Apparatus for heating polymer-containing stream

Abstract

The utility model relates to equipment for heating polymer-containing material flow. Specifically, an apparatus for heating a polymer-containing stream transferred from a polymerization reactor to a gas separation vessel, the apparatus comprising a polymerization reactor, a gas separation vessel, and a plurality of parallel heaters through which the polymer-containing stream can be transferred from the reactor to a single gas separation vessel, each heater comprises a transfer line for the flow and means for heating the transfer line, in which three parallel heaters, which are open between the polymerization reactor and the gas separation vessel, and a fourth heater, which is closed between the polymerization reactor and the gas separation vessel, are provided, and in which the third heater is closed between the polymerization reactor and the gas separation vessel. The ratio Do / Di of the outlet diameter Do to the inlet diameter Di of each transfer line is between 1.2 and 10.

Description

Technical Field

[0001] This invention relates to an apparatus for improving the degassing of polymers, particularly olefin polymers. Background Technology

[0002] Equipment used for the polymerization of olefins is well known, in which olefin monomers, and optionally olefin comonomers, are often polymerized in the presence of a catalyst and / or diluent. The polymer is extracted from the polymerization reactor along with reagents and inert hydrocarbons. For economic, safety, and environmental reasons, the reagents and hydrocarbons need to be recovered. Equipment used for these steps typically involves depressurization and volatilization of the polymer stream after extraction from the polymerization reactor. Volatilization requirements are highest in processes where the polymer extracted from the reactor has a high content of absorbed or free liquid hydrocarbons. These are typically processes for producing polymers with significantly low-density components or amorphous phases (where hydrocarbon absorption is high) and / or processes for producing polymers in the presence of liquid hydrocarbons (active or inert).

[0003] Over the years, the maximum capacity of commercial-scale plants has steadily increased, and with this increased productivity, the potential cost impact of unreliability in any part of the process has also increased significantly, affecting not only the polymer unit itself but also upstream and downstream units. Simultaneously, growing operational experience has led to operations with increasingly higher solids concentrations (loadings) of the polymer extracted from the reactor. Increased solids concentrations in slurry polymerization units are typically achieved by increasing circulation rates, for example, through higher reactor power requirements, as illustrated by EP432555 and EP891990. This increase in solids loading is desirable because it increases reactor residence time for a fixed reactor volume and also reduces downstream diluent handling and recycling requirements. However, product transfer becomes more problematic at high solids loadings, and careful design and operating practices are required to avoid polymer fouling and clogging issues that would not occur at lower solids loadings.

[0004] During the depressurization and devolatization of the polymer stream extracted from the polymerization reactor, and due to this depressurization and devolatization, the polymer temperature decreases. It is well known that the process of devolatization and desorption from polymers is significantly enhanced by maintaining the polymer at the highest possible temperature. Therefore, in slurry loop processes, the transfer line between the polymerization reactor and the gas separation (degassing) vessel for the polymer stream is often heated. As an example of a typical process, in WO04 / 031245 and WO05 / 044871, the take-off line from the loop polymerization reactor includes a flash line containing the extracted slurry, which is surrounded by a conduit supplied with a heating fluid, such as low-pressure steam, to provide indirect heating of the slurry. However, it is also well known that the viscosity and susceptibility of the polymer transferred, causing agglomeration and / or scaling in the transfer lines and vessels, generally increases with temperature, and as mentioned above, the problem of scaling or agglomeration becomes more pronounced with increasing solid loads in transfer systems currently in use. Therefore, the transfer lines from the polymerization reactor need to be carefully designed to achieve sufficient heating to aid in the removal of volatiles without the risk of solid polymer scaling or clumping.

[0005] The devolatilization of the polymer stream causes the liquid phase of the stream to evaporate and generate a gas phase, resulting in an increase in volume and a corresponding increase in flow velocity in the transfer line. However, if the velocity becomes too high, it can exceed the speed of sound (the speed of sound in the medium), causing flow interruption. On the other hand, if the initial velocity is too low, there is an increased risk of solid polymer fouling or agglomeration as mentioned above.

