US20030165390A1

US20030165390A1 - Continuous conveying process and device for shear-sensitive fluids

Info

Publication number: US20030165390A1
Application number: US08/581,589
Authority: US
Inventors: Peter Jahn; Otto Schmid; Adolf Schmidt
Original assignee: Individual
Current assignee: Bayer AG
Priority date: 1993-07-23
Filing date: 1994-07-11
Publication date: 2003-09-04
Also published as: JPH10508073A; EP0710328B1; DE59404484D1; DE4324777A1; WO1995003491A1; EP0710328A1; ES2108486T3

Abstract

The invention relates to a process for continuously conveying shear-sensitive fluids, in particular polymer latices or plastics dispersions, which is accompanied by the taking of samples and the processing and analysis thereof, to a device for implementing the process and to a new double-long-stroke reciprocating pump.

Description

The invention relates to a process for continuously conveying shear-sensitive fluids, in particular polymer latices or plastics dispersions, a device for implementing the process and also a new double-long-stroke reciprocating pump.

It is known that when subject to shear—when being conveyed for example—polymer latices or plastics dispersions can easily coagulate—ie, from the finely dispersed fluids solid substances (coagulate) are precipitated which can coat or obstruct conveying elements and pipelines. This coagulate can also be deposited on metering probes of all types which are immersed in the latex or which come into contact with latex and can falsify or hinder ongoing measurements. Latices which, with a view to the quality of the product, have to be produced with a minimum of emulsifier are in particular highly prone to form coagulate.

Known conveying devices are rotary, diaphragm or reciprocating pumps. Rotary pumps consist of stator (casing) and rotor (running wheel). As a result of the high speed of rotation of the running wheel the product to be conveyed is accelerated radially from the centre of the rotor (central point) and pressed through the pressure connection of the casing by the corresponding centrifugal forces at the outer diameter of the running wheel. As a rule, rotary pumps require a speed of rotation of more then 500 rpm. At a lower speed no product is conveyed with this system. By virtue of their design characteristics rotary pumps have large clearance volumes and are not self-priming.

Diaphragm pumps and reciprocating pumps are displacement pumps which operate as rotary pumps at lower frequencies. The lowest frequency is around 30 strokes/min. The conveying motion of the piston or of the diaphragm, which is also moved on the drive side by a piston, is intermittent—pulsations occur, so that, considered in the short term, it is not possible to speak of a continuous conveying or transport of product. The pulsation is pressure-dependent and has an effect on the constancy of conveying and the dosing precision.

Long-stroke reciprocating pumps are known as single-stroke reciprocating pumps, but they are unsuitable and too expensive for the continuous dosing of small quantities (see Menges, Recker, Carl-Hanser-Verlag; Munich, Vienna 1986, Automatisieren in der Kunststoffverarbeitung, pages 318-320).

A disadvantage with these conveying implements is that, for example, rotary pumps can only convey at a high speed of rotation and can build up pressure. As a result of the high circumferential velocities on the rotor, product particles are very strongly sheared at the gaps in the pump head, so that the particles undergo a change. Owing to a high clearance volume a rotary pump is unsuitable for conveying small quantities of product with subsequent sampling. For example, it is not possible to withdraw a representative sample quantity of, for example, 10 ml from a reactor and to make inferences therefrom as to reaction conditions in the actual reactor. Piston pumps or diaphragm pumps operate at stroke frequencies that are too high, and by virtue of their design characteristics they have many narrow gaps in which shearing of product or particles occurs. An additional factor which alters the product is the inconstant speed of the displacement elements. On pipelines or pump-head parts that have been touched by product the irregular delivery rate brings about various frictional forces which cause product shearing in the microparticle range. With the known pump designs the dwell-time of the product in the pump head is not finite or not precisely defined, as all pump heads have a large clearance volume. Large proportions of a suction-stroke volume remain in the pump head for a long time, with each new suction stroke only a partial mixing with old product occurs, so that product particles are subjected to shear stress over a long period. A further disadvantage is that the piston pumps or diaphragm pumps have no positive valves (ball valves) which, depending on suction or compression process, open a large, free cross-section. The non-synchronicity of suction process and compression process has a further disadvantageous effect. With these pump designs an absolute synchronous operation of suction side and compression side is not possible, even if an improvement in the conveying can be partially achieved by means of multiple heads. By virtue of the rotational acceleration of the control gearing and the associated conversion of rotary motion into reciprocating motion, the individual processes that have to be considered in the individual pump head are always irregular and are the reason for the primary pulsation in the flow being transported.

