FLASH EXTRUSION VALVE AND PROCESS FOR USING IT

Abstract: A combination valve is disclosed which is useful in flash extrusion of polymer-liquid mixtures or polymer solutions. The combination valve comprises an automatically actuated main process fluid valve (13, 14) and an automatically actuated prime-flush valve (25, 26) in communication with the main process fluid channel (24) just downstream from the main process fluid valve. In operation the prime-flush valve is opened at start up to supply fluid under pressure to the main process fluid channel so that solvent does not evaporate and leave solid polymer to plug the system when the main process control valve is first opened. A shutdown, just as the main process control valve is being closed the prime-flush valve is opened to supply fluid to the system and remove polymer therefrom before it can solidify and plug the system thereby interfering with the next start up.

Description: TITLE FLASH EXTRUSION VALVE AND PROCESS FOR USING IT FIELD OF THE INVENTION BACKGROUND OF THE INVENTION The present invention relates to an improved valve for utilizing an inert gas or optionally a liquid solvent at start up and shutdown of a flash extrusion process which utilizes a polymer and solvent at an elevated temperature and pressure to produce foams, sponges, plexifilament strands, etc. PRIOR ART U.S. 3,227,784 discloses a process for producing molecularly oriented microcellular or plexifila entary products by extrusion of a polymer solution. U.S. 3,461,193 discloses a start up procedure and valve arrangement for flash-extrusion of solutions of a polymer. The procedure involves pressuring the extrusion apparatus downstream from the polymer solution control valve with an inert gas to prevent premature flashing of the solvent and consequent solidification of the polymer. SUMMARY OF THE INVENTION The present invention relates to a combination valve and process for utilizing such valve in flash extrusion of polymer-liquid mixtures or polymer solutions. As used herein flash extrusion means a process wherein solvent immediately flashes when a solvent-polymer mixture or polymer solution passes through a restricting orifice or series of orifices to form a foam, sponge or fibrous structure. Typical of processes that can be benefitted by the present invention is the process disclosed by Blades and White in U.S. 3,227,784, however, the present valve and process of the present invention is beneficial for any flash extrusion process as well as other polymer extrusion process requiring valve or equipment to be flushed immediately upon shutdown. The apparatus of the present invention (combination valve) yields consistent repeatable start up of flash extrusion processes. In addition, this apparatus provides the means of shutting down the flash extrusion process with a minimum of polymer residue in valve and flash extrusion hardware by allowing the injection of inert gas or solvent close to the process valve prior to allowing pressure to fall to a level that allows polymer to solidify in the downstream hardware. This short flush at the time the process valve is closed makes clean up and restart of the flash extrusion hardware easier. BRIEF DESCRIPTION OF THE DRAWING The drawing is a cross-section of the combination valve forming part of the present invention which valve is used in the process of the present invention. DETAILED DESCRIPTION OF THE DRAWING Referring now to the drawing, the combination valve of the present invention is depicted generally at 11. Combination valve 11 contains two valves contained in one valve body 12. The first valve is a process fluid valve fitted with valve seat 13 which cooperates with linearly actuated valve stem 14 to provide off-on control of the process fluid (polymer-solvent mixture or polymer solution) in process fluid feed stream 15. Valve stem 14 is connected to piston 16 actuated automatically by being urged toward the closed position by air pressure in chamber 17, defined by air pressure cylinder 18 and piston 16, or by being urged toward the open position by air pressure in chamber 19 defined by air pressure cylinder 18 and piston 16. The position of valve stem 14 can be visually determined by the position of indicator 21 on scale 22. Valve stem 14 can be manually locked in the closed but not open position or its maximum opening preset by means of manual control means 23. Process fluid channel 24 makes a substantially right angle bend just downstream from valve seat 13. At the right angle bend in process fluid channel 24 combination valve 11 is fitted with prime-flush valve seat 25. Valve seat 25 in cooperation with linearly actuated valve stem 26 opens and closes communication between solvent or gas channel 27 and process fluid channel 24. Valve stem 27 is urged toward the open position by means