US3817045A

US3817045A - System for dispensing carbon dioxide

Info

Publication number: US3817045A
Application number: US00294734A
Authority: US
Inventors: A Muska
Original assignee: Airco Inc
Current assignee: Airco Inc
Priority date: 1972-10-03
Filing date: 1972-10-03
Publication date: 1974-06-18
Anticipated expiration: 1991-06-18
Also published as: JPS4975485A; FR2201445B1; BE800095A; DE2327128B2; DE2327128A1; FR2201445A1; GB1433789A; CA990515A

Abstract

A system for dispensing carbon dioxide, particularly a slurry of particulate solid carbon dioxide dispersed in liquid carbon dioxide, so as to produce solid carbon dioxide in a form suitable for use as a refrigerant. The carbon dioxide is expanded from a source at a pressure above 60 p.s.i.g. to a zone at atmospheric pressure (0 p.s.i.g.) through a positive displacement device which transports a discrete, isolated charge of carbon dioxide from the high pressure source to the atmospheric zone without significant pressure drop. The head pressure on the device, the quantity of liquid carbon dioxide transported, slippage in the device, etc., are controlled to provide for reliable dispensing of slurry, e.g., without plugging of the system.

Description

United States Patent .[191

Muska [111 3,817,045 [451 June 18, 1974 SYSTEM FOR DISPENSING CARBON DIOXIDE [75] Inventor: Allen V. Muska, Berkeley Heights,

[73] Assignee: Airco, Inc., Montvale, NJ.

[22] Filed: Oct. 3, 1972 [21] App]. No.: 294,734

[52] US. Cl. 62/10, 62/35 [51] Int. Cl F25j 1/00 [58] Field of Search 62/10, 35, 39, 12, 41

[56] References Cited UNITED STATES PATENTS 579,866 3/1897 Elworthy 62/38 2,011,550 8/1935 Hasche 62/10 2,011,551 8/1935 Hasche 62/10 2,084,474 6/1937 Booth r 62/10 3,354,662 11/1967 Daunt i 62/10 3,395,546 8/1968 Sherlock 62/10 FOREIGN PATENTS OR APPLICATIONS 298,910 1/1930 Great Britain 62/10 Primary Examiner--Norman Yudkoff Assistant ExaminerArthur Purcell Attorney, Agent, or Firm-Roger M. Rathbun; H. Hume Mathews; Edmund W. Bopp [5 7] ABSTRACT A system for dispensing carbon dioxide, particularly a I slurry of particulate solid carbon. dioxide dispersed in liquid carbon dioxide, so as to produce solid carbon dioxide in a form suitable for use as a refrigerant. The carbon dioxide is expanded from a source at a pressure above 60 p.s.i.g. to a zone at atmospheric pressure (0 p.s.i.g.) through a positive displacement device which transports a discrete, isolated charge of carbon dioxide from the high pressure source to the atmospheric zone without significant pressure drop. The head pressure on the device, the quantity of liquid carbon dioxide transported, slippage in the device, etc., are controlled to provide for reliable dispensing of slurry, e.g., without plugging of the system.

10 Claims, 2 Drawing Figures GAS PELLETIZER P E LLETS PATENTEDJUNI 81914 3 a 1 7; 0 15 SLURRY I4 GAS PELLETIZER PE LLETS 7 l SYSTEM FOR DISPENSING CARBON DIOXIDE This invention relates to a system for dispensing carbon dioxide under high pressure and particularly to a system for dispensing slurries of particulate solid carbon dioxide dispersed in liquid carbon dioxide, to produce solid carbon dioxide for use in refrigeration or other systems.

