Method and apparatus for producing a controlled atmosphere

Abstract: A method and apparatus for producing a controlled atmosphere in an area (1). The method includes the intermittent removal of oxygen, carbon dioxide, water vapor and ethylene from the area. An apparatus to practice the method is disclosed that includes a compressor (15) to increase the pressure of the gases present which are then separated by diffusion across membranes in the gas separator (23).

Description: Field of Invention: This invention pertains to the production of controlled atmospheres particularly controlled atmospheres for the preservation of vegetable material and with still greater particularity to the production and maintenance of oxygen and carbon dioxide depleted atmospheres for the long term refrigerated storage of produce. Background of Invention : It has been known for some time that produce stored under refrigeration maintains its freshness for a greater time than that stored at ambient temperatures. For example, the refrigerated shipment of produce from California to the East Coast has been done since the 1920's. It is not as widely known but is familiar to those in the business that controlling the atmosphere where storage is being undertaken extends the lifetime of the produce. An important example is the storage of apples and other fruit in Washington State from one fall harvest to another. To provide proper storage the ripening process must be retarded. During the ripening process the starches in the fruit are converted to sugars through natural respiration processes. Ethylene gas is released and accelerates the process. With refrigeration alone the ripening process continues at a slower rate unless the temperature is reduced to a point where the fruit freezes resulting in considerable deterioration. To remedy this problem it has been found desirable to place the fruit into a dormant stage by placing it in a reduced oxygen atmosphere and a lowered temperature. Reduction of temperature is commonly done with large refrigeration units both in the prior art and the invention. There are several methods in present use for production of the proper atmosphere. The simplest method is flooding the area with nitrogen separated by liquidation of air. The nitrogen is supplied either in compressed or liquid form. While this method has the advantage of simplicity it is quite expensive to maintain the proper atmosphere and thus not met with widespread adoption. The method in most common use in the Washington State apple business is to burn out the oxygen and remove combustion products. To accomplish this end propane is injected into an incoming air stream and ignited, generally by a catalytic burner. The resulting products of combustion are nitrogen, water vapor and carbon dioxide. The carbon dioxide is harmful to the produce and must be removed. The removal of carbon dioxide is commonly done by devices called scrubbers. One type of scrubber uses sodium or potassium hydroxide solutions to absorb the carbon dioxide forming sodium or potassium carbonate as a waste product. Similarly, calcium hydroxide (lime) has been used to remove the carbon dioxide. These chemical scrubbers are expensive to maintain and require constant addition of reagents with removal and storage of wastes. Problems with scrubbers have led to large scale introduction of molecular sieves as removal means. A molecular sieve will absorb a gas such as carbon dioxide at one temperature then release the carbon dioxide at an elevated temperature. In operation the sieve is cycled between absorption and regeneration states requiring heating and cooling at each step. The primary problems with the use of catalytic burners and molecular sieves are related to cost of operation. These are largely related to the rising cost of energy. Since the atmosphere is reused the input of atmosphere to the catalytic burner is of the order of 0° C. This atmosphere must be heated to the ignition point of the propane mixture even in a catalytic burner. The cost of the energy to do so is not unsubstantial. In addition the molecular sieve must be heated on its regeneration cycle further increasing energy consumption. There must be an excess of oxygen to ensure formation of carbon dioxide in the burner rather than carbon monoxide. This requirement puts an upper limit on the possible oxygen reduction in this type of system. Finally, this type of a system does not remove ethylene gas formed from the fruit which accelerates ripening. Accordingly, there is a demand for an atmosphere control system which 1) operates at a lower temperature than existing systems, 2) controls oxygen content to less than 2.5% to reduce the conversion of starch to sugar, 3) removes almost all carbon dioxide from the atmosphere, and 4) removes ethylene gas from the atmosphere. "The Packer" (23rd May 1987) discloses a semi-permeable membrane-based system for creating a controlled atmosphere for the storage of apples. This document does not disclose the removal of ethylene or the control of oxygen, water vapour, or ethylene by recirculation. FR-A-1 590 579 describes an apparatus for controlling the atmosphere within a container for storing, for example, vegetable produce. In this apparatus the air is exhausted from the storage area by means of fans which draw the exhausted air over a gas separator which comprises a selectively permeable membrane. The method and apparatus of the present invention includes the features set out in claims 1 and 4 respectively. By compressing the air before it is passed through the gas separator it is possible to provide a relatively simple system which effectively controls the atmosphere in a storage area. Such a system can remove almost all of the oxygen from the atmosphere as well as carbon dioxide without producing any carbon dioxide products itself as in the prior art. A method and apparatus in accordance with the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic piping diagram of the invention. Figure 2 is a flow chart of the invention. Figure 1 is a schematic drawing of the system. The storage area 1 is indicated by a rectangle. In actual use area 1 could be a warehouse or room that is so constructed as to be airtight. The storage area has its walls treated so that no movement