EP0710328B1 - Continuous conveying process and device for shear-sensitive fluids - Google Patents

Abstract

A process is disclosed for continuously conveying shear-sensitive fluids, in particular polymer lattices or plastic dispersions when sampling, processing and measuring samples. Also disclosed is a device for carrying out the process, as well as a new double-long-stroke reciprocating piston pump.

Description

The invention relates to a method for the continuous delivery of shear-sensitive fluids, in particular polymer latices or plastic dispersions, a device for carrying out the method and a new double-length reciprocating piston pump.

It is known that polymer latices or plastic dispersions under shear stress e.g. can easily coagulate during funding, i.e. Solid masses (coagulate) separate out from the finely dispersed fluids, which can occupy or clog conveying elements and pipes. This coagulum can also settle on sensors of all kinds that dip into latex or come into contact with latex and falsify or prevent ongoing measurements. Especially latices, which have to be produced with a minimum of emulsifier out of consideration for the product quality, tend to form coagulate very easily.

Known conveyors are centrifugal, diaphragm or piston pumps, centrifugal pumps consist of a stator (housing) and a rotor (impeller). Due to the high speed of the impeller, the product to be conveyed is radially accelerated from the center of the rotor (fulcrum) and pressed by the corresponding centrifugal forces on the outer diameter of the impeller through the pressure port of the housing. As a rule, centrifugal pumps need a speed of more than 500 / min. At this lower speed, product delivery is no longer possible with this system. Due to the design features, centrifugal pumps have large dead space volumes and are not self-priming.

Diaphragm and piston pumps are positive displacement pumps that operate at lower frequencies than centrifugal pumps. The lowest frequency is 30 strokes / min. The conveying movement of the piston or membrane, the drive side also being moved by a piston is jerky, pulsations occur, so that one cannot speak of continuous product conveyance or transport in the short-term view. The pulsation is pressure-dependent and affects the delivery consistency and dosing accuracy.

Long-stroke piston pumps are known as single-stroke piston pumps, but are unsuitable and too expensive for the continuous metering of small quantities. (See Menges, Recker, Carl-Hanser-Verlag; Munich, Vienna; 1986, Automation in plastics processing, pp. 318-320).

A disadvantage of these conveyors is that, for example, centrifugal pumps can only deliver and build up pressure at high speed. Due to the high peripheral speeds on the rotor, product particles are very strongly sheared at the pump head gaps, so that a particle change takes place. Due to the high dead space volume, a centrifugal pump is unsuitable for conveying small quantities of products with subsequent sampling. For example, it is not possible to take a representative sample amount of, for example, 10 ml from a reactor and to infer reaction conditions in the actual reactor. Piston or Diaphragm pumps operate at too high stroke frequencies and, due to their design features, have many narrow gaps in which product or particle shear occurs. The non-constant speed of the displacement elements also has a product-changing effect. The uneven conveying speed causes different frictional forces on product-contacting pipelines or pump head parts, which cause product shear in the microparticle range. The residence time of the product in the pump head is not finite or not precisely defined in the known pump designs, since all pump heads have a high dead space volume. Large portions of a suction stroke volume remain in the pump head for a long time, with each new suction stroke only partial mixing with the old product takes place, so that product particles are subjected to shear stress for a long time. Another disadvantage is that the piston or diaphragm pumps have no positively controlled valves (ball valves) that open a large, free cross-section depending on the suction or pressure process. Another disadvantage is the fact that the suction and pressure processes are not synchronized. With these pump designs there is an absolute synchronous Working from the suction or pressure side is not possible, even if an improvement in the conveyance can be achieved by multiple heads. The individual processes that have to be considered in the individual pump head are always uneven due to the rotational acceleration of the control gear and the associated conversion of rotation into stroke movement and are the reason for the primary pulsation in the delivery flow.

Another disadvantage of known pump designs is that the dead space in the pump head increases as the volume flow becomes smaller. This means that the actual promoting displacement body changes its way of work in proportion to the delivery rate.

Single-piston pumps are very large, are very expensive and do not work continuously. The constructions do not allow self-priming, so that pumps are also required on the suction side. Their large dead space makes them z. B. Not suitable for sampling. Even with conceivable twin piston pumps, synchronous operation of the suction and pressure processes is not possible.

A delivery device is known from US Pat. No. 5,066,199, in which fluid delivery is made possible by means of a double piston pump in cooperation with two pairs of valves controlled by gravity. This device has the disadvantage that the gravity valves used in this case no longer close securely, particularly when the fluid to be pumped is of higher density. The construction of the double-piston pumps used has proven to be a particular disadvantage, in which the return of the pistons is to be achieved by means of a return spring which, even with the greatest possible compression, still includes an undesired dead space. Otherwise, the use of the control disk used in the device prevents the pump bodies from being completely emptied.