Copending U.S. Pat. application Ser. No. 258,962 filed June 2, 1972, incorporated herein by reference, is directed to a novel cryogenic material, more particularly, a pumpable slurry of finely divided, particulate solid carbon dioxide dispersed in liquid carbon dioxide. This slurry is a mixture of solid and liquid carbon dioxide containing up to about 85 percent solids by weight, with the solid being in the form of finely divided, dispersed particles so that the mixture is fluid and pumpable. Generally, the solids are in excess of about 10, or 14 percent by weight and it is desired to have as high a concentration of solids as possible and still have a pumpable slurry. Solids concentrations in excess of 50 percent by weight are preferred. The slurry is a two .phase mixture that can exist at any pressure and temperature along the solid-liquid equilibrium curve for carbon dioxide. The lowest pressure on this equilibrium curve is 75 p.s.i.a. (6O p.s.i.g.) and the lowest tempera ture is 70F., which pressure and temperature occur at the triple point, the only point on the curve where all three phases of carbon dioxide, i.e., gas, liquid, and solid, can exist simultaneously. The solid particles in the slurry have a maximum diameter on the order of 3mm. or less and show little, or no tendency to agglomerate on standing.

While this slurry has significantadvantages over liquid carbon dioxide as a refrigerant, there are problems in dispensing the slurry and, at the same time, recovering the total refrigeration available in the slurry which are not presented in dispensing liquid carbon dioxide. Normally, dispensing of liquid carbon dioxide is performed in a device known as a snow horn which typically consists of a thin plate having a number of tiny orifices (e.g., 4 or 5) and a horn to which the plate is at tached. With a completely liquid system, it is necessary only to release pressure on the liquid in order to recover the refrigeration.

' Upon such a reduction of pressure, as the triple point is approached, the system contains liquid and gas, and, as the triple point is passed, it contains gas and solid. Therefore, liquid, typically available commercially at 300 p.s.i.g., is dispensed by allowing it to expand through the orifices of the snow horn and form gas and solid. The gas serves to keep the orifices clear of solid and the horn serves mainly to slow the gas. In reducing pressure upon a slurry as it is dispensed, however, the existing solid particles remain unchanged (except that their temperature decreases) and the liquid is con verted to gas and solid, thereby increasing the total amount of solids in the system. While U.S. Pat. No. 3,660,985 discloses a system using very high pressures, 250 to 300 p.s.i.g., to handle carbon dioxide slush of low, i.e., less than 50 percent solids content in a snow horn, the existing solids in the slurry and the increased total solids in the carbon dioxide dispensed render conventional snow horns unreliable as dispensing devices for a slurry because such solids frequently cause plugging, particularly at lower pressures. Therefore, in order to recover the total refrigeration from a slurry of solid and liquid carbon dioxide, it is necessary to carefully control dispensing of the slurry.

In accordance with this invention an effective and reliable ,means has been designed to dispense carbon dioxide in the form of a slurry of solid particles dispersed in liquid having solids concentrations up to about percent by weight. In accordance with this invention, carbon dioxide slurry, at a pressure in excess of 60 p.s.i.g., is fed into a positive displacement device hav ing a dynamic seal (sliding or rotating) that separates the high pressure carbon dioxide source from a low pressure discharge zone and is transported therein to the discharge zone which is at atmospheric pressure (0 p.s.i.g.). While the use of an expansion engine to make and maintain a slush cryogen such as hydrogen is shown in US. Pat. No. 3,395,546, the use of such engine is to form slush from slush whereas in the present invention the liquid carbon dioxide in the slurry is converted to gaseous and solid carbon dioxide so as to provide solid carbon dioxide refrigerant.

Positive displacement devices suitable for use in this invention must accept at the entrance port of the device a discrete charge of slurry from the high pressure side into an isolation chamber and transport the charge in the isolation chamber without significant gas and solid formation in the chamber, i.e., with an essentially negligible or insignificant pressure drop, to the low pressure atmospheric discharge or exit port of the device.

