diameter. In contrast, trays are a less expensive internal structure, have a lower likelihood of performance degradation due to uneven flow distribution, less fouling by insoluble polymers, and are easier to clean for polymers. Having packing above the feed stream and trays below the feed stream is a preferred design for the distillation finishing column 1. This is because this configuration provides some benefits such as a lower pressure and a lower liquid holdup due to packing while providing the tray where fouling by polymers is most likely to occur.

[0018] As shown in Figure 1, to further reduce the styrene concentration in the residue using the current system, typically the heat input to the residue reboiler 4 is increased. However, doing so can cause an overly high residue temperature, which can decompose the polymerization inhibitor in the residue. Instead, as shown in Figure 2, it is disclosed that styrene is stripped from the residue using gas in an additional downstream stripping column 6, and the gas with the recovered styrene is returned to the styrene monomer purification finishing system. The present disclosure further provides a cost-effective means for reducing the loss of styrene to the residue in a styrene plant without having an excessive temperature that can degrade the polymerization inhibitor.

[0019] Figure 2 shows a styrene monomer purification finishing and stripping system 200 constructed in accordance with an embodiment of the present disclosure. The styrene monomer purification finishing and stripping system 200 of Figure 2 includes the system 100 of Figure 1, and supplies a high-temperature liquid residue stream 21 of the net liquid stream 17 from the residue of the flash drum 5 to the stripping tank 6, where it is brought into contact with a stripping gas 22 to remove a portion of the styrene from the liquid. In Figure 2, the stripping gas 22 is shown as being supplied separately from the high-temperature liquid residue stream 21. Supplying the liquid 21 to the top and the gas 22 to the bottom is typically only applicable when the stripping tank 6 is a stripper column, i.e., a tank having trays or packing (internal element 18) described in more detail below. As an alternative embodiment, as indicated by the dashed arrow, the stripping gas 22 can be supplied into the high-temperature liquid residue stream 21 and then supplied to the stripping tank 6. The gas mixture containing the styrene 23 recovered from the stripping tank 6 is sent to the distillation finishing column 1. A portion of the liquid residue bottom stream of the stripping tank (schematically depicted by line 20) is optionally recycled to the starting point of the styrene prefractionation system, and the remaining liquid is the net heavy byproduct residue stream 19 from the styrene monomer process. The stripping tank includes an outlet in fluid communication with the liquid residue bottom stream 20.

[0020] Since the styrene monomer purification finishing and stripping system 200 operates under vacuum, the system design typically allows for air leakage from the atmosphere to the equipment and piping, uses a vacuum system to remove the air, and maintains a constant vacuum pressure at the top of the distillation finishing column. The flow rate of air, natural gas, or nitrogen gas required within the stripping tank 6 can be made small relative to the typical allowable amount of air leakage into the design. Gas injection reduces the air leakage capacity of the vacuum system, but the reduction in capacity is typically minimal, and compared to the prior art system of Figure 1, the styrene monomer purification finishing and stripping system of Figure 2 typically does not require additional equipment.

[0021] The stripping gas 22 may be steam. In this case, the styrene monomer product 12 contains dissolved water. If the amount of steam injected is sufficiently large, a separated liquid water phase can be formed in an overhead reflux drum (not shown), or in some cases, in any tank used for storing or transporting styrene. The aqueous phase containing the styrene product typically needs to be separated and removed before using the styrene.

[0022] Using the stripping tank 6, the heat input to the styrene monomer finishing and stripping system 200 is lower than that of the prior art system of FIG. 1, even though the recovery level of styrene from the net residue is higher. Utilizing air in the stripping tank 6 instead of nitrogen or natural gas has the further advantage of reducing the loss of styrene to polymers in the system because the presence of oxygen enhances the effectiveness of the polymerization inhibitor 4-tert-butylcatechol (TBC) typically added to the distillation finishing column reflux 14. The size of the stripping tank 6 and associated pumps is very small compared to the equipment in a prior art styrene monomer finishing system.