The invention has for its object to promote shear-sensitive fluids, in particular polymer dispersions, continuously, with little pulsation and gently, so that the fluids remain unaffected in their phase state by the promotion, and in particular with dispersions there is no phase separation or coagulation. In particular, it should be made possible to convey a polymer latex from a reactor into a pumping loop and back, avoiding the formation of coagulate or new particle formation. The invention is also based on the object of providing a device which enables this support to be carried out gently and over a long period of time without malfunction and reliably. In particular, the device should enable the fluid between different parts of the system, for. B. a reactor and a measuring loop with different pressure ratios, gently transfer back and forth. This object is achieved in that the fluid is sucked in and pumped by means of a dead space-free, low-pulsation double-length reciprocating piston pump and dead space-free valves are used in the piping system through which the fluid flows. This double-length reciprocating pump sits z. B. in a pressure loop, a defined amount of sample is transferred through a lock or an overflow valve, which serves as a connecting element to a parallel measuring circuit with a different pressure level, and in the parallel measuring circuit of a second long-stroke piston pump of the same type the actual on-line measuring devices is fed.

The invention relates to a method for the continuous delivery of shear-sensitive fluids, in particular polymer latices or plastic dispersions, with a viscosity of up to 100,000 mPa.s at a delivery rate of 10 ml / h to 100 l / h, characterized in that the fluid has at least one Dead space-free valve is sucked in by a piston of a double-long stroke piston pump, while synchronously fluid from the second piston chamber is also released to the delivery side via at least one further dead space-free valve and, after the second piston chamber has been completely emptied, the first valve is opened on the delivery side and closed on the suction side, while the second valve is discharged is closed and opened on the suction side and the direction of movement of the pistons is reversed synchronously, the pistons being seated on a spindle, the direction of movement of the pistons and the position of the valves being controlled via contact switches, and that in the piston chamber according to E Remaining volume remaining is less than 1% of the stroke volume. A preferred embodiment is characterized in that the fluid is conveyed in a pumping loop which is connected to a reactor is coupled. Sensors or sensors or internals can be located in this pump-around loop or parallel to it. Examples of such sensors are temperature sensors, pH electrodes, conductivity electrodes, NIR light guide probes, vibrating U-tubes for density measurements, refractometers, ultrasonic measuring heads or devices for calorimetry. The sensors or measuring devices mentioned are not occupied or blocked by the circulating fluid (for example a latex). In principle, it is possible to incorporate further mixing devices, such as static mixers or heat exchangers, into the pumping loop, which are not occupied or blocked due to the continuous pusation-free and low shear-stressing conveying device. Another preferred variant of the method is characterized in that the fluid is passed through an overflow valve or a lock into a region of reduced pressure (for example a secondary pumping loop). A variant of this method is particularly preferred, in which the overflow valve or the lock is connected as a coupling between the primary and secondary pump circuit. With this variant it is possible to branch off defined sample volumes of the fluid from the main flow line or a primary pumping circuit and, for example, to conduct a measurement under reduced pressure.

The invention is explained in more detail below by way of example using an exemplary embodiment illustrated in the drawings.

Fig. 1

Schema der erfindungsgemäßen Förderung von scherempfindlichen Fluiden

Fig. 2

schematische Abbildung des Förderverfahrens als Teil einer Umpumpschleife und zum Auskoppeln von Polymerisat

Fig. 3

Aufbau der erfindungsgemäßen Doppellanghubkolbenpumpe in einer Seitenansicht

Fig. 4

erfindungsgemäße Doppellanghubkolbenpumpe in Draufsicht.

Show it

Fig. 1

Scheme of the promotion of shear sensitive fluids according to the invention

Fig. 2

schematic representation of the conveying process as part of a pumping loop and for coupling out polymer

Fig. 3

Construction of the double-length reciprocating piston pump according to the invention in a side view

Fig. 4

Double-length reciprocating piston pump according to the invention in plan view.