It is important that the charge be physically isolated from at least one of the entrance and exit ports at all times so there is no continuous path through the device from the high pressure side to the low pressure side. Also, the device must discharge on the low pressure side with a minimum of restriction and each isolation chamber in the device must discharge substantially completely to avoid the return of solid to the high pressure side in the isolation chamber with attendant buildup of solid in the chamber. The gas generated in the device is often sufficient to remove the solids from the isolation chamber, but it is desirable to also design the geometry, size and shape, of the chambers and exit port of the device to allow for easy removal of the solids. Removal of the solids can be further assisted by a downward orientation of the exit port although usually the gas generated and geometry are sufficient. if necessary, mechanical devices such as air jets or plungers operating from within the chamber can be used to clear the chamber. Leakage past the dynamic seals in the device from the high to the low pressure sides must be low. Heat flow into the dispenser from the exterior atmosphere during dispensing is preferably minimized by insulating the body of the device.

The positive displacement device can be a reciprocating, e.g., piston or plunger device, or a rotary, e.g., gear, double helix screw or sliding vane device, but in all cases, the liquid slurry is expanded through the positive displacement device under controlled conditions. Unlike the snow horn which operates at constant enthalpy as the pressure is lowered, the device of this invention operates between constant enthalpy and constant entropy. Ideally, the expansion should take place at constant entropy, however, this is not possible in a practical expansion device, because constant entropy expansion corresponds to percent efficiency. If no mechanical work is performed, the engine has 0 percent efficiency. The greater the expansion energy that is converted into mechanical work, the greater is the refrigeration effect in the exiting carbon dioxide stream. Practical devices will have efficiencies between these two extremes, typically 30 to 90 percent. The effect of this difference in operation between the displacement device of this invention and the snow horn is to produce more solids from the same amount of liquid in the displacement device as compared to the snow horn because some energy is taken out of the fluid as work. The calculated or theoretical percent solid carbon dioxide formed during the expansion of carbon dioxide from various initial conditions to atmospheric pressure is shown by the following table:

The difference in solid carbon dioxide produced represents a significant economic advantage to this invention. Accordingly, while this invention is primarily di rected to the dispensing of a slurry of liquid and solid carbon dioxide, it can also be used to advantage to dispense liquid carbon dioxide and gain additional refrigeration per unit of liquid.

Most positive displacement devices have leakage of the liquid being transported between the displacement element, e.g., screw, gear, vane, etc., and the casing which contains the displacement element, sometimes called lip, which reduces the desired isolation of the charge being transported from the high pressure source to the low pressure discharge zone. Such slippage is small and effectively moves in the direction of the displacing element because of the pressure differential across the element. If the pressure on the liquid slipping in the displacement device is too close to the triple point, additional solids are produced at the area of slipping due to reduction in pressure below the triple point because of slippage. Slippage is a problem in the present invention because essentially only liquid carbon dioxide slips and the slippage has a tendency to cause plugging of the device. In a gear device, for example, it packs the area between the teeth with solid carbon dioxide and prevents the dev ice from transporting the carbon dioxide feed. Also, slippage across the face of the gears allows formation of solids between the face and the gear casing and prevents the gears from turnmg.

The amount of leakage, or slip per unit of time for a given displacement device is dependent mainly upon the pressure drop across the device and is relatively constant for a given pressure drop regardless of the rotational speed of the device. To alleviate the problems caused by slippage, and consequent formation of solids in the device, the head, or inlet pressure of the device and the quantity of liquid transported in the device are controlled. Leakage (slip) is tolerable up to about 50 percent but is preferably below about percent of the flow through the device. In general, the head pressure on the device is maintained above the triple point pressure, i.e., 60 p.s.i.g. and when a slurry is being dispensed, a sufficient pressure, generally above p.s.i.g., is used to force the solids in the slurry through the device. The device is particularly suited for dispensing carbon dioxide at pressures below 150 p.s.i.g. Although the slurry can exist at pressures above the triple point of carbon dioxide, there is no advantage to using a pressure above that necessary to avoid plugging of the device because the enthalpy of the slurry is always that of the triple point and the higher pressure offers no refrigeration advantage. Furthermore, the use of higher pressures will increase slippage by increasing the pressure drop. The quantity of slurry transported in the device is maintained sufficiently high to supply enough liquid to compensate for the liquid lost by slippage. Since the amount of material lost by slippage in a given device per unit of time is dependent mainly upon the pressure drop across the device and is relatively constant regardless of the rotational speed of the device, slippage will add a relatively constant amount of solid to the liquid of the slurry internally of the device, although the total quantity of slurry, and therefore liquid, transported per unit of time in the device can be increased by speeding up the device. The total quantity of slurry transported is always maintained sufficiently high that the total amount of liquid passing through the device, to which the slippage solids are added, is such that the concentration of solids in the slurry internally of the device is insufficient to cause plugging. While this internal concentration of solids in the slurry is desirably below about percent by weight, it is obvious that both the desired head pressure and the quantity of slurry desired will be dependant to a large extent upon the particular device being used, e.g., gear, vane, etc., and will vary with both the type of device as well as the size of the device, but in each instance the specific limits can be quickly determined within the above criteria.