A further disadvantage with known pump designs is that as the volumetric flow becomes smaller the clearance volume in the pump head becomes greater. This means that the actual conveying displacement body changes its working stroke in proportion to the flow rate.

Single-piston pumps are of very large construction, are very expensive and do not work continuously. The designs do not permit self-priming so that pumps are required in addition on the suction side. Their large clearance volume makes them unsuitable for sampling, for example. Even in the case of twin-piston reciprocating pumps that are conceivable, synchronous operation of suction and compression processes is not possible.

The object underlying the invention is to convey shear-sensitive fluids, in particular polymer dispersions, in continuous, low-pulsation and gentle manner so that the fluids remain unaffected in their phase condition by the conveying, and in particular in the case of dispersions no phase separation or coagulation occurs. In particular it is intended to make it possible to convey a polymer latex out of a reactor into a recirculating loop and back, whereby the formation of coagulate or the re-forming of particles is avoided. The object further underlying the invention is to make available a device which enables this conveying in a manner which is gentle and which over a long period is trouble-free and operationally dependable. In particular, the device should make it possible to transport the fluid back and forth in gentle manner between various parts of the plant, for example a reactor and a metering loop with differing pressure conditions. This object is achieved in accordance with the invention in that the fluid is aspirated and pumped by means of a double-long-stroke reciprocating pump which is clearance-free and has low pulsation and that, in the system of pipes that is flowed through, use is made of clearance-free valves. This double-long-stroke reciprocating pump is located, for example, in a pressure loop, whereby through a sluice or an overflow valve which serves as connecting element to a metering circuit with a different pressure level arranged parallel, a defined sample quantity is transported and is supplied to the actual on-line gauges in the parallel metering circuit by a second long-stroke reciprocating pump of identical design.

The invention provides a process for continuously conveying shear-sensitive fluids, in particular polymer latices or plastics dispersions, 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 by a piston of a double-long-stroke reciprocating pump, while fluid is synchronously emitted from the second piston chamber likewise via at least one additional clearance-free valve towards the conveying side and after total evacuation of the second piston chamber the first valve is opened on the emission side and is closed on the suction side while the second valve is closed on the emission side and is opened on the suction side and the direction of motion of the pistons is synchronously reversed. A preferred embodiment is characterised in that the fluid is conveyed in a recirculating loop which is coupled to a reactor. Within this recirculating loop, or parallel to the latter, metering probes or sensors or built-in assemblies may be located. Examples of such metering probes are temperature sensors, pH-electrodes, conductivity electrodes, NIR light-guide probes, oscillating U-tubes for density measurements, refractometers, ultrasound metering heads or devices for calorimetry. The stated metering probes or gauges are not coated or obstructed by the fluid which is conducted in the circuit (eg, a latex). In principle it is possible to introduce into the recirculating loop additional devices for thorough mixing, such as static mixers or heat exchangers which, by virtue of the conveying device which is constantly pulsation-free and which exerts a low degree of shear, are not coated or obstructed. A further preferred variant of the process is characterised in that the fluid is conducted through an overflow valve or a sluice into a region of reduced pressure (eg, a secondary recirculating loop). Particularly preferred is a variant of this process in which the overflow valve or the sluice is connected by way of coupling between primary and secondary recirculating circuits. With this variant it is possible to branch defined sample volumes of the fluid out of the principal-flow line or a primary recirculating circuit and, for example, to cause it to be metered under reduced pressure.

