Method to separate water and caprolactam from steam stripping polycaprolactam

Abstract

The present invention provides methods and systems for depolymerization and purification of polycaprolactam-containing materials using steam stripping.

Description

Attorney Docket No. VTIP-A1034-PCTCustomer No. 95,450METHOD TO SEPARATE WATER AND CAPROLACTAM FROM STEAM STRIPPING POLYCAPROLACTAMBACKGROUND OF THE INVENTIONField of the Invention

[0001] The present invention relates to the field of chemical processing methods and systems. More specifically, the present invention relates to depolymerization of nylon 6 to caprolactam in a process aided by stripping steam.Description of Related Art

[0002] Poly caprolactam, also referred to as nylon 6, is a common thermoplastic used in the automotive industry, electronics, and textiles and is produced from the monomer caprolactam. The production of nylon 6 generates some equilibrium oligomers that are separated from the polymer before end use. Manufacturing processes using nylon 6 also generate unusable scrap nylon 6 material. Chemical recycling is commonly practiced to convert oligomers yielded from nylon 6 production and scrap nylon 6 from manufacturing processes to the monomer caprolactam.

[0003] The most common method to chemically recycle nylon 6 and its oligomers involves introducing a melted mixture of the polymer and oligomers into a vessel and contacting the melt with steam. A vapor exits the reactor containing both caprolactam and steam. Steam introduces water into the melt and reduces the molecular weight, which speeds the caprolactam forming reaction. Additionally, the steam acts as a stripping agent by diluting the partial pressure of caprolactam in the vapor and thus its equilibrium concentration in the melt. The continuous removal of caprolactam from the reactor allows the reaction to proceed to completion and very high yields of caprolactam can be obtained.

[0004] Steam stripping processes are described by several patents, such as US5169870, US3182055A, US5556890, US6087494A, US5681952A, US5990306A, and US3939153. The processes typically involve contacting melted nylon 6 or oligomers with steam at a temperature between 250 °C - 350 °C for some amount of time.

[0005] A variety of catalysts are employed to facilitate the process. For example, US5169870, US3182055A, and US5556890 describe using steam stripping with an H3PO4 catalyst. US6087494A describes using an NaOH catalyst. US5681952A, US5990306A, and US3939153 describe steam stripping without the use of a catalyst. Dmitrieva et al. describe several different catalysts and operating conditions employed industrially for steam stripping depolymerization ofAttorney Docket No. VTIP-A1034-PCT Customer No. 95,450 nylon 6 and its oligomers (Dmitrieva, L A. et al. Regeneration of £-caprolactam from wastes in the manufacture of polycaproamide fibres and yarns. Fibre Chemistry. 1986;17(4):229-241).

[0006] US5681952A describes the benefit of the steam stripping reaction under positive pressure. A pressure is identified for a given steam rate and temperature where the rate of caprolactam produced in the vapor phase is maximized. This optimum pressure is a function of the reaction temperature and steam rate but occurs between 500 kPa and 1,500 kPa under the conditions evaluated. This fundamental trait of an optimal positive reaction pressure for steam stripping depolymerization can be extended to catalyzed steam stripping.

[0007] Following the steam stripping reactor the vapor containing steam and caprolactam must be processed such that the caprolactam is separated from the water. This is typically done by distillation where both a crude caprolactam product and liquid water are yielded.

[0008] US4107160A describes an improvement to the conventional separation of water and caprolactam vapor from the reactor. It describes the vapor from the reactor entering a distillation column where a crude caprolactam product is produced from the bottom of the column and the water exits the top of the column as vapor. The water vapor or steam is then circulated back to the reactor by means of a compressor. Keeping the water in the vapor phase prevents having to vaporize the water after it is condensed. The electrical energy required to recycle the steam by compression is an order of magnitude lower than the energy to vaporize water once it is condensed.

[0009] It is advantageous to implement such a configuration where water or steam is kept in the vapor phase after caprolactam is separated and the water vapor is recycled by compression. It is also advantageous to operate the steam stripping reaction at an optimal positive pressure; however, operating the water-caprolactam distillation column at positive pressures results in high temperatures in the column. The temperature of the water-caprolactam distillation column can be reduced by lowering its operating pressure, but a lower pressure increases the pressure differential a steam compressor must provide to circulate steam back to the reactor and lowers its attractiveness.

[0010] It is well known that, in separations where caprolactam is evaporated, the separation is sensitive to high temperatures because caprolactam decomposes and begins to polymerize. Such separations are described by US5990306A, W02000050394A2, US5556890A, US4326925A, US4457807, US4720328A, US3145198, and US5458740A. It can be appreciated that, in the absence of water, it is preferential to keep caprolactam in the liquid phase below a temperature ofAttorney Docket No. VTIP-A1034-PCTCustomer No. 95,450 approximately 140 °C - 160 °C for extended periods of time. Caprolactam has a normal boiling temperature of 270 °C, so it can be understood the bottom product concentrated in caprolactam from a water-caprolactam separation operating above atmospheric pressure is going to be significantly above 140 °C where decomposition and hydrolysis of caprolactam can be expected if liquid residence time in the column is more than a few seconds.

