POLYMERIZATION PLANT

Field of the invention

[0001] The present disclosure relates to an apparatus and method for withdrawing samples from a unit of a polymerization plant. More particularly, the disclosure relates to an apparatus and a method for withdrawing samples of polymer from a unit of a plant for the polymerization of olefins.

Background of the invention

[0002] The polymerization of olefins, e.g. ethylene, propylene, butene and/or higher olefins, is carried out in polymerization plants comprising several units, including reactors, transfer lines, filters, and tanks. To control the polymerization process it can be necessary or useful to withdraw samples of the olefin polymer from one or more of such units and analyse the polymer. Therefore, one or more units of the polymerization plant are typically equipped with means for withdrawing samples from such units.

[0003] WO 2005/080441 Al describes a method for improving a polymerization reaction by taking out and analyzing samples from one or two polymerization reactors. The sampling system comprises conduits to remove samples from the reactor and transfer them to a flash tank, from which the sample is transferred to a couple of sample receivers. The system includes valves controlling the flow of the sample from the reactors to the samples receivers and degassing of the samples.

[0004] Withdrawal of material from a polymerization reactor is a critical operation since it requires acting on a nozzle or port provided in the reactor wall. During sampling, the line downstream of the sampling port or nozzle is at the same pressure of the reactor, thus it requires blowing down the sample to atmospheric pressure while guaranteeing safety of the operation. Moreover, safety must be guaranteed for any maintenance activity to be performed on any component of the sampling system.

There is a need, therefore, for an improved apparatus and method for withdrawing samples from a reactor, or any other unit of a polymerization plant, which ensures optimum performance and safety of operation.

Summary of the invention

[0005] According to a first aspect, the present disclosure addresses this need by providing an apparatus for withdrawing a sample from a unit of a polymerization plant provided with a sampling nozzle, said apparatus comprising a sampling valve suitable to be connected to said sampling nozzle, a transfer conduit operatively connected at one end to said sampling valve and at the opposite end to a first sample collection tank provided with a gas discharge conduit and a solid discharge port, characterized in that said apparatus further comprises:

a first control valve mounted on said transfer conduit downstream of said sampling valve, controlling the flow of said sample into said transfer conduit; a second control valve mounted on said transfer conduit downstream of said first control valve and upstream of said first sample collection tank, controlling the flow of said sample into said first sample collection tank;

a third control valve mounted on said gas discharge conduit of said first sample collection tank;

a second sample collection tank, located below said first sample collection tank and connected to said first sample collection tank via a fourth control valve mounted on said solid discharge port, said fourth control valve being provided with a locking device;

a solid discharge conduit connected to a discharge port at the bottom of said second sample collection tank;

a fifth control valve mounted on said solid discharge conduit, said fifth control valve being provided with a locking device;

a sixth control valve, mounted on said solid discharge conduit downstream said fifth valve; and

a flushing system for said conduits and said valves;

said locking devices of said fourth and fifth control valves being operated by a single key extractable only when said locking devices are locked, whereby opening of one of said fourth and fifth valves is possible only when the other of said fourth and fifth valve is closed.

[0006] According to another aspect, the present disclosure provides a method for withdrawing samples from a unit of a polymerization plant provided with a sampling nozzle, wherein said unit operates at a pressure above the atmospheric pressure, by means of an apparatus comprising:

a sampling valve suitable to be connected to said sampling nozzle;

a transfer conduit operatively connected at one end to said sampling valve and at the opposite end of a first sample collection tank provided with a gas discharge conduit and a solid discharge port;

a first control valve mounted on said transfer conduit downstream said sampling valve, controlling the flow of said sample into said transfer conduit;

a second control valve mounted on said transfer conduit downstream of said first control valve and upstream of said first sample collection tank, controlling the flow of said sample into said first sample collection tank;

a third control valve mounted on said gas discharge conduit of said first sample collection tank;

a second sample collection tank, located below said first sample collection tank and connected to said first sample collection tank via a fourth control valve mounted on said solid discharge port, said fourth control valve being provided with a locking device;

