Efficient chiller for a supercritical fluid chromatography pump

Abstract

Methods and systems for pumping compressible fluids in high pressure applications such as high-pressure liquid chromatography (HPLC) or supercritical fluid chromatography (SFC) applications are disclosed. An improved cooling device for a pump head for use in a supercritical fluid chromatography (SFC) system is described. A system for chilling a pumping system, includes a Peltier cooling element in thermal contact with a pump head, wherein the cooling element chills the pump head and a mobile phase fluid flowstream prior to the mobile phase fluid entering the pump; a fluid-cooled heat exchanger, attached to the Peltier cooling element, which removes heat from the cooling element using a circulating fluid; and a second heat exchanger which cools the circulating fluid.

Claims

1. A supercritical fluid chromatography (SFC) system, comprising: a pumping system configured to drive supercritical carbon dioxide as a mobile phase through a stationary phase for separating compounds of a sample introduced into the mobile phase downstream from the pumping system; and a mobile phase supply configured to supply carbon dioxide gas to the pumping system, the mobile phase supply comprising a reservoir containing a liquid layer of liquefied carbon dioxide and a head space of carbon dioxide gas separate from the liquid layer, wherein the pumping system is in fluid communication with the head space and is configured to sample the carbon dioxide gas from the head space without sampling the liquefied carbon dioxide from the liquid layer.

2. The SFC system of claim 1, wherein the pumping system comprises a pump head, and further comprising a pump head chiller assembly configured to liquefy the carbon dioxide gas supplied from the mobile phase supply and deliver liquefied carbon dioxide to the pump head.

3. The SFC system of claim 2, wherein the pump head chiller assembly comprises: mobile phase tubing comprising a mobile phase inlet fluidly communicating with the mobile phase supply, a mobile phase outlet fluidly communicating with the pump head, and a multi-turn tubing section between the mobile phase inlet and the mobile phase outlet, wherein the mobile phase tubing is configured to flow a mobile phase through the mobile phase tubing and toward the pump head; and a Peltier element comprising a cool side and an opposing hot side and mounted near the pump head such that the cool side removes heat from the pump head, wherein the mobile phase tubing is in thermal contact with the cool side such that the carbon dioxide gas is liquefied in the mobile phase tubing.

4. The SFC system of claim 3, wherein the pump head chiller assembly has a configuration selected from the group consisting of: the pump head chiller assembly comprises a chiller plate arranged on the cool side and mounted in thermal contact with the pump head such that the chiller plate removes heat from the pump head, wherein the mobile phase tubing is interposed between and in thermal contact with the cool side and the chiller plate such that the Peltier element and the chiller plate are configured to liquefy the carbon dioxide gas in the mobile phase tubing, and the cool side is in thermal contact with the chiller plate such that the Peltier element removes heat from the chiller plate; the pump head chiller assembly comprises a first heat exchanger arranged on the hot side and configured to remove heat from the Peltier element; and both of the foregoing.

5. The SFC system of claim 2, comprising a fluid-sealed, thermally insulated compartment enclosing the pump head and the pump head chiller assembly.

6. The SFC system of claim 2, comprising a first heat exchanger configured to remove heat from the pump head chiller assembly by circulating a coolant, and a second heat exchanger configured to cool the coolant.

7. The SFC system of claim 2, wherein: the pump head is a first pump head configured for pressurizing the liquefied carbon dioxide and delivering the pressurized, liquefied carbon dioxide to the second pump head; and the pumping system further comprises a second pump head configured for metering the flow of the pressurized, liquefied carbon dioxide to the stationary phase.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram of a conventional simplex reciprocating pump.

(2) FIG. 2 is a diagram of a conventional duplex reciprocating pump.

(3) FIG. 3 is a diagram of and exploded view of a representative embodiment of the chiller assembly.

(4) FIG. 4 is a diagram of the side view of the representative embodiment of the chiller assembly.

(5) FIG. 5 a diagram of the front view of the assembled chiller assembly.

(6) FIG. 6 is a right profile view of the assembled pump drawer showing the radiator, circulator pump, and enclosed chiller assembly and pump head.

(7) FIG. 7 is a front view of the pump drawer showing the chiller assembly mounted behind the pump head within a vapor tight enclosure.

(8) FIG. 8 is a left side profile of the pump drawer showing the pump drive mechanism positioned outside the vapor tight enclosure.

