Polymer stream transfer

The invention relates to a device for improving the degassing of polymers, in particular olefin polymers.

[0002] The polymerization of olefin in which the olefin monomer and optional olefin comonomer are polymerized, usually in the presence of a catalyst and / or diluent, is well known. The polymer is discharged from the polymerization reactor together with reactants and inert hydrocarbons. Reagents and hydrocarbons should be recovered for economic, safety and environmental reasons and many ways to achieve this are known in the art. These methods generally involve expanding and removing volatiles from the polymer-containing stream after it has been discharged from the polymerization reactor. The evaporation requirements are greatest in the methods in which the polymer discharged from the reactor has a high absorbed or free content of liquid hydrocarbons.

[0003] The maximum efficiency of commercial scale installations has steadily increased over the years, and as the production rate has increased, the potential cost contribution due to unreliability in any part of the process has also increased, affecting not only the polymer unit itself but also the units above and below . At the same time, the increasing experience in processing led to the treatment of increasingly higher concentrations (loads) of solids in the polymer discharged from the reactors. The increase in solids concentration in the suspension polymerization units is usually achieved with increased circulation speed, achieved, for example, by a reactor with a higher power demand, as illustrated for example in EP 432555 and EP 891990. This increase in solids loading is desirable, because it increases the residence time in the reactor for a constant volume of the reactor, and also reduces the subsequent treatment of the diluent and the recycle requirements. However, the transfer of a product with high solids loads is more problematic and precise design and processing methods are required to avoid polymer sticking and blocking problems that are not experienced with a lower solid load.

[0004] During and as a result, the method of expanding and removing volatiles from the polymer stream discharged from the polymerization reactor, the polymer temperature is lowered. It is well known that the method of removing volatiles and desorbing the polymer is significantly enhanced by keeping the polymer at as high a temperature as possible. Thus, in the suspension loop processes, a transfer line between the polymerization reactor and the expansion tank (degassing) stream of the polymer is usually heated. As an example of a typical method in WO 04/031245 and

WO 05/044871 a receiving line from a loop polymerization reactor has an outflow line carrying the removed slurry, surrounded by a conduit which receives a heated fluid, such as low pressure steam, to provide an indirect heating of the slurry. However, it is also well known that the viscosity and susceptibility of the transferred polymer to agglomerate and / or stick to transfer lines and reservoirs generally increases with temperature, and jamming and agglomeration problems become more important as the weight load increases in the currently used transfer system as mentioned above.

A further example of a method for transferring a polymer slurry is described in US 4126743, which describes a continuous method of introducing slurry into consecutive heating zones, the second zone having a larger diameter than the first. It has been reported that the pressure in the reactor is preferably 10-30 kg / cm² g, such as the pressure at the inlet to the first heating zone, indicating that there is no pressure drop between the reactor and the first heating zone. The pressure at the outlet of the first heating zone, as indicated, is preferably 5-27 kg / cm² g, and the pressure at the outlet of the second heating zone is preferably 0 / 1-7 kg / cm² g.

[0006] The removal of volatiles from the polymer stream causes evaporation of the liquid phase of the stream, which results in an increase in the capacity on the transfer line and the resulting increase in the flow rate. However, if the speed becomes too high, it may exceed the speed of sound (the speed of the sound in the center), which leads to the interruption of the flow. On the other hand, if the initial rate is too low, there is an increased risk of jamming or agglomeration of the solid polymer as mentioned above.

[0007] It is further taken into account that in large installations the transfer lines must be very long in order to allow sufficient heating to occur and the length may sufficiently influence the spatial planning of the installation. This can cause various problems, such as the space occupied by the equipment in the installation and the control of conditions inside the line. It is often necessary to heat a significant proportion of the transfer line length to meet the heat input requirements. Thus, it will be appreciated that ensuring that the polymer stream reaches the degassing tank at the desired temperature and pressure and with minimum jamming / agglomeration is a significant technical challenge.

The invention aims to optimize the heating of the polymer during its transfer from the reactor to the degassing tank and at the same time to maintain a reliable product transfer through the particular construction of the transfer line between the polymerization reactor and the degassing tank for the polymer stream.

Accordingly, in a first aspect, the invention provides a method of heating a stream comprising a polymer transferred from a polymerization reactor to a degassing vessel, comprising passing the stream through a heater comprising a transfer line for the stream and means for heating the transfer line, wherein the pressure drop in the preheater is between 5% and 50%, preferably between 10 and 35%, of the total pressure drop between the polymerization reactor and the entrance to the degassing tank.

