WO2006048408A2

WO2006048408A2 - Loop reactor for emulsion polymerisation

Info

Publication number: WO2006048408A2
Application number: PCT/EP2005/055620
Authority: WO
Inventors: David Charles Adams
Original assignee: Akzo Nobel Coatings International B.V.
Priority date: 2004-11-04
Filing date: 2005-10-28
Publication date: 2006-05-11
Also published as: WO2006048408A3

Abstract

A polymerisation reactor comprising a circulation loop with an inlet for raw material, driving means for circulating a reactor charge within the circulation loop, and an outlet connected to a discharge line for the discharge of polymer emulsion. The circulation loop includes a line section by-passing the circulation pump. The discharge line intersects the by-pass line. The outer end of the discharge line comprises an inlet for a cleaning pig while the other outer end leads to a collection tank. A shut-off valve is located between the cleaning pig inlet and the intersection of the discharge line and the loop. At the point of intersection the discharge line offsets the tube of the circulation loop at a distance greater than the radius of the discharge line.

Description

LOOP REACTOR FOR EMULSION POLYMERISATION

The present invention relates to a polymerisation reactor comprising one or more circulation loops with one or more inlets for raw material, driving means for circulating a reactor charge within the circulation loop, and an outlet connected to a discharge line for the discharge of polymer emulsion.

WO 00/07177 discloses a loop reactor for emulsion polymerisation. The loop reactor comprises a circulation pump and a tubular circulation loop connecting the pump's outlet to its inlet. Water, monomers, and stabilisers are continuously fed to the loop and circulated and polymer emulsion is continuously drawn off. The reactor is particularly suitable for the production of polymers derived from vinyl and/or acrylic monomers, used for instance in paints or adhesives.

A problem encountered in polymerisation processes employing a tubular reactor is the formation of deposits from the reaction products on the internal wall of the reactor. These deposits lead to a need for an increased delivery pressure from the circulation pump and impair heat transfer from the reaction medium to, e.g., a coolant in a jacket surrounding the reactor tube, thus leading to higher and often deleterious reactor temperatures or else necessitating an increased coolant circulation rate, a lower coolant temperature, or a reduced rate of production. Fouling also reduces the reactor volume and increases the shear on the emulsion. This shifts the process conditions, which may have been optimised on a clean reactor. In any case, product properties will drift, nullifying the advantages of consistency of production expected from continuous reactors. In WO 00/07177 cleaning pigs are used for cleaning the inside of the reactor tubes. The cleaning pigs have a diameter which is about the inner diameter of the reactor tube. The pigs are launched from a pig station and propelled through the loop by the polymerising emulsion. To discharge polymer emulsion from the reactor, loop reactors are provided with a discharge line leading to collection tanks. The discharged polymer emulsion still comprises a certain monomer content. In order to reduce this monomer content, the discharge line can be extended to increase the reaction time for the final traces of monomer. Fouling also occurs in the discharge line. As a consequence, also the discharge line needs to be cleaned on a regular basis, although generally not as often as the reactor tube itself. Cleaning of the discharge line is usually done by steam cleaning, which is dangerous, or by solvent cleaning, which is not environment-friendly. Moreover, these methods mean loss of production time.

The object of the invention is to provide a loop reactor which can be cleaned more effectively and completely in a convenient way. A further object of the invention is to construct a loop reactor that can be cleaned completely by pigging.

The object of the invention is achieved by a loop reactor having a discharge line which intersects the circulation loop. One of the outer ends of the discharge line comprises an inlet for cleaning pigs, while the other outer end leads to a collection tank. This way, the path of the pig cleaning the by-pass crosses the path of the pig cleaning the reactor tube when it enters the outer end discharge line. The cleaning pig passes through the complete discharge line. The need for solvent cleaning of any part of the discharge line is thus eliminated.

Near the outer end where the pig is inserted, the discharge line can comprise a valve for holding back polymer emulsion. To enable the insertion of a pig, a screw junction can for instance be used. Optionally, pressurised water may be used to launch the pig. After crossing the intersection with the circulation loop, the flow of polymer emulsion drives the cleaning pig forward. At the end of the discharge line, e.g., at a collection tank, the pig enters a trap in the discharge line and can be recovered and returned to the start position. The closed loop reactor can comprise a reactor tube of which at least a substantial part forms a helical coil. In comparison to the common trombone arrangement of the continuous tube (as disclosed in, e.g., M. Wilkinson and K. Geddes, "An award winning process," Chemistry in Britain, pp. 1050-1053, December 1993), the shape is more appropriate for pigging in that the pig is not forced to make sharp turns, thus reducing the wear of the pig and allowing the use of longer pigs. Furthermore, uncooled joints, which are one of the origins of wall fouling, can be avoided by using a helically coiled continuous tube.