[0006] Another consideration is that in large plants, transfer lines must be very long to allow for adequate heating, and their length can be significant enough to impact plant space planning. This presents various challenges, such as the footprint of hardware within the plant and the control of conditions within the lines. Often, a large portion of the transfer line length needs to be heated to meet heat input requirements. Therefore, it will be recognized that ensuring the polymer flow reaches the gas separation vessel at the desired temperature and pressure with minimal fouling / caking is a significant technical challenge. Utility Model Content

[0007] This invention provides an apparatus that can provide optimized heating to polymers during transfer from a reactor to a gas separation vessel while maintaining reliable product transfer, particularly in processes with multiple parallel heating transfer lines between the polymerization reactor and the gas separation vessel.

[0008] Therefore, in a first aspect, the present invention provides an apparatus for heating a polymer-containing stream transferred from a polymerization reactor to a gas separation vessel, the apparatus comprising a polymerization reactor, a gas separation vessel, and a plurality of parallel heaters, through which the polymer-containing stream can be transferred from the reactor to a single gas separation vessel. Each heater includes a transfer line for the stream and a device for heating the transfer line. Three parallel heaters are provided that are open between the polymerization reactor and the gas separation vessel, and a fourth parallel heater is provided that is closed between the polymerization reactor and the gas separation vessel. Furthermore, the outlet diameter D of each transfer line is... o Its inlet diameter D i The ratio D o / D i Between 1.2 and 10. Attached Figure Description

[0009] Figure 1 The apparatus according to the present invention is schematically shown for heating a polymer-containing stream transferred from a polymerization reactor to a gas separation vessel. Detailed Implementation

[0010] It will be appreciated that, when using the equipment, the polymer-containing stream is continuously conveyed through some form of piping from the moment it leaves the polymerization reactor until it enters the gas separation vessel. To achieve the objectives of this invention, at least three parallel heaters are provided that are open between the polymerization reactor and the gas separation vessel, and at least one parallel heater is closed between the polymerization reactor and the gas separation vessel. In this document, the term "heater" as used herein may include, within its scope, multiple heating sections connected in series. In this invention, each heater includes a transfer line, which is part of a conduit that connects the reactor to the gas separation vessel and can be heated. Thus, the heater / transfer line is a section of conduit extending from the beginning of a heating section (or, if more than one, the first of the heating sections) to the end of the heating section (or the last of the heating sections). It will be appreciated that there may be sections of the heater / transfer line that are not heated during use. For example, typically, the heater / transfer line may include a series of heating sections connected by non-heated sections (e.g., bends and elbows). These are still part of the heater / transfer line as defined herein. It will also be recognized that sections of the pipe before the start of a heating section (or, if there is more than one, the first of the heating sections) and after the end of a heating section (or the last of the heating sections) are typically not provided with individual heating. (In use, they can, of course, be at high temperatures due to the heat transferred through the flow through the pipe.)

[0011] In the equipment of this utility model, the outlet diameter D of each transfer pipeline is... o Its inlet diameter D i The ratio D o / D i The ratio is between 1.2 and 10. Typically, the ratio is at least 1.3, and usually at least 1.4. However, the ratio is preferably no more than 4, more preferably no more than 2, and most preferably a maximum of 1.9.

[0012] We found that increasing the diameter along the length of the transfer line allows the heater to accommodate a wider range of polymer-containing flow rates. Specifically, a relatively small diameter at the inlet allows for relatively high velocities even at low flow rates, thus reducing the risk of fouling; while a relatively large diameter at the outlet avoids the risk of velocities exceeding the speed of sound even at high flow rates. This capacity range is particularly valuable during start-up and shutdown operations. To further reduce the risk of downstream blockage, the outlet diameter D of the transfer line is also preferred. o Smaller than the solid outlet of the gas separation container. (D) o Defined as the inner diameter of the transfer pipeline at its outlet, and D i It is the inner diameter of the transfer pipeline at its inlet, wherein the outlet and inlet of the transfer pipeline are defined as described above.

