PUMPING OF LIQUEFIED GAS

Abstract: A device for pumping liquefied gas, comprises a pump body (3) having a cylindrical bore (5), a piston (15) arranged in the bore of the pump body and axially movable between a front and a back position; a drive means (19, 21) for movement of the piston between the front and back positions. Liquefied gas is flown into the pump chamber as the piston is moved from the front to the back position and is pumped from the pump chamber as the piston is moved from the back to the front position. The pump includes a control means (23) arranged to control (i) the movement of the piston from the front to the back position to be slow enough to substantially avoid boiling of the liquefied gas flown into the pump chamber; and (ii) the movement of the piston from the back to the front position at a variable velocity.

Description: PUMPING OF LIQUEFIED GAS TECHNICAL FIELD The present invention generally relates to pumping of liquefied gas, and particularly to a device and a method, respectively, for pumping of liquefied gas, preferably at high pressures, and to a system and a method, respectively, for the manufacturing of polymer products, comprising said device and said method, respectively, for pumping of liquefied gas. BACKGROUND OF THE INVENTION Known pumps for pumping of liquefied gas at high pressures comprise different piston and membrane pumps . These pumps operate continuously, often using several pistons or membranes driven by a common motor. Further, so called syringe pumps are known for instance within the field of HPLC (High Pressure Liquid Chromatography), which also are used for the pumping of liquefied gas, for instance carbon dioxide, at high pressure, though in very small amounts. Also these pumps are aimed for continuous pumping often during very long time (up to several hours). Drawbacks of these known pumps are that they are not suitable for pumping during short periods of time to there in-between not pump anything at all. This is of course solvable by using a continuous pump, which is provided with a controllable valve for controlling the amount of pumped liquid in the disclosed manner. This results however in two further problems; firstly it will be unduly costly as the pump consumes energy continuously, and secondly the pump system is supplied with energy in the form of heat, which may cause cavitation of the carbon dioxide, whereby no accurately controlled amount of liquid can be pumped. The problems of cavitation, and thereby deteriorated accuracy and precision of the amount of liquid pumped, can also arise due to other reasons such as heat transmission between different gas amounts and sudden pressure falls. For instance, EP 0 501 806 Al discloses a piston pump for continuous pumping of liquefied carbon dioxide (at pressures up to above 500 bar), which has a liner of low thermal conductivity provided along the inner surfaces of the pump cylinder to prevent heat arising during compression of the liquefied carbon dioxide from being transmitted to the pump cylinder. The heat remains in the carbon dioxide, which is pumped away and leaves the pump during compression. When a subsequent pumping cycle is initiated the newly carbon dioxide intaken is not exposed to any remaining heat and will thus stay in liquefied phase. If the pressure of the carbon dioxide source is 60-70 bars there is however a risk of cavitation since the carbon dioxide at this pressure is very close to its boiling point. This problem is solved by means of using a precooler. Another manner of avoiding boiling of carbon dioxide in the pump cylinder includes to circulate a cooling medium, for instance a liquefied gas, cooling water, or glycol around the cylinder to cool the same, see for instance the US patents 2,439,957 and 2,439,958. These ways of avoiding boiling of carbon dioxide in the pump cylinder are unnecessary complicated, space demanding and/or costly. Further, they do not solve the above-mentioned problems of continuous energy consumption during discontinuous use of the pump. SUMMARY OF THE INVENTION It is thus an object of the present invention to provide a device and a method, respectively, for pumping of an accurately and precisely controllable amount of liquefied gas, which are in lack of at least some of the problems associated with prior art. A further object of the invention is to provide a device and a method, respectively, for pumping of an accurately and precisely controllable amount of liquefied gas, wherein cavitation of the liquefied gas is essentially avoided. Yet a further object of the invention is to provide a device and a method for discontinuous pumping of an accurately and precisely controllable amount of liquefied gas. Still a further object of the invention is to provide a device and a method for pumping of an accurately and precisely controllable amount of liquefied gas, wherein the pump velocity is variably controllable during a single piston stroke. Yet a further object of the invention is to provide a system and a method for the manufacturing of polymer products comprising said device and method, respectively, for pumping of liquefied gas . These and other objects