of air or nitrogen pressure in chamber 28, defined by air pressure cylinder 29 and piston 30 or urged toward the closed position by air pressure in chamber 31 defined by air pressure cylinder 29 and piston 30. DETAILED DESCRIPTION The "Combination Valve" as disclosed herein consists of two valves in one valve body. One valve (Process Valve) serves as on-off control of the process fluid in addition to controlling the pressure of process fluid. The second valve (Prime-Flush) serves as on-off control for an inert gas or solvent that is utilized when starting and shutting down flash extrusion process. Valve body has steam passages (not shown in the drawing) for maintaining the valve at process temperature. The Process Valve has an air operated actuator that permits remote and automatic operation. Valve stem and seat are designed to provide pressure control based on signal from pressure transducer located unstream from the extrusion orifice. Pressure signal is supplied to a valve positioner mounted on the valve body. Pressure at the extrusion point is a critical process parameter. The Prime-Flush Valve allows for the injection of an inert gas or solvent downstream of the Process Valve to permit raising the pressure between the Process Valve and the extrusion point to a level that prevents polymer separating and fouling the hardware when the process is started by opening the Process Valve. The Prime-Flush Valve is located close to the Process Valve to allow polymer to be flushed from the valve when necessary to shut off polymer flow; flush must take place prior to pressure falling to the point that solvent flashes and polymer solidifies in valve and flash extrusion hardware. The minimum pressure to obtain a satisfactory flush with a minimum amount of inert gas or solvent is a function of the polymer, solvent, concentration, and temperature. The inert gas or solvent valve is designed with an actuating cylinder so that it can be operated remotely and sequenced automatically. An inert gas is preferred to solvent for priming the flash extrusion hardware because: 1. It has a low heat capacity and it can be used unheated with no significant impact on process hardware temperature. 2. It does not mix with and dilute the solution causing the initial flash extrusion product to be very poor quality. However, priming using solvent has been done successfully. Solvent does a better job of flushing hardware at time of shutdown than an inert gas but the inert gas does a satisfactory job and avoids the need for supplying both inert gas and solvent to this valve. Automatic sequencing of the two actuators of the "Combination Valve" is desirable but not required. Automation simplifies operator tasks and allows starting and stopping polymer flow with the minimum amount of inert gas or solvent. The preferred sequence for operation of the "Combination Valve" is: Process Start Up 1. Operator initiates automatic start up sequence 2. Prime-Flush valve opens to inert gas source that is at a pressure slightly higher than the process supply pressure. 3. Process valve opens as soon as pressure downstream of valve exceeds normal operating pressure. Prime-Flush valve closes rapidly at time Process Valve is opened. 4. Process Valve is placed in automatic control. Process Shut Down 1. Operator or on line sensor initiates automatic shutdown sequence. 2. Process Valve closes and Prime-Flush valve opens rapidly at the same time so that flush proceeds without a loss in pressure. 3. Timer starts when Prime-Flush valve opens. 4. Fush Valve closes after a pre-set time (1-100 seconds) .

Claims: CLAIMS 1. A combination valve comprising a first valve comprising a linearly actuated valve stem which cooperates with a valve seat to open and shut a channel adapted to feed a flash extrusion process fluid, immediately downstream from said first valve in said channel, a second valve, in communication with said channel, comprising a valve seat and a linearly actuated valve stem adapted open and shut a communication between a pressurized stream of prime-flush fluid and said channel. 2. The combination valve of claim 1 wherein said second valve is located at a bend in said channel. 3. A process of starting up and shutting down a flash extrusion process wherein a flash extrusion process stream is shut by a valve, as said valve is shut a pressure is applied to the process fluid channel downstream by means of a prime-flush fluid fed to said process fluid channel through a prime-flush fluid valve, and when shutting down the system opening the prime-flush fluid valve to flush the system at the time process fluid valve is closed.

Inventors: DEMPSEY JAMES J, DEMPSEY, JAMES, J.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1989
Main classification: B29C67
IPC codes: B01F5, B29B7, B29C37, B29C44, B29C47, F16K1, F16K11