Method and apparatus for dampening flow variations and pressurizing carbon dioxide

Abstract

An apparatus is provided for maintaining a steady flow rate and pressure of a carbon dioxide stream at high pressure when a low-pressure source of the carbon dioxide varies with time. Liquid level in an accumulator that is sized to accommodate variations in supply rate is controlled by sub-cooling of liquid entering the accumulator and heating in the accumulator, the sub-cooling and heating being controlled by a pressure controller operable in the accumulator.

Claims

1. Flow-connected apparatus for decreasing fluctuations in rate of flow of a stream of carbon dioxide, wherein a pressure of the stream of carbon dioxide is at or above a triple point pressure, wherein the stream of carbon dioxide is from an intermittent or variable rate source of carbon dioxide, comprising: a first heat exchanger configured to subcool the stream of carbon dioxide; a flow isolation device configured to prevent a backflow of carbon dioxide to the source of carbon dioxide; an accumulator connected to the first heat exchanger, wherein the accumulator contains a vapor phase and a liquid phase of carbon dioxide; a mister system coupled to the first heat exchanger wherein the mister system is located inside the accumulator, and wherein the mister system is configured to provide sub cooled carbon dioxide to a vapor space in the accumulator; a heat source for supplying heat flux in the first heat exchanger and the accumulator, a pressure controller configured to maintain a set pressure in the accumulator by regulating a valve, wherein the valve regulates heat flux into the accumulator such that a portion of the liquid phase carbon dioxide vaporizes when there is a net negative flow of carbon dioxide into the accumulator and regulating heat flux into the first heat exchanger such that a portion of the carbon dioxide is liquefied when there is a net positive flow of carbon dioxide into the accumulator, wherein the net negative flow of the carbon dioxide is indicated by the volume of the liquid phase inside the accumulator falling, wherein the net positive flow of the carbon dioxide is indicated by the volume of the liquid phase inside the accumulator rising; upper and lower liquid level controls in the accumulator, for determining an accumulator volume in the accumulator between the liquid level controls, the accumulator volume selected to accommodate predicted variations of output rate from the source of carbon dioxide; a conduit for carrying heated fluid, the conduit disposed between or below the liquid level controls in the accumulator, flow through the conduit being controlled by the pressure controller responsive to pressure in the accumulator; a pump connected to the accumulator for pumping liquid carbon dioxide, wherein the pump removes carbon dioxide from the apparatus and is connected to a pipeline or well, wherein a speed of the pump is controlled by an average flow rate from the source of carbon dioxide, and a second heat exchanger connected in between the accumulator and pump wherein the second heat exchanger is configured to densify the liquid.

2. The apparatus of claim 1 wherein the heat source is a heat pump containing a refrigerant.

3. The apparatus of claim 1 wherein the heat source is provided by an outside process.

4. The apparatus of claim 2 wherein the refrigerant is capable of carbon dioxide liquefaction at the pressure of the source of carbon dioxide.

5. The apparatus of claim 1 further comprising an additional heat exchanger and refrigeration downstream of the pump.

6. The apparatus of claim 1 wherein the flow isolation device comprises a throttle, a check valve, a snap acting valve, or a combination thereof.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 illustrates one embodiment of apparatus used to decrease variations of flow rate of carbon dioxide supplied for pumping to high pressure for injection into wells, a pipeline or other uses.

(2) FIG. 2 shows a flow chart of the disclosed method for maintaining a steady stream of carbon dioxide from a source having variations in flow rate.

DETAILED DESCRIPTION OF THE INVENTION

(3) Referring to FIG. 1, variable-rate or intermittent carbon dioxide source 10 uses a batch process, regeneration process or other process that results in varying output rates of carbon dioxide. Source 10 may be based on adsorption-desorption, de-sublimation-sublimation, or other processes. The pressure of CO2 from source 10 is greater than, or is compressed to be equal to or greater than, the triple point pressure (75.12 psia). Preferably, the pressure is less than the critical pressure, but the pressure may be as high as about 2000 psi. Intermittent flow isolation device 11 may be used to prevent backflow to source 10. This device may be a throttle, check or snap acting valve or it may be controlled by pressure controller 11a. The CO2 may be any in any combination of phases (solid, liquid and gas). Heat exchanger 12 may be a shell and tube, counter-flow or any type heat exchange device. The CO2 may be cooled or heated (depending on the phases of CO2 from source 10) in heat exchanger 12 to liquefy CO2 or densify any supercritical CO2 and sub-cool the liquid, using external heat pump 16. The heat pump may include a compressor and condenser and may use a refrigerant selected to optimize the vaporization and liquefaction of CO2 at any application-specific pressure. The refrigerant supply is controlled by temperature control valve 13b2. Alternatively, the heat pump may include heat sinks and heat sources from outside processes, such as adsorption and desorption separation of CO2 to supply source 10. The outside processes may be synchronized to accommodate the need for alternating heat flux in the disclosed apparatus. Alternatively, a heat storage device may be used to provide a thermal capacitance suitable for specific application alternating heat flux requirements.