[0011] This invention addresses the issue of the high temperature in a high-pressure water- caprolactam separation and avoids the decomposition and hydrolysis of caprolactam, while enabling the circulation of water in the vapor phase by means of compression.SUMMARY OF THE INVENTION

[0012] In one embodiment, this invention improves a system capable of depolymerizing nylon 6 by steam stripping. In this system, nylon is converted to caprolactam. Examples of nylon 6 steam stripping depolymerizations can be found in US5169870, US3182055A, US5556890, US6087494A, US5681952A, US5990306A, and US3939153. In embodiments, the steam stripping depolymerization reaction can be enhanced using a catalyst such as H3PO4, H3BO3, NaOH, or other steam stripping catalyst. In embodiments, this invention can be applied to either an uncatalyzed steam stripping reaction or a catalyzed steam stripping reaction. The steam stripping reactor yields a vapor product that contains a mixture of water and caprolactam, along with some impurities and degradation products. The vapor product goes to a water-caprolactam separation device, such as a distillation column, where the bulk of water in the product is separated from caprolactam. In this embodiment, water essentially free of caprolactam leaves the top of the water-caprolactam separation device as a vapor. Most of the water vapor from the water-caprolactam separation device is recycled to the steam stripping reactor by means of a compressor and some of the water vapor goes to a water purification system to remove accumulating impurities before being recycled to the steam stripping reactor. A caprolactam concentrated liquid exits the bottom of the water- caprolactam separation device that may contain some water. In embodiments, there is no heating element at the bottom of the water-caprolactam separation device. The caprolactam concentrated liquid may go through any number of purification treatments to improve it to a polymer grade quality and be further separated to remove the residual water, which may be recycled back to the water-caprolactam separation device.

[0013] In embodiments, the system described by this invention uses a steam stripping reaction system that contacts superheated steam with a nylon 6 or oligomer melt, typically with a mass ratioAttorney Docket No. VTIP-A1034-PCT Customer No. 95,450 between 0.5 and 5 at a temperature between 300 °C and 400 °C and a pressure between 300 kPa and 3,000 kPa for uncatalyzed steam stripping and a temperature between 230 °C and 330 °C and a pressure between 100 kPa and 2,000 kPa for catalyzed steam stripping.

[0014] In embodiments, the water-caprolactam separation device operates below, but favorably close to, the steam stripping reaction pressure. The water-caprolactam separation device may operate between 150 kPa and 2,000 kPa for an uncatalyzed steam stripping reaction and between 100 kPa and 1,500 kPa for a catalyzed steam stripping reaction. At these conditions, the caprolactam free water vapor from the water-caprolactam separation device may be 100 °C to 210 °C at the bottom.

[0015] In embodiments, the period of time the caprolactam rich liquid is maintained at elevated temperatures is minimized by design innovations applied to the water-caprolactam separation device. One innovation is the utilization of structured packing internals in the separation device. Structured packing allows good contact between the liquid and vapor phase in the column, allowing the separation of water from caprolactam without a large liquid holdup. The residence time of the liquid in the column is well known to be on the order of seconds. The other innovation is the immediate cooling of liquid in the bottom of the column. Normal operation of a distillation column requires maintaining a certain holdup in the bottom of the column of a couple minutes or longer for process control. This embodiment involves the utilization of a cooling heat transfer element inside the column where this holdup exists that cools the bottom liquid to a temperature between 140 °C - 160 °C. Applying these two innovations limits the time the caprolactam liquid spends at temperatures above 140 °C - 160 °C to less than a minute.

[0016] In embodiments, most of the vapor from the top of water-caprolactam separation device is recycled to the steam stripping reactor by means of a steam compressor. The inlet pressure of the steam compressor is close to the operating pressure of the water-caprolactam separation device. In embodiments, the outlet pressure of the steam compressor is sufficient to circulate the steam back to the steam stripping reactor. The pressure differential provided by the steam compressor is equal to the difference in pressure between the steam stripping reactor and the water-caprolactam separation device plus the hydraulic losses of steam through a steam super heater to heat steam above the stripping reactor temperature, control valves, and line losses and may be between 20 kPa and 150 kPa. In embodiments, the water-caprolactam separation device pressure allows the steam compressor to operate with a compression ratio between 1.05 and 2.5.Attorney Docket No. VTIP-A1034-PCT Customer No. 95,450

[0017] In embodiments, the operation of the water-caprolactam separation device at elevated temperature provides high temperatures in the condensing heat exchanger at the top of the water-caprolactam separation device and the cooling element at the bottom of the water- caprolactam separation device. The temperatures are high enough that useful heat can be recovered from these cooling heat exchangers. In this embodiment, these two heat exchangers generate steam or other heating utility that can be used as a heat source elsewhere in the process.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings illustrate certain aspects of implementations of the present disclosure and should not be construed as limiting. Together with the written description the drawings serve to explain certain principles of the disclosure.

[0019] FIG. 1 is a diagram that depicts a steam stripping reaction system and a water-caprolactam separation device according to embodiments of the invention.

[0020] FIG. 2 is a diagram that depicts a portion of a water-caprolactam separation device according to embodiments of the invention.