[0010] We have found that because the pressure at the cylinder outlet usually depends on the conditions below, the excessively high pressure drop in the preheater means the higher pressure at the cylinder inlet and thus the higher starting temperature of the polymer-containing stream. The higher the initial temperature of the polymer-containing stream, the smaller the temperature difference between the flow and the heater itself, and thus the less efficient heating of the stream.

[0011] The polymer-containing stream may be removed from the polymerization reactor continuously. The stream may or may not contain an active polymer.

[0012] This will be appreciated that the polymer-containing stream passes through a certain form of the pipeline all the way from the moment the polymerization reactor leaves the entrance to the degassing tank. For the purposes of this invention, it is recognized that the heater comprises a portion of the pipeline from the beginning of the heating section (or the first of the heating section) to the end of the heating section (or the last of the heating section). In this context, the term "heater", as used hereinafter, includes within its scope the possibility of several heaters connected in series. The outlet of the preheater (or heat transfer line) is considered to be the end of the line heating section and the inlet of the preheater is considered to be the beginning of the line heating section, the line heating section comprising a single heater or a plurality of preheaters in series. Through the "line"

[0013] The pressure drop across the transfer line per unit length is preferably between 0.01 bar / ma 0.2 bar / m, preferably between 0.0125 bar / ma 0.1 bar / m, most preferably between 0.0125 bar / m 0.04 bar / m. Excessive pressure drops on the transfer line are not desirable because they can lead to choked flow conditions.

[0014] A typical pressure at the cylinder inlet, Pi is 5-30 bar, preferably 10-25 bar. Higher Pi is undesirable because it means less pressure difference between the inlet of the preheater and the outlet of the reactor, thus ensuring a still high temperature of the stream containing the polymer at the inlet of the preheater. This reduces the temperature differences between the polymer-containing stream and the heater, as a result of which the heat transfer is reduced.

[0015] The outlet pressure, Po, is usually from 1.5-12 bar, preferably from 7-11 bar.

It is advantageous if between the exit from the polymerization reactor and the entrance to the degassing tank the polymer-containing stream passes through a pressure control valve which introduces a pressure drop which is usually between 45% and 90%, preferably between 60% and 80%, of the total pressure drop between the polymerization reactor and the entrance to the degassing tank. A typical pressure drop in the pressure control valve is between 10 and 30 bar, more typically between 15 and 25 bar. Preferably, the pressure control valve is arranged between the reactor and the inlet of the preheater, in which case the pressure drop in the pressure control valve is the difference between the pressure at the reactor outlet and Pi.

Typically, the ratio of the pressure drop across the pressure control valve to the pressure drop in the preheater is between 0.1 and 6, preferably between 0.1 and 2, and even more preferably between 0.2 and 2, e.g. between 0.2 and 0. 5.

[0018] In the process according to the invention, it is advantageous if the temperature of the polymer-containing stream at the exit from the preheater is maintained above the dew point of the non-polymeric stream portion, preferably from 5-80 ° C, most preferably from 10-30 ° C above the dew point. The temperature of the inner surface of the transfer line (sidewall temperature of the process) at any point along its length is preferably maintained at 5 ° C or more below the softening point of the polymer, more preferably 10 ° C or more below the softening point. The softening point of the polymer is defined as the Vicat softening point according to ASTM D1525, ISO 306. In case the heater is placed between the polymerization reactor and the degassing tank, the temperature of the inner surface of the transfer line can be maintained above the reactor temperature.³ the process side wall temperature is usually adjusted between 75 and 130 ° C, preferably between 85 and 105 ° C. For a polymer having a density of 955-965 kg / m³ the process side wall temperature is usually adjusted between 80 and 135 ° C, preferably between 95 and 110 ° C.

[0019] The Vicat softening point according to ASTM D1525, ISO 306, is the temperature at which the flat needle penetrates the polymer sample to a depth of 1 mm at a load of 10 N. The temperature reflects the softening point expected when the material is used in elevated temperature applications. The test specimen, which has a thickness between 3 mm and 6.5 mm and at least 10 mm in width and length, is placed in a testing device (e.g. ROSAND ASP 6 HDTNICAT System) such that a penetrating needle whose top has a surface in cross section transverse 1 mm², located on the surface of the sample at least 1 mm from the edge. A load of 10 N is applied to the sample. The sample is then lowered to an oil bath at 23 ° C. [Bath temperature] is increased at a rate of 50 ° C per hour until the needle enters 1 mm; the temperature at which this occurs is Vicat's softening temperature.

The outlet temperature of the transfer line - especially the temperature of the inner wall of the line at its outlet immediately below the last heated part - is preferably maintained as a temperature higher than the temperature of the inner wall of the line at its inlet immediately above the first heated part, more preferably at least 5 ° C above the inlet temperature.