of atmosphere from outside controlled area can enter. The walls are sealed to floor which is also vapor tight. All wall penetrations are also vapor tight. The doors are sealed to the walls. Storage area 1 is equipped with bubble valves (not shown) to prevent the pressure from being 0.75 inches of water (187 Pa) greater or less than ambient atmospheric pressure. The exit 2 of storage area 1 is connected to an outlet line 3. Outlet line 3 is connected to an electrically operated valve 4. A second electrically operated air valve 6 is connected 7 downstream of valve 4. Valve 6 is also connected to the outside atmosphere 8. An intake bleeder adjustment valve 5 is connected in parallel to valve 6 to provide a constant flow of atmosphere. Valves 4 and 6 operate to control pressure in area 1 and indirectly control gas content. The solenoids 9,11 of valves 4,6 are connected to a pressure sensing device 10 which senses the pressure in area 1 via a conduit 13. Pressure sensor 10 may be a photohelic device. A common setting for pressure sensor 10 is for 1 inch of water (249 Pa) column of positive pressure to open solenoid 9 and close solenoid 11 of valve 6 and for 1 inch (249 Pa) of negative pressure to close solenoid 9 of valve 4 and open solenoid 11 of valve 6. This controls the gas pressure in storage area 1. The result is that when the pressure is too low in area 1 air is drawn through inlet 8 and valve 6 into the system. A second pressure sensor 20 is connected to shut down the system if pressure is excessive. Air cleaner 12 is provided to remove particulate material from the airstream. From point 7 the piping enters the high pressure compressor packing 15. Packing 15 increases gas pressure to as much as 300 pounds per square inch (2068 kPa) although the system will operate with a pressure as low as 20 pounds per square inch (130 kPa). A typical size of compressor is from 10 to 80 horse power (7457 to 59656W) with a 40 horse power (29828W) unit used in the preferred embodiment. A suitable compressor unit is sold as a Sullar Series 10B 40 hp (29828W) unit by Central AG & Manufacturing. Compressor unit 15 includes an air cleaner 12 to eliminate oil and/or other contaminants from the gas stream. An O bleeder adjustment valve 14 is connected to output 16 of compressor 15. The outlet of valve 14 enters an air cooler 17. The outlet of air cooler 17 exits to the storage space 1 via a motor operated valve 18. A second air cooler inlet 19 is also connected to the output of compressor 15. The output of air cooler 19 and the output of compressor 15 are further connected to a three way valve 21. The output of valve 21 is connected to a coalesing filter and trap 22 provided to remove oil and particulate matter from the compressor from the gas stream. The output of filter 22 enters the gas separator unit 23. Gas separator unit 23 is a commercially available gas separator unit which includes an assembly of semi permeable cylindrical membranes in a metallic shell. Such units are commonly used in petro chemical processing. The particular unit used was manufactured by Monsanto Company of St. Louis, Missouri and was sold under the trademark "Prism". Other similar gas separator units could be substituted by a person of average skill in the art. The gas separator unit 23 has two outputs. The first output 24 is called the permiate or fast gas output. Output 24 is connected to a valve 26 and allows exit of carbon dioxide (CO₂), water vapor (H₂O) , oxygen (O₂) and ethylene gases which are then vented to the outside atmosphere. A moisture separator and trap 27 removes the moisture from this waste gas before venting. The second output 28 is the non-permeate or slow gas output. Output 28 is connected to the input of air cooler 17. The primary constituent of output 28 is clean dry nitrogen (N₂). The invention further includes an electronic control system. The primary component of the control system is a programable controller 30. Programable controller 30 can be a programable personal computer such as a PC/XT such as that manufactured by International Business Machines or equivalent. Controller 30 also includes necessary instructions or software for operation of the system. Controller 30 is connected to suitable sensors to sense the status of the system. A pressure transducer 31 is connected to the output of the gas separator 28 to sense the pressure of the gas leaving separator 28. A temperature probe 32 is connected between filter 22 and gas separator 23 to sense the temperature of the gas stream at this point. An oxygen/carbon dioxide sensor 33 is connected to a selection valve 34 to sense the relative percentages of carbon dioxide and oxygen in storage area 1 and the output of gas separator 28. Inputs 31,32 and 33 are all connected through suitable interfaces to the programable controller 30. Programable controller 30 processes the information provided by sensors 31,32 and 33 and controls the operation of three-way valve 21 and motor operated valve 18. In this manner the content of the atmosphere in storage area 1 is monitored and controlled. The system is provided with pressure gauges 36 and 37 which monitor the input and output pressure of the gas separator 28. A temperature indicator 38 tells the operator what the temperature of the gas stream at the input of gas separator 28 is. A second pressure indicator 39 gives an indication of the pressure at the output of valve 21. The result is that storage area 1 is constantly flooded with a gas that is largely nitrogen but includes controlled amounts of oxygen and water vapor. Oxygen, ethylene, and carbon dioxide either separated by the ripening process or from leaks are constantly removed by the system. The gas mixtures obtainable with this apparatus are unobtainable with any other commercially available apparatus. The apparatus reduces oxygen content below 2% of the mixture. Tests conducted on the system show the most beneficial range to be between 1.5 and 2.5% with the best preservation at the low end of the range. Conventional equipment provides an oxygen range of just below 5%. Apples stored in a 1.5% oxygen environment retain 83% of initial firmness after the storage season compared with 73% with conventional burner equipment. Tests done at Michigan State University are even more impