Process and apparatus for separating diluent from polymer solids

IS 2 340 993 T3

DESCRIPTION

Process and apparatus for separating diluent from polymer solids.

Field of the invention

The present process and apparatus relate to suspension polymerization, in particular to the removal of the diluent from the polymer solids produced in suspension polymerization.

Background of the invention

Polyolefins such as polyethylene and polypropylene and other polymers can be prepared by polymerization in particulate form, also called suspension polymerization. In this technique, feedstocks such as monomer and catalyst are introduced into a reaction zone, and a fluid suspension comprising solid polyolefin particles is formed in a liquid medium in the reaction zone.

In loop reactors, the various feedstocks can be introduced into the cycle reaction zone in various ways. For example, the monomer and catalyst can be mixed with varying amounts of diluent prior to introduction to the cycle reaction zone. In the reaction zone of the cycle, the monomer and the catalyst are dispersed in the fluid suspension. The fluid suspension circulates through the cycle reaction zone, and the monomer reacts on the catalyst in a polymerization reaction. The polymerization reaction produces solid polymer particles in a liquid medium, for example a liquid diluent used to form the fluid suspension.

In order to recover the polymer from a particulate polymerization process, it is necessary to separate the polymer solids from the liquid diluent that constitute the effluent slurry withdrawn from the reactor. Typical separation systems include a reduction in pressure so that the liquid diluent vaporizes. The vaporized diluent comes out of an upper portion of a vent tank. The polymer remains solid, and is recovered through a lower portion of the flash zone. Other polymer recovery processes use multi-stage flash vaporization systems. For example, a first intermediate pressure flash zone and a second lower pressure flash zone. The temperature and pressure of the first flash zone are such that they will vaporize a greater amount of the diluent and this vapor can be condensed without compression by heat exchange with a fluid having a temperature, for example, in the range from about 40 ° F to about 130 ° F (4-54 ° C). The polymer particles from the first vaporization stage are subsequently subjected to a lower pressure vaporization stage to vaporize the additional remaining diluent.

In a polymer recovery system that uses one or more vaporization chambers, it has been common to have some diluent incorporated into the polymer exiting the vaporization chambers. It is desirable, however, to obtain the polymeric product substantially free of diluent. If the polymer leaving the vaporization chamber contains an excess amount of diluent, the polymeric product may be sticky and subsequently clog the lines and cause restrictions. Additionally, in many polymer production processes, polymer solids after the vaporization chamber undergo further processing to remove residual and incorporated diluents. Examples of such processing include purge zones, conveyor belt dryers, and other drying systems known in the art.

One method of removing additional amounts of the diluent after the vaporization chamber includes passing the polymer solids through a purge zone, in which a non-combustible gas is used to remove the diluent.

In a typical recovery system, when the polymer moves from a higher pressure zone to a lower pressure zone, it is important to maintain a pressure seal between the zones. Otherwise, the pressure will balance between the two zones, or the diluent or other material from the upper pressure zone could move to the next zone or the material from the lower pressure zone could move to the higher pressure zone. It is desirable to minimize the amount of gaseous or liquid diluent that passes into a purge zone to reduce the effort to purge said diluent and to ensure the production as a final product of the polymer that is essentially free of said diluent. Also, because at least a portion of the flash zone gas is often recycled to the reactor, it is desirable to prevent it from entering the flash zone.

One technique for maintaining a pressure seal between an upper pressure zone and a lower pressure zone includes the use of a "fluff chamber" or "balance vessel" between the zones. The foam chamber generally operates at a normal level of 75 to 85 percent of its polymer solids capacity. This level is desired to minimize the amount of hydrocarbon gas that should be in the space above the polymer solids.

In association with a foaming chamber, the valves are used to control the flow of polymeric solids (often called "foaming" or "flakes" or "powder") into and out of the foaming chamber in a batch type process. For example, in some recovery systems, when the foaming chamber inlet valve is open, polymer solids from the vaporization chamber or other higher pressure tank

ES 2 340 993 T3 pass into the foaming chamber, and the outlet valve of the foaming chamber is closed to maintain a pressure seal. When the foaming chamber outlet valve is open, the polymer solids exit the foaming chamber, and the pressure within the foaming chamber is released, but the foaming chamber inlet valve is closed to maintain a Pressure seal between flash zone and foaming chamber. In this scheme, the foaming chamber inlet and outlet valves do not open simultaneously. Keeping one of the valves always closed prevents the loss of the high pressure diluent into a downstream container.

As polymerization reactors get larger and production rates increase, the foaming chamber valves must also be larger and / or cycle more frequently, presenting cost and maintenance issues.

Brief summary of the invention

As an aspect of the present invention, there is provided a process for the suspension polymerization of olefins and for the separation of the resulting solid olefinic polymer particles from the liquid medium used for the suspension, as defined in claim 1. In this process , the olefin monomer is polymerized to produce a fluid suspension comprising the liquid diluent and the polymer solids. For example, the olefin monomer may itself be the liquid medium (such as in certain polypropylene processes), or an olefin monomer such as ethylene may be polymerized in a separate diluent liquid such as isobutane. The present invention will be described with reference to the polyolefin process using a liquid diluent that is added to the monomer, but the present invention is not limited to such a process.

The fluid suspension withdrawn from the reaction zone passes into an intermediate pressure zone under conditions of pressure and temperature such that the diluent is separated from the polymeric solids with a stream of steam. The pressure and temperature conditions are such that the vapor stream can be condensed without compression as described in more detail below. The polymer solids and any remaining diluent (including the incorporated diluent) are subsequently removed from the intermediate pressure zone and passed to a downstream purge zone. Various techniques are described herein for transferring polymer solids from the intermediate pressure zone to the downstream zone. The techniques can facilitate the polymer solids to be held at a desired level or for a desired time in the intermediate pressure zone. Optionally, the polymeric solids can pass substantially continuously from the intermediate pressure zone to the purge zone.

The process includes removing the polymer solids from the intermediate pressure zone and transferring the polymer solids to a purge zone without passing through a flash zone. Additionally, the process includes removing a portion of the polymeric solids from the intermediate pressure zone, controlling the level of the polymeric solids in the intermediate pressure zone, and adjusting the extraction of the polymeric solids from the intermediate pressure zone at response to the controlled level. Additionally, the process may include holding the polymer solids in the intermediate pressure zone for a characteristic average residence time sufficient to remove substantially all of the unincorporated diluent, extracting the polymer solids from the zone, and transferring the polymer solids to a purge zone to remove substantially all of the incorporated diluent.

As another aspect of the present invention, there is provided an improved apparatus for separating diluent from polymer solids, as defined in claim 12. The apparatus comprises an intermediate pressure chamber fluidly connected (although the connection may be indirect) to a purge column, without having a low pressure vaporization chamber between them. The apparatus includes a level sensor and an outlet valve. The vapor from the diluent or other polymerization medium is separated from the polymer solids in the chamber. The chamber has an inlet for receiving a fluid suspension comprising the diluent and polymer solids and an outlet for discharging a concentrated intermediate product, such as substantially dry polymer solids or a concentrated suspension. The chamber also has a gas outlet to discharge the vaporized diluent. The level sensor is in contact with the chamber to detect the level of polymer solids in the chamber. The outlet valve is fluidly connected to the outlet. The level sensor is operatively connected (via a computer or other controller) to the outlet valve so that the outlet valve is manipulated in response to the sensed level of polymer solids. The apparatus also includes a purge column fluidly connected to the outlet valve so that the purge column receives the polymeric solids from the intermediate pressure chamber.

The apparatus for separating the diluent from the polymer solids may comprise an intermediate pressure chamber in which the diluent is separated from the polymer solids, a fluid passage connected at one end to a lower portion of the intermediate pressure chamber, and a purge column connected to an opposite end of the fluid passage. Such apparatus does not include a low pressure vaporization chamber.

Several innovative techniques are provided to transfer the polymer solids to the purge column from the intermediate pressure chamber. One or more transport tanks may be located after the intermediate pressure chamber. The transport tank uses the force of high pressure gas, instead of or in addition to the force of gravity, to transport the material. A similar technique can be combined with a carrier tank or its function with the intermediate pressure chamber. In this technique, the evaporation gas can be used as a high pressure gas. Still another technique involves the use of cycle valves and a foam chamber. Innovative techniques facilitate the present invention.