DETAILED DESCRIPTION

(9) It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the apparatus, system, and method of the present invention, as presented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention.

(10) Referring to FIGS. 3, 4 and 5, a schematic illustration of a representative pump head chiller assembly 48 is displayed. A cold plate or chiller plate 50 is attached to a Peltier element 54 with associated electrical connectors 56. The chiller plate 50 is formed out of copper but could be aluminum or other appropriate materials. Chiller plate 50 is mounted near or behind a pump head through a plate extension 52. In one embodiment, the chiller plate 50 is mounted perpendicular to the pumping axis onto a housing frame via a support plate 66. A labyrinth of bent or serpentine stainless steel tubing 58 (hereinafter “tubing”) is embedded in or located on or near the chiller plate 50. The tubing 58 has an inlet 62 and an outlet 60. A brazed cooler block 70 of a first heat exchanger mounts to the Peltier element 54 using an appropriate mounting technique such as seating compound such as ceramique thermal seating compound. The Peltier element 54 is positioned approximately to the center of cooler block 70. Inlet 72 and outlet 74 adapters for cooler block 70 connect tubing for fluid flowstreams to and from a second heat exchanger.

(11) A spacer plate 64 constructed of plastic or other insulating material is located between the support plate 66 and the chiller plate 50. Spacer plate 64 thermally isolates the chiller plate 50 from radiative heating. The support plate 66 and the cooler block 70, on either side of chiller plate 50 also serve as structural support for pump head chiller assembly 48. The pump head chiller assembly 48 is held together via screws 68 or other appropriate attaching elements.

(12) Referring again to chiller plate 50, the side of the chiller plate 50 against which the Peltier element 54 would be pressed comprises the tubing 58 that is pressed into channels within chiller plate 50. A thermal compound is applied to the tubing 58 and the chiller plate 50 to ensure good thermal contact with both the aluminum (or copper) metal in the chiller plate 50 and the stainless steel of the tubing 58. Carbon dioxide gas enters the tubing 58 at inlet 62 and is pre-chilled and liquefied before it reaches the base of the pump via outlet 60. Accordingly, the chiller plate 50 and the tubing 58 may be part of a prechiller assembly. A temperature sensor is located in the chiller plate 50 immediately behind the pump head. Thus the chiller plate 50 can maintain good thermal contact with and cool both a pump head and the tubing 58 for the flowstream. The cooler block 70 is a liquid heat exchanger, and may be constructed of copper clad materials, coated stainless steel, or brass. Pre-chilling the carbon dioxide flowstream via thermal contact between the chiller plate 50 and the pump head enhances the ability to maintain the carbon dioxide fluid in a liquid state while in the pump during compression and delivery.

(13) The chiller plate 50 nominally operates near about negative 10 degrees Celsius (° C.), for example. The CO.sub.2 flowstream in gaseous state enters the tubing 58 at the inlet 62, turns to liquid state in the chilled tubing 58, exits at the outlet 60, and enters into the pump head as chilled liquid. The present embodiments for the pump head chiller assembly 48 operate not only at sub-ambient temperatures, but also at a sub-freezing point of water. Conventional pump designs, whether liquid or Peltier chilled, have always avoided going below 0 degrees C. because of the problem of building up of ice onto the components, as described above. The present invention isolates the heat exchange components of the cooler block 70, except for the inlet 72 and outlet 74 adaptors, within a vapor tight compartment 78 (hereinafter “compartment”, FIGS. 6-8) that prevents infiltration of atmospheric moisture. The flat face of the cooler block 70 is mounted in direct contact with the warm side (or “hot” side) of the Peltier element 54. Circulation of coolant within the cooler block 70 maintains the Peltier “hot” surface at just a few degrees Celsius above room temperature. Since efficiency of the Peltier element 54 depends on the delta T (change in temperature) that currently exists across the Peltier element 54, keeping the temperature of the warm side as low as possible allows lower working temperatures on the cold side of the Peltier element 54.