[0021] For the method according to the invention, it is desirable to maintain a sufficiently high average speed of the polymer-containing stream to avoid jamming or blocking. By "medium" is meant the average speed over the entire flow cross-section at any point along the transfer line. Thus, it is advantageous if the average rate at the inlet Vi is at least 2 m / s, preferably at least 5 m / s and more preferably at least 8 m / s. Typical values ​​for Vi range from 3-20 m / s. In addition, it is desirable to keep the speed below the speed of sound. Accordingly, it is advantageous if the average velocity at the outlet of the Vo is below 80 m / s, preferably below 70 m / s. Preferably, Vo is at least 20 m / s; typical values ​​for Vo are from 30-80 m / s. It is beneficial

The method of the invention results in a heating of the polymer-containing stream such that at least 90 mol%, preferably at least 98 mol% and most optimally 100 mol%. liquid hydrocarbons discharged from the treatment in the polymerization reactor evaporates before entering the degassing tank. The degassing tank operates at a pressure between 6 bar and 12 bar while maintaining a pressure drop over the length of the preheater of less than 0.5 bar per ton of polymer, most preferably between 0.1 bar / ta 0.3 bar / t. It has been found that this optimized low pressure drop per production unit can be easily operated even at high loads of solid bodies at the entrance to the preheater. It is advantageous if the solids content of the polymer-containing stream is between 35 wt.%. a 70% weight, most preferably between 50 wt.%. and 65% by mass when the stream enters the preheater and is also preferred when the flow rate at the entry to the preheater does not differ by more than 15%, preferably not more than 5% in any 30-second period. One way by which this can be achieved is to use continuous rather than discontinuous dialing from the polymerization reactor. This action with a high load of solid bodies combined with the expanding diameter of the heater allows to minimize the pressure drop in the heater. this can be achieved by using continuous rather than discontinuous dialing from the polymerization reactor. This action with a high load of solid bodies combined with the expanding diameter of the heater allows to minimize the pressure drop in the heater. this can be achieved by using continuous rather than discontinuous dialing from the polymerization reactor. This action with a high load of solid bodies combined with the expanding diameter of the heater allows to minimize the pressure drop in the heater.

[0023] The average Reynolds number in the cross section of the stream at any point along the length of the heater transfer line should always be above 500,000, preferably between 1 million and 10 million, most preferably between 1.8 million and 5 million.

[0024] Heating and decompression of the polymer stream as it passes the transfer line to the degassing tank causes progressive vaporization of the liquid from the stream and the consequent increase in line speed. When developing a transfer line to ensure efficient and reliable polymer transfer and heat transfer, the opposite requirements must be met. While high speeds increase heat transfer and generally minimize jamming, they also lead to large pressure drops along the line. It is therefore important to be able to minimize the length of the transfer line and the required heat transfer surface while providing the polymer after sufficient removal of the volatiles at an acceptable temperature.

Regarding the design of the preheater itself, it is advantageous if the ratio of the outlet diameter of the transfer line To to the diameter of its inlet Di, To / Di, is above 1, preferably between 1.2 and 10. It is usually at least 1.3, and commonly at least 1.4. However, this ratio is preferably not more than 4, and more preferably no more than 2, with a maximum of 1.9 being most preferred. We have found that increasing the diameter of the transfer line along its length allows the heater to receive a greater range of the flow rate of the polymer-containing stream. The relatively small inlet diameter allows a relatively high speed even at low flow rates, reducing the risk of jamming; on the other hand, the relatively larger average of the outlet avoids the risk of exceeding the speed of sound by speed even at high flow rates. Having such a capacity range is especially valuable during start-up and shut-down operations. In order to reduce the risk of blockages, it is also advantageous if the outlet diameter of the transfer line is smaller than the outlet for the solids in the degassing tank. It is defined as the inner diameter of the transfer line at its outlet, and Di is the internal diameter of the transfer line at its inlet, the outlet and inlet of the transfer line being defined as previously described. In order to reduce the risk of blockages, it is also advantageous if the outlet diameter of the transfer line is smaller than the outlet for the solids in the degassing tank. It is defined as the inner diameter of the transfer line at its outlet, and Di is the internal diameter of the transfer line at its inlet, the outlet and inlet of the transfer line being defined as previously described. In order to reduce the risk of blockages, it is also advantageous if the outlet diameter of the transfer line is smaller than the outlet for the solids in the degassing tank. It is defined as the inner diameter of the transfer line at its outlet, and Di is the internal diameter of the transfer line at its inlet, the outlet and inlet of the transfer line being defined as previously described.