A positive drive is provided for the device of this invention; however, once the device is started, there is little, or no, need to provide positive drive since the head pressure of the carbon dioxide feed will provide the energy necessary to drive the device. The positive drive can be used to advantage to slow the device during dispensing and prevent uncontrolled operation which could damage it. Also, the positive drive provides a means of controlling the amount of carbon dioxide dispensed since, in a positive displacement device, there is a direct relation between the amount of carbon dioxide transported and the rotational speed of the device. Accordingly, the device can also function as a metering device.

The rate of production of solid carbon dioxide is also a factor in causing plugging of the device. The discharge opening of the device in all instances must be of sufficient size to accommodate the solids and gas generated during discharge and, preferably, the discharge opening is oriented to discharge the solid generated downward. It is also desirable to remove particles having a size larger than the isolation chambers of the dispensing device from the slurry. For this reason a screen is normally placed in the feed line to the device. Since the maximum particle size in the slurry produced in accordance with copending US. Pat. application Ser. No. 258,962 is about 1 to 3 millimeters, particle size is not a significant problem.

This invention will bevmore fully described hereinafter with reference to the drawings in which:

FIG. 1 is a flow sheet of a commercial system in accordance with this invention; and

FIG. 2 is a schematic illustration of the preferred em bodiment of the dispenser of this invention utilizing meshed gears.

Referring to FIG. 1, in the system in accordance with this invention, carbon dioxide, for example, a slurry of carbon dioxide produced in accordance with copending application Ser. No. 258,962 is fed through line to dispenser 12. FIG. 1 schematically illustrates a system for the production of carbon dioxide pellets, a commercially available form of carbon dioxide for use in refrigeration systems such as large freezer lockers. In this system the dispenser 12 produces solid carbon dioxide in the form of snow and carbon dioxide gas. The gas is separated in a gas separator 14 and the snow is pelletized in a conventional pelletizer unit 16. Carbon dioxide snow can also be used as a refrigerant, if desired.

Dispenser 12 is a positive displacement device in accordance with this invention and can be any one of several forms including meshed gears, double helix screw,

sliding vane s, etc., which generally are characterized in that the carbon dioxide is supplied to one-side of the device at a high pressure in slurry form, and the device transports the carbon dioxide in discrete, isolated charges at a controlled rate to a zone at essentially atmospheric pressure, thereby converting the liquid in the slurry to solid and gaseous carbon dioxide. As discussed above, for proper operation of the dispenser, it is desirable that slippage in the dispenser, head pressure on the dispenser, rotational speed of the dispenser, etc., be controlled.

FIG. 2 illustrates a preferred dispenser incorporating two

meshed gears

102 and 104 mounted in casing 103, gear 102 being driven by motor 106. In this dispenser, carbon dioxide is introduced from a source, e.g., line 10 at point A in the direction of arrow 110 at a pressure in excess of 60 p.s.i.g., and often up to 150 p.s.i.g., and is transported to point B which is at atmospheric pressure, essentially 0 p.s.i.g. The carbon dioxide is transported through the dispenser in isolation chambers, e.g., 114, 116 formed between the teeth of gears.l02 and 104, to point B in the direction of

arrows

112 and 113. .The product exiting at point B comprises, for example, approximately 18 percent gas and 82 percent solids by weight then a 60 percent slurry is provided at point A. As discussed above, a small amount of the carbon dioxide will leak, or slip through space 118 between the gear teeth and casing 108 in the direction of arrow 120, e.g., from area 114 to 116, increasing the solids concentration in area 114. Slippage also occurs across the face of gears I02 and 104 at each end. Such slippage is kept to a minimum by control of tolerances in the gears and by control of pressure drop, etc. Screen 122 keeps larger particles from entering casing 108.