[0013] The inner diameter D of each transfer pipeline at its inlet i Preferably at least 20 mm, and more often at least 40 mm, for example between 40 and 200 mm, or between 50 and 100 mm. As already mentioned, the inner diameter D of each transfer line at its outlet... o It can be determined by its relationship with diameter D i The ratio is limited. In absolute terms, it is preferably less than 200 mm, and more preferably less than 150 mm, for example, between 75 and 150 mm. Each transfer line can also be characterized by its average inner diameter between its inlet and its outlet, which we will refer to as D. ave D ave Preferably, the diameter is at least 20 mm, and more preferably at least 40 mm, for example, between 60 and 150 mm. The most preferred average inner diameter is between 60 mm and 100 mm.

[0014] As mentioned, a relatively small diameter at the inlet allows for relatively high velocities even at low flow rates, thus reducing the risk of scaling; while a relatively large diameter at the outlet avoids the risk of velocities exceeding the speed of sound even at high flow rates. In use, this device allows the average velocity of the polymer-containing flow to be maintained sufficiently high to avoid the risk of scaling or clogging. We have found that the design of the various transfer lines according to this invention allows for reliable operation while maintaining V o / Vi The ratio is controlled within preferred limits. For V i Typical values ​​are 3-20 m / s, and for V o The typical value is 30-80 m / s.

[0015] Preferably, each heater is at least 20m in length.

[0016] The length L of each heater and therefore each transfer line is preferably at least 30m, more preferably at least 50m, but often does not exceed 800m. A preferred length range is 100m to 500m, more preferably 200m to 450m.

[0017] Preferably, the length L of the transfer pipeline is proportional to its average inner diameter D. ave The ratio L / D ave The value is 500 to 10,000, preferably 1,500 to 7,000, and more preferably 2,000 to 5,000. For a transfer pipeline consisting of multiple sections, each with a different diameter, D ave It is the average inner diameter of those sections, weighted by their lengths. Alternatively, it can be calculated by referring to the total internal volume V of the pipeline, where V = (πD) ave 2 .L) / 4.

[0018] Preferably, the increase in the inner diameter of the transfer line occurs in discrete steps rather than continuously. Typically, there are one, two, or three increases in diameter along the length of the transfer line.

[0019] Preferably, one or all of the sections of the transfer lines are installed substantially vertically (or vertically) rather than horizontally, resulting in a smaller footprint in the plant: in this configuration, the first section of the line preferably has its inlet at the bottom. Thus, in use, the initial flow of material through the transfer lines is upward. Preferably, less than 20%, most preferably less than 10%, of the length of each transfer line is horizontal, and best of all, the lines are configured to have substantially no horizontal sections. In one embodiment, at least the inlets and outlets of each heated transfer line are vertically oriented, such that flow through the inlet is upward and flow from the outlet is downward. In one embodiment of the invention, each transfer line includes a series of sections connected by elbows, typically U-shaped, causing the line to fold back one or more times. The advantage of this configuration is that it makes the transfer lines more compact in the plant. The sections between the elbows are often straight. The elbows can be heated like the rest of the line, but often they are not heated to simplify the configuration of the heater. It is also generally preferred that any expansion of the pipeline diameter occurs in the unheated section of the pipeline; therefore, the pipeline sections can have different diameters, with the diameter increase occurring at one or more elbows, preferably at the elbow outlet, so that the velocity decreases at the elbow outlet rather than at its inlet, and most preferably at the elbow outlet at the top of the vertically heated section. The design of the expansion sections and elbows in the transfer pipeline is crucial for reliable operation without scaling. The number of vertical or horizontal sections between the elbows that make up the various main transfer pipelines can be 2 to 10, but 3 to 7 sections are more common.

[0020] The elbows of transfer lines can have varying degrees of curvature. The radius of the curve defined by the elbow can be expressed as a multiple of the line's diameter D at that point. Elbows typically have radii between 3D and 30D, with 5D-20D being most preferred to ensure reliable operation without scaling while minimizing the line's footprint. As previously mentioned, elbows are preferably U-shaped, but alternatives allowing for smooth flow paths, such as L-shaped elbows, are not excluded. Clearly, transfer lines formed by segmentation can employ a mixture of elbows of the types described above, or indeed elbows at other angles, such as 60° or 120°.