IS 2 340 993 T3

Brief description of the drawings

Fig. 1 shows a suspension polymerization system comprising an intermediate pressure tank and a purge column.

Fig. 2 shows a suspension polymerization system comprising an intermediate pressure tank, cycle valves, a foaming chamber, a purge column, and pressure equalization lines.

Fig. 3 shows a diluent separation system comprising a carrier tank after an intermediate pressure tank.

Detailed description of the invention

The present invention separates substantially all of the unincorporated diluent from the polymeric solids in an intermediate pressure zone and transfers the polymeric solids from the intermediate pressure zone to a downstream purge zone. The use of a low pressure flash zone is avoided.

Polymeric solids can be maintained at a desired level in the intermediate pressure zone. Maintaining the polymer solids at a desired level in the intermediate pressure zone can increase the residence time of the polymer solids in this zone. Increasing this residence time allows more diluent (preferably most of the diluent or substantially all of the diluent) to separate from the polymer solids in the intermediate pressure zone, which reduces or eliminates the need for a vaporization zone. low pressure instantaneous.

The present invention is applicable to any suspension polymerization in a liquid medium. The invention is particularly applicable to polymerizations of olefins in a liquid diluent in which the resulting polymer is generally insoluble under the conditions of polymerization. More particularly the invention is applicable to any olefin polymerization in a batch reactor that uses a diluent to produce a suspension of polymeric solids and liquid diluent. Suitable olefin monomers are 1-olefins having up to 8 carbon atoms per molecule and no branching closer to the double bond than the 4-position. The invention is particularly suitable for the copolymerization of ethylene and a higher 1-olefin such as 1-butene, 1-pentene, 1-hexene, 1-octene and 1-decene. For example, copolymers can be made from ethylene and 0.01 to 10 weight percent, alternatively 0.01 to 5 weight percent, alternatively 0.1 to 4 weight percent, olefin. based on total weight of ethylene and comonomer. Alternatively, sufficient comonomer can be used to give the above-described amounts of comonomer incorporation into the polymer.

Suitable diluents for use as a liquid medium are well known in the art and include hydrocarbons, which are inert and liquid under the reaction conditions. Suitable hydrocarbons include isobutane, propane, npentane, i-pentane, neopentane, and n-hexane, isobutane is especially preferred. Additional details regarding the batch reactor apparatus and polymerization processes can be found in US Pat. Nos. 4,674,290, 5,183,866, 5,455,314, 5,565,174, 5,624,877, 6,005,061, 6,045,661, 6,051,631, 6,114,501, 6,262,191, and 6,420,497.

Furthermore, the present techniques for controlling heavy components can be employed when the unreacted monomer is the liquid medium for polymerization. For example, the present techniques can be used for the polymerization of propylene where propylene is the liquid medium and no inert diluent is present in substantial amount. You can even use a diluent for the catalyst. By way of illustration, but not as a limitation, the present invention will be described in relation to a process for obtaining polyethylene using an inert diluent as the liquid medium, but it is understood that the present invention can also be employed when the monomer is used as the medium. liquid and can take the place of the diluent in the following descriptions.

Suitable catalysts for olefinic suspension polymerizations are well known in the art. Particularly suitable is chromium oxide on a support such as silica, as widely described in US Patent No. 2,825,721 (March 1958). Reference herein to silica supports is also meant to encompass any known silica-containing support such as, for example, silica-alumina, silicatitania and silica-alumina-titania. Any other known support such as aluminum phosphate can also be used. The invention is also applicable to polymerizations using organometallic catalysts including those frequently referred to in the art as Ziegler catalysts (or Ziegler-Natta catalysts) and metallocene catalysts.

Additional details regarding the loop reactor apparatus and polymerization processes can be found, for example, in U.S. Patent Nos. 4,424,341, 4,674,290, 5,183,866, 5,455,314, 5,565,174, 5,624,877, 6,005,061, 6,045,661, 6,051,631, 6,114,501, and 6,420,497.