(4) Sub-cooled liquid (below saturation temperature) from heat exchanger 12 passes to accumulator 13, where it flows (preferably as a spray through mister system 13a) into the vapor space. The level of heavier phase carbon dioxide may vary between 13a1 and 13a2, which define the bottom and top of the accumulator volume in accumulator 13. Accumulator volume is selected to accommodate the variations in output rate of source 10. Level controls 13a1 and 13a2 may be used to shut-down an upset condition and/or to adjust to more gradual changes to average flow of source 10. Level controls 13a1 and 13a2, pressure controller 13b, coil 19 and sub-cooled liquid flowing into accumulator 13 are used to maintain the liquid level between level controls 13a1 and 13a2. Pressure controller 13b, which may work in conjunction with temperature controller 12b, controls heat flux of sub-cooled liquid by valve 13b2 and heat flux through coil 19 by valve 13b1. Heat medium fluid or refrigerant enters coil 19 at 16a. The heat flux may be supplied from heat pump 16 or another source, such as a CO2 recovery process using adsorption and desorption (not shown). Pressure controller 13b throttles valve 13b2 such that sub-cooled fluid flowing through mister system 13a cools the vapor in 13, liquefying enough vapor to offset the volume of net positive influx of liquid into accumulator 13. Pressure controller 13b throttles heat flow into the saturated liquid section of accumulator 13 to vaporize sufficient liquid to offset the net negative liquid influx. If there is a net positive flow of CO2 into accumulator 13, pressure is maintained in accumulator 13 by cooling vapor to liquefy a portion of the vapor to offset the reduction of the vapor space volume (rising liquid level). If there is a net negative flow of CO2 into accumulator 13, pressure is maintained by heating the saturated liquid section such that sufficient liquid is vaporized to offset the increase in vapor space volume (falling liquid level).

(5) Pump 15 may be a conventional pump, such as a multistage centrifugal pump. It may be used to pump liquid CO2 to a pipeline or well or other use. The CO2 may be further densified at heat exchanger 14, which may use refrigerant from heat pump 16, ambient air or other means, to increase the Net Positive Suction Head to prevent cavitation or increase efficiency of pump 15. Temperature control is provided at valve 14b, controlled by temperature controller 14a. Further cooling may be provided at heat exchanger 17 to increase the efficiency of a downstream pipeline or injection well. Equipment may be industry-standard. One of the important features of the apparatus described herein is the ability to pump dense or liquid carbon dioxide from the apparatus at a steady rate and without the inefficiency and high cost of compression of gas while avoiding problems of control and wear caused by cycling of the CO2 pump.

(6) Referring to FIG. 2, the steps of the method for supplying carbon dioxide at a steady rate from a source producing carbon dioxide at a varying or intermittent rate are shown. An intermittent or varying rate source of carbon dioxide at a pressure at or above its triple-point pressure is supplied. If the source originally does not produce CO2 at a pressure at or above the triple-point pressure, the CO2 pressure is increased to that pressure. The stream is then cooled or heated to a temperature sufficient to produce sub-cooled liquid carbon dioxide. The stream is then conveyed to an accumulator, where the temperature of the sub-cooled carbon dioxide is controlled by a pressure controller responsive to pressure in the accumulator. Heat flux may also be supplied to the accumulator by a fluid flowing through a conduit or coil in the accumulator at a rate controlled by the pressure controller responsive to pressure in the accumulator. A conduit may be any type of heat transfer device, including electric heaters and other conventional devices, with appropriate controls for the heat transfer device. A pump removes the dense or liquid carbon dioxide from the accumulator at a steady rate determined by the average flow rate of the stream entering the accumulator.

(7) Although the present invention has been described with respect to specific details, it is not intended that such details should be regarded as limitations on the scope of the invention, except to the extent that they are included in the accompanying claims.