a solid discharge conduit connected to a discharge port at the bottom of said second sample collection tank;

a fifth control valve mounted on said solid discharge conduit, said fifth control valve being provided with a locking device; a sixth control valve, mounted on said solid discharge conduit downstream said fifth valve; and

a flushing system for said conduits and said valves;

said locking devices of said fourth and fifth control valves being operated by a single key extractable only when said locking devices are locked;

said method comprising the steps of:

a. setting all said valves in the closed position and keeping said single key of the locking devices of said fourth and fifth control valves inserted in the locking device of said fifth control valve;

b. opening said first, second and third control valves and flushing said valves, said transfer conduit, said first sample collection tank and said gas discharge conduit with a flush gas provided by said flushing system;

c. closing said third control valve and opening said sampling valve to withdraw a sample of a polymerization material from said unit of a polymerization plant and transferring it via said transfer conduit to said first sample collection tank;

d. detecting that said sample is present in said first sample collection tank;

e. opening said third control valve to remove gas and vaporizable liquid from said sample collected in said first sample collection tank;

f. extracting said single key from the locking device of said fifth control valve;

g. inserting said single key in the locking device of said fourth control valve and opening said fourth control valve, whereby said sample collected in said first sample collection tank is discharged by gravity in said second sample collection tank;

h. closing said fourth control valve and extracting said single key from the locking device of said fourth control valve;

i. inserting said single key in the locking device of said fifth control valve and opening said fifth control valve; and

j. opening said sixth control valve mounted on said solid discharge conduit, whereby said polymerization material collected in said second sample collection tank is discharged through said discharge conduit.

[0007] In the present description the term“units of a polymerization plant” means any device or equipment that contains the polymer which is being formed, or is already formed as a result of the polymerization reactor, both in a rest state or in a transfer state. It includes reactors, transfer lines, filters, and tanks, without being limited to these. When operated, the unit is at a pressure Pi above the pressure P ₂ of the apparatus for withdrawing a sample. When the apparatus for withdrawing a sample is not in fluid communication with the unit of the polymerization plant, the pressure P ₂ of the apparatus is typically the atmospheric pressure, or slightly above.

In the present description, the term“sampling valve” means a type of valve that allows taking a representative portion of a solid or of a mixture of a solid with a liquid and/or a gas for the purpose of testing. [0008] In the present description, the term“control valve” means a valve that allows controlling the flow of a material by varying the size of the flow passage as directed by a signal from a controller.

Description of the figure

[0009] Fig. 1 is a schematic representation of an apparatus for withdrawing samples from a unit of a polymerization plant according to an embodiment of the present disclosure.

Detailed description of the invention

[0010] The disclosure is now described with reference to Fig. 1, in which 10 designates a portion of polymerization unit containing an olefin polymer, for example polyethylene or polypropylene.

[0011] The polymerization unit 10 is, for example, a polymerization reactor of any type, such as a reactor for slurry polymerization or for gas-phase polymerization, including loop reactors. Unit 10 operates typically at a pressure Pi above the atmospheric pressure. If unit 10 is a polymerization reactor, it may operate at pressures of up to 10 MPa, depending on the type of polymerization process and reactor.

[0012] Unit 10 is provided with a sampling nozzle 12 from which a sample of the polymer or of the polymer slurry can be withdrawn for the purpose of testing. Nozzle 12 is preferably located in a point of the reactor in which the polymer is in a densified form or is settled. If the reactor is a gas-phase reactor of the type having interconnected polymerization zones, where the growing polymer particles flow through a first polymerization zone (riser) under fast fluidization or transport conditions, leave said riser and enter a second polymerization zone (downcomer), the sampling nozzle 12 is located in the downcomer.

[0013] The nozzle 12 is typically formed or equipped with a reinforcing pad to ensure stability and sealed connection between unit 10 and the sampling apparatus.

Withdrawal of samples is carried out by an apparatus that comprises at one end a sampling valve 14 adapted to be connected to the sampling nozzle 12.

[0014] Sampling valve 14 is preferably a piston valve. It is directly coupled to the sampling nozzle 12 so that, when the valve is closed, the piston element is pushed against the wall of the unit and the end surface of the piston element is substantially aligned with this wall. Therefore, no section of valve can be filled by the polymerization material, thus assuring a continuous operability of the sampling nozzle. This is an important advantage compared to other types of valves, particularly when the unit from which the sample is withdrawn is a reactor. In fact, if another type of valve is used, a recess on the reaction wall is formed and the growing polymer particles may enter part of the valve body, with undesirable polymerization and consequent clogging of the valve itself.

[0015] Sampling valve 14 is actuated by an operator by means of a servopneumatic device 15, for instance by pressing a button on a control panel. Valve 14 remains in the open position until the operator keeps the button pressed, and switches to the closed position when the operator releases the button. Since the pressure IF of the sampling apparatus is well below the pressure Pi of the unit 10, the transfer of material is caused by the difference of pressure. The pressure IF of the sampling apparatus is typically the atmospheric pressure, or slightly above the atmospheric pressure. The duration of the sampling step is determined manually by the operator. This ensures flexibility and safety of the sampling step.

[0016] The sample withdrawn by means of a sampling valve 14 enters transfer conduit 16 via a first control valve 24 mounted on said transfer conduit 16 downstream the sampling valve 12. Control valve 24 is preferably a ball valve, more preferably a full-bore ball valve. It allows for controlling the flow of the sample and, when it is in the closed position, it ensures that no material flows into the transfer conduit 16. A ball valve provides a higher degree of certainty of blocking the passage of material than a piston valve, particularly when the pressure within unit 10 is well above the atmospheric pressure, for example 4-5 MPa, as it occurs in many polymerization processes.

[0017] A second control valve 26 is mounted on the opposite end of transfer conduit 16, downstream of the first control valve 14 and upstream of a first sample collection tank 18.

As with control valve 24, control valve 26 is preferably a ball valve, more preferably a full-bore ball valve. The association of the first and second control valves 24 and 26 ensures, when both are in the closed position, that no material flows from the transfer conduit 16 toward the first sample collection tank 18, thus allowing, for example, to carry out maintenance activities in the sample collection tank 18.

[0018] A fluid communication unit 10 and the first sample collection tank 18 is thus established only when the sampling valve 14, the first control valve 24 and the second control valve 26 are open. In this condition the pressure within the first sample collection tank 18 is the same as the pressure within unit 10, namely Pi = P ₂ .

The first sample collection tank 18 comprises a top portion provided with a gas discharge conduit 20, and a bottom portion provided with a solid discharge port 22.

A third control valve 28 is mounted on gas discharge conduit 20 of the first sample collection tank 18. By opening this valve it is possible to discharge pressurized gases from the sample transferred to collection tank 18. As with control valve 24 and 26, control valve 28 is preferably a ball valve, more preferably a full-bore ball valve. The solid fraction of the sample contained in collection tank 18 can be removed through the discharge port 22.

[0019] The gas discharge conduit 20 is also equipped with a differential pressure gauge 21 upstream of control valve 28, to measure the pressure in the collection tank 18. Differential pressure gauge 21 is of the type having a sensor cell containing a membrane that adheres to the wall of the gas discharge conduit 20, so that substantially no discontinuity is formed on the wall of conduit 20, and no deposit of polymer occurs.

The gas discharge conduit 20 is also equipped with a filter 29 downstream of control valve 28, to remove solid particles entrained by the gas leaving collection tank 18. The gas stream is then preferably transferred to a blow-down low-pressure system 31 located downstream of filer 29, to which other discharge gases are transferred to be treated or eliminated, for example by means of a flare.

[0020] The discharge port 22 of the first sample collection tank 18 is equipped with a fourth control valve 32. As with control valve 24, 26 and 28, the fourth control valve 32 is preferably a ball valve, more preferably a full-bore ball valve. A second sample collection tank 30 is located below the first sample collection tank 18 and is connected to it via the fourth control valve 32, described above. The fourth control valve 32 is operated manually and is provided with a locking device, as will be described in the following.