[0026] The inner diameter D of the transfer line is preferably at least 20 mm and more usually between 40 mm and 200 mm. The most preferred are internal diameters between 60 mm and 150 mm.

[0027] The length L of the heater, and hence the transfer line, is preferably at least 20 m, more preferably at least 30 m, but usually not more than 600 m. A preferred length range is 50 m to 500 m, more preferably 70 m to 300 m m.

[0028] It is advantageous if the ratio of the length L of the transfer line to its average internal diameter D.sub.red. L / D.sub.red is between 500 and 10,000, preferably between 1500 and 3500, and more preferably between 2000 and 3000. If the transfer line is constructed of several sections, each with a different diameter, Dśred. is the average of the inside diameter of these sections weighted by the length of each section; alternatively, it can be calculated in relation to the total internal capacity of the V line, where V = (% D average.².L) / 4.

[0029] If the diameter of the transfer line increases in its length, it is preferred that the growth occurs in separate stages rather than continuously. There is usually one, two or three increases in diameter over the length of the pipe.

[0030] It is advantageous if one or all of the line sections are generally in a vertical position than a horizontal one, so that the line occupies a smaller space in the installation: in this configuration, the first section of the line preferably has its inlet at the bottom so that the initial flow of material the transfer line goes up. It is advantageous if less than 20%, most preferably less than 10% of the length of the transfer line is horizontal, and optimally the line is constructed essentially without horizontal sections. In one embodiment, at least the inlet and exit of the heated transfer line are oriented vertically so that the inlet flow through the line is upward and the outlet flow through the line is down. In one embodiment of the invention, the transfer line comprises a series of sections connected by means of arches (elbows), which usually have a U-shape such that the line doubles one or more times. The advantage of this configuration is that thanks to it the transfer line is more compact in the installation. The sections between the knees are usually simple. The arches can be heated just like the rest of the line, but usually - in order to simplify the design of the heater - they are not heated. In general, it is also advantageous if each expansion of the line diameter occurs in unheated sections of the line; thus, the line sections may have different diameters, wherein the increase in diameter occurs in one or more elbows, preferably at the elbow exit, so that the speed decreases rather at the elbow exit than at its inlet, and most preferably at the elbow exit at the top of the vertically heated section. The development of the expansion section and arcs in the transfer line is crucial for reliable operation without jamming. The number of vertical and horizontal sections between the elbows that make up the entire transfer line can be from 2 to 10, although it is more commonly 3 to 7 sections.

[0031] The elbows of the transfer line may have a different degree of curvature. The radius of the curve defined by the elbow can be expressed as a multiple of the diameter D of the line at this point. The bends usually have a radius between 3D and 30D, with 5D-20D being the most advantageous for providing reliable operation without jamming, while minimizing the space occupied by the line. As previously stated, the elbows are preferably U-shaped, although alternative options such as L-shaped elbows are not excluded, which provide a smooth flow path. Of course, the transfer line formed from the section may comprise a mix of the above elbow types or indeed elbows with different angles, such as 60 ° or 120 °.

[0032] It has been found that the length of any section of the transfer line extension should be greater than 0.25D, preferably between 0.5D and 10D, most preferably between 0.75D and 3D. Preferably, the expansion section is placed immediately above or below the elbow, preferably immediately below the elbow. It is also advantageous if the extension is concentric, although other extension geometries are also possible.

[0033] The total appropriate heat transfer surface of the transfer line, which is the area of ​​its outer surface in contact with the heating means, is preferably at least 0.5 m² heat transfer area per tonne / hour polymer production, usually between 0.7 and 10, more preferably between 1 and 5, most preferably between 1.5 and 3.5 heat transfer surfaces per tonne / hour. polymer production.

It is advantageous if the inlet of the preheater is at approximately the same level as the output of the polymerization reactor to which it is connected, preferably the transfer line from the polymerization reactor to the inlet of the preheater is substantially horizontal.

[0035] It is most advantageous when the output of the transfer line (at the entry point of the degassing tank) is at a higher level than the inlet of the transfer line and / or the outlet of the polymerization reactor.

[0036] Means for heating the transfer line usually include a mantle surrounding the line. The heating jacket may be in the form of an electric heater, but it is advantageous if it is in the form of a concentric tube surrounding the line through which the heating fluid is passed. The most commonly used heating fluid is water vapor. It has been found that the conditions can best be optimized by using a chilled steam as a heating medium, especially when the maximum saturation temperature is 0-30 ° C and preferably no more than 10 ° C below the softening point of the heated polymer. Regardless of the form of the jacket, it may provide the same heat input along the entire length of the transfer line or it may provide differential heating in different parts of the line. It is also possible that parts of the line (such as arches) are unheated, as discussed above. We have found that the optimal heat flow along the transfer line is achieved by means of such a study in which the temperature of the heating medium (or the temperature of the inner wall of the line) is higher at the inlet of the line than at its exit. Accordingly, since the vapor fraction in the stream containing the polymer increases as it passes through the line, it is preferred that the temperature of the heating medium (or the temperature of the inner wall of the line) decreases. This can be achieved by continuous grading or in several separate stages by means of sections of different temperature. However, the most preferred is a jacket that operates at different temperatures in different parts of the line, typically by independently providing a heating medium to each section where a different temperature is required. that the optimal heat supply along the transfer line is achieved by means of such a study in which the temperature of the heating medium (or the temperature of the inner wall of the line) is higher at the inlet of the line than at its exit. Accordingly, since the vapor fraction in the stream containing the polymer increases as it passes through the line, it is preferred that the temperature of the heating medium (or the temperature of the inner wall of the line) decreases. This can be achieved by continuous grading or in several separate stages by means of sections of different temperature. However, the most preferred is a jacket that operates at different temperatures in different parts of the line, typically by independently providing a heating medium to each section where a different temperature is required. that the optimal heat supply along the transfer line is achieved by means of such a study in which the temperature of the heating medium (or the temperature of the inner wall of the line) is higher at the inlet of the line than at its exit. Accordingly, since the vapor fraction in the stream containing the polymer increases as it passes through the line, it is preferred that the temperature of the heating medium (or the temperature of the inner wall of the line) decreases. This can be achieved by continuous grading or in several separate stages by means of sections of different temperature. However, the most preferred is a jacket that operates at different temperatures in different parts of the line, typically by independently providing a heating medium to each section where a different temperature is required.

[0037] In a preferred embodiment of the invention, the transfer line is heated by means of a concentric pipe using steam as a heating medium. The outlet temperature of the transfer line is preferably controlled by the steam flow rate: for a given water vapor temperature this has the advantage of allowing wall temperature control of the transfer line to provide a lower temperature at low polymer flow rate and higher temperature at a higher flow rate when the rate is higher .

[0038] One way of further increasing the temperature of the polymer-containing stream itself at the outlet of the transfer line (in addition to increasing the energy input to the heater) is to increase the solids content in the stream. This can be achieved by increasing the solids content of the effluent from the polymerization reactor and / or by means of a solids concentration device above the transfer line. Solids can carry more heat than liquid or gaseous components of the stream, thus requiring less inflow from the transfer line heater to achieve the desired temperature.

[0039] The use of a solids concentration device above the transfer line with a diluent above (as described in our patent EP 1 118 624) is a preferred embodiment of the invention and enables minimization of the monomer concentration in the transfer line, thereby reducing the risk of jamming.

[0040] It is advantageous if the pipe can easily be separated at the length of the heater to facilitate cleaning. Preferably the pipe has flanges at intervals of 5-15 m. Where heating is carried out by means of a jacket comprising a heating fluid, it is advantageous if the heating fluid does not cover any of the flanges.

[0041] In order to maximize the heat transfer to the polymer-containing stream, the pipe is preferably made of a material having a thermal conductivity above 30 Wm.^-2K^-1, preferably above 40 Wm^-2K^-1. The pipe is usually seamless, although a line weld pipe is advantageous when a large heat transfer is required.

[0042] It is advantageous if the entire polymer-containing stream exiting the polymerization reactor passes through a single transfer line according to the invention. Such a transfer line may be fed with one or more discharge lines from the reactor. The stream discharged from the reactor can be concentrated before passing through the transfer line, preferably by means of gravity or centrifugation, most preferably using a hydrocyclone. It is also within the scope of this invention to provide multiple parallel transfer lines for receiving a polymer-containing stream, each being arranged according to the invention. In such a design, not all transfer lines must operate at any given time. In a further embodiment, the polymerization reactor has a plurality of discharge lines, each has its own transfer line. The invention also includes within its scope the use of single or parallel solid-state devices, wherein a typical arrangement is one solidifying device located above each transfer line.

[0043] In an embodiment with a parallel heater, it is preferred that when two heaters operate, the average flow rate at any cross-section of the transfer line of each heater is maintained between 2 and 100 m / s, most preferably between 10 and 70 m / s. The operation of each transfer line can be monitored by parameters, including steam flow to the heating jacket to measure heat input (nominal capacity) to the flow, differential pressure in the heater and measurement from the reactor pressure valve to measure the flow or flow rate to each transfer line, compound between the steam flow and the outlet temperature for each heater, mass balance of the reactor to calculate the total flow to all heaters and the difference between the evaporation temperature at the cylinder outlet and the process flow dew point. The pressure drops in the transfer lines of each preheater are preferably substantially the same as in the single heater embodiment as described above.

[0044] In embodiments with both a single heater and a plurality of parallel heaters, the flow rate of the polymer-containing stream discharged from the polymerization reactor is preferably controlled using a pressure or flow control valve, most preferably placed between the solids con- centrating device and the transfer line heater inlet. The control valve is designed to exhibit a pressure drop between 45% and 90%, most preferably 50% and 80% pressure loss between the reactor and the entrance to the first tank below. The heated transfer line is preferably developed so that it has a pressure drop between 5% and 75%, most preferably between 10% and 35% pressure drop between the reactor and the entrance to the degassing tank.

[0045] The polymer-containing stream may comprise a gaseous component as well as a liquid component. Typically, the water vapor fraction in the liquid component of the polymer-containing stream at the inlet of the preheater varies from 5 to 60 mole%. In one preferred embodiment of the invention in which the pressure or flow control valve is above the preheater, the water vapor fraction in the stream at the cylinder inlet is between 25 and 60 mol%. The water vapor fraction in the liquid stream component at the heater outlet can vary from 70 to 100 mole%, typically from 95-100 mole%, most preferably above 99 mole%.

[0046] This invention can be used in any polymerization process (e.g., gas phase, slurry or solution) containing a polymer stream that needs to be heated to evaporate the liquid during depressurization.

[0047] The methods for copolymerizing olefins in the suspension phase are well known in the art. Such methods may, for example, be carried out by introducing a monomer or a co-monomer into a stirred tank or a continuous loop reactor containing a polyolefin and a polymerization catalyst. The reactor is usually controlled to achieve the desired melt index and polymer density at optimal production and temperature.

[0048] In polymerization processes in a polyethylene slurry, the polymer is typically discharged from a polymerization reactor with substantial amounts of liquid hydrocarbons, and thus the invention is particularly suitable for such methods. The suspension in such reactors usually comprises a particulate polymer, hydrocarbon diluent (s), (co) monomer (s), catalyst, chain terminators such as hydrogen and other reactor additives. In particular, the suspension will contain from 20-75, preferably from 30-70 weight percent based on the total weight of the particulate polymer suspension, and from 8025, preferably from 70-30 weight percent based on the total weight of the suspending medium suspension, wherein the suspending medium is the sum of all liquid components in the reactor and includes a diluent, an olefin monomer and any additives; the diluent may be a neutral diluent or it may be a reactive diluent, such as a liquid olefin monomer. When the main diluent is an inert diluent, the olefin monomer will typically contain from 2-20 wt%, more particularly from 4-10 wt%. suspension.

[0049] The polymerization is usually carried out at temperatures in the range of 50-125 ° C and under a pressure in the range of 1-100 bar. The catalyst used can be any catalyst usually used for olefin polymerization, such as chromium oxide, Ziegler-Natta or metallocene type catalysts. The resulting suspension containing polymer and diluent, and in most cases a catalyst, monomer and olefin comonomer, can be collected intermittently or continuously, using concentration devices, such as hydrocyclones or settling tanks, to minimize the amount of fluids discharged with the polymer.

This invention particularly relates to the polymerization in loop reactors in which the suspension circulates in the reactor usually by means of a pump or a mixer. Liquid-filled loop reactors are particularly known in the art and are described, for example, in US 3152872, US 3242150 and US 4613484. The loop reactor has a continuous tubular structure comprising at least two, e.g. four, vertical sections and at least two, e.g. four, horizontal sections. The heat of the polymerization is usually removed by indirect exchange with a cooling medium, preferably water, in jackets surrounding at least part of the tubular loop reactor. The capacity of the loop reactor may vary, but is usually in the range of 20 to 170 m³.

[0051] In commercial installations, the particulate polymer is separated from the diluent in such a way that the diluent is not exposed to contamination, allowing the diluent to be returned to the polymerization zone with minimal, if any, purification. Separation of the particulate polymer produced in the method of the invention from a diluent can usually be carried out by any method known in the art, e.g., i) employing discontinuous vertical settlers so that the flow of the slurry through their opening provides a zone in which which polymer particles may drop to some extent from the diluent or (ii) discharge product continuously through one or more discharge openings, the distribution of which on the loop reactor may be any, but it is usually adjacent to the distal end of the horizontal loop section. As previously discussed, the treatment of high concentrations of suspended solids in large size reactors minimizes the amount of the main diluent discharged from the polymerization loop. The use of concentration devices for the discharged polymer slurry, such as hydrocyclones (single or in the case of a larger number of hydrocyclones in parallel or in series) further enhances the recovery of the diluent in an energy-saving manner, as it avoids a significant pressure drop and evaporation of the recovered diluent.

[0052] The drained and preferably concentrated polymer suspension is usually expanded prior to transfer via the heater of the invention to the first expander.

[0053] The diluent and any monomer vapor recovered in the first expander is typically concentrated, preferably without recompression, and reused in the polymerization process. The pressure in the first expander is usually adjusted to allow condensation with an easily accessible cooling medium (e.g., cooling water) of substantially all of the expansion vapor prior to any recompression. The pressure in the first expander is generally in the range of 2-25 bars, more typically from 5-20 bars, and most often from 6-11 bars. The solid material recovered from the first expander usually passes to a second expander to remove residual volatiles. Alternatively, the slurry may pass to a lower pressure expander than in the above-described expander, whereby recompression is required for recovering the diluent.

[0054] More specifically, an example of a type of polymerization process for which the invention is particularly useful is the continuous polymerization of olefins, preferably alpha-olefins, in the reaction zone, preferably an elongated, tubular, closed loop. Olefine (olefin) is added continuously and contacted with a catalyst in a hydrocarbon diluent. The monomer (s) polymerizes and forms a suspension of particulate polymer suspended in a polymerization medium or diluent. The rate of discharge of the polymer product is controlled via a valve above the preheater according to the invention.

[0055] The concentration of solids in the slurry in the reactor will usually be above 20% by volume, preferably about 30% by volume, e.g. from 20-40% by volume, preferably from 25-35% by volume, with% volume being [(total volume) suspension volume of suspending medium) / (total volume of suspension)] x100. The solids concentration, measured as a mass percentage, which is equivalent to that measured as a volume percent, will vary depending on the polymer produced, but more specifically depending on the diluent used. If the polymer produced is polyethylene and the diluent is an alkane, e.g. isobutane, it is preferred that the solids concentration is above 30 wt%, in particular above 40 wt%, e.g. in the range of 40-60 wt%, preferably from 45-55% by mass relative to the total weight of the slurry. We have found that for high solids loads, especially above 40% by mass, reliable product discharge and heating between the polymerization reactor and the degassing tank (as shown by clamping, flow variations and / or heat transfer) can be maintained within acceptable processing limits using a preheater according to the invention.

[0056] This type of method can optionally be carried out in a multi-reactor system. The second or each subsequent reactor in the multi-reactor system may be another loop reactor or any olefin polymerization reactor, e.g. a fluidized bed reactor. However, usually the second or each subsequent reactor in the multi-reactor system is another loop reactor. Such multi-reactor systems can be used to produce monomodal or multimodal, preferably multimodal, polymers.

In the case of a plurality of reactors in the series, the first reactor of the series is fed with a catalyst or prepolymer and optionally a cocatalyst in addition to the diluent and the monomer, and each subsequent reactor is fed with at least a monomer, in particular ethylene, and a suspension obtained from the preceding reactor in series, wherein this mixture contains a catalyst and a mixture of polymers made in the preceding reactor in series. It is possible to feed the second reactor and / or, where appropriate, at least one of the subsequent reactors with fresh catalyst and / or cocatalyst. However, it is more common to introduce the catalyst and cocatalyst exclusively into the first reactor.

[0058] In the case where the plant comprises 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 reactors adjacent or non-adjacent. The hydrogen concentration is maintained at (i) low (or zero) level in the reactor (s) producing components of high molecular weight, e.g. the hydrogen percentage is between 0-0.1% by volume, and on ( (ii) very high levels in the reactor (s) producing components of low molecular weight, e.g. hydrogen percentage between 0,5 to 2,4% vol. The reactors can be operated in the same way to obtain substantially the same melt index of polymer in subsequent reactors.

[0059] When such reactor systems produce polymers with a molecular weight below 50 kilodaltons or above 150 kilodaltons, in the past, particular problems have been faced with reactor jamming and agglomeration in the heater between the polymerization reactor and the degassing tank. These problems can increase in the heater due to the high concentration of solids in the polymer. This is another problem that can be mitigated by using a preheater according to the invention.

EXAMPLE 1 [0060] A polymerization reactor that polymerizes ethylene and hexene comonomer in a isobutane diluent operates at a pressure of 41 bar at a temperature of 95 ° C. From it, the polymer-containing stream is withdrawn, which is in the form of a suspension whose liquid component essentially contains 91 mol%. Isobutane, 8 mol%

unreacted ethylene and 1 mol% hexene. The solids content of the suspension is about 40% by weight, including polyethylene with a density of 940 kg / m³as well as some unused catalyst. These conditions are the same as in Example 1.