[0021] FIG. 3 is a diagram that depicts a portion of a water-caprolactam separation device according to embodiments of the invention.DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

[0022] The depolymerization of nylon 6 or oligomers of nylon 6 by contact with stripping steam produces a caprolactam product that is separated from the reactor in the vapor phase. The primary motivation for depolymerizing nylon 6 to caprolactam is to convert byproduct oligomer, polymer scrap, and / or post-consumer waste into a form that can be more easily purified of impurities and then repolymerized into a virgin quality product.

[0023] US5681952A describes the benefit of the steam stripping reaction under positive pressure, which increases caprolactam in the vapor product and reduces the concentration of impurities, such as the cyclic dimer of caprolactam, that are yielded in the vapor product. The vapor product, primarily a mixture of water and caprolactam, must ultimately be separated so water can be recycled to the steam stripping reactor, and caprolactam can be yielded as a product. This separation is done in a water-caprolactam separation device, such as a distillation column. It is energetically most efficient to operate the column at as high of a pressure as possible and produceAttorney Docket No. VTIP-A1034-PCT Customer No. 95,450 a caprolactam free water vapor product from the top of the column. The water vapor can be recycled using a very low amount of electrical energy to compress the steam.

[0024] The problem with operating the water-caprolactam column at high pressures is it results in high temperatures in the column. A column operating above atmospheric pressure is going to result in the caprolactam concentrated bottom liquid temperature being well above the 140 °C - 160 °C temperature that is well known to be the upper operating temperature limit for separations where caprolactam is vaporized because of issues with degradation and hydrolysis of caprolactam. The degradation and hydrolysis of caprolactam is a function of time and temperature. The preferred embodiment solves the problems that have, until now, prevented the water-caprolactam column from operating at elevated pressure by minimizing the amount of time caprolactam rich liquid in the column is at elevated temperature to several seconds, thus severely mitigating the degradation of caprolactam in the column. The preferred embodiment additionally takes advantage of the heat integration and process intensification potentials provided by operating the water-caprolactam column at higher pressure.

[0025] FIG. 1 illustrates an embodiment of the present invention as applied to the nylon 6 polymer and nylon 6 oligomer depolymerization process where melted polymer and oligomers are contacted with steam. A caprolactam product is yielded in the vapor phase, followed by separation and purification steps to isolate the caprolactam product and recycle water to the steam stripping reactor. FIG. 1 is not meant to cover all potential implementations of the inventions.

[0026] A feed stream of nylon 6 polymers, oligomers, nylon 6 containing post-consumer waste, or combinations thereof, 100, is fed to a steam stripping reactor system, 101. The feed stream, 100, may contain between about 10 wt% and 100 wt% of caprolactam forming material, such as about 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 50 wt%, 55 wt%, 60 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, 95 wt%, or 99 wt% caprolactam forming material. The feed stream, 100, additionally comprises between 0 wt% and 90 wt% water, such as about 1 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 50 wt%, 55 wt%, 60 wt%, 70 wt%, 75 wt%, 80 wt%, or 85 wt% water. The feed stream, 100, can further comprise between 0 wt% and 70 wt% of other materials, such as backing material used in nylon 6 carpeting or glass fibers used in some reinforced nylon 6 products, such as about 1 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 50 wt%, 55 wt%, 60 wt%, or 65 wt% of other materials. The feed stream, 100, enters the reactor system, 101, in the liquid phase, normallyAttorney Docket No. VTIP-A1034-PCT Customer No. 95,450 from an extruder and is at a temperature between about 220 °C and 350 °C, such as about 230 °C, 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, 290 °C, 300 °C, 310 °C, 320 °C, 330 °C or 340 °C.

[0027] The feed stream, 100, is contacted with superheated steam, 415, in the reactor system, 101. The superheated steam, 415, is at a temperature equal to or greater than the temperature of the stripping reactor system, 101, and enters the reactor at a temperature between about 250 °C and 450 °C, such as about 260 °C, 270 °C, 280 °C, 290 °C, 300 °C, 310 °C, 320 °C, 330 °C, 340 °C, 350 °C, 360 °C, 370 °C, 380 °C, 390 °C, 400 °C, 410 °C, 420 °C, 430 °C, and 440 °C. The total steam rate to the reactor system, 101, ranges between a mass ratio of 0.5 and 5 by mass of caprolactam forming material (e.g., nylon 6) relative to the feed stream rate of the feed stream, 100,. The steam flow may be divided into different sections of the reactor system, 101.

[0028] In embodiments, heat is required in the reactor system, 101, to overcome the heat of vaporization for caprolactam. Heat can be supplied by the superheated steam, 415, jacketed heating in the reaction system, 101, or external heat exchangers with circulation loops, or some combination thereof. The reactor system, 101, ranges between about 300 °C and 400 °C, such as about 310 °C, 320 °C, 330 °C, 340 °C, 350 °C, 360 °C, 370 °C, 380 °C, or 390 °C. The reactor system, 101, operates at a pressure between about 300 kPa and 3,000 kPa, such as about 400 kPa, 450 kPa, 500 kPa, 550 kPa, 600 kPa, 650 kPa, 700 kPa, 750 kPa, 800 kPa, 850 kPa, 900 kPa, 950 kPa, 1,000 kPa, 1,100 kPa, 1,200 kPa, 1,300 kPa, 1,400 kPa, 1,500 kPa, 1,600 kPa, 1,800 kPa, 2,000 kPa, 2,200 kPa, 2,400 kPa, 2,600 kPa, or 2,800 kPa, for uncatalyzed steam stripping. The reactor system, 101, operates at a temperature between 230 °C and 330 °C, such as about 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, 290 °C, 300 °C, 310 °C, or 320 °C, and a pressure between 100 kPa and 2,000 kPa, such as about 150 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa, 400 kPa, 450 kPa, 500 kPa, 550 kPa, 600 kPa, 650 kPa, 700 kPa, 750 kPa, 800 kPa, 850 kPa, 900 kPa, 950 kPa, 1,000 kPa, 1,100 kPa, 1,200 kPa, 1,300 kPa, 1,400 kPa, 1,500 kPa, 1,600 kPa, 1,700 kPa, or 1,800 kPa, for catalyzed steam stripping. The temperature may vary at different sections of the reactor system, 101.

[0029] The reactor system, 101, may consist of a single continuous stirred tank reactor, multiple stirred tank reactors in series, a baffled reactor, a horizontal paddle reactor, or combinations thereof.

[0030] A catalyst feed, 104, may be added to the reaction system, 101. The catalyst feed, 104, may comprise an acid, such as H3PO4, a base, such as NaOH, or one or more other known steamAttorney Docket No. VTIP-A1034-PCT Customer No. 95,450 stripping catalyst(s) at a concentration between about 0.1 wt% and 10 wt%, such as about 0.5 wt%,1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, or 9 wt%, relative to the rate of caprolactam forming material in the feed stream 100. A residue stream, 103, depleted of caprolactam forming material and containing non-reactive and non-volatile impurities, degradation products, reaction catalyst, and some caprolactam and caprolactam forming material exits the stripping reactor system, 101. The recovery of caprolactam in the vapor product, 102, relative to the caprolactam forming material in the feed stream, 100, may range between about 80% and 99%, such as about 82%, 85%, 88%, 90%, 92%, 95%, or 98%. The vapor product, 102, exiting the reactor system, 101, contains between about 15 wt% and 65 wt% caprolactam, such as about 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, or 60 wt% caprolactam, between about 30 wt% and 85 wt% water, such as about 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, or 80 wt%, and between 0.1 wt% and 5 wt% volatile impurities, such as ammonia, volatile degradation byproducts, aminocaproic acid, and the cyclic dimer of caprolactam, such as about 0.5 wt%, 1 wt%, 1.5 wt%,2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, or 4.5 wt% impurities.

[0031] In embodiments, the system comprises a water-caprolactam separation device, 200, such as a distillation column. The vapor product, 102, enters the bottom of the water-caprolactam separation device, 200. The water-caprolactam separation device, 200, contains between about3 and 10 equilibrium stages, such as about 4, 5, 6, 7, 8, or 9 equilibrium stages, and has structured packing internals specifically designed to minimize liquid hold up time. The water-caprolactam separation device, 200, internals are designed such that the average liquid residence time inside the column above the feed stage is less than one minute and preferentially less than 15 seconds, such less than about 5 seconds, 10 seconds, 20 seconds, 25 seconds, 30 seconds, 35 seconds, 40 seconds, 45 seconds, 50 seconds, or 55 seconds. The water-caprolactam separation device, 200, may operate between about 150 kPa and 2,000 kPa for an uncatalyzed steam stripping reaction, such as about 175 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa, 400 kPa, 450 kPa, 500 kPa, 550 kPa, 600 kPa, 650 kPa, 700 kPa, 750 kPa, 800 kPa, 850 kPa, 900 kPa, 950 kPa, 1,000 kPa, 1,100 kPa, 1,200 kPa, 1,300 kPa, 1 ,400 kPa, 1,500 kPa, 1,600 kPa, 1,700 kPa, or 1,800 kPa. For a catalyzed steam stripping reaction, the water-caprolactam separation device, 200, may operate between about 100 kPa and 1,500 kPa, such as about 125 kPa, 150 kPa, 175 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa, 400 kPa, 450 kPa, 500 kPa, 550 kPa, 600 kPa, 650 kPa, 700 kPa, 750 kPa, 800 kPa,Attorney Docket No. VTIP-A1034-PCT Customer No. 95,450850 kPa, 900 kPa, 950 kPa, 1,000 kPa, 1,100 kPa, 1,200 kPa, 1,300 kPa, 1,400 kPa, and at a pressure between about 10 kPa and 1,000 kPa lower than the stripping reactor system, 101, such as about 25 kPa, 50 kPa, 75 kPa, 100 kPa, 150 kPa, 200 kPa, 250 kPa, 300 kPa, 350 kPa, 400 kPa, 450 kPa, 500 kPa, 600 kPa, 700 kPa, 800 kPa, or 900 kPa lower than the stripping reactor system, 101