To a reactor 1 for the emulsion polymerization of butadiene, which works under a pressure of> 5 bar, a pumping loop 2 is connected, which contains a new double-length reciprocating piston pump 3 with positively controlled inlet and outlet valves 4, 5 as a conveying device, and an overflow valve 6, which the primary circuit 2 connects to the secondary circuit 7. The newly developed double-length reciprocating piston pump is shown in FIGS. 3 and 4. Your two pistons 8 and 9 are driven by an angular stroke gear 10 with an upstream control gear 11. The stroke volume of the pump heads can be adjusted by means of a ring 12, which at the same time assumes the task of actuating a contact switch 13 for changing the direction of rotation and for switching the fittings. Double seals and support rings are placed on the top of each piston to seal the housing. The head seal of the pistons enables the design of a low dead space pump head regardless of the stroke volume. The aspirated product volume is displaced quantitatively from the pump head during conveying. While on the one hand the reaction mixture is slowly sucked in by the piston 23, for example, the opposite piston 24, which is guided over the same spindle 14 as the piston 23, presses the previously sucked reaction mixture quantitatively out of the Pump head. The double-long stroke piston pump according to the invention is self-priming and self-venting at a pulsation frequency of less than 10 strokes per minute. The dead space of the pump is less than 1% of the pump head volume. It is possible to work with the pump at a pressure of up to 300 bar and a temperature of -100 to + 250 ° C. With the pump according to the invention, fluids containing solids can also be pumped if the sedimentation time of the solid is greater than the residence time of the fluid in the pump head. In a preferred embodiment, the spindle of the pump has an additional anti-rotation device.

The double-length reciprocating piston pump 3 enables a partial flow of the reaction volume from the reactor 1 to be pumped around in a manner that is gentle on the product. Using the device according to the invention, 100 ml / h of butadiene polymer were pumped over 100 h at a pressure of 5 bar and a temperature of 50 ° C. without deposition or coagulum formation.

In a preferred embodiment, the pumping loop 2 is connected to a vacuum vessel 13 via an overflow valve 6. The overflow valve 6 prevents spontaneous expansion of the liquid monomers contained in the sample amount. This prevents uncontrolled foaming. The vacuum vessel 13 has a defined volume and is evacuated to a preselected negative pressure, for example 50 mbar, via a control. When the negative pressure is reached, the control switches a valve 25 in the pumping loop into delivery, so that the double-long stroke piston pump 3 pumps the defined volume against the valve 25 in delivery and increases the system pressure in the pumping loop. The overflow valve 6 allows the sample amount to pass into the vacuum vessel at a previously set pressure which is above the reactor pressure. As soon as the sample quantity has been discharged from the pumping loop, the control gives the command to open the valve 25 so that the pumping circuit is put into operation again. The injected sample produces a pressure increase in the vacuum vessel, which consists of a calibrated, cylindrical measuring vessel 13 and an expansion vessel 15. The expansion tank is preferably designed so that when a low-boiling component of the multi-phase fluid is released, the max. resulting pressure does not exceed 1 bar absolutely. The pressure increase is in turn increased by evaporating components of the sample. If the pressure in the measuring vessel no longer changes after a certain time (e.g. <30 min), the pressure difference is calculated and, together with the temperature, the volume of the sample and the volume of the vacuum container, the monomer concentration is determined and thus conclusions can be drawn about the current product composition in the Hit reactor. If the pressure-generating component of the reactor sample is isolated, the remaining, non-evaporated sample amount is automatically compared with the specified target sample amount. If the measured sample volume is below the setpoint, the vacuum vessel is evacuated again and a further sample is requested from the double-length reciprocating piston pump. This sub-process is repeated until the sufficient amount of sample is reached. The vacuum vessel 13 is then aerated with inert gas and the rest of the sample is pumped into a measuring loop. The measuring circuit 7 is equipped with a single long-stroke piston pump 16 in order to supply measuring sensors with product. The single long-stroke piston pump, like the double-long stroke piston pump, is equipped with positively controlled valves 17 and 18, respectively. If the isolated sample is sucked out of the vacuum vessel, the valves 17, 18 switch over to the actual measuring circuit. As a result, the vacuum vessel between valve 6 and valve 17 is temporarily excluded from the rest of the process. Now a cleaning process of the degassing cell, consisting of a rinsing and drying process, can run automatically in parallel to the other automated process. The rinsing process is necessary in the case of upstream sample preparation processes in order to clean parts wetted by the product so that there are no falsifications of measurements during subsequent measurements. The rinsing and drying process is initiated by valves 20 and 21, respectively. After the rinsing liquid has been introduced into the vacuum vessel, the valve 22 opens in order to allow the amount of detergent introduced to flow away into a coupled collecting vessel. The rinsing process can optionally be repeated several times depending on the product properties. After rinsing has ended, a drying process takes place, which is initiated by valve 21. The drying process is only necessary if the remaining amount of rinsing liquid, which adheres to the inner walls, does not evaporate due to the subsequent evacuation and thereby falsifies the initial measured value for the determination of the low boiler. In this measuring loop more Analysis or measuring devices are installed to determine different properties of the degassed sample.