A number of experimental runs have been made to dispense a slurry using a device as schematically shown in FIG. 2. The device used had an inlet 54 inch diameter, two gears l i inch diameter, and a discharge opening in theform of a rectangle (having rounded comers) about I /4 inch by 2 inches. Slurry entering the device was, in the latter runs, passed through a screen having a maximum opening of 0.095 inch; the largest dimension between the gear teeth was aboutOLl inch. The solids concentration of the slurry ranged'from 10 to 63 percent and the head pressure on the device was between and 140 psi. g. Before transfer of the slurry was effected, the device was cooled by feeding liquid carbon dioxide at 80 to 100 p.s.i.g. from a storage tank through the dispenser and forming snow. No plugging occurred during these cooling periods demonstrating the suitability of the device for dispensing liquid carbon dioxide. As the device operates between constant enthalpy and constant entropy, more snow is produced from the liquid upon dispensing through this device than through a conventional snow horn.

Table I contains from three representative runs made with the equipment, and operated as described above.

A careful analysis of 34 runs with the device of FIG. 2 revealed the following significant parameters considered essential to the dispensing of a slurry, particularly a slurry of high solid concentration, e.g., 50 percent or greater. Firstly, it is necessary to transport the slurry from the high pressure source to the exit at atmospheric pressure in relatively small discrete charges isolated at all times from one of the high pressure source and the exit with essentially negligible pressure drop during the isolation. Secondly, the slippage in the dispensing device must be kept to a minimum. Additionally, the head pressure on the device must be over 60 p.s.i.g., preferably over 80 p.s.i.g., and the quantity of slurry per unit of time should be sufficient to accommodate any solids produced by slippage. Since large particles in the slurry feed could clog the device, it is desirable to place a screen at the entrance of the dispensing device to filter the slurry of such particles and for the device to have a large exit opening.

It is claimed:

1. A method for continuously dispensing a pumpable carbon dioxide slurry containing about 10 to percent by weight solid consisting essentially of particles of solid carbon dioxide dispersed in liquid carbon dioxide at a pressure of above 80 p.s.i.g. from a closed container at said pressure to a zone at essentially atmospheric pressure, which method comprises admitting a charge of said carbon dioxide slurry in sequence to the chambers in a positive displacement metering device having a plurality of such chambers by placing said chambers in sequence in direct communication with said closed container to fill said chambers with carbon dioxide slurry at the same pressure as the pressure in said closed container, isolating the charge in each such chamber from said closed container, thereafter placing each such chamber in sequence in direct communica- 7 tion with said atmospheric pressure zone to which the carbon dioxide slurry is to be dispensed, thereby allowing the charge to expand into said zone and cause the liquid in the charge to flash to solid and vapor and simultaneously deliver into said zone the solid mixed with the vapor, and thereafter admitting to said chambers additional charges of said carbon dioxide slurry to continuously dispense said slurry, the quantity of said slurry being transported in said chambers from the container to the atmospheric pressure zone being sufficient, in combination with the pressure on the slurry, that the total amount of liquid passing through the device is such that the concentration of solids in the slurry internally of the device is insufiicient to cause plugging of the device.

2. The method of claim 1 wherein the concentration of solids in the slurry internally of the device is below about 85 percent by weight.