[0021] It has been found that the length of any expansion section of the transfer pipeline should be greater than 0.25D, preferably between 0.5D and 10D, and most preferably between 0.75D and 3D. Preferably, the expansion sections are located directly upstream or downstream of the elbow, preferably directly downstream of the elbow. It is also preferred that the expansions be concentric, but other expansion geometries are also feasible.

[0022] The total specific heat transfer area of ​​all transfer pipelines, i.e., the outer surface area in contact with the heating device, is preferably less than 2m² per ton / hour of polymer production. 2 The heat transfer area is typically between 0.1 and 1 m². 2 More preferably, the polymer production per ton / hour is between 0.2 and 0.5 m³ / h. 2 The heat transfer area between them.

[0023] Preferably, for each heater, the heater inlet is at a higher height than the outlet of the polymerization reactor to which it is connected.

[0024] Preferably, for each heater, the outlet of the transfer line (at the gas separation vessel inlet) is at a higher height than the inlet of the transfer line and / or the outlet of the polymerization reactor.

[0025] Devices used for heating individual transfer lines often include a jacket surrounding the line. Preferably, the heater jacket is in the form of a concentric tube surrounding the line. In use, the heating fluid is conveyed through the jacket. The most commonly used heating fluid is steam. Regardless of its form, the jacket can provide the same heat input along the entire length of the transfer line, or it can provide differential heating at different sections of the line. It is also possible, as discussed above, that sections of the line (e.g., bends) are not heated. We have found that the optimal heat input along the length of the transfer line of the active heater is achieved using a design such that the temperature of the heating medium (or the inner wall temperature of the line) is higher at the inlet of the line than at its outlet. Therefore, in use, and as the vapor fraction of the polymer-containing flow increases as it travels along the line, it is preferable to reduce the temperature of the heating medium (or the inner wall temperature of the line). This can be done in a continuous, stepped manner, or in multiple discrete steps through sections of different temperatures. However, it is most preferred that the jacket operates at different temperatures in different sections of the line, often by providing independent heating medium supplies for the sections requiring different temperatures.

[0026] In operation, the individual transfer lines in the active heater can be heated by concentric tube jackets using steam as the heating medium. The outlet temperature of the individual transfer lines in the active heater is preferably controlled using the steam flow rate: for a given steam temperature, this has the benefit of being able to control the transfer line wall temperature, ensuring lower temperatures at low polymer flow rates and higher temperatures at higher flow rates at higher velocities.

[0027] Providing a solids concentration device upstream of the transfer line is a preferred embodiment of the device of this invention. An example of such a device is described in our patent EP1118624. In use, this allows the monomer concentration in the transfer line to be minimized, thereby reducing the risk of scaling.

[0028] Preferably, the transfer line is easily detachable along the length of the heater for easy cleaning. Preferably, the transfer line is fitted with flanges at 5-15m intervals. When using a jacket containing the heating fluid for heating, it is preferable that the heating fluid does not cover any flanges.

[0029] The tube is preferably made of material with a strength greater than 30 W / m. -2 K -1 Preferably greater than 40Wm -2 K -1 The tubes are made of materials with high thermal conductivity. They are typically seamless, but welded tubes are preferred in applications requiring high heat transfer.

[0030] In use, heating is typically applied such that the temperature of the polymer-containing flow at the heater outlet in each active heater is higher than the dew point of the non-polymer portion of the flow, and the temperature of the flow along the length of each active heater remains below the softening point of the polymer, wherein the softening point of the polymer is defined as the Vicat softening temperature under a 10 N load according to ASTM D1525, ISO 306.

[0031] Preferably, the temperature of the polymer-containing flow at the outlet of each active heater is 5-80°C higher than the dew point of the flow, and most preferably 10-30°C higher.

[0032] Preferably, the temperature of the inner surface of the transfer line along the length of each active heater is kept below the softening point of the polymer.