The invention is elucidated in greater detail below with reference to an example of an embodiment represented in the drawings. [0011]
Shown are: [0012]
FIG. 1 a scheme of the conveying, in accordance with the invention, of shear-sensitive fluids [0013]
FIG. 2 a schematic illustration of the conveying process as part of a recirculating loop and for decoupling polymer [0014]
FIG. 3 structure of the double-long-stroke reciprocating pump according to the invention in a side view [0015]
FIG. 4 a double-long-stroke reciprocating pump according to the invention in top view.[0016]
Connected to a [0017] reactor 1 for the emulsion polymerisation of butadiene which operates under a pressure of >5 bar is a recirculating loop 2 which contains a new double-long-stroke reciprocating pump 3 with positive input and output valves 4, 5 as conveying device and also an overflow valve 6 which connects the primary circuit 2 to the secondary circuit 7. The newly developed double-long-stroke reciprocating pump is illustrated in FIGS. 3 and 4. Its two pistons 8 and 9 are driven via an angle-stroke gearing 10 with control gearing 11 connected upstream. The stroke volume of the pump heads can be adjusted by means of a ring 12 which simultaneously fulfils the function of actuating a contact switch 13 for changing the direction of rotation and for the change-over of the armatures. Double seals and support rings are placed on the head of each piston in such a manner as to seal in relation to the casing. The head seal of the pistons enables the formation of a low-clearance pump head irrespective of the extent of piston displacement. The product volume aspirated is displaced quantitatively out of the pump head during the conveying. While on the one hand the reaction mixture is, for example, slowly aspirated by the piston 23, the opposite piston 24, which is guided via the same spindle 14 as the piston 23, pushes previously aspirated reaction mixture quantitatively out of the pump head. The double-long-stroke reciprocating pump according to the invention is self-priming and self-ventilating at a pulsation frequency of less than 10 strokes per minute. The clearance of the pump amounts to 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 +125° C. With the aid of the pump according to the invention it is also possible for fluids containing solid to be pumped if the sedimentation-time of the solid is greater than the dwell-time of the fluid in the pump head. In a preferred embodiment the spindle of the pump has an additional anti-torsion device.
The double-long-[0018] stroke reciprocating pump 3 makes it possible to recirculate a partial-flow quantity of the reaction volume from the reactor 1 in a manner which at no time harms the product. With the aid of the device according to the invention 100 ml/h of butadiene polymer were recirculated by pumping for over 100 h at a pressure of 5 bar and a temperature of 50° C. without sedimentation or coagulation.
In a preferred embodiment the [0019] recirculating loop 2 is connected to a vacuum vessel 13 via an overflow valve 6. The overflow valve 6 prevents a spontaneous expansion of the liquid monomers contained in the sample quantity. As a result, an uncontrolled formation of foam is prevented. The vacuum vessel 13 has a defined volume and is evacuated via a control system to a preselected reduced pressure, for example 50 mbar. Once the reduced pressure has been reached, the control system switches a valve 25 located in the recirculating loop into the closed position so that the double-long-stroke reciprocating pump 3 pumps the defined volume against the valve 25 that is in the closed position and increases the system pressure within the recirculating loop. The overflow valve 6 allows the sample to pass into the vacuum vessel at a previously adjusted pressure which is above the reactor pressure. As soon as the sample has been channelled out of the recirculating loop the control system gives the command to open the valve 25 so that the recirculating circuit is again put into operation. The injected sample generates an increase in pressure in the vacuum vessel, which consists of a calibrated, cylindrical metering vessel 13 and an equalising vessel 15. The equalising vessel is preferably so designed that upon expansion of a low-boiling component of the multi-phase fluid the maximum pressure arising definitely does not exceed 1 bar. The increase in pressure is in turn intensified by components of the sample which evaporate. If after a certain time (eg, <30 min) the pressure in the metering vessel no longer changes, then the pressure difference is calculated and together with the temperature, the volume of the sample and the volume of the vacuum vessel the monomer concentration is determined and an inference is thereby made as to the present composition of the product in the reactor. If the pressure-generating component of the reactor sample is isolated, the remaining, unevaporated sample is automatically compared with the prespecified target sample quantity. If the metered sample volume is below the target value, the vacuum vessel is again evacuated and a further sample is required from the double-long-stroke reciprocating pump. This partial process is repeated until the sample quantity is sufficient. Then the vacuum vessel 13 is ventilated with inert gas and the rest of the sample is pumped into a metering loop. The metering circuit 7 is equipped with a single-long-stroke reciprocating pump 16 in order to provide metering probes with product. The single-long-stroke reciprocating pump is equipped, like the double-long-stroke reciprocating pump, with positive valves 17 and 18. Once the isolated sample has been aspirated out of the vacuum vessel, the valves 17, 18 switch over to the actual metering circuit. As a result, the vacuum vessel between valve 6 and valve 17 is temporarily excluded from the remaining process. Now a process for cleaning the degassing cell can proceed automatically, consisting of flushing and drying processes in parallel with the other automated process. In the case of sample-processing operations upstream the flushing process is necessary in order to clean parts that have been moistened with product, in order that no falsification of measured values can occur in subsequent measurements. The flushing and drying processes are initiated via the valves 20 and 21 respectively. After the flushing liquid has been introduced into the vacuum vessel the valve 22 opens in order to allow the quantity of flushing medium that has been channelled in to drain off into a coupled collecting vessel. The flushing process may optionally be repeated several times depending on product characteristics. After the flushing is over, a drying process takes place which is initiated via the valve 21. The drying process is only required when the remaining quantity of flushing liquid that adheres to inner walls does not evaporate as a result of the subsequent evacuation and thereby falsifies the initial measured value for the determination of the low-boiling component. It is possible for further analytical and metering devices to be installed within this metering loop in order to determine different properties of the degassed sample.