3. The method of claim 2 wherein the pressure drop in said chamber during the isolation of the charge is negligible.

4. The method of claim 2 wherein the slippage of the device is less than percent of the flow through the device.

5. The method of claim 2 wherein the slippage of the device is less than 50 percent of the flow through the device.

6. The method of claim 2 wherein the pressure in the closed container is below 150 p.s.i.g.

7. The method of claim 6 further including filtering the carbon dioxide feed to the device to remove solid carbon dioxide having a size larger than the chamber within the device.

8. The method of claim 6 wherein the solid phase carbon dioxide comprises 50 to 85 percent by weight of the slurry.

9. A method for continuously dispensing a carbon dioxide slurry containing about 50 to 85 percent by weight solid consisting essentially of particles of solid carbon dioxide dispersed in liquid carbon dioxide at 'a pressure of about p.s.i.g. from a closed container to an atmospheric pressure zone which method comprises admitting a charge of said carbon dioxide slurry in sequence to each chamber in a positive displacement metering device having a plurality of said chambers by placing said chambers without restriction in sequence in direct communication with said closed container to fill said chambers with carbon dioxide slurry at the same pressure as the pressure in said closed container, isolating the charge in each said chamber from said closed container, and thereafter placing each said chamber in sequence in direct communication with said atmospheric pressure zone to which the carbon dioxide slurry is to be dispensed without restriction, thereby allowing the charge to expand into said zone and cause the liquid in the charge to flash to solid and vapor and simultaneously deliver into said zone the solid mixed with the vapor, the pressure drop in each said chamber during isolation of the charge being negligible, and thereafter admitting in sequence to said chamber additional charges of said carbon dioxide slurry to continuously dispense said slurry, the quantity of slurry being transported from the container to the atmospheric pressure zone being sufi'icient that the total amount of liquid passing through the device, in combination with the pressure, is such that the concentration of solids in the slurry internally of the device is insufficient to cause plugging of the device.

10. The method of claim 9 wherein the pressure in the closed container is below about l50 p.s.i.g.

Claims

2. The method of claim 1 wherein the concentration of solids in the slurry internally of the device is below about 85 percent by weight.
3. The method of claim 2 wherein the pressure drop in said chamber during the isolation of the charge is negligible.
4. The method of claim 2 wherein the slippage of the device is less than 10 percent of the flow through the device.
5. The method of claim 2 wherein the slippage of the device is less than 50 percent of the flow through the device.
6. The method of claim 2 wherein the pressure in the closed container is below 150 p.s.i.g.
7. The method of claim 6 further including filtering the carbon dioxide feed to the device to remove solid carbon dioxide having a size larger than the chamber within the device.
8. The method of claim 6 wherein the solid phase carbon dioxide comprises 50 to 85 percent by weight of the slurry.
9. A method for continuously dispensing a carbon dioxide slurry containing about 50 to 85 percent by weight solid consisting essentially of particles of solid carbon dioxide diSpersed in liquid carbon dioxide at a pressure of about 80 p.s.i.g. from a closed container to an atmospheric pressure zone which method comprises admitting a charge of said carbon dioxide slurry in sequence to each chamber in a positive displacement metering device having a plurality of said chambers by placing said chambers without restriction in sequence in direct communication with said closed container to fill said chambers with carbon dioxide slurry at the same pressure as the pressure in said closed container, isolating the charge in each said chamber from said closed container, and thereafter placing each said chamber in sequence in direct communication with said atmospheric pressure zone to which the carbon dioxide slurry is to be dispensed without restriction, thereby allowing the charge to expand into said zone and cause the liquid in the charge to flash to solid and vapor and simultaneously deliver into said zone the solid mixed with the vapor, the pressure drop in each said chamber during isolation of the charge being negligible, and thereafter admitting in sequence to said chamber additional charges of said carbon dioxide slurry to continuously dispense said slurry, the quantity of slurry being transported from the container to the atmospheric pressure zone being sufficient that the total amount of liquid passing through the device, in combination with the pressure, is such that the concentration of solids in the slurry internally of the device is insufficient to cause plugging of the device.
10. The method of claim 9 wherein the pressure in the closed container is below about 150 p.s.i.g.