[0033] By maintaining the temperature of the polymer-containing flow at each heater outlet / outlet above the flow's dew point, but with the inner surface of the transfer line below the polymer's softening point, it is possible to ensure that all liquid in the flow has evaporated by the time it reaches the heater outlet, while simultaneously minimizing the risk of fouling. The temperature of the inner surface of the transfer line can also be maintained above the reactor temperature.

[0034] In use, the polymer-containing stream is extracted from the polymerization reactor before entering the heaters. Therefore, in the apparatus of this invention, each heater inlet is directly connected to the polymerization reactor, and each heater outlet is directly connected to a gas separation vessel. The gas separation vessel can and often is located upstream of the final polymer processing and extrusion section or upstream of another polymerization reactor.

[0035] During use, the typical pressure P at the heater inlet i The pressure at the outlet is 15-30 bara. o(This is also the pressure at the inlet to the gas separation vessel) typically 1.5-12 bara, preferably at least 5 bara, for example 5-12 bara, and most preferably 5-11 bara. In use, the pressure drop in each active heater is preferably at least 1000 kPa. Preferably, the pressure drop in each heater is in the range of 1000 kPa to 3000 kPa, for example 1000 kPa to 2000 kPa.

[0036] In this invention, a single common gas separation container has multiple parallel heaters connected to it.

[0037] The pressure drop in each active heater is typically between 25% and 95% of the total pressure drop between the polymerization reactor and the inlet to the gas separation vessel, preferably between 40% and 90%. More generally, preferably, a pressure control valve is provided between the polymerization reactor and the gas separation vessel, and more typically before the heater. In use, the polymer-containing flow is conveyed through this valve, which introduces a pressure drop. This pressure drop is typically between 5% and 75% of the total pressure drop between the polymerization reactor and the inlet to the gas separation vessel, preferably between 10% and 60%. The typical pressure drop across the pressure control valve is between 2 and 30 bar, more often between 5 and 25 bar. Preferably, each of the plurality of parallel heaters has a separate pressure control valve located in the conduit between the polymerization reactor and the heater. (In this case, the pressure drop across the pressure control valve is the reactor outlet pressure plus the pressure P at the inlet of the subsequent active heater.) i The difference between them.

[0038] In operation, the performance of each transfer line can be monitored using the following parameters: steam flow rate at the location of the heating jacket or steam valve to measure the heat input (load) to the flow; pressure difference across the heater and reactor pressure valve output to measure the flow rate or flow ratio entering each transfer line; the relationship between steam flow rate and outlet temperature for each heater; reactor mass balance to calculate the total flow rate entering all heaters; and the difference between the steam temperature at the heater outlet and the dew point of the process flow. The pressure drop across each heater's transfer line is preferably substantially the same.

[0039] In this invention, a fourth heater is provided that is shut off between the polymerization reactor and the gas separation vessel. This can be put into use if needed, and particularly when at least one of the active heaters must be shut down.

[0040] The device of this invention can be used in any polymerization process (e.g., gas phase, slurry, or solution) containing a polymer stream that requires heating during decompression to cause liquid evaporation.

[0041] Preferably, the apparatus is used to heat a polymer-containing stream (slurry) extracted from a slurry-phase polymerization process. Processes for olefin copolymerization in a slurry phase are well known in the art. For example, such processes can be carried out by introducing monomers and comonomers into a stirred tank or a continuous loop reactor containing polyolefins and a catalyst for polymerization. The reactor is typically controlled to achieve the desired melt index and density for the polymer at optimal production and temperature.

[0042] Polyethylene slurry polymerization processes typically extract polymers from a polymerization reactor while generating significant amounts of liquid hydrocarbons, and therefore the equipment of this invention is particularly suitable for such processes. The slurry in such reactors typically comprises particulate polymer, hydrocarbon diluent, (copolymer)monomer, catalyst, chain terminator (e.g., hydrogen), and other reactor additives. Specifically, the slurry will contain 20-75% by weight, preferably 30-70% by weight, of particulate polymer based on the total weight of the slurry, and 80-25% by weight, preferably 70-30% by weight, of a suspension medium, which is the sum of all fluid components in the reactor, and includes a diluent, olefin monomer, and any additives; the diluent can be an inert diluent or it can be an active diluent, such as liquid olefin monomer. When the primary diluent is an inert diluent, the olefin monomer typically constitutes 2-20% by weight of the slurry, more specifically 4-10% by weight.