are according to the present invention attained by devices and methods as claimed in the appended patent claims . An advantage of the present invention is that the amount of liquefied gas, which is pumped during a single piston stroke, is accurately and precisely controllable, wherein also the pump velocity can be altered during the piston stroke per se. Further advantages of the invention, and features of the same, will be apparent from the description of the preferred embodiments . SHORT DESCRIPTION OF THE DRAWINGS The invention is further disclosed below with reference to Figs. 1-5, which are shown solely in order to illustrate the invention and shall in no way limit the same. Fig. 1 shows schematically, in cross section, a device for pumping of liquefied gas according to an embodiment of the present invention. Fig. 2 shows schematically a block scheme of a system for manufacturing of polymer products, where the system comprises the device for pumping of liquefied gas shown in Fig. 1. Figs. 3a-c are diagrams showing typical values in arbitrarily units of the amount of liquefied gas pumped per time unit as a function of time for the pump device shown in Fig. 2 (solid lines). Fig 3c also shows typical values of the amount of plasticized polymer fed by time unit as a function of time for the system for manufacturing of polymer products as shown in Fig. 2 (dashed line). Fig. 4 shows schematically, in cross section, a device for pumping of liquefied gas according to a further embodiment of the present invention. Fig. 5 is a diagram illustrating typical values in arbitrarily units of the amount of liquefied gas pumped per time unit as a function of time for the device shown in fig. 1 (solid line) and for the device shown in Fig. 4 (dashed line). DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the following description, with the purpose of explaining and not limiting the invention, specific details are given such as particular applications, techniques etc. in order to give a thorough understanding of the invention. It shall, however, be apparent for the man skilled in the art that the invention can be practiced in other versions than these. The invention will now be described closer with reference to Fig. 1, which in a schematic cross sectional view, illustrates a first embodiment of a pump 1 according to the present invention. The pump comprises a pump body or a cylinder 3 having a cylindrical bore 5, which constitutes a pump chamber. The pump chamber 5 is provided with an inlet 7, which is connected to a source of liquefied C0 2 (not shown in Fig. 1). The source may be a conventional gas bottle with liquefied carbon dioxide at room temperature and at a pressure at approximately 60 bars, but is preferably a ring conduit system, wherein carbon dioxide can be stored in a more controlled manner. At the inlet 7 there is arranged a non-return valve 9, which prevents carbon dioxide from flowing out from chamber 5 through the inlet. In the forward direction the valve 9 opens preferably at a pressure of about 0.5 bar . The pump chamber is further provided with an outlet 11, through which the carbon dioxide is pumped from the chamber. At the outlet 11 there is provided a valve 13, for instance a spring closing non-return valve, which preferably opens at ' a predetermined pressure, for instance 80 bars in the forward direction and is completely closed in the reversed direction. It shall however be appreciated by the man skilled in the art that other valves, for instance remotely controlled valves, can be used to achieve a similar functionality. The present invention includes of course use of such valves. The pump device 1 further includes a piston or needle 15 arranged in the cylindrical bore 5 of the pump body, which is axially movable between a front position and a back position, which is indicated in Fig. 1 by means of a bi-directional arrow 17. When the piston is moved backwards the volume of the pump chamber is increased and when the piston is moved forward the volume of the pump chamber is decreased. The volume change is given by the piston displacement, in Fig. 1 indicated by L, for a given cross sectional area of the piston 15. In order to operate at high pressure the pump comprises a sealing (not shown in Fig. 1) for sealing between the piston and the cylinder wall. It is of course important that the sealing has a good resistance against permeability and/or diffusion of carbon dioxide, such that the risk that the sealing expands and disables pumping functionality is very low. A preferable version of the pump device 1 manages a pressure of up to 500 bars. The piston is driven by a motor 19, suitably a linear electrical motor via a transmission medium, in Fig. 1 schematically indicated by 21. The transmission can for instance be hydraulic or pneumatic. It shall, however, be apparent to the man skilled in the art that any given motor having power transmission capable of achieving a linear movement of piston 15 is usable in the present invention. For instance, a servomotor provided with a rack gearing can be used for driving of piston 15. The motor 19 is controlled by means of a control computer 23 provided