The circulation loop may for example include a line section by-passing the circulation pump, e.g. as is described in WO00/07177. Such intersection by- passes cleaning pigs around the circulation pump. In such embodiment, the discharge line can intersect the circulation loop at the by-pass section.

To prevent sticking of the cleaning pig at the point of the intersection, the circulation loop can offset the discharge line at a distance greater than the radius of the discharge line. An optimum was found in the shortest distance between the outline of the loop line and the centre line of the discharge line being 20 - 30 % of the outer diameter of the discharge line, and the diameter of the circulation loop line being 12 - 20 % smaller than the diameter of the discharge line.

A pig can be launched at intervals ranging from approximately 1 to approximately 24 hours, preferably from approximately 4 to 8 hours. Apart from more effective cleaning of the tube walls, regular disturbance of slow moving or static layers of emulsion polymer close to the tube walls will prevent the onset of a stationary layer of polymer which builds up over a period of several months and eventually plugs the pipe.

Some typical monomers suitable for use in the present polymerisation process include, e.g., butyl acrylate, methyl methacrylate, styrene, vinyl acetate, Veova® 9, Veova® 10, Veova® 11 (all three ex Shell), ethyl acrylate, 2-ethyl hexyl acrylate, ethylene, and vinyl chloride. The addition reaction is initiated by radicals to give a dispersion of high-molecular weight polymer particles, usually of 50 to 3000 nm diameter, suspended in a medium in which the polymer is insoluble, usually water. Common free radical generators include the sodium, potassium, and ammonium salts of peroxodisulphuric acid, e.g. ammonium peroxodisulphate. Alternatively, redox couples can be used. These consist of an oxidising agent and a reducing agent. Commonly used oxidisers are the salts of peroxodisulphuric acid and t-butyl hydroperoxide and hydrogen peroxide itself. Reducers are sodium sulphite, sodium metabisulphite, sodium formaldehyde sulphoxylate, and sodium dithionate.

Polymerisation of monomers can take place in aqueous suspension and, in that case, raw materials are preferably provided by separate feed streams. These streams introduce fresh monomer and an aqueous solution of stabilisers known as the water phase or, e.g., a pre-emulsion of monomer and water and an aqueous solution in a separate small stream. At the start of the reaction the reactor is filled with water phase made up in a solution tank. Other fillings are also possible, particularly finished emulsion polymer (of the same or different composition) from a previous run, optionally diluted to any concentration.

Agitation in the reactor is provided by the in-line circulation pump. Shortly after the feed streams start to flow, the monomers begin to react and heat is generated. The temperature is stabilised by cooling means, usually by controlled circulation of a cooling fluid (e.g. water) through a cooling jacket. The product flows to the cooling tank, where residual monomer converts to polymer. After cooling, the emulsion polymer is filtered to remove any oversize particles or gritty material in the strainer and transferred to the product storage tank.

Optionally, the polymerisation process may be carried out under pressure, for instance under a pressure of 10 to 150 bar. Alternatively, the polymerisation may be carried out at ambient pressure.

Pigs can for example be made of soft or semi-hard natural or synthetic material, e.g. rubber or polyurethane. Also, pigs or scrapers with (flexible) metal parts or metal brushes as well as combinations of metal and a soft or semi-hard natural or synthetic material can be used. Of the numerous possible shapes, cylinders, cylinders with round edges as well as cylindrical bodies with thick lips and/or thick strips on the outer circumference are particularly suitable. Dumb-bell shaped cylindrical pigs have two scraping surfaces, as opposed to spherical surfaces, which have only one.

The design of the pig and the materials used depend, among other things, on the type of deposit and the tolerances and radii of the reactor tube.