[0021] The second sample collection tank 30 has a bottom portion provided with a solid discharge port 34, connected to a solid discharge conduit 36 via a fifth control valve 38, also provided with a locking device, and a sixth control valve 40, mounted on said solid discharge conduit downstream of said fifth valve 38.

[0022] The fifth control valve 38, as with the fourth control valve 32, is operated manually and is provided with a locking device. As with the other control valves, the fourth and fifth control valves 32, 28 are preferably ball valves, more preferably full-bore ball valves.

[0023] By opening the fourth control valve 32 the sample contained in the first sample collection tank 18 is discharged by gravity into the second sample collection tank 30, and by opening the fifth control valve 38 and the sixth control valve 40 the sample contained in the second sample collection tank 30 is discharged from the sampling apparatus to the external environment, for example into a suitable container or line for carrying out the analysis.

[0024] To ensure safety of operation, the locking devices of the fourth 32 and fifth 38 control valves are operated by a single key, which is extractable only when the locking device is locked. The single key typically has the shape of a card bearing a code that is recognized by the locking device.

[0025] Therefore, opening of the fifth control valve 38 requires the use of the same key used to lock the fourth valve 32, which can be extracted from the locking device of the fourth valve 32 only if the latter is closed. Thus, the fourth control valve 32 and the fifth control valve 38 are operated manually, and opening the fifth valve 38 is possible only when the fourth valve 32 is closed, and vice versa.

[0026] The features described above for the fourth 32 and fifth 38 control valves prevent both valves being open. Since at least one of these valves must be closed, this ensures that the sampling apparatus, and the unit 10, are always isolated from the external environment, even if the sixth valve 40 is inadvertently open.

[0027] The fourth 32 and fifth 38 control valves provided with locking devices operated by a single key are known in the field also as“car-sealed valves”. These valves are commercial devices available on the market and known to persons skilled in designing, manufacturing and operating industrial plants. Therefore, they are not described in detail here.

[0028] The sixth control valve 40 is actuated by an operator by means of a servopneumatic device 41, for instance by pressing a button on a control panel. Valve 40 remains in the open position until the operator keeps the button pressed, and switches to the closed position when the operator releases the button. Therefore, the amount of the sample withdrawn is determined manually by the operator. This ensures flexibility and safety of the sampling step, like for sampling valve 14. [0029] The apparatus for withdrawing samples according to the disclosure is provided with a flushing system for cleaning conduits and valves from deposits of polymer and residues of liquid and gas.

[0030] The flushing system comprises sources 42 and 44 of a cleaning gas, for example nitrogen, suitable to be circulated under pressure into the conduits, valves, tanks and filters of the sampling apparatus. Source 42 is connected via a flush conduit 43 to a first flush valve 45, which is mounted upstream of sampling valve 14. Source 44 is connected via a flush conduit 46 to a second flush valve 48, which is in turn connected to the bottom of the second sample collection tank 30 via line 49.

[0031] In an alternative embodiment, there can be just one source of nitrogen connected to lines 43 and 46. In any embodiment, the cleaning gas introduced into line 43 cleans conduit 16 and the valves mounted on it, and the cleaning gas introduced into line 46 cleans the first and second sample collection tank 18 and 30, as well as conduit 20 and the valves mounted on it.

[0032] According to another aspect, the present disclosure provides a method for withdrawing samples from a unit of a polymerization plant by means of a sampling apparatus as defined above.

[0033] As said above, the pressure Pl within unit 10 is above the atmospheric pressure. If unit 10 is a polymerization reactor, it may operate at pressures of up to 10 MPa, depending on the type of polymerization process. In many polymerization processes the pressure is about 4-5 MPa. When the sampling valve 14 and the first control valve 24 are open, the pressure within conduit 16 of the sampling apparatus is the same as the pressure within unit 10, and when the second control valve 26 is also open, the pressure in the first sample collection tank is the same as the pressure of unit 10.