[0032] The vapor from the top of the water-caprolactam separation device, 200, is partially condensed by a condenser, 201. The vapor and liquid phase are separated by a drum, 202, with the liquid being refluxed back to the water-caprolactam separation device, 200. The reflux rate can be set to control the composition of caprolactam in the vapor distillate, 210, to between about 1 ppmw and 5,000 ppmw, such as about 5 ppmw, 10 ppmw, 25 ppmw, 50 ppmw, 75 ppmw, 100 ppmw, 150 ppmw, 200 ppmw, 300 ppmw, 400 ppmw, 500 ppmw, 750 ppmw, 1,000 ppmw, 1,500 ppmw, 2,000 ppmw, 2,500 ppmw, 3,000 ppmw, 3,500 ppmw, 4,000 ppmw, or 4,500 ppmw, and / or the composition of water in the bottom liquid, 205, to between about 1 wt% and 30 wt%, such as about 2 wt%, 5 wt%, 10 wt%, 15 wt%, 20 wt%, or 25 wt%. The temperature of the vapor distillate, 210, is between about 100 °C and 200 °C, such as about 110 °C, 120 °C, 130 °C, 140 °C, 150 °C, 160 °C, 170 °C, 180 °C, or 190 °C. The condenser, 201, preferentially generates a heating utility, such as steam at a temperature between about 90 °C and 190 °C, such as about 100 °C, 110 °C, 120 °C, 130 °C, 140 °C, 150 °C, 160 °C, 170 °C, or 180 °C, that is utilized elsewhere in the process.

[0033] A concentrated liquid comprising caprolactam is collected in a pool, 209, at the bottom of the water-caprolactam separation device, 200. A holdup time is maintained in this pool, 209, for process control purposes to maintain a level of liquid and prevent vapor from flowing out of the bottom of the column. The holdup time of the pool, 209, may be between about 1 minute and 30 minutes, such as about 2 minutes, 3 minutes, 5 minutes, 7 minutes, 10 minutes, 12 minutes, 15 minutes, 18 minutes, 20 minutes, 22 minutes, 25 minutes, or 28 minutes. The temperature of the concentrated liquid comprising caprolactam entering from above the pool, 209, from the water-caprolactam separation device, 200, is between about 150 °C and 300 °C, such as about 160 °C, 170 °C, 180 °C, 190 °C, 200 °C, 210 °C, 220 °C, 230 °C, 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, or 290 °C. The temperature of the pool, 209, is maintained between about 100 °C and 160 °C, such as about 110 °C, 120 °C, 130 °C, 140 °C, or 150 °C, such as by cooling by a heat transfer element, 203. The heat transfer element, 203, preferentially generates a heating utility such as steam at a temperature between about 90 °C and 150 °C such as about 100 °C, 110 °C, 120 °C,Attorney Docket No. VTIP-A1034-PCT Customer No. 95,450130 °C, or 140 °C, that is utilized elsewhere in the process. A feed of inert gas, 204, such as nitrogen, is introduced above the pool, 209, and below the reaction vapor feed, 102, to prevent vapor from the water-caprolactam separation device, 200, condensing into the subcooled pool, 209.

[0034] A bottom liquid, 205, from the pool, 209, at the bottom of the water-caprolactam separation device, 200, contains between about 70 wt% and 99 wt% caprolactam, such as about 75 wt%, 80 wt%, 85 wt%, 90 wt%, or 95 wt% caprolactam, between about 1 wt% and 30 wt% water, such as about 5 wt%, 10 wt%, 15 wt%, 20 wt%, or 25 wt% water, and between about 0.1 wt% and 5 wt% impurities, such as about 0.5 wt%, 1 wt%, 1 .5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, or 4.5 wt% impurities, and goes to a caprolactam purification unit, 300. Caprolactam purification is known to industry and can be done by one or more of distillation of caprolactam, crystallization, extraction, oxidation, and hydrogenation. US5990306A, US5556890A, and US5169870 describe different methodologies for purifying caprolactam to polymer grade purity using different purification operations in different ordering.

[0035] In embodiments, the caprolactam purification unit, 300, produces a polymer grade caprolactam product, 302, as a liquid above 68 °C or as a solid below 68 °C with a total impurity concentration between about 1 ppmw and 1,000 ppmw, such as about 5 ppmw, 10 ppmw, 25 ppmw, 50 ppmw, 75 ppmw, 100 ppmw, 150 ppmw, 200 ppmw, 250 ppmw, 300 ppmw, 350 ppmw, 400 ppmw, 450 ppmw, 500 ppmw, 550 ppmw, 600 ppmw, 650 ppmw, 700 ppmw, 750 ppmw, 800 ppmw, 850 ppmw, 900 ppmw, or 950 ppmw. The caprolactam purification unit, 300, separates impurities, which are represented by the impurities stream, 301, which may comprise one or more waste products separated in the caprolactam purification unit 300. The caprolactam purification unit may separate a recycling stream, 304, concentrated in oligomers and caprolactam forming material along with some impurities. The caprolactam forming materials in the recycling stream, 304, may be preferentially recovered by recycling to the reactor system, 101. Water stream, 303, is separated in the caprolactam purification unit, 300, which is recycled to the water-caprolactam separation device, 200. The water stream, 303, contains between about 1 wt% and 80 wt% caprolactam, such as about 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, or 75 wt% caprolactam, and is fed to the water-caprolactam separation device, 200, at a temperature between about 40 °C and 200 °C, such as about 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, 140 °C,Attorney Docket No. VTIP-A1034-PCT Customer No. 95,450150 °C, 160 °C, 170 °C, 180 °C, or 190 °C. The water stream, 303, is introduced into the water- caprolactam separation device, 200, preferentially by an upward facing liquid distributor above the reaction vapor product, 102.