In suspension polymerizations in a cycle reaction zone, removal or extraction of the suspension can be performed on an intermittent basis using settling columns as is well known in the art. The slurry can also be continuously stirred, which can be done even more conveniently, for example, by operating the reactor at high concentrations of solids. In addition, when the extracted portion of the suspension has a high concentration of solids, this benefits the separation procedure since it is necessary to separate

ES 2 340 993 T3 less polymer diluent. The solids concentration of the extracted portion will generally be at least about 35 weight percent, more preferably at least 50 weight percent, even more preferably 50 weight percent to 70 weight percent. Additional details regarding the continuous removal of a slurry having a high concentration of solids from a cycle reaction zone are available in U.S. Patent No. 6,239,235.

During the recovery of the polymer from a continuous suspension cycle process, it is advantageous to recover the diluent at a sufficiently high pressure (in other words, an intermediate pressure) so that it can be condensed without compression in a heat exchanger at a temperature normally available with cooling tower water. Compression requires energy and capital investment. An intermediate pressure is normally lower than the pressure within the loop reactor but higher than atmospheric pressure. Additional details regarding the use of an intermediate pressure vent tank are available in US Patent No. 4,424,341. However, the polymer has typically been subjected to low pressure at some point during the recovery process in order to minimize the amount of residual diluent and other reaction components left on the polymer prior to extrusion or other processing.

One technique for transferring polymer solids from an intermediate pressure zone to a lower pressure zone is to use two valves with a swelling chamber between the valves. Said system operates by (1) opening the upper valve when the lower valve is closed; (2) flow of the polymeric solids into the foaming chamber; (3) closing the upper valve while the lower valve remains closed, until the level of polymer solids in the foaming chamber reaches a desired level, preferably as complete as possible. The top valve can be closed based on time or level measurement; (4) from this moment the lower valve opens while the upper valve remains closed, in this way the polymer and any gas (and any non-vaporized diluent or other liquid) go down to a zone or lower pressure vessel, such as as a purge column; and (5) closing the lower valve when the level of polymer solids in the foaming chamber reaches a desired level, preferably when the foaming chamber is substantially empty. The sequence of opening and closing the valves can also be based on time or a level measurement. Process steps (1) to (5) can be repeated as necessary to move the polymer solids from the intermediate pressure zone to the lower pressure zone. The system can also be operated so that the foam chamber is depressurized before the lower valve opens and is repressurized before the upper valve opens.

This technique comprising the foaming chamber and the valves is efficient since it reduces or minimizes the pressure loss in the upper pressure zone due to the transfer of polymeric solids. It is typically more efficient (i.e. less high pressure low gas at lower pressure per unit amount of polymer) than a continuous system because the present system always has a closed valve between the high pressure zone and the low pressure zone. The continuous process uses an open pathway or valves that open and thus have some constant leakage of high pressure gas and liquid (if any) between the zones. Maintaining a polymer level in the intermediate pressure tank above the open valve can help prevent pressure loss, but can increase the tendency of the valve to plug due to limited valve movement. Also, even with the polymer level maintained, there is still some high pressure gas leakage. In the present system, even when valves are provided, it is also advantageous to maintain a level in the high pressure zone during the time the top valve is open. The flow of the high pressure gas to the low pressure zone is slowed by the flow through the winding path through the level of the polymer particles above the valve, so the valve can leak when it is closed.

Surprisingly, it may also be advantageous to provide a relatively small flow path for gas from a high pressure vent tank to the foaming chamber and / or from the foaming chamber to the purge column. While these flow paths cause some pressure loss, they can reduce the pressure drop across the ball valves so that the ball valves can be rotated easily, and wear on the valves and actuators is reduced.

The foaming chamber can be designed and constructed to prevent polymer build-up or sticking in it. One technique to avoid polymer build-up or sticking is to include a rotating mixer or scraper. A rotary mixer mixes the polymer solids, and the mixing prevents the polymer solids from remaining in a steady state too long. A rotary scraper scrapes down the sides of the foaming chamber and removes polymer solids that are set to the sides. Another technique to prevent build-up or plugging of polymer solids is to provide the foaming chamber with a surface finish that facilitates uniform flow of polymer solids from the reactor. An advantage of providing uniform flow through the foaming chamber is that the polymeric solids do not segregate and the polymer particles have the same residence time in the foaming chamber, which promotes a uniform product composition. The foaming chamber may have a surface finish and a sloping bottom that is adapted for the type of resin, to ensure flow of the polymer solids out of the foaming chamber. For example, the foaming chamber may have a surface finish of polished carbon steel, stainless steel, or other suitable material of construction for the pressure vessel.

FIG. 1 shows a loop reactor 10 having major segments 12, minor top segments 14, and minor bottom segments 16. These minor top and bottom segments define the upper and lower zones of the flow. The upper and lower minor segments can be horizontal segments that join

ES 2 340 993 T3 major segments per elbow. Alternatively, the upper and / or lower segments can be continuous curved structures, such as two joined elbows. The reactor can be cooled by conventional means such as double pipe heat exchangers 18. Each segment is connected to the next segment by a smooth bend or bend and thus provides a continuous flow path substantially free of internal obstructions. The polymerization mixture circulates by means of a motor-driven impeller (not shown) 24. An elongated hollow end for continuous output of an intermediate product suspension is generally referred to as reference character 34. Continuous output mechanism 34 is sample located adjacent to a downstream end of one of the lower horizontal reactor sections 16 and adjacent to a connector elbow.

The continuous outlet end is shown at the downstream end of a lower horizontal segment of the loop reactor. Another possible location is at the bottom of the elbow of a major descending segment (based on the direction of flow). The location can be exactly before the point of introduction of the catalyst to allow the fresh catalyst the maximum possible time in the reactor before first passing an exit point. However, the continuous outlet end can be located on any segment or on any elbow. As already indicated, however, the present invention applies equally to alternative outlet systems, such as settling columns and / or non-batch discontinuous outlet systems.

Preferably, much of the liquid diluent vaporizes rapidly at the continuous outlet end as pressure drops, and the remaining liquids are vaporized as heat is added and pressure further drops in the evaporation lines before entering the tank. intermediate pressure. The intermediate product suspension (reactor effluent) passes from the continuous outlet end 34 via evaporation line 36 to the intermediate pressure chamber 28. In some embodiments, water, hydroxide, alcohol, or other catalyst deactivators are generally injected through the water injection conduit 26 to "destroy" the catalyst and / or cocatalyst or render it inactive. Because these are catalytic poisons, they must be removed or at least essentially removed from any material recycled to the reactor.

The evaporation line 36 is surrounded by a heat exchange conduit 40 that is provided with a heated fluid, which provides indirect heating to the material (suspension, vapor, liquid and solids) in the evaporation line 36. This constitutes a "heater evaporation line ”that heats the reactor effluent, or at least prevents excessive cooling of the effluent, before the effluent passes into the intermediate pressure vaporization chamber 28. Surrounding conduit 40 may be essentially the same length as evaporation line 36 or it may be of shorter sections.

In some systems that use an evaporation line heater, some or all of the diluent will instantly vaporize in evaporation line 36 before reaching intermediate pressure chamber 28. However, the terms "vaporization chamber" and "tank Vent ”are still frequently used for the tank that follows the evaporation line, where the vaporized diluent is separated from the polymer solids. The "vent tank" or "vaporization chamber" is still used even though there is little or no "flash" in the vent tank if all or substantially all of the diluent has already been vaporized in the evaporation line. In current designs that have evaporation lines that discharge at higher pressures and without downstream drying devices, attempts are made to design the evaporation lines so that there is little or no pressure drop when entering the vent tank. Usually the pressure drop in the evaporation line increases along its distance, as it progresses further from the loop reactor. The steam becomes less dense with the reduction in pressure and the speed of the material increases. Consequently the pressure drop per unit distance increases to the highest point as the material enters the vent tank. Pressure drops and flash vaporization can occur throughout the entire evaporation line heater. Flash vaporization can also occur, although it is not preferred herein, in the vaporization chamber, but preferably in essentially all liquids that vaporize prior to entering the vaporization chamber.

Maintaining the polymer solids in a vent tank under intermediate pressure allows time for the hydrocarbons incorporated in the polymer solids to escape and equilibrate with the concentration in the vapor space. The present process and apparatus can be applied to systems in which all, substantially all, substantially none, or some portion between all and none of the diluent is vaporized in evaporation line 36 prior to introduction into intermediate chamber 28. Accordingly, the present process and apparatus can be applied to systems in which all, substantially all, substantially none, or some portion between all and none of the diluent actually vaporizes in vaporization chamber 28.