The metered sample remainder can be supplied from the metering loop into the [0020] recirculating loop 2 via an additional three-way valve and thereby conducted back into the reactor as seed polymer. The conveying of the polymer within the metering loop is preferably also carried out with the aid of an additional conveying device according to the invention, in particular with the aid of a second pump according to the invention. With the aid of the process according to the invention it is possible to achieve a fully automatic sampling and determination of the current monomer concentration during a pressure polymerisation by means of the stated pressure-difference measurement. The monomer concentration determined in this connection can be used by way of control variable for the dosing of added monomer or initiator. It proves to be a particular advantage that by virtue of the gentle conveying and the further treatment, within the metering loop for example, the metered polymer can be supplied again to the reaction mixture in the reactor without impairing the quality of the product. A variant with a view to the transport of polymer into the metering loop consists in sealing off a defined volume of the recirculating loop 2 which is filled with reaction mixture and at the same instant opening a loop bypass so that the product flow within the recirculating loop is not interrupted. The quantity of reaction mixture under pressure in the sluice is then spontaneously discharged into the vacuum vessel. Then the sluice is switched into the flow of reaction mixture again and the loop bypass is closed. In this connection the sluice replaces the overflow valve 6.
It is possible to complete the demonomerisation or processing of the reaction mixture in the vacuum vessel by repeated evacuation and subsequent ventilating with inert gas, whereby in the case of foaming products, for example, light barriers can be used as foaming guards. [0021]
Typical measurements which are made, for example, on demonomerised latex within the course of the metering loop are density, refractive-index, NIR, ultrasound, pH and conductivity measurements. Furthermore, measurements of light extinction (where appropriate at various wavelengths) or a determination of particle size with the aid of laser correlation spectroscopy can be carried out on demonomerised latex which has been diluted in defined manner. By means of a combination of the data from these measurements it becomes possible to determine the kinetics of the polymerisation while the reaction is in progress and to implement a control of the process. [0022]
The invention further provides a low-pulsation, clearance-free, double-long-stroke reciprocating pump with a pump capacity of 10 ml/h to 100 l/h for the conveying of shear-sensitive fluids having a viscosity of up to 100,000 mPa.s, exhibiting two [0023] pistons 8, 9 on a common drive spindle 14, an angle-stroke gearing 10 with control gearing 11 connected upstream or a hydraulic gearing 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 disc 27 and also double seals on the head of the piston chambers.
The pump is driven, for example, via a reduction gearing with angle-stroke gearing connected in series which converts the rotary motion into rotation-free reciprocating motion of the [0024] pistons 8, 9. This has the advantage that the suction-piston and compression-piston heads are mounted on one spindle and the double reciprocating pump can convey continuously. By means of this arrangement, compression piston and suction piston run in absolutely synchronous manner. In each case the side of the respective suction-piston head facing away from the product triggers the switch in order to change over from the compression process to the suction process. The design may also be such that the pressure head, rather than the suction head, triggers the change-over. In this case the switching sensor would have to be placed on the pump-head side rather than on the gearing side. The reciprocating rod, which is a threaded rod (piston rod) on which the two piston heads are located, is for example provided with a groove extending axially, with which an anti-torsion device preferably engages in order to avoid rotation of the reciprocating rod. The piston head is provided with at least one elastic seal with respect to the piston casing and at least one guide ring so that the aspirated product can also be displaced totally out of the piston-head casing in the course of the pressure process.
Suction and compression chambers are, for example, connected to one another via two capillaries in which two positive three-way valves (ball valves) ([0025] 4, 5) are placed. The three-way valves provide for a separation of the two chambers. Instead of the two three-way valves, four single valves can also be provided. The valves preferably consist of ball valves in order to ensure a shear-free passage of product. Each piston has a product inlet opening and a product outlet opening arranged vertically above one another, the inlet at the bottom and the outlet at the top, in order to enable self-ventilating and self-priming of the pump. In a particular embodiment a telescopic spindle (length-adjustable spindle) is employed in order to be able, in the case of extremely small piston displacements, to generate a pump circulation and to keep the residual volume low. The double-long-stroke reciprocating pump can discharge the pump head in remainder-free manner irrespective of the flow rate. By reason of the extremely low stroke frequency (eg, max. 2 strokes per minute), a low-pulsation conveying of product takes place, the product is conveyed in laminar manner, a plug flow occurs in the capillaries or pipelines. At constant pressure the laminar conveying of a product is a prerequisite for the shear-free transport of latices. The pump drive and the pump-head volume should preferably be so designed that a switching cycle of greater than 5 minutes is maintained. As a result, shear-free pumping, of the polymer latices or plastics dispersions for example, is ensured. These frequency characteristics describe a pulsation-free (low-pulsation) conveying system which can pump shear-sensitive products over a long period.
By virtue of its simple design the pump can convey continuously extremely small quantities, from 10 ml/h, or can also aspirate defined quantities in the ml-range and can be used as sampling apparatus with discontinuous or continuous processes. The pump can function as a sluice, since compression side and suction side are separated from one another, it can transport product quantities from a region of low pressure or a region of excess pressure into a region of low pressure or a region of excess pressure respectively. By reason of the clearance-free design, no back-mixing occurs with temporally older product quantities. [0026]
A particular advantage of the process with the double-long-stroke reciprocating pump is that with the adjusted sample quantity of, for example, 10 ml a defined product quantity per stroke is conveyed and can be transported at any time into, for example, a metering circuit. This sample quantity or the maximal stroke distance is capable of being adjusted with an adjusting nut on the piston spindle inside the piston casing. [0027]
The piston heads and pump casing may consist of suitable metallic and/or non-metallic materials. In special embodiments the piston casing may also be lined with glass or ceramic sleeves. The piston casing can be heated or cooled in simple manner. [0028]
By virtue of the separation of compression side and suction side, arbitrary positive or negative pressure differences can exist between these sides. The pump can be operated at a temperature of −100° C. to +200° C. It is usual to employ the pump in a temperature range of about −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 clearance freedom, preferably amounts to less than 1%. [0029]