[0036] The vapor distillate, 210, from the water-caprolactam separation device, 200, splits between a first vapor feed, 207, which is routed to a compressor, 410, and a second vapor feed, 206, which is directed to a water purification unit, 400. The compressor, 410, increases the pressure of the first vapor feed, 207, to a pressure between about 20 kPa and 100 kPa above the pressure of the reactor system, 101, such as about 30 kPa, 40 kPa, 50 kPa, 60 kPa, 70 kPa, 80 kPa, or 90 kPa above the pressure of the reactor system, 101. The compression ratio of the compressor, 410, is preferentially between about 1.05 and 2.5, such as about 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, or 2.4. Circulation by compression does not provide release of non-volatile impurities from the process, which leads to accumulation. The split of second vapor feed, 206, to the water purification unit, 400, provides an outlet for these accumulating impurities. The rate of flow of the second vapor feed, 206, to the water purification can be controlled by one or more valve, 208, to control the concentration of water and impurities in the stripping steam, 413. One or more valve, 208, can directly or indirectly control the composition of impurities in the water vapor feed based on measurements at sensor, 412. In embodiments, the sensor, 412, is any sensor, instrument, analyzer, or combination thereof capable of directly or indirectly determining the presence and / or amount of one or more component(s) in the composition, and / or ratio of components. The concentration of water in the stripping steam, 413, can be controlled between 95 wt% and 99.99 wt%, such as about 97 wt%, 98 wt%, 99 wt%, or 99.9 wt%.

[0037] The control loop depicted in FIG. 1 represents how the fraction of flow to the water purification unit, 400, controls the concentration of impurities in the stripping steam 413, but should not exclude other methods of controlling or limiting the impurity concentration in the stripping steam, 413, by way of recycling the vapor split fraction as steam, 406.

[0038] The water purification unit, 400, is designed to separate impurities present, which can be classified as impurities lighter than water, 401, and impurities heavier than water, 402. In embodiments, the water purification unit, 400, comprises two distillation columns with one separating impurities more volatile than water, 401, and the second separating impurities heavier than water, 402, including caprolactam. The water purification unit, 400, can also use different purification methods, such as adsorption, or can be designed as a single divided wall column. TheAttorney Docket No. VTIP-A1034-PCT Customer No. 95,450 water purification unit, 400, preferentially removes between about 90% and 99.99% of both light impurities, 401, and heavy impurities, 402, present in the second vapor feed, 206. Some purified water, 403, from the purification unit, 400, may be purged depending on the content of water in the feed stream, 100. The remaining water to be recycled, 404, is a liquid between about 40 °C and 150 °C and goes to a vaporizer, 405. The vaporizer, 405, evaporates the water to be recycled, 404, at a pressure between about 200 kPa and 3,000 kPa, such as about 250 kPa, 300 kPa, 400 kPa, 500 kPa, 600 kPa, 700 kPa, 800 kPa, 1,000 kPa, 1,200 kPa, 1,400 kPa, 1,600 kPa, 1,800 kPa, 2,000 kPa, 2,200 kPa, 2,400 kPa, 2,600 kPa, or 2,800 kPa, and at least about 20 kPa to 100 kPa higher pressure than the reactor system, 101, pressure, such as about 30 kPa, 40 kPa, 50 kPa, 60 kPa, 70 kPa, 80 kPa, or 90 kPa higher. The steam, 406, from the vaporizer, 405, is at a temperature between about 120 °C and 230 °C, such as about 130 °C, 140 °C, 150 °C, 160 °C, 170 °C, 180 °C, 190 °C, 200 °C, 210 °C, or 220 °C.

[0039] Steam, 406, from the vaporizer, 405, is combined with steam, 411, from the compressor, 410, and enters the super heater, 414. This super heater, 414, heats the recycled steam, 413, to a temperature between about 250 °C and 450 °C, such as about 260 °C, 270 °C, 280 °C, 290 °C, 300 °C, 310 °C, 320 °C, 330 °C, 340 °C, 350 °C, 360 °C, 370 °C, 380 °C, 390 °C, 400 °C, 410 °C, 420 °C, 430 °C or 440 °C. The steam, 415, temperature is preferentially between about 10 °C and 150 °C higher than the operating temperature of the reactor system, 101, such as about 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C, 110 °C, 120 °C, 130 °C, or 140 °C higher .

[0040] In embodiments, the time a caprolactam rich liquid in the water-caprolactam column, 200, is at elevated temperature is minimized to well under one minute using the cooling of the pool, 209, at the bottom of the water-caprolactam separation device, 200. A problem that can result from cooling the pool, 209, is the vapor pressure of the liquid becomes less than the pressure in the water-caprolactam separation device, 200. Warm vapor from above the pool, 209, could condense into the pool, 209, causing some of the vapor flow downward directly into the pool, 209, rather than traveling through the water-caprolactam separation device, 200, internals and providing the intended separation between caprolactam and water. This problem can be solved by injecting a vapor that does not condense at the temperature of the pool, 209, and the pressure inside the water-caprolactam separation device, 200.Attorney Docket No. VTIP-A1034-PCT Customer No. 95,450

[0041] FIG. 2 depicts the details of bottom of the water-caprolactam separation device, 200 in FIG. 1, where an inert gas is injected above the liquid pool to prevent vapor entering the column from directly condensing into the pool, 209.