[0043] Polymerization in slurry processes is typically carried out at temperatures ranging from 50 to 125°C and pressures ranging from 10 to 100 bara. The catalyst used can be any catalyst typically used for olefin polymerization, such as chromium oxide, Ziegler-Natta, or metallocene catalysts. The product slurry, containing the polymer and diluent, and in most cases the catalyst, olefin monomers, and comonomers, can be discharged intermittently or continuously, optionally using a thickening device, such as a hydrocyclone or settling leg, to minimize the amount of fluid extracted along with the polymer.

[0044] This invention specifically relates to a polymerization reactor in a loop reactor, wherein a slurry is circulated within the reactor by a pump or agitator. Liquid full-loop reactors are particularly well known in the art and are described, for example, in US3152872, US3242150, and US4613484. The loop reactor has a continuous tubular configuration, comprising at least two (e.g., four) vertical sections and at least two (e.g., four) horizontal sections. Heat of polymerization is typically removed by indirect exchange with a cooling medium (preferably water) in a jacket surrounding at least a portion of the tubular loop reactor. The volume of the loop reactor can vary, but is typically between 20 and 170 m³. 3Within the range.

[0045] In commercial plants, particulate polymers are separated from diluents in such a manner that the diluent is not exposed to contamination, allowing it to be recycled back to the polymerization zone with minimal remediation (if any). Separation of particulate polymers from diluents can typically be achieved by any method known in the art; for example, it can involve (i) the use of discontinuous vertical settling legs, such that the slurry flow across their openings provides a region from which polymer particles can settle from the diluent, or (ii) continuous extraction of the product via one or more extraction ports, the location of which can be anywhere on the loop reactor, but often adjacent to the downstream end of the horizontal section of the loop. As discussed above, operation of large-diameter reactors with high solids concentrations in the slurry minimizes the amount of primary diluent extracted from the polymerization loop. The recovery of diluents is further enhanced in an energy-efficient manner by using a concentration device, such as a hydrocyclone (single or in the case of multiple hydrocyclones, in parallel or series), on the extracted polymer slurry, as significant pressure drops and evaporation of the recovered diluent are avoided.

[0046] The extracted, and preferably concentrated, polymer slurry is often depressurized before being transferred via the heater of this invention to a gas separation vessel (also often referred to herein as a flash vessel).

[0047] The diluent and any monomer vapors recovered in the flash vessel are typically condensed, preferably without recompression, and reused in the polymerization process. The pressure in the flash vessel is often controlled to allow substantially all flash vapors to be condensed with an readily available cooling medium (e.g., cooling water) before any recompression. Specifically, the pressure is typically in the range of 2-25 bara, more typically at least 5 bara, for example 5-20 bara, and most commonly 5-11 bara. Solid material recovered from the flash vessel is often transferred to a second flash vessel or a washing vessel to remove residual volatiles. Alternatively, the slurry may be transferred to a flash vessel with a lower pressure than the flash vessels mentioned above, requiring recompression to condense the recovered diluent. The use of a high-pressure flash vessel is preferred.

[0048] More specifically, an example of a polymerization process type to which this invention is particularly applicable is the continuous polymerization of olefins (preferably α-monoolefins) in a reaction zone (preferably an elongated tubular closed loop). The olefins are continuously added to a hydrocarbon diluent and contacted with a catalyst within the hydrocarbon diluent. The monomers are polymerized to form a slurry of solid particulate polymer suspended in a polymerization medium or diluent. The extraction rate of the polymer product is controlled by one or more valves upstream of the active heater of this invention.