with suitable software via signals over a control conduit 25. Particularly, the pump device may be arranged such that movement of piston 15 is achieved depending on amplitudes or amplitude variations of said signals. Further, motor 19 and/or control computer 25 are/is provided with A/D and D/A converters, if needed (not shown in Fig. 1). The computer has further a bi-directional interface 27 for communication for instance with an operator of the pump, or with another control system such as for instance a control computer for control of a manufacturing process of micro- cellular polymer products. Such an example is closer described below with reference to Fig. 2. The function of the pump will in the following be briefly described, starting from a position where piston 15 is fully moved forward and where the volume of the pump chamber, called dead volume, is minimum, and assuming that this dead volume includes liquefied carbon dioxide. In order to fill the chamber with liquefied carbon dioxide the force, with which the piston is held in its advanced position, is decreased such that the pressure of the carbon dioxide source will be sufficient in order to have liquefied carbon dioxide flowing into chamber 5 and press the piston backwards. The force to balance piston 15 during the intake phase is needed to safeguard that the intake of carbon dioxide is performed slowly. In such manner the pump 1 is not self-priming. The slower the carbon dioxide is allowed to flow into the pump chamber the smaller amount of gas is obtained in the chamber and the better accuracy and precision regarding the amount of carbon dioxide intaken is obtained. In practice, the intake phase would endure for a time of from a few to some tens of seconds and up to minutes, but it certainly depends on the particular application. In order to perform pumping, i.e. piston advancing, more frequently a multi-piston pump may be utilized, which will be closer described with reference to Fig. 4 below. When the pump has sucked a predetermined amount of liquefied carbon dioxide (i.e. the piston has been moved backwards a corresponding length) , the pump is ready for pumping of the carbon dioxide. Pumping of the amount of liquefied carbon dioxide is controlled by means of software in any suitable manner. The velocity, at which the amount of carbon dioxide is pumped, can be controlled in a predetermined manner, such that an accurately and precisely controlled amount of liquefied carbon dioxide can be delivered at an exact correct point of time. The pump according to the present embodiment is thus operating with a single piston stroke, which can be repeated subsequent to the slow piston movement backwards . In this respect a discontinuous pumping function is obtained. A particular version of the pump manages to pump up to 2 g/s during the piston advancement at a pressure up to 500 bars (this pressure shall thus be overcome in order to pump the carbon dioxide through the outlet) . The maximum amount, which can be pumped (during a single piston stroke), is given by the increase of the volume of the chamber when the piston is moved backwards, which for a given cross sectional area of the piston is given by the piston movement length L. In this respect, it is very important that the piston is moved backwards slowly in order to secure that the carbon dioxide is not boiling (i.e. that cavitation is not achieved). Thus, the piston is preferably moved at a velocity such that liquefied gas is flown into the pump chamber at a rate slower than 10, 5, 3 or 2 g/s. In one version of the invention, liquefied gas is flown into the pump chamber at a rate of about 1 g/s. There will always be a two phase flow within the chamber, but very small amounts of gas will not impose any severe problem since partly the amount of carbon dioxide intaken will only be somewhat smaller (given by the density of the gas compared to the density of the liquid), partly this amount of gas is directly condensed when the pressure is increased when the piston is moved forward. The pump may further comprise a position sensor 29, which in any suitable manner, measures the exact position of the piston 15 and transmits a position signal via a signal conduit 31 to the control computer 23. The present invention thus provides for very accurate and precise control of the amount of carbon dioxide pumped during a single piston stroke and this control can even be enhanced by means of providing the control computer with this feedback whereby the position signal can, if the sensed position of the piston is not matching the computed position in the control computer, compensate the control of the movement of the piston in real time . Further, the carbon dioxide source is preferably localized, whether it is a bottle or a ring conduit system, above, particularly high above, the pump per se. In this respect the hydrostatic pressure, which arises in the liquid column, achieved, prevents boiling of the carbon dioxide or at least reduces the amount of carbon dioxide, which boils. In an alternative version, a cooling medium, particularly liquefied carbon dioxide, cooling water or glycol, is led through passages (not shown in Fig. 1) along the outsides of the pump body in order to cool the same and further minimize the risk of boiling. In Fig. 2, a system 41 for injection molding of micro-cellular plastic details in a cavity 43 by means of an injection tool 45, is shown. The system comprises a device 47 for plasticizing and feeding of a polymer, including an inlet for supply of polymer raw material (indicated by arrow 49) and a screw for feeding and plasticizing (schematically indicated by an arrow 51), where the screw is driven by a motor 53 controlled by a computer 55 or other control means provided with suitable software. The plasticized polymer is further fed to a mixing chamber or mixer 57, wherein liquefied carbon dioxide is supplied in a controlled manner, which is described further below. The polymer and the carbon dioxide are mixed, and the mixture is further fed to a piston system 59 for increase of pressure. Finally, when the detail is to be molded, a valve 61 is opened and a mixture is allowed to be injected (foamed) into the cavity 43 of the molding tool 45 for the manufacturing of a micro-cellular plastic detail, i.e. a plastic detail having very small gas-filled micro-cells, which typically are in the size of a few micrometer or smaller. Such plastic details are of substantially lower weight than corresponding homogeneous details, at the same time as the mechanical strength can be equally good, or in some respects even better. The carbon dioxide is introduced by means of the pump shown in Fig. 1. In Fig. 2 the pump 1 is only shown briefly comprising pump 3, pump piston 15, motor 19 and control computer 23. The inlet of the pump is connected to a source of carbon dioxide 63 and the outlet thereof is connected to the mixing chamber 57 via a non-return valve 65. This non-return valve 65 safeguards that no polymer can flow in the reversed direction, i.e. towards the outlet of the pump. Further, there is a bi-directional communication interface 67 between the control computer 23 of the pump and the control computer 55 of the system for control of device 47 for plasticizing and feeding raw polymer. In an alternative version the control computer 23 and 55 are constituted by a single control computer, and in a further alternative version also motors 19, 53 are constituted by a single motor. Pumping of carbon dioxide is performed during a limited part of a cycle of plastic molding, where a cycle typically lasts approximately 60 - 150 s. Preferably, pumping is performed (advancement of the pump piston) during typically about 20 s, where the pump velocity is typically in the order of 2 g/s of liquefied carbon dioxide. In this respect there is about 40 — 130 s for intake (back movement of piston) of liquefied carbon dioxide in the pump. This time may for some processes be fully acceptable (such that the carbon dioxide will not boil), but in other circumstances it is insufficient. In such manner a multi- piston pump may be used. It shall further be appreciated by the man skilled in the art that the pump according to the present invention may be used in a plurality of different cyclic manufacturing techniques for manufacturing of polymer products . By means of the software controlled motor the pumping velocity of the pump may according to the present invention be varied during the piston stroke. Figs. 3a-c are thus diagrams illustrating typical values in arbitrary units of the amount. of liquefied gas pumped per time unit as a function of time for the pump device shown in Fig. 2 (solid lines). In Fig. 3a the result of pumping at a constant pumping velocity is shown, in Fig. 3b the result of pumping at a pumping velocity varying according to a step function is shown and in Fig. 3c the result of pumping at a pumping velocity varying continuously — firstly an increasing velocity and then a decreasing velocity — during a piston stroke, is shown. Fig. 3c also shows typical values of the amount of plasticized polymer fed per time unit in an arbitrary scale as a function of time for the system for manufacturing of polymer products shown in Fig. 2 (dashed line). By means of communication interface 67 synchronization may thus be performed between feeding of polymer and supply of liquefied carbon dioxide. Note that the pump according to the present invention not only provides for an accurate and precise control of the pumping velocity but also provides for flexible and variable advancement of the piston, for instance according to the functions shown in Figs. 3a-c. With reference now to Fig. 4, which schematically, in a cross sectional view, shows a device for pumping a liquefied gas, a further embodiment of the present invention will be depicted. The device comprises four pumps 81-87 arranged side by side. Each one of the pumps 81-87 is in principal constituted by a pump as the one shown in fig. 1. However, this four-pump system is utilizing a common motor for power transmission and a common control computer (not shown in Fig. 4). Further, the four-pump system comprises a supply conduit system 89, which in a parallel manner connects a source of liquefied carbon dioxide (not shown in Fig. 4) to the respective inlets of pumps 