[0023] Continuing to refer to FIG. 2, the stripping vessel 6 can be either a simple gas-liquid separation drum or a stripper column having contacting internals 18 below the liquid feed stream. In terms of performance, the stripper column is preferred over the simple drum. In a stripper column where the liquid from the residue flash drum is introduced at the top and stripping gas is added at the bottom of the stripper column, contacting internals 18 such as trays or packing effect countercurrent stripping of styrene from the liquid, which can significantly reduce the amount of gas required for a particular amount of styrene removal from the residue. The gas strips a portion of the styrene from the residue and returns it to the distillation finishing column 1 via a return line 23 to the optimal inlet point. Since the gas dilutes the styrene in the vapor phase from the stripping vessel 6, the equilibrium temperature of the liquid from the residue flash drum 5 decreases when it enters the stripping vessel 6 and contacts the stripping gas. This decrease in liquid temperature provides the heat required to vaporize the styrene in the liquid from the residue flash drum 5.

[0024] Three tables with simulation results for a styrene monomer purification finishing system in a baseline process (Table 1), a process with an embodiment of the stripping vessel as a drum (simple gas-liquid separator) (Table 2), and an improved process with a further embodiment of the stripping vessel as a stripper column (Table 3) are provided below. All of these simulations are based on the production of a final styrene monomer product of 125,000 lb / hr and a distillation finishing column having packing above the feed stream and trays below the feed stream. For the results in Tables 2 and 3, air was used as the stripping gas. Using nitrogen instead of air gives substantially the same results since air is mainly composed of nitrogen (79 volume %) and has very similar physical properties. Steam and natural gas have different physical properties than nitrogen or air, but since the gas flow is very small relative to the liquid feed stream to the stripping vessel, an equimolar flow of steam or natural gas has essentially the same effect on the equilibrium as nitrogen or air.

[0025] Furthermore, in the above process, there is no need to heat the stripping gas. Since the flow rate of the stripping gas is small compared to the flow rate of U.S. Patent No. 3,515,647, the advantage of heating the gas can be ignored.

[0026] Table 1 shows the results for the prior art process shown in Figure 1 when reducing the styrene concentration in the net residue. As shown, the temperature of the residue in the residue flash drum rises significantly as the styrene content in the residue decreases and the load on the residue reboiler increases. Using 5.0 wt% styrene in the residue, the residue flash drum is operated at a temperature of about 324°F (162°C). To reduce the styrene concentration in the residue to 2.0 wt%, the residue flash drum temperature needs to be raised to 358°F (181°C). However, at least one commercially available inhibitor will decompose at this temperature. Therefore, reducing this yield loss to 2.0 wt% in the residue flash drum results in an unduly high temperature that degrades the polymerization inhibitor in the residue.

[0027]

Table 1

[0028] Table 2 shows the results for the improved process of the present disclosure according to an embodiment of a single-stage stripping drum using air as the stripping medium. Compared to Table 1, for all concentrations of styrene in the net residue in Table 2, the temperature of the residue flash drum is lower and the temperature of the stripping drum is lower than the temperature of the residue flash drum. Even though the separation of the system is improved, the load on the residue reboiler actually decreases slightly at the lower styrene concentration in the net residue. The gas can be supplied directly to the stripping drum as shown in Figure 2, but it is preferred to supply the gas to the residue line feeding the drum as shown by the dashed line in Figure 2. The results using nitrogen, natural gas, or steam as the stripping gas are nearly identical for the same molar flow rate of the stripping gas.

[0029]

Table 2

[0030] Table 3 shows the results for a further embodiment of the improved process of the present disclosure having a stripper column as a stripping tank using air as the stripping gas. The stripper column has a plurality of stages, the liquid feed stream from the residue flash drum enters at the top of the stripper column, and air enters at the bottom of the stripper column. Comparing Table 2 and Table 3 for the case of having 2.0 wt% styrene in the net residue, it is shown that by moving from a 1-stage stripping drum (the 4th data column in Table 2) to a 2-stage stripper column (the 3rd data column in Table 3), the required stripping air is reduced by 64% (from 88.9 lb / hr to 32.3 lb / hr). The last three columns in Table 3 show that by adding additional stripper stages, the stripping gas consumption is reduced, or the amount of styrene lost in the final residue with the same flow rate of stripping gas is decreased.