[0042] The reactor vapor, 1102, enters the bottom of the column, 1200. Water recycled from a downstream caprolactam purification unit, 1303, enters the column, 1200, above the reactor vapor, 1102, and is preferentially fed using an upward facing distributor, 3030, so that liquid from 1303 has sufficient contact with the reactor vapor, 1102, and does not bypass the vapor and go directly to the bottom liquid pool, 1209. The liquid from the column, 1200, ultimately collects in the bottom liquid pool, 1209. An inert gas, 1204, is introduced to the column, 1200, preferentially by a distributor, 2040, and preferentially as close to the pool, 1209, liquid level as possible. The inert gas, 1204, is fed at a rate that creates a continuous flow of vapor moving upward away from the pool, 1209, such that it prevents reactor vapor, 1102, from reaching the surface of the pool, 1209, by diffusion. The inert gas, 1204, is introduced at a rate that provides a superficial velocity between about 0.01 and 3 ft / s, such as about 0.05 ft / s, 0.1 ft / s, 0.2 ft / s, 0.3 ft / s, 0.5 ft / s, 0.75 ft / s, 1 ft / s, 1.5 ft / s, 2 ft / s, or 2.5 ft / s, in the column, 1200, below the reactor vapor, 1102, feed. In embodiments, a cross-sectional area reducing element, 1250, may be utilized that allows liquid to drain into the pool, 1209, and reduces the cross-sectional area above the pool, 1209, to reduce the flow of inert gas, 1204, into the column, 1200. The cross-sectional area reducing element, 1250, is embodied by any design where the cross-sectional area above the liquid pool, 2209, is reduced from the cross-sectional area of the column, 1200. The cross-sectional area reducing element, 1250, may reduce the cross-sectional area between about 20% and 99%, such as about 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or 97%, compared to the cross-sectional area of the column, 1200.

[0043] The heating element, 1203, is in the bottom pool, 1209, and cools the liquid and maintains the pool at a temperature between about 100 °C ant 160 °C, such as about 120 °C, 130 °C, 140 °C, or 150 °C. A cooling fluid, 2030, such as condensate water, enters the heating element, 1203, to remove heat and exits, 2031, for example as steam that can be utilized elsewhere in the process.

[0044] FIG. 3 depicts an alternate configuration of the heating element (203 in FIG. 1) that also is reflective of an embodiment of this invention.Attorney Docket No. VTIP-A1034-PCT Customer No. 95,450

[0045] In this configuration, a heat transfer element is not located in the liquid pool, 2209, which may be due to limited space given the column, 2200, diameter, or poor heat transfer without circulation or agitation. Instead, some liquid is drawn from the pool, 2209, and circulated by a pump, 2230, through a cooling heat exchanger, 2203, that operates equivalently to the heat transfer element, 203 in FIG. 1, and the heat transfer element, 203, and / or the cooling heat exchanger, 2203, can be used by alone or in combination with one another. The cooled liquid is then returned to the liquid pool, 2209. It is preferential to locate the take off and return for the circulation loop at as high a level in the liquid pool as practical given the tendency for lower temperature liquid to have higher density and move toward the bottom of the pool.

[0046] The present invention has been described with reference to particular embodiments having various features. In light of the disclosure provided above, it will be apparent to those skilled in the art that various modifications and variations can be made in the practice of the present invention without departing from the scope or spirit of the invention. One skilled in the art will recognize that the disclosed features may be used singularly, in any combination, or omitted based on the requirements and specifications of a given application or design. When an embodiment refers to “comprising” certain features, it is to be understood that the embodiments can alternatively “consist of’ or “consist essentially of’ any one or more of the features. Any of the methods disclosed herein can be used with any of the compositions disclosed herein or with any other compositions. Likewise, any of the disclosed compositions can be used with any of the methods disclosed herein or with any other methods. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention.

[0047] It is noted in particular that where a range of values is provided in this specification, each value between the upper and lower limits of that range is also specifically disclosed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range as well. The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. It is intended that the specification and examples be considered as exemplary in nature and that variations that do not depart from the essence of the invention fall within the scope of the invention. Further, all of the references cited in this disclosure are each individually incorporated by reference herein in their entireties and as such are intended to provideAttorney Docket No. VTIP-A1034-PCT Customer No. 95,450 an efficient way of supplementing the enabling disclosure of this invention as well as provide background detailing the level of ordinary skill in the art.

Claims

Attorney Docket No. VTIP-A1034-PCTCustomer No. 95,450CLAIMS1. A method for depolymerizing polycaprolactam-containing material comprising: contacting a poly caprolactam material with stripping steam in a depolymerization reactor to produce a vapor product comprising water and caprolactam; separating the vapor product to provide a water product and a caprolactam product; purifying the caprolactam product, such as to produce a polymer grade quality caprolactam; and optionally purifying the water product to remove one or more impurities such that the purified water product can be recycled to the depolymerization reactor.

2. The method of claim 1, wherein the polycaprolactam material is obtained from extracted oligomer from nylon 6 production, scrap polymer from manufacturing processes, post-consumer waste nylon, or some mixture of different caprolactam comprising materials.