[0061] The slurry from the reactor passes through the pressure control valve to lower the pressure before entering the preheater of the invention. The conditions at the inlet to the heater are as follows:

Temperature: 82.4 ° C. Pressure Pi: 17.4 bar

Vi speed: 10.7 ms^-1

Reynolds number: 1.72 million

The concentration of solids is 40% by mass; from the remaining liquid phase, 40% by weight is water vapor and 60% by weight. liquid.

[0062] The heater has a length of 187 m and has 3 straight and vertical sections, each 58 m long, connected by 180 ° bends; elbows together constitute 13 m from the total length and are not heated. All sections have an internal diameter of 78 mm and a wall thickness of 5.5 mm, which gives an L / Dsred value. approximately 2397. The thermal conductivity of the wall in the whole pipe is 46.4 W / mK. The heating element is in the form of concentric outer tubes extending along 26 m of each section through which the cooled steam passes.

[0063] The suspension passes through the heater at a rate of 15 tons / hour. The length and diameter of the pipe, the heat input in the heater from the heating medium to the slurry, the slurry speed and its initial solid content, all are calculated to ensure that heat transfer to the slurry during its passage through the heater is sufficient to ensure that until the slurry exits from the pre-heater the liquid phase will evaporate completely. The suspension leaves the heater at a temperature of 76 ° C, a pressure Po of 9 bar and a speed Vo of 63.3 m / s (Vo / Vi = 5.9), with a Reynolds number of 3.3 million. This equates to a pressure drop in the preheater of 0.045 bar / m. At this pressure, the dew point of the steam would be around 60.8 ° C, so the stream is 15 ° C above the dew point,

[0064] The temperature of the internal wall of the preheater is between 89 ° C and 93 ° C over the entire length of the preheater; this is compared to the polymer softening point of about 128 ° C. The coefficient of heat transfer from the suspension through the heater wall was calculated at 984 W / m²K.

EXAMPLE 2 [0065] In this Example, the heater has a lower L / D-ratio, but also an increasing diameter.

The polymerization reactor, which polymerizes the ethylene-hexene comonomer in the isobutane diluent, operates at a pressure of 40 bar at a temperature of 95 ° C. From it, the polymer-containing stream is withdrawn, which is in the form of a suspension whose liquid component essentially contains 91 mol%. Isobutane, 8 mol% unreacted ethylene and 1 mol% hexene. The solids content of the suspension is about 40% by weight, including polyethylene with a density of 940 kg / m³as well as some unused catalyst.

[0067] The slurry from the reactor first passes through the hydrocyclone to concentrate the solids up to 50% by weight, and then the pressure control valve to lower the pressure before entering the preheater of the invention. The conditions at the inlet to the heater are as follows:

Temperature: 76 ° C. Pressure Pi: 14.4 bar

Vi speed: 16.6 ms^-1Reynolds number: 2 million

The concentration of solids is 50% by mass; from the remaining liquid phase, 40% by weight is water vapor and 60% by weight. liquid.

[0068] The heater has a length of 152 m and comprises 5 straight and vertical sections, each 26 m long, connected by 180 ° bends; elbows together constitute 22 m from the total length. Each of the first three sections has an internal diameter of 78 mm, while each of the other two sections has an inside diameter of 102 mm. A single increase in diameter occurs at the exit of the elbow connecting the third and fourth sections. Thus, the heater has a Do / Di value of 1.33 and a L / Dsred value. approximately 1730. The wall thickness of a 78 mm diameter pipe is 5.5 mm and the wall thickness of a 102 mm pipe is 6.0 mm. As in Example 1, the thermal conductivity of the wall in the whole pipe is 46.4 W / mK, and the heating element is in the form of concentric outer tubes extending along each 58 m straight section,

[0069] The suspension passes through the heater at a rate of 20 tons / hour. Pipe length and diameter, heat input from the preheater, slurry velocity and its initial solids content, all calculated to ensure that heat transfer to the slurry as it passes through the preheater is sufficient to ensure that the liquid phase evaporates until the slurry exits the preheater completely. The suspension leaves the heater at a temperature of 80 ° C, a pressure of 10 bar and a velocity of Vo of 30 m / s (Vo / Vi = 1.78), with a Reynolds number of 2.7 million. This equals the pressure drop in the heater of 0.03 bar / m. At this pressure, the steam dew point would be around 65 ° C, so the stream is 15 ° C above the dew point,

[0070] The temperature of the internal wall of the preheater is between 89 ° C and 93 ° C over the entire length of the preheater; this is compared to the polymer softening point of about 128 ° C. The coefficient of heat transfer from the suspension through the heater wall was calculated at 600 W / m²K.

Mirosława Ważyńska Patent attorney