[0031]

Table 3

[0032] By stripping styrene from the residue using gas in the stripping tank and returning the gas having the stripped styrene to the distillation finishing column, the loss of styrene to the residue is significantly reduced without the need to operate at a higher temperature. Adding a stripping tank not only increases styrene recovery but also actually reduces the heat input to the styrene monomer purification finishing system, which is an unexpected result in the distillation system because the component separation is more complete.

[0033] The systems and methods of the present disclosure provide means for economically recovering styrene from a heavy byproduct stream in a styrene plant without exceeding the temperature degradation limit of the polymerization inhibitor, as described above and shown in the drawings. Each of the above systems and methods is based on a single distillation finishing column, followed by a residue flash drum, and followed by a stripping tank having a stripping gas. Although the apparatus and methods of the present disclosure have been illustrated and described with reference to embodiments, those skilled in the art will readily understand that changes and / or modifications can be made to those upstream of the stripping tank without departing from the spirit of the present disclosure.

Claims

1. A method for improving the recovery of styrene, comprising: supplying a high-temperature liquid residue stream from a styrene monomer purification finishing system to a stripping tank, the high-temperature liquid residue stream containing styrene and a compound less volatile than styrene; introducing a gas into the stripping tank to strip a portion of the styrene as vapor to produce a vaporized styrene portion; returning the vaporized styrene portion and the gas to the styrene monomer purification finishing system; recovering at least a portion of the vaporized styrene portion into a styrene monomer product; producing a final liquid residue stream having a lower concentration of styrene than the high-temperature liquid residue stream supplied to the stripping tank from the bottom of the stripping tank; A method comprising the above steps.

2. The method according to claim 1, wherein the stripping tank operates at a temperature lower than that of the high-temperature liquid residue stream.

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

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

5. The method according to claim 1, wherein the stripping tank operates under vacuum.

6. The method according to claim 1, further comprising recycling a portion of the liquid residue stream from the bottom of the stripping tank back to the starting point of a styrene preliminary fractionation system upstream of the styrene monomer purification finishing system.

7. The method according to claim 1, wherein the liquid residue stream exiting the stripping tank is at a temperature of about 250°F to 340°F.

8. The method according to claim 1, wherein introducing the gas into the stripping tank includes at least one of (i) supplying the gas to the stripping tank separately from the liquid residue stream, or (ii) supplying the gas to the high-temperature liquid residue stream and then to the stripping tank.

9. The method according to claim 1, wherein supplying the high-temperature liquid residue stream includes supplying the high-temperature liquid residue stream to the top of the stripping tank, and introducing the gas includes introducing the gas to the bottom of the stripping tank.

10. A system for improving the recovery of styrene from a residue stream, comprising a stripping tank configured and adapted to receive a high-temperature liquid residue feed stream from a styrene monomer purification finishing system.

11. The system according to claim 10, further comprising a return conduit connected to the stripping tank and configured and adapted to return a portion of the styrene recovered from the high-temperature liquid residue stream to the styrene monomer purification finishing system upstream of the stripping tank.

12. The system according to claim 10, wherein the stripping tank is a drum, and the drum is configured and adapted to directly receive stripping gas into the drum or to receive the stripping gas in a high-temperature liquid residue feed line and then into the drum.

13. The system according to claim 10, wherein the stripping tank is a stripper column having packing and / or trays, the top of the stripper column being configured and adapted to receive the high-temperature liquid residue feed stream, and the bottom of the stripper column being configured and adapted to receive a stripping gas feed stream.

14. The system according to claim 10, wherein the stripping tank is a drum without internal elements for improving gas-liquid contact.

15. The system according to claim 10, wherein the stripping tank includes internal elements for improving gas-liquid contact.

16. The system according to claim 15, wherein the internal element in the stripping tank is a distillation tray.

17. The system according to claim 15, wherein the internal element in the stripping tank is distillation packing.

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

19. The system according to claim 10, wherein the stripping tank is configured and adapted to facilitate contact between a high-temperature liquid residue feed stream from a styrene monomer purification finishing system and a gas for recovering a portion of the styrene contained in the liquid feed stream. ​

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