The measured sample residue can be fed from the measuring loop into the pumping loop 2 via a further three-way valve and can thus be returned to the reactor as a seed polymer. The polymer is preferably conveyed in the measuring loop with the aid of a further conveying device according to the invention, in particular with the aid of a second pump according to the invention. With the aid of the method according to the invention it is possible to achieve a fully automatic sampling and determination of the current monomer concentration during a pressure polymerization by means of the pressure difference measurement mentioned. The monomer concentration determined here can be used as a control variable for the metering of monomer or initiator addition. It proves to be a particular advantage that the measured polymer due to the gentle conveyance and further treatment, e.g. in the measuring loop, the reaction mixture can be fed back into the reactor without impairing the product quality. A variant for transferring polymer into the measuring loop is to shut off a defined volume of the pumping loop 2 filled with reaction mixture and to open a loop at the same time so that the product flow in the pumping loop is not interrupted. The pressurized amount of reaction mixture in the lock is then emptied spontaneously into the vacuum vessel. The lock is then switched back into the reaction mixture stream and the loop bypass is closed. The lock replaces the overflow valve 6.

It is possible to complete the demonomerization or preparation of the reaction mixture in the vacuum vessel by multiple evacuation and subsequent aeration with inert gas, with light barriers as foam monitors e.g. can be used for foaming products.

Typical measurements, for example on demonomerized latex in the course of the measuring loop, are density, refractive index, NIR, ultrasound, pH and conductivity measurements. Furthermore, light extinction measurements can be carried out on defined, diluted, demonomerized latex (if necessary at different wavelengths) or a particle size determination using laser correlation spectroscopy. A combination of the information from these measurements makes it possible to determine the kinetics of the polymerization and to carry out a process control while the reaction is in progress.

The invention also relates to a device for carrying out the method according to the invention, comprising a double-long-stroke piston pump 3 with two pistons 8 and 9, an intake line and an output line, at least two valves 4 and 5, in particular three-way valves, which define the respective piston chamber 23 or 24 connect the pistons 8 and 9 to the suction line or the discharge line, characterized in that the valve 4 and the further valve 5 are dead space-free valves which are controlled via a respective contact switch 26 or 13 of the double-long stroke piston pump 3, that the pistons 8 and 9 are synchronously movable, and the change in the direction of movement of the pistons 8, 9 is used to control the valves 4 and 5, and that the remaining volume of the piston spaces 23 and 24 is less than 1% of the stroke volume.

Particularly suitable as a double-length reciprocating piston pump is a low-pulsation, dead-space-free double-length reciprocating piston pump with a delivery rate of 10 ml / h to 100 l / h for conveying shear-sensitive fluids with a viscosity of up to 100,000 mPa.s upstream control gear 11 or a hydraulic gear for driving the pistons 8, 9 a ring 12 for adjusting the stroke volume of the pump heads 23, 24 a contact switch 26 for changing the direction of rotation of an initiator disk 27 and double seals on the head of the piston chambers.

The pump is driven, for example, via a reduction gear with an angular stroke gear connected in series, which converts the rotary movement into a rotation-free stroke movement of the pistons 8, 9. This has the advantage that the suction and pressure piston head can be mounted on a spindle and the double piston pump can continuously deliver. With this arrangement, the pressure and suction pistons run absolutely synchronously. The side of the respective suction piston head facing away from the product releases the Switch off to switch from printing to suction. The design can also be designed so that the print head triggers the switchover instead of the suction head. In this case, the switch sensor would not have to be placed on the transmission side but on the pump head side. The lifting rod, which is a threaded rod (piston rod) on which the two piston heads are seated, is provided, for example, with an axially extending groove, into which an anti-rotation lock preferably engages in order to avoid rotation of the lifting rod. The piston head is provided to the piston housing with at least one elastic seal and at least one guide ring, so that the product sucked in can also be completely displaced from the piston head housing during the printing process.

Suction and pressure chambers are e.g. connected to each other via two capillaries in which two positively controlled three-way valves (ball valves) (4, 5) are placed. The three-way valves separate the two chambers. Instead of the two three-way valves, four individual valves can also be provided. The valves preferably consist of ball valves in order to ensure a shear-free passage of the product. Each piston has a product inlet and outlet, they are arranged vertically one below the other, the inlet at the bottom and the outlet at the top to enable self-ventilation and self-priming of the pump. In a special embodiment, a telescopic spindle (length-adjustable spindle) is used in order to be able to generate a pump circuit with the smallest stroke volume quantities and to keep the remaining volume small. The double-length reciprocating piston pump can completely drain the pump head regardless of the flow rate. Due to the extremely low stroke frequency (e.g. max. 2 strokes per minute), low-pulsation product delivery occurs, the product is delivered laminar, there is a plug flow in the capillaries or pipelines. The laminar conveyance of a product at constant pressure is a prerequisite for the shear-free transport of latices. The pump drive and the pump head volume should preferably be designed so that a switching cycle of more than 5 minutes is maintained. This ensures shear-free pumping e.g. the polymer latices or plastic dispersions guaranteed. These frequency characteristics describe a pulsation-free (low-arm) delivery system that can pump shear-sensitive products over a long period of time.