(14) The cold side of the Peltier element 54 functions to transfer heat away from the chiller plate 50, from the gas-state CO.sub.2 entering the tubing 58 to convert the gas-state CO.sub.2 to liquid-state CO.sub.2 in the tubing 58, and from the back of the pump head 80 (FIGS. 7 and 8) via the plate extension 52 to carry away the adiabatic heat of compression, as pressure in the system reaches up to 400 bar. Temperature fluctuations depend on workloads for the system. With no load, temperatures reach in a range of about negative 15 to negative 20 degrees C., with standard operation loads temperatures reach a range of about negative 12 to negative 5 degrees C., and with very high loads and with very high ambient temperatures, the system can operate at about +5 to 6 degrees C. above zero and still meet the demands of the pump which is operating/drawing at about 5 ml/minute, for example. A typical operating range is well outside the marginal zone for operation.

(15) FIGS. 6, 7, and 8 illustrate embodiments for a pump head chiller assembly 48 mounted to a pump frame housing 76. The pump head chiller assembly 48 is clamped between the pump head 80 and an insulating spacer 94, all of which are placed inside of the sealed compartment 78, which is enclosed and insulated. Using the pump head chiller assembly 48 in such an arrangement, the pump head 80 of pump 82 is exposed and can be removed for servicing without loosening the chiller plate 50.

(16) To seal the interior components of the compartment 78, there is a foam vapor barrier provided by foam backing 84. The embodiment uses a specially designed insulating spacer 94 positioned behind the chiller plate extension 52 both to seal to against the foam backing 84 and provide pressurized contact between the chiller plate extension 52 and the pump head 80. In addition, the back plate of the cooler block 70, is designed so that when the pump head 80 is secured, the inlet and outlet ports of the cooler block 70 protrude through the foam backing 84 and connect to heat exchanger low pressure tubing 86. The compartment 78, which may be plastic, excludes air, and thus moisture, from interior components, and the foam backing 84 is closed cell foam, which excludes water. Thus, the compartment 78 excludes nearly all ambient water vapor from reaching the chiller plate 50 and the pump head 80. Air inside the compartment 78 has no, or virtually no, air exchange with external ambient air. Ambient air containing water vapor can only contact the chiller plate 50 and the pump head 80 when a front panel or cover of the compartment 78 is removed for servicing or maintaining the components. Once the pump head chiller assembly 48 begins operation, any moisture trapped in the air within the compartment 78 will be deposited as a thin film of ice on the chiller plate 50 and the pump head 80 but is not replaced. While the compartment 78 remains sealed, no additional condensation can occur on the pump head chiller assembly 48 or the pump head 80, thus no additional loss of efficiency is encountered. Liquid condensation resulting in accumulation of water on and around the pump head chiller assembly 48 is also avoided.

(17) As shown in FIG. 7, removing the front panel of the compartment 78 exposes the pump head 80. The check valve holders are directly exposed, allowing normal replacement of the check valves for inlet and outlet flow to and from the pump 82. The pump head 80 can be removed by loosening two exposed nuts, exactly as is done with the pump head chiller assembly 48, allowing easy replacement of the pump main piston seal. The check valves and pump seals can be replaced for routine maintenance without removing the pump head chiller assembly 48.

(18) As shown in FIG. 6, the system further comprises the second heat exchanger, which may include a radiator 88 and fan, installed distal to the pump head chiller assembly 48 in the pump frame housing 76, and configured to provide ambient cooling (i.e., heat exchange) of the circulating coolant. The cooler block 70 of the liquid first heat exchanger is configured to remove heat from the hot side of the Peltier element 54 (FIGS. 3 and 4). The inlet and outlet side of the cooler block 70 penetrates the rear wall of the compartment 78, through the pump frame housing 76 so that inlet and outlet flowstreams of the circulating fluid (coolant) in the low pressure tubing 86 can be attached. A heat exchanger circulating fluid such as a propylene glycol/water mixture is circulated through the cooler block 70 by a small centrifugal pump 90, such as a commercial liquid circulating pump of the type used to cool computers. The heat exchanger is hermetically sealed to prevent water from leaking outside of its components. The heat exchanger fluid is then pumped through a high surface area, car-like, radiator 88 located some distance from the pump 82. Room temperature air is blown through the radiator 88 by the large, dedicated fan. The air flow from the fan does not strike the chiller plate 50 or the pump head 80 due to the compartment 78. There can be no condensation on the low pressure tubing 86 for the circulating fluid, or the radiator 88 or the small centrifugal pump 90 since they are all warmer than room temperature. Due to the efficiency of how water carries away heat, the heat exchanger fluid does not vary its temperature significantly, and in some embodiments remains in the range of about 28 to 30 degrees C. during its entire operation. An alternative to water cooling is using air. However more power is generally used in a Peltier device with air cooling which may bring the air in the heat exchange system up to about 35 or 40 degrees C. and therefore maintaining a low temperature of about zero or negative 10 degrees C. at the pump head 80 would be much less efficient. Water cooling helps maintain the cooling effects of the Peltier device so that it shifts down the effective range of average cooling temperature and thereby the cooling capacity of the embodiment.