81-87, and an outlet conduit system 91, which in a parallel manner connects the respective outlets of the pumps 81-87 to a common outlet conduit. The motor and the control computer are arranged to perform pumping cycles with respective pump 81-87 according to the inventive method depicted with reference to Fig. 1. Preferably, the pumping system is arranged with the same phase delay between each pump (as illustrated in Fig.4), such that the system is performing as a single pump of the kind shown in Fig. 1, but which is four times as fast. This embodiment of pumping system is particularly useful in a system for injection molding of a micro-cellular plastic detail where the cycle times are very short. In a further version the pistons are completely in phase such that the system is performing as a single pump of the kind shown in Fig. 1, but which has four times higher pumping capacity per stroke. The present invention puts no limitations as regards the phase delays between the pistons in the system. Fig. 5, finally, is a diagram illustrating typical values in arbitrary units of the amount of liquefied gas pumped per time unit as a function of time for the device shown in Fig. 1 (solid line) and for the device shown in Fig. 4 (dashed line). It is seen that the frequency of the piston strokes are increased proportionally with the number of pistons . The present invention thus described solves the problems associated with prior art. It is certainly not limited to the embodiments described above and illustrated in the drawings, but can be modified within the scope of the appended patent claims . Particularly, the pump may be used in any application having a need of a discontinuous, accurate and precise supply of liquefied gas.

Claims: CLAIMS 1. A device for pumping of an accurately and precisely controllable amount of liquefied gas, comprising a pump body (3) having a cylindrical bore (5), which constitutes a pump chamber; a piston (15) arranged in the bore of the pump body and axially movable between a front and a back position; a drive means (19, 21) for movement of said piston between said front and back positions; - an inlet (7) to said pump chamber, which inlet is connectable to a source of liquefied gas; and an outlet (11) from said chamber, wherein liquefied gas from said source can be flown into the pump chamber through said inlet while the drive means moves said piston from the front position to the back position and liquefied gas can be pumped out from said pump chamber through said outlet while the drive means moves said piston from the back position to the front position, characterized in a control means (23) arranged to control said drive means to move said piston from the front position to the back position sufficiently slow in order to substantially avoid boiling of the liquefied gas flown into the pump chamber; and - move said piston from the back position to the front position at a variably controllable velocity, whereby the amount of liquefied gas which is pumped through the outlet is accurately and precisely controllable. 2. The device as claimed in claim 1 wherein the drive means is arranged to control the drive means to move said piston from said back position to said front position at a velocity such that liquefied gas is flown into the pump chamber at a rate slower than 10 g/s. 3. The device as claimed in claim 1 wherein the control means is arranged to control said drive means to move said piston from said back position to said front position at a velocity such that liquefied gas is flown into the pump chamber at a rate slower than 5 g/s. 4. The device as claimed in claim 1 wherein the control means is arranged to control said drive means to move said piston from said back position to said front position at a velocity such that liquefied gas is flown into the pump chamber at a rate slower than 3 g/s, preferably slower than 2 g/s, and more preferably at a rate of about 1 g/s. 5. The device as claimed in any of claims 1-4 wherein said device is arranged for discontinuous pumping of said liquefied gas. 6. The device as claimed in any of claims 1-4 wherein said device is a single-stroke type of pump. 7. The device as claimed in any of claims 1-6 further comprising a position sensor (29) connected to said control means by means of a single conduit (31), wherein said position sensor is arranged to measure the position of the piston and transmit the measured position to the control means . 8. The device as claimed in any of claims 1-7 further comprising a passage along the outside of the pump body arranged for transport of a cooling medium, particularly a liquefied gas, and thus for cooling of said pump body. 9. The device as claimed in any of claims 1-8 wherein the drive means comprises a linear motor. 10. The device as claimed in any of claims 1-9 wherein the drive means is arranged for hydraulic driving of the piston. 11. The device as claimed in any of claims 1-10 wherein said inlet is arranged for connection to a source of liquefied gas, wherein said source is located above, particularly very above, said pump body. 12. The device claimed in any of claims 1-11 wherein said inlet is provided with a valve which, in a direction towards the pump chamber, is arranged to open at a predetermined pressure and which, in a direction from the pump chamber, is closed. 