Due to its simple design, the pump can deliver small quantities, continuously from 10 ml / h, or suck in defined quantities in the ml range and be used as a sampler in discontinuous or continuous processes. The pump can act as a sluice because the pressure and suction sides are separate, it can transfer product quantities from a vacuum or pressure range to a vacuum or pressure range. Due to the dead space-free design, there is no backmixing with older product quantities.

A particular advantage of the method with the double-length reciprocating piston pump is that with the set sample amount of e.g. 10 ml, a defined amount of product per stroke is conveyed and at any time e.g. can be transferred into a measuring circuit. This sample amount or the max. The stroke can be adjusted with an adjusting nut on the piston spindle inside the piston housing.

The piston heads and pump housings can consist of suitable metallic and / or non-metallic materials. In special embodiments, the piston housing can also be lined with glass or ceramic sleeves. The piston housing can be easily heated or cooled.

Due to the separation of the pressure and suction side, there can be any positive or negative pressure differences between them. The pump can be operated at a temperature of -100 ° C to + 200 ° C. It is common to use the pump in a temperature range from around -20 to + 150 ° C, in the case of latices in the range from +2 to + 100 ° C. The ratio of stroke volume to residual volume in the pump head, which is a measure of the freedom from dead space, is preferably less than 1%.

Claims (8)

1. Process for continuously conveying shear-sensitive fluids having a viscosity of up to 100,000 mPa.s with a pump capacity of 10 ml/h to 100 l/h, characterised in that the fluid is aspirated via at least one clearance-free valve (4) by a piston (8) of a double-long-stroke reciprocating pump (3) while fluid is synchronously emitted by the second piston (9) from the second piston chamber (24) likewise via at least one additional clearance-free valve (5) towards the conveying side and after evacuation of the second piston chamber (24) the valves (4) is opened on the emission side and closed on the suction side while the additional valve (5) is closed on the emission side and opened on the suction side and the direction of motion of the pistons (8, 9) is synchronously reversed, whereby the pistons (8) and (9) are located on one spindle (14), whereby the direction of motion of the pistons (8, 9) and the positions of the valves (4, 5) are controlled via contact switches (26) and the residual volume still remaining in the piston chamber after evacuation amounts to less than 1 % of the stroke volume.
2. Process according to Claim 1, characterised in that the shear-sensitive fluid is conveyed in a recirculating loop (2) which is coupled to a reactor (1).
3. Process according to Claims 1 and 2, characterised in that the shear-sensitive fluid is a polymer latex or a plastic dispersion.
4. Process according to Claims 2 and 3, characterised in that the fluid is pumped into the region of reduced pressure via an overflow valve (6) or a sluice which is connected between the recirculating circuit (2) and a region of reduced pressure.
5. Process according to Claims 3 to 4, characterised in that the region of reduced pressure is part of a metering loop (7) which is parallel to the recirculating loop (2) and by way of metering devices comprises at least one calibrated vacuum vessel with pressure gauge for determining the monomer concentration of the polymer latex.
6. Device for implementing the process according to Claim 1, comprising a double-long-stroke reciprocating pump (3).with two pistons (8) and (9), a suction line and an emission line, at least two valves (4) and (5), in particular three-way valves, which connect the respective piston chamber (23) or (24) of the pistons (8) and (9) to the suction line or the emission line, characterised in that the valve (4) and the additional valve (5) are clearance-free valves which are controlled via a respective contact switch (26) or (13) of the double-long-stroke reciprocating pump (3), in that the pistons (8) and (9) are capable of being moved synchronously and the change in the direction of motion of the pistons (8, 9) is used for controlling the valves (4) and (5), and in that the residual volume of the piston chambers (23) and (24) amounts to less than 1 % of the stroke volume.
7. Device according to Claim 6, characterised in that it is connected to a recirculating loop (2) which is coupled to a reactor (1).
8. Device according to Claim 7, with an overflow valve (6) or a sluice on the emission side of the conveying device in the recirculating loop for transporting fluid in a region of reduced pressure.

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