Claims

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, positive 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, positive valve (5) towards the conveying side and after total evacuation of the second piston chamber (24) the valve (4) is opened on the emission side and closed on the suction side while 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 a spindle (14).

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 plastics 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 claim 3 to 4, characterised i 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 exhibits at least one calibrated vacuum vessel with pressure gauge for determining monomer concentration of the polymer latex.

6. Device for implementing the process according to claim 1, exhibiting a double-long-stroke reciprocating pump (3), at least one clearance-free, three-way valve (4) which is positively controlled via a contact switch (26) of the double-long-stroke reciprocating pump (3) and at least one clearance free, three-way output valve (5) which is likewise positively controlled via a contact switch (26) of the double-long-stroke reciprocating pump (3).

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 into a region of reduced pressure.

9. Low-pulsation, clearance-free, double-long-stroke reciprocating pump with a pump capacity of 10 ml/h to 100 l/h for conveying shear-sensitive fluids having a viscosity of up to 100,000 mPa.s, exhibiting two pistons (8, 9) on a common drive spindle (14), an angle-stroke gearing (10) with control gearing (11) connected upstream 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, an initiator disc (27) and also double seals on the head of the pistons (23, 24).

10. Double-long-stroke reciprocating pump according to claim 9, characterised in that the spindle (14) exhibits an anti-torsion device.

11. Double-long-stroke reciprocating pump according to claims 9 and 10, characterised in that the spindle (14) is a telescopic spindle and replaces the adjusting nut (12).

12. Double-long-stroke reciprocating pump according to claims 9 to 11, characterised in that the movement of the piston is controlled via a hydraulic system instead of the angle-stroke gearing (10) with control gearing (11) connected upstream.