13. The device as claimed in any of claims 1-12 wherein said outlet is provided with a valve which, in a direction from the pump chamber, is arranged to open at a predetermined pressure and which, in a direction towards the pump chamber, is closed. 14. The device as claimed in any of claims 1-13 wherein the liquefied gas is carbon dioxide. 15. The device as claimed in any of claims 1-14 wherein said outlet is connectable to a system for manufacturing of polymer products . 16. A pump system comprising at least two (81, 83, 85, 87) of the device as claimed in any of claims 1-15. 17. The pump system as claimed in claim 16, wherein the inlets of all devices are connected by means of a supply conduit system (89) and the outlets of all devices are connected by means of an outlet conduit system (91). 18. A method for pumping of an accurately and precisely controllable amount of liquefied gas by means of a pump comprising a pump body (3) having a cylindrical bore (5), which constitutes a pump chamber, a piston (15) arranged in the bore of the pump body and axially movable between a front and a back position, an inlet (7) to said pump chamber, which inlet is connectable to a source of liquefied gas, and an outlet (11) from said chamber, wherein said method comprises the steps of: flowing liquefied gas from said source into the pump chamber through said inlet when said piston is moved from the front position to the back position; and pumping liquefied gas out of the pump chamber through said outlet when said piston is moved from the back position to the front position, characterized by that: said piston is moved from the front position to the back position sufficiently slow in order to substantially avoid boiling of the liquefied gas flown into the pump chamber; and said piston is moved from the back position to the front position at a variably controllable velocity. 19. The method as claimed in claim 18 wherein said piston is moved from the front position to the back position at a velocity such that liquefied gas is flown into the pump chamber at a rate slower than 10 g/s. 20. The method as claimed in claim 18 wherein said piston is moved from the front position to the back position at a velocity such that liquefied gas is flown into the pump cnamber at a rate slower than 5 g/s. 21. The method as claimed in claim 18 wherein said piston is moved from the front position to the back position at a velocity such that liquefied gas is flown into the pump chamber at a rate slower than 3 g/s, preferably slower than 2 g/s, and more preferably at a rate of about 1 g/s. 22. The method as claimed in any of claims 18-21 wherein the steps of flowing liquefied gas from said source into the pump chamber and pumping liquefied gas out of the pump chamber are repeated to thereby provide for a discontinuous pumping f nctionality. 23. A system for cyclic manufacturing of polymer products comprising a device (47) for plasticizing and feeding of a polymer, a piston pump (1) connected to a source (63) of liquefied gas, a mixing chamber (57), and a molding tool (45) having a molding cavity (43), wherein, for each manufacturing cycle, the plasticizing and feeding device is arranged for reception of a polymer, for plasticizing of the polymer and for feeding of the plasticized polymer, the pump is arranged for pumping of a liquefied gas, the mixing chamber is arranged for the reception of said plasticized polymer and the liquefied gas, and for mixing of these, and the molding tool is arranged for reception of the mixture and for forming of the polymer product, characterized in that the pump is constituted by a device according to any of the claims 1-14, wherein the device is arranged to only strike once per manufacturing cycle. 24. The system as claimed in claim 23 wherein the pump is arranged for intake of liquefied gas during a first time period of each cycle and for pumping of liquefied gas during a second time period of each cycle, where said first time period is longer than said second time period. 25. A method for cyclic manufacturing of polymer products comprising the steps of: - providing a polymer; plasticizing said polymer, supplying an amount of liquefied gas to said polymer; mixing said amount of liquefied gas and said polymer; and injecting said mixture in the cavity (43) of a molding tool and wherein the mixture is allowed to solidify characterized by that said amount of liquefied gas is supplied to the polymer through pumping according to the method as claimed in any of claims 18-22. 26. The method as claimed in claim 25 wherein liquefied gas is flown into the piston pump during a first time period of each cycle, and wherein liquefied gas is pumped from said piston pump during a second time period of each cycle, where said first time period is longer than said second time period.

Inventors: ORSEN MIKAEL, ORSEN, MIKAEL
Applicants: AGA AB, ORSEN MIKAEL, AGA AKTIEBOLAG, ORSEN, MIKAEL
Application year: 2001
Main classification: F04B15
IPC codes: F04B15, F17C9