[0034] The method for withdrawing samples from unit 10 comprises a sequence of steps.

[0035] In step (a) all the valves of the apparatus are set in the closed position. The single key of the locking devices of the fourth (32) control valve and fifth (38) control valve is inserted in the locking device of the fifth (38) control valve.

[0036] In step (b) the first (24), second (26) and third (28) control valves are opened. By operating the flushing system, the flush gas is released from source 42 into line 43, and, by opening valve 45, into transfer conduit 16, the first sample collection tank (18) and the gas discharge conduit 20. The flush gas cleans the apparatus of deposits and residues and prepares it for carrying out sampling.

[0037] In step (c) the third valve (28) is closed and the sampling valve (14) is opened for a sufficient time to withdraw a sample of a polymerization material from unit 10, as a consequence of Pl being greater than P2. The sampling valve 14 is actuated by an operator by means of servopneumatic device 15, by pressing a button on a control panel. The sampling valve 14 remains in the open position until the operator keeps the button pressed, so that the duration of the sampling step is determined manually by the operator. This ensures flexibility and safety of the sampling step. Since the first (24) and second (26) control valves are opened, the sample is transferred into the first sample collection tank 18. [0038] Step (d) comprises detecting that the sample of polymerization material is actually present in the first sample collection tank 18. This is done by checking the differential pressure gauge 21 mounted on the gas discharge conduit 20, which measures the pressure in the sample collection tank 18.

[0039] In step (e) the control valve 28 is opened to remove gas and vaporizable liquid from the sample present in the first sample collection tank 18. This leaves in tank 18 only the solid part of the sample, namely the polymer. Also, the pressure P2 in the tank 18 is about atmospheric pressure. The gases and vaporizable liquid released from tank 18 are filtered by filter 29 and transferred to a low-pressure blow down system 31, and eventually to a flare.

[0040] In step (f) the said single key of the locking device of the fifth (38) control valve is extracted.

[0041] In step (g) the said single key is then inserted in the locking device of the fourth (32) control valve, so that this fourth valve 32 can be opened. This causes discharging by gravity the sample in the second sample collection tank 30.

[0042] In step (h) the fourth control valve 32 is closed and the single key is extracted from the locking device of the fourth valve 32.

[0043] In step (i) the single key is inserted in the locking device of the fifth control valve 38, so that the fifth control valve 38 can be opened.

[0044] In step (j) the sixth control valve 40 mounted on the solid discharge conduit 36 is opened. This causes discharging by gravity the sample in the external environment through the discharge conduit 36. Since the sixth control valve 40 is actuated by an operator by means of a servopneumatic device 41, for instance by pressing a button on a control panel, valve 40 remains in the open position until the operator keeps the button pressed, and switches to the closed position when the operator releases the button. Therefore, the amount of the sample withdrawn is determined manually by the operator. This ensures flexibility and safety of the sampling step, like for sampling valve 14.

[0045] The method of the disclosure requires performing the withdrawal of samples through a predetermined sequence, thus preventing or minimizing human or system errors.

[0046] The apparatus and the method of using it are such that between the unit 10 of the polymerization plant and the external environment multiple valves are present. This is an important feature to prevent accidents due to possible exposure of humans to a reaction environment. In particular:

the sampling valve 14 is associated with the first control valve 24 and control valve 26, thus ensuring isolation of unit 10 from the first sample collection tank 18;

the sampling valve 14 is actuated by an operator by means of a servopneumatic device, so that the duration of the sampling step is determined manually by the operator; the fourth 32 and fifth 38 control valves are equipped with locking devices operated by a single key, thus ensuring that at least one of them is closed;

a sixth control valve 40 is associated with the fifth control valve 38, thus ensuring isolation of the second sample collection tank 30 from the external environment; and the sixth control valve 40 is controlled by a servopneumatic device 41 actuated by an operator, so that the amount of the sample withdrawn is determined by the operator.

[0047] Therefore, use of a defined sequence of steps combined with locking devices associated to selected valves opened only by a single key mitigates risks of human error and provides optimum safety of operation.