In the process and apparatus illustrated in FIG. 1, vaporization chamber 28 is an intermediate pressure chamber, eg, the intermediate pressure chamber of a two-stage flash vaporization system discussed in Hanson et al. US Patent No. 4,424,341. Preferably, the intermediate pressure chamber or zone can be operated at a pressure within the range of 689-10342 kPa (7-105 kg / cm²), preferably 862-1896 kPa (8.8-19 kg / cm²), more preferably 1034-1724 kPa (10.5-17.6 kg / cm²) or 896-1586 kPa (9.1-13.4 kg / cm²). The intermediate pressure chamber or zone can operate at a temperature within the range of 100-250 ° F (37.8121 ° C), preferably 130-230 ° F (54.4-110 ° C), more preferably 150-210 ° F (65.6-98.9 ° C) or 170-200 ° F (76.693.3 ° C). The preceding values ​​can be approximate. In general, the narrower ranges are particularly suitable for polymerizations using 1-hexene comonomer and isobutane diluent, and the wider ranges are particularly suitable for higher 1-olefin comonomers and hydrocarbon diluents.

IS 2 340 993 T3

Most of the non-solid components of the polymerization effluent will form a vapor which is extracted via the evaporation gas line 42. This vapor (or evaporation gas) is mainly the diluent isobutane. It also contains most of the unreacted ethylene and 1-hexene monomers. Polymeric solids containing a minor amount of solvent and incorporated monomers can be continuously removed via the solids outlet 44 which is followed by a solids outlet check valve 46.

The intermediate pressure chamber 28 is equipped with a polymeric solids level sensor 38. The level sensor can be any sensor used to measure the level of solids in a tank, including vibration point level sensors, reflectometric devices capacity and time domain (TDR) that have direct contact with the material being measured, as well as ultrasonic and radar technologies that use non-contact devices. The level sensor can be a point level measurement sensor or a continuous level measurement sensor. Nuclear gamma devices are also suitable and can be mounted to measure the level over a wide range.

The flow rate of polymeric solids being drawn from intermediate pressure chamber 28 via solids outlet 44 is controlled by manipulation of control valve 46. Manipulation of the control valve can be accomplished by establishing a first signal representative of the actual level of polymer solids in vaporization chamber 28; establishing a second signal representative of a desired level of polymer solids in vaporization chamber 28 (which may be predetermined or entered manually or automatically calculated based on other parameters); comparing the first signal and the second signal and establishing a third signal that responds to the difference between the first signal and the second signal; and manipulation of the control valve that responds to the third signal. The first signal is obtained from the level sensor while the second signal is preferably an input based on the desired level of polymer solids. Any type of suitable control system can be used to manipulate the control valve, including stand-alone or integrated controller systems.

The control system can be configured to manipulate the control valve so that the residence time of the polymer solids is kept at a desired level. The residence time of the polymeric solids is preferably kept substantially from zero to 2 minutes. Alternatively, the residence time of the polymeric solids is preferably kept in the range of 10 seconds to 30 minutes. Alternatively, the residence time of the polymeric solids is preferably kept in the ranges of 30 to 90 minutes or 30 to 120 minutes.

By maintaining a desired level of olefinic polymer particles in an intermediate pressure zone, the residence time of polymer solids, which is the average amount of time that a polymer particle spends in the intermediate pressure zone, can be controlled. Increasing the residence time of the polymer solids allows more diluent, including more built-in diluent, to be vaporized and / or separated from the polymer solids, thereby increasing the purity and processability of the polymer exiting the zone. Additionally, by maintaining a desired level of polymer solids in the intermediate pressure zone, a pressure seal can be created between the zone and downstream equipment. Additionally, operating and maintenance costs are reduced by providing a pressure seal between the intermediate pressure zone and the purge zone that does not require the use of on / off valves. Additionally, the need to separate the foaming chamber can be eliminated. The pressure seal may reside at the level of the polymer solids to restrict the flow of diluent gas or liquid (if present) outside of the intermediate pressure zone. The polymeric solid particles can substantially close most of the flow path (cross-sectional area) available to the diluent. However, it is contemplated that a small proportion of the flow path may be available through small gaps between adjacent particles. This small continuous flow can reduce the ultimate recovery efficiency of the diluent in the intermediate pressure zone.