[0049] The solids concentration in the slurry in the reactor will typically be higher than 15% by volume, for example 15-35% by volume, preferably 18-30% by volume, where volume% is [(total volume of slurry - volume of suspension medium) / (total volume of slurry)] x 100. The solids concentration measured as a weight percentage (equivalent to the solids concentration measured as a volume percentage) will vary depending on the polymer produced, but more specifically on the diluent used. When the polymer produced is polyethylene and the diluent is an alkane (e.g., isobutane), it is preferred that the solids concentration based on the total weight of the slurry be higher than 30% by weight, particularly higher than 35% by weight, for example in the range of 35-50% by weight. We have found that for high solids loadings, particularly above 40% by weight, reliable product extraction and heating (as demonstrated by scaling, flow rate variations, and / or heat transfer) between the polymerization reactor and the gas separation vessel can be maintained within acceptable operating limits using the equipment of this invention.

[0050] Slurry polymerization processes can be carried out in multi-reactor systems. The second reactor or any subsequent reactor in a multi-reactor system can be another loop reactor or any reactor used for olefin polymerization, such as a fluidized bed reactor. However, the second reactor or any subsequent reactor in a multi-reactor system is often another loop reactor. Such multi-reactor systems can be used to produce monophasic or multiphasic, preferably multiphasic, polymers.

[0051] In the case of multiple reactors in series, in addition to diluent and monomer, the first reactor in the series is supplied with a catalyst or prepolymer and optionally a co-catalyst, and each subsequent reactor is supplied with at least monomer (particularly ethylene) and a slurry from the previous reactors in the series, the mixture comprising a mixture of catalyst and polymer produced in the previous reactors in the series. It is feasible to supply fresh catalyst and / or co-catalyst to at least one of the second reactor and / or (if appropriate) subsequent reactors. However, it is more common to introduce the catalyst and co-catalyst separately into the first reactor.

[0052] In plants comprising at least two reactors in series, the polymer with the highest melt index and the polymer with the lowest melt index can be produced in two adjacent or non-adjacent reactors in series. Hydrogen is maintained at (i) a low (or zero) concentration in the reactor producing the high molecular weight component, for example, a hydrogen percentage between 0 and 0.1 vol%, and at (ii) a very high concentration in the reactor producing the low molecular weight component, for example, a hydrogen percentage between 0.5 and 2.4 vol%. The reactors can be operated similarly to produce substantially the same polymer melt index in consecutive reactors.

[0053] When such reactor systems produce polymers with molecular weights less than 50 kDalton or greater than 150 kDalton, particular problems of reactor scaling and agglomeration have been observed in the heater between the polymerization reactor and the gas separation vessel. These problems can be exacerbated by high polymer solids concentrations in the heater. This is another problem that can be improved by using the device of this invention.

[0054] This utility model can be referenced. Figure 1 To explain, Figure 1 An apparatus for heating a polymer-containing stream transferred from a polymerization reactor (1) to a gas separation vessel (2) is shown. The apparatus includes the polymerization reactor (1), the gas separation vessel (2), and a plurality of parallel heaters through which the polymer-containing stream can be transferred from the reactor (1) to the gas separation vessel (2). Each heater includes a transfer line for the stream, which includes valves (4a, 4b, 4c, 4d) and devices (3a, 3b, 3c, 3d) for heating the transfer line. Specifically, three parallel heaters are provided that are open between the polymerization reactor (1) and the gas separation vessel (2) due to the presence of three open valves (4a, 4b, 4c), and a fourth heater is provided that is closed between the polymerization reactor (1) and the gas separation vessel (2) due to the presence of a closed valve (4d).

[0055] Example 1

[0056] Operation as follows Figure 1 The apparatus shown is used to polymerize ethylene and hexene comonomers in isobutane diluent to produce a slurry containing a polyethylene copolymer.

[0057] Four parallel extraction lines are provided, each including a pressure control valve, followed by a heater, and each extraction line is individually connected to the lower part of the polymerization reactor. Three of the lines are open, while the fourth is closed.

[0058] In this specific example, each heater is 382m in length and comprises nine straight and vertical sections, each 38m long, connected by 180° elbows; the elbows together account for 40m of the total length and are not heated. The first two vertical sections are formed by 3" (76.2mm) outer diameter tubes, and the remaining sections are formed by 4" (101.6mm) outer diameter tubes. The inner diameters of these sections are 71.2mm and 95.6mm, respectively, thus D o / D i The value is 1.34. The average inner diameter across each heater is 0.09 m, and the L / D ratio is 4244.