(19) Regarding gaseous entry into the system, CO.sub.2 in gaseous state enters from a remote supply through an on/off valve, passes temperature and pressure sensors, and flows through a tube to a bulkhead fitting through the pump frame housing 76 into the compartment 78. Once in the compartment 78, the flowline enters at inlet 62 into the tubing 58 in the front of the chiller plate 50, then the bottom of the tubing 58 goes into an entry tube to the pump head 80. Because of the negative 10 degrees C. nominal temperature of chiller plate 50, CO.sub.2 rapidly liquefies prior to reaching the outlet 60 of the tubing 58, when it enters the pump head 80. Thus, CO.sub.2 is a gas when it enters the heat exchanger/Peltier tubing 58 and exits as a liquid ready for compression.

(20) Within the chiller plate 50, there is a limited volume of the tubing 58, for example, about 40 to 45 cm of tubing, so it can provide a few hundred microliters of total volume in the tubing 58. An alternative embodiment for a chiller plate 50/tubing 58 arrangement is to use a high surface-area plate and expose the plate to a cavity or volume. A larger volume with a high surface area plate, such as with fins, placed against a Peltier unit, creates a larger volume of gas in the liquid, which can be siphoned out of the bottom of the unit. Another alternative embodiment is to coil the tubing continuously, instead of a serpentine arrangement. A further alternative uses two Peltier units instead of just one for a chiller arrangement to extend the volume that can be liquefied.

(21) After compression at the pump head 80, the flowstream of compressed CO.sub.2 flows through a tube in the compartment 78 to a bulk-head fitting though the pump frame housing 76, and enters pulse dampener 92. The pulse dampener 92 may be a 25-mL empty volume, for example, located inside the pump frame housing 76. After passing a final pressure sensor, the CO.sub.2 flowstream is ready for transfer to a metering pump or other downstream system process.

(22) Overall, the pump head chiller assembly 48 and the compartment 78 creates a very efficient chilling system over prior systems. Smaller Peltier cooling units may be used on SFC systems than have been used in prior systems, yet the smaller units provide colder temperatures for cooling the pump heads and allow unequivocal condensation of adequate vapor to meet all the liquid needs of the system.

(23) An additional, high value use of the chiller plate 50 is as a condensing unit to liquefy compressible fluids supplied to the booster in vapor form. Such capability creates a much broader variety of sources of the working fluid. A premier example is liquefying CO.sub.2 from a lower purity source such as a beverage grade CO.sub.2 reservoir. By sampling from the gas state of the tank rather than the liquid state, the CO.sub.2 is actually distilled, which removes nonvolatile impurities from the working fluid. Purity of the CO.sub.2 working fluid can be elevated well above the purity of traditional high purity CO.sub.2 grades such as SFC or SFE grades costing at least an order of magnitude more. By sampling from a high pressure cylinder, the CO.sub.2 pressure is already very near the room temperature gas-liquid equilibrium pressure. As a result, only the heat of vaporization needs to be removed (a few watts of cooling per gram) to form liquid CO.sub.2. From this point, lowering the temperature further, e.g. below 10° C., gives sufficient margin to prevent cavitation of the liquid CO.sub.2 during the aspiration portion of the piston stroke.

(24) Another advantage of delivering vapor CO.sub.2 to the pump is the dramatically reduced cost of distributing a modest pressure gas stream throughout a laboratory or process site as opposed to delivering high pressure liquefied gas. If the prechiller is capable of cooling CO.sub.2 below −20 degrees C. the pressures of CO.sub.2 available to most dewar cylinders and bulk tank installations become available as sources. Hence a high power prechiller can truly lower the operating cost of the CO.sub.2 supply as well as allow its safe transport though low pressure piping within a facility. The economics are largely driven by the relative cost of bulk beverage grade CO.sub.2 at less than $0.10 per pound compared to SFC grade CO.sub.2 at more than $7.00 per pound—a 70-fold increase.

(25) Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.