The polymeric solids are removed via the solids outlet 44 and into a purge column 48, or purge zone. The purge gas normally enters the bottom of the purge column and exits the top of the purge column along with any purged diluents and monomers. While any suitable non-combustible gas can be employed as the purge gas, a purge gas consisting essentially of nitrogen is preferred.

The present process and apparatus can also be used in a system that includes a foaming chamber that uses on / off valves to maintain a pressure seal between the intermediate pressure zone and the purge zone. In such a system, a flash zone outlet line (flash zone outlet conduit) can be used as the primary and secondary flash zone outlet line.

In some systems, polymer solids can be continuously drawn out of the intermediate pressure zone. The process may include controlling the flow rate of the polymeric solids through the solids outlet from the intermediate pressure zone by manipulating a solids outlet control valve. The process may also include the steps of establishing a first signal representative of the actual level of the polymer solids in the intermediate pressure zone; establishing a second signal representative of a desired level of the polymer solids in the intermediate pressure zone; comparing the first signal and the second signal and establishing a third signal that responds to the difference between the first signal and the second signal; and manipulating the solids outlet control valve in response to the third signal.

IS 2 340 993 T3

Fig. 2 shows a system that includes a foaming chamber and cycle valves. In this system the loop reactor 10 comprises larger segments 12 joined by upper curved segments 14a and lower curved segments 16a. Foam chamber 50 is located after intermediate pressure chamber, or more particularly after control valve 46 for solids outlet 44. The control valve 46 may be a cycle ball valve located upstream of the foam chamber 50, and a second cycle ball valve 52 can be located downstream of the foam chamber 50. The foam chamber 50 can act as a descent chamber, to lower the pressure of the transferred material from the intermediate pressure chamber 28.

Cycle ball valves 46 and 52 operate so that the upper valve opens while the lower valve is closed. During this period, the foaming chamber 50 is filled with polymeric solids from the intermediate pressure vaporization chamber 28, at a desired level or quantity of polymeric solids, not to exceed the maximum allowable capacity. When the desired level or amount is reached, the upper valve 46 closes and the lower valve 52 opens, and the polymer solids are transferred to a relatively lower pressure vessel, such as a purge column 48. These steps are repeated as appropriate to transfer material from intermediate pressure chamber 28 to purge column 48.

The opening and closing of the valves may be based on a level controller or gauge 54 associated with the foam chamber. The valves can be operated so that the foam chamber is greater than 85 percent full, alternatively greater than 95 percent full, alternatively substantially full. Pressure drop and wear on valves 46 and 52 can be minimized by maintaining a first gas passage 56 for gas from intermediate pressure chamber 28 to foaming chamber 50 and a second gas passage 58 from gas chamber. puff 50 to the lower pressure destination 48. This batch transfer process produces less gas leakage to the lower pressure destination 48 than a continuous process.

The intermediate pressure chamber can be supported on the purge column. The solids exiting the intermediate pressure chamber are brought into the purge column by gravity through the foaming chamber isolated by two valves. The foam chamber has a cleaning filter and valves to isolate the operational valves for maintenance. The structure supports the intermediate pressure chamber, the foaming chamber, and the operating and isolation valves in the air.

Alternatively, the design can be modified to replace the foaming chamber between the intermediate pressure zone and the purge zone with a carrier tank, which is another technique for transferring polymer solids from an intermediate pressure zone to a pressure zone. lower. Carrier tanks are also known as dump tanks or as a solid pump (see Zenz, "Fluidization and Fluid-Particle Systems" 1989, p. 665). A carrier tank can be used in place of the foaming chamber and the valves between the intermediate pressure zone and the purge zone (or other lower pressure zone). In the present improvement, the transport tank can be combined with the chamber containing the intermediate pressure zone, with the upper half of the chamber being a high-efficiency cyclone. A metal filter can be used on top of the cyclone.