3. The method of claim 1 or 2, wherein a catalyst, such as H3PO4, H3BO3, NaOH, or other steam stripping catalyst, is added to the depolymerization reactor.

4. The method of claim 3, wherein the vapor product contains between 15 wt% and 65 wt% caprolactam, between 30 wt% and 85 wt% water, and between 0.1 wt% and 5 wt% volatile impurities, such as ammonia, volatile degradation byproducts, aminocaproic acid, and / or a cyclic dimer of caprolactam.

5. The method of any of claim 4, wherein the separation of the vapor product is achieved by distillation performed by feeding the vapor product, optionally by way of a vapor product feed, into a distillation column which is operably connected to a condenser.

6. The method of claim 5, wherein: the distillation column lacks a reboiler; the vapor product is introduced into the bottom of the distillation column; and / or a vapor distillate is yielded, such as from the top of the distillation column, wherein the vapor distillate comprises the water product.Attorney Docket No. VTIP-A1034-PCTCustomer No. 95,4507. The method of claim 5, wherein the distillation column contains structured packing internals that are capable of providing minimum liquid hold up and an average liquid hold up time of less than 1 minute, such as less than 30 seconds or less than 15 seconds.

8. The method of claim 5, wherein the pressure of the distillation column is between 100 kPa and 2000 kPa.

9. The method of claim 5, wherein the pressure of the distillation column is between 10 kPa and 1,000 kPa lower than the pressure of the depolymerization reactor.

10. The method of claim 5, where the condenser operates to generate a reflux rate in the distillation column to control the content of caprolactam in the vapor distillate to between 1 ppmw and 1,000 ppmw.

11. The method of claim 5, where the condenser operates to generate a reflux rate in the distillation column to control the concentration of water in a liquid exiting the bottom of the distillation column to between 1 wt% and 30 wt%.

12. The method of claim 5, wherein a cooling fluid is supplied to the condenser to generate a heating utility, such as steam, that can supply heat elsewhere in the process at a temperature between 90 °C and 190 °C.

13. The method of claim 5, wherein a liquid collects in a pool in the bottom of the distillation column, such as below the vapor product feed, optionally having an average holdup time between 1 minute and 1 hour.

14. The method of claim 13, wherein the liquid pool in the distillation column bottom is cooled to a temperature between 100 °C and 160 °C.Attorney Docket No. VTIP-A1034-PCT Customer No. 95,45015. The method of claim 14, wherein the cooling is performed in whole or part by a heat transfer element inserted into the distillation column, wherein the heat transfer element is completely or partially submerged in the pool.

16. The method of claim 14, wherein the cooling is performed in whole or part by withdrawing liquid from the pool and using a pump to circulate the liquid through a cooling heat exchanger located outside the distillation column before returning the cooled liquid to the pool.

17. The method of claim 15, wherein the heat transfer element and / or the heat exchanger comprise a cooling fluid capable of removing heat from the pool and capable of generating a heating utility, such as steam, that can supply heat elsewhere in the process at a temperature between 90 °C and 150 °C.

18. The method of claim 13, wherein an inert vapor is injected, such as by way of an inert vapor feed, above the pool and below the vapor product feed to prevent the vapor product feed from contacting the pool.

19. The method of claim 18, wherein the inert vapor is any compound or mixture that does not condense at the temperature of the pool and the pressure of the distillation column.

20. The method of claim 19, wherein the inert vapor is nitrogen.

21. The method of claim 18, wherein the inert vapor maintains a superficial velocity between 0.01 ft / s and 3 ft / s at some point in the distillation column between the pool and the vapor product feed.

22. The method of claim 18, wherein a cross-sectional area reducing element is located above the inert vapor feed and below the vapor product feed.Attorney Docket No. VTIP-A1034-PCT Customer No. 95,45023. The method of claim 22, wherein the cross-sectional area reducing element reduces the cross-sectional area to between 20% and 99% of the distillation column above the reaction vapor product feed.

24. The method of claim 6, wherein at least a portion of the vapor distillate from the distillation column goes to a compressor.

25. The method of claim 24, wherein the compressor increases the pressure of the vapor distillate to between 10 kPa and 100 kPa above the operating pressure of the depolymerization reactor.

26. The method of claim 24, wherein the compressor has a compression ratio between 1.05 and 2.5.

27. The method of claim 24, wherein an outlet of the compressor is heated above the operating temperature of the depolymerization reactor and recycled as steam for the depolymerization reactor.

28. The method of claim 6, wherein at least a portion of the vapor distillate is a split fraction of the vapor distillate from the distillation column and is directed to a water purification unit.

29. The method of claim 28, wherein the water purification unit removes between 90% and 99.9% of impurities more volatile than water.

30. The method of claim 28, wherein the water purification unit removes between 90% and 99.9% of impurities less volatile than water.31 . The method of claim 28, wherein purified water produced from the water purification unit is vaporized and combined with vapor from an outlet of the compressor.Attorney Docket No. VTIP-A1034-PCTCustomer No. 95,45032. The method of claim 28, wherein the split fraction of vapor distillate from the distillation column going to the water purification unit is used to control the purity of the stripping steam used in the depolymerization reactor.

33. The method of claim 1 , wherein the stripping steam comprises water at a concentration that is controlled to between 95 wt% and 99.9 wt%.

34. A system configured to perform the method of claim 1.

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