The invention is further illustrated by the drawings. In the drawings:

Figure 1 : shows a section of a loop reactor according to the invention; Figure 2: shows in cross-section the intersection of the discharge line and the loop tube in the reactor of Figure 1 ; Figure 3: shows an alternative embodiment of the intersection of Figure 2. Figure 1 shows a part of a loop reactor 1 comprising a circulation pump 2 connecting one end 3 of the loop 1 to the other end 4 of the loop 1. The loop reactor 1 further comprises a by-pass line 5 for by-passing cleaning pigs around the circulation pump 2. The by-pass line 5 includes a pig station 6 between two shut-off valves 7, 8. Past the valve 8, the by-pass line 5 turns back to the main loop line. A discharge line 9 for discharge of polymer emulsion intersects the by¬ pass line 5. The reactor 1 further comprises a monomer inlet 10 and a water phase inlet (not shown). Water phase and monomer are continuously supplied, while at the same rate polymer emulsion continuously overflows from the reactor 1 via the discharge line 9 to a collection tank (not shown). The pig station 6 comprises a drain 11 and is connected to the discharge line 9 via a rerouting line 12, which is provided with a shut-off valve 13. A further shut-off valve 14 is located in the discharge line 9, between the connection with the rerouting line 12 and the intersection with the by-pass line 5. A coil-shaped reactor tube (not shown) connects outer end 15 to outer end 16 to form a closed circuit.

During normal use of the reactor 1 , a cleaning pig is stored in the pig station 6. Valve 14 and the water phase feed line are open, while valves 7, 8, and 13 are closed. Emulsion is discharged from the loop via the by-pass line 5 and the discharge line 9 to a collection tank.

To clean the loop reactor, a cleaning pig is launched from the pig station 6. Electro-magnetic pig detectors open and close valves to direct the returning pig into the by-pass line 5, by-passing the circulation pump 2 and returning it to the pig station 6, where it remains until a next launch is initiated. In its route, the cleaning pig does not clean the discharge line 9. This line is pigged separately. A pig is introduced into the outer end 17 of the discharge line 9. This end 17 is provided with a valve 18 and connected to a mains water line. To launch the pig, valve 18 is opened and water is let in to push the pig along the discharge line 9 until it crosses the by-pass line 5. After that, valve 18 is closed and the pig is driven forward by the flow of discharged emulsion.

Figure 2 shows, in cross-section, the by-pass line 5 crossing the discharge line 9. The discharge line 9 offsets the by-pass line 5. In Figure 2, this offset is such that the centre line of the loop line is a tangent of the outline of the discharge line 9. The intersecting parts are in open connection. The diameter of the discharge line 9 is slightly smaller than the diameter of the by-pass line 5.

The intersection must take up sufficient area through which the emulsion can flow. Moreover, the pig must not stick or be held up as it passes the junction. With these requirements in mind an optimum was found as shown in Figure 3, by off-setting the centre line 19 of the by-pass line 5 from the parallel tangent 20 of the outline of the discharge line 9 at a distance which is about a quarter of the outer diameter of the by-pass line 5. In this embodiment, the diameter of the by¬ pass line 5 is about 6/5 of the diameter of discharge line 9. Alternatively, the diameter of the discharge line can be larger than the diameter of the by-pass line.

Claims

1. A polymerisation reactor comprising one or more circulation loops with one or more inlets for raw material, driving means for circulating a reactor charge within the circulation loop, and an outlet connected to a discharge line for the discharge of polymer emulsion, characterised in that the discharge line intersects the circulation loop, and in that one of the outer ends of the discharge line comprises an inlet for a cleaning pig while the other outer end leads to a collection tank.

2. A reactor according to claim 1 , characterised in that the circulation loop includes a line section by-passing the circulation pump and in that the discharge line intersects the circulation loop at the by-pass section.

3. A reactor according to claim 1 or 2, characterised in that a shut-off valve is located between the cleaning pig inlet and the intersection of the discharge line and the loop.

4. A reactor according to any one of the preceding claims, characterised in that at the intersection the discharge line offsets the tube of the circulation loop.

5. A reactor according to any one of the preceding claims, characterised in that at the intersection, the circulation loop offsets the discharge line at a distance greater than the radius of the discharge line.

6. A reactor according to claim 5, characterised in that the shortest distance between the outline of the loop line and the centre line of the discharge line is 20 - 30 % of the outer diameter of the discharge line, and in that the diameter of the circulation loop line is 12 - 20 % smaller than the diameter of the discharge line.