Fig. 3 shows the arrangement including a carrier tank after an intermediate pressure tank. In Fig. 3, a stream comprising polymer solids, vaporized diluent and / or liquid, and other components of a suspension polymerization are incorporated through inlet 60 to an intermediate pressure tank 62. The outlet of this tank 62 It is equipped with a manual door 64 that can be closed when you want to seal the outlet for an unusual purpose. Manual door 64 will generally remain open during normal operation, during which an outlet valve 66 regulates the flow of material out of tank 62. Material (polymer solids or a concentrated slurry) passes through valve 66 into the conveyor tank. 68. A vent line 70 is provided between the transport tank 68 and the intermediate pressure tank to allow easier filling of the transport tank 68 and avoid back pressure that could impede the flow of material. Vent line 70 is equipped with vent valve 72. When the transport tank 68 is substantially full or the material present there reaches a desired level, the vent valve 72 and the outlet valve 66 are closed and sealed. High pressure gas, such as pressurized air, is gradually incorporated to the top of the conveyor tank 68, and the material is forced by the gas pressure through the outlet of the conveyor 74 to the next stage of processing. In this way, the material of the intermediate pressure tank 62 can be transported mainly by a force other than gravity such as the pressure of the high pressure gas, and it is not necessary to suspend the intermediate pressure tank 68 above the ground. The material from the transport tank 68 will normally pass into a receiver tank 76 where the material is separated from the high pressure gas. The level of the material in the transport tank 68 can be detected by a level detector 78. The high pressure gas can be supplied by an air compressor 80.

Several advantages are obtained with these designs that employ transport tanks: (1) Structure and cost are minimized since the intermediate pressure tank can be located closer to the ground level, (2) The valves for the transport tank are Smaller and less expensive than the valves associated with the foaming chamber, (3) Heat can be added to the foaming in the carrier tank line that could decrease the amount of hydrocarbon transferred.

Another option is to effectively combine the intermediate pressure tank and the carrier tank as a single tank. The upper part of the tank could be designed as a high efficiency cyclone and a metal filter could be used on the upper part of the cyclone. When the lower valve of the transport tank opens, the material is

ES 2 340 993 T3 is carried by the evaporating gas from the high pressure vent tank at a gauge pressure of approximately 135 psi (930 kPa) to the top of the purge column. Polymer transport can take place by dense phase transport until the line is clear. Alternatively, the conveyor line could operate continuously with the conveyor valves fully or partially open for coarse flow adjustment. Fine adjustment of the flow and speed of solids transport can be obtained by varying the amount of reinforcing gas added to the dense phase conveyor line. This gas velocity can be controlled with the level at the bottom of the conveyor. Consequently, this eliminates the cyclical movement of the very large valves normally used below the vaporization chamber and also the cyclical movement of the 2 transport tanks.

Alternatively or additionally, two carrier tanks can be used in combinations of vent tanks and closing chamber cyclones. A replacement cyclone transporter tank would allow cleaning of polymer fragments, should they occur. Polymeric solids and diluent from the reactor discharge can enter one of the transport tanks tangentially. Polymeric solids can fall to the bottom and gas can flow out of the top. When a sufficient level is reached in the carrier tank, the flow can switch to the other carrier tank. The valve at the bottom of the first carrier tank opens and the dense phase flow of the slurry is sent to the top of the purge column. When the conveyor empties of solids, indicated by a characteristic sound or a level detector, the lower valve closes. When the second tank is full, the flow from the reactor switches back to the first tank and the second tank empties. Adjusting the time or controlling the level in the tank would prevent the tank from being completely empty and leading to the release of high pressure evaporation gas, so that the ball valve at the bottom of the conveyor sends the minimum of gas evaporation to the purge column.

The advantages of a system combining the intermediate pressure tank and the transport tank include: (1) the structure of the vaporization chamber and the purge column can be minimized. Plants without high pressure vent tanks can be retrofitted with high pressure flash vaporization more economically (2) Cleaning filters can be located at ground level in a location away from the reactor and finish. (3) The valves for the conveyor will probably be much smaller than the valves for the closing chamber. (4) The smaller valve would not have to cycle frequently to open and close and consequently would have a much higher vapor factor. This is a way to continuously control the flow of solids and gas without a cyclic valve in suspension service. (5) Heat can be added to the fluff in the transfer line to the purge column by using more heaters from the evaporation line.