[0059] In operation, in each open line, the slurry from the reactor is pressure-reduced by passing through a pressure control valve before entering the heater. The slurry is at a pressure of 45 bara when it enters the extraction line (i.e., before the pressure control valve). The discharged polymer-containing stream is initially in slurry form with a solids content of approximately 40% by weight, and its liquid composition includes 91 mol% isobutane, 8 mol% unreacted ethylene, and 1 mol% unreacted hexene.

[0060] The pressure drop across each pressure control valve (in each operating line) is 14 bar, and the pressure at the heater inlet is 30 bar.

[0061] At the outlet of each heater, the pressure P o The pressure is 10 barg, which corresponds to the pressure in the gas separation vessel. Here, the slurry liquid completely evaporates, meaning the stream containing polyethylene solids and evaporated liquid enters the gas separation vessel. This is equivalent to a pressure drop of 20 bar across the heater, corresponding to 0.052 bar / m, and also to 59% of the total pressure drop between the reactor and the gas separation vessel.

Claims

1. An apparatus for heating a polymer-containing stream, said apparatus comprising a polymerization reactor, a gas separation vessel, and a plurality of parallel heaters through which said polymer-containing stream can be transferred from said polymerization reactor to a single gas separation vessel, each heater comprising a transfer line for said polymer-containing stream and means for heating said transfer line, wherein, The system provides three parallel heaters that are open between the polymerization reactor and the gas separation vessel, and a fourth heater that is closed between the polymerization reactor and the gas separation vessel, wherein the outlet diameter D of each transfer line is... o Its inlet diameter D i The ratio D o / D i Between 1.2 and 10.

2. The apparatus of claim 1, wherein, Each heater is at least 20 m in length.

3. The apparatus of claim 1, wherein, The outlet of each transfer line is directly linked to the single gas separation vessel.

4. The apparatus of claim 1, wherein, A pressure control valve is provided before each heater.

5. The apparatus of claim 1, wherein, The internal diameter D of each transfer line is at least 20 mm.

6. The apparatus of claim 1, wherein, The length L of each heater, and thus of each transfer line, is at least 30 m.

7. The apparatus of claim 6, wherein, The length is 100 m to 500 m.

8. The apparatus of claim 1, wherein, The length L of the transfer pipeline and its average inner diameter D ave The ratio L / D ave The range is from 500 to 10,000.

9. The apparatus of claim 1, wherein, The diameter increases along the length of the slurry heater in discrete steps rather than continuously.

10. The apparatus of claim 1, wherein, One or all of the sections of the transfer line are upright rather than horizontally mounted.

11. The apparatus of claim 10, wherein, The first section of the transfer line has its inlet at the bottom, so that the initial flow of material through the transfer line is upwards.

12. The apparatus of claim 1, wherein, Less than 20% of the length of each transfer line is horizontal.

13. The apparatus of claim 1, wherein, Each transfer line comprises a series of sections connected by bends (elbows), typically U-shaped, so that the transfer line folds back on itself one or more times.

14. The apparatus of claim 13, wherein, The number of vertical or horizontal sections between the elbows making up each total transfer line can be between 2 and 10.

15. The apparatus of claim 1, wherein, The internal diameter D of each transfer line is between 40 mm and 200 mm.

16. The apparatus of claim 1, wherein, The length L of each heater, and thus of each transfer line, is at least 50 m.

17. The apparatus of claim 16, wherein, The length is 200 m to 450 m.

18. The apparatus of claim 1, wherein, The length L of the transfer pipeline and its average inner diameter D ave The ratio L / D ave The range is from 1,500 to 7,000.

19. The apparatus of claim 1, wherein, The length L of the transfer pipeline and its average inner diameter D ave The ratio L / D ave The range is 2000 to 5000.

20. The apparatus of claim 1, wherein, Less than 10% of the length of each transfer line is horizontal.

21. The apparatus of claim 20, wherein, The transfer line is configured without horizontal sections.

22. The apparatus of claim 13, wherein, The number of vertical or horizontal sections between the elbows making up each total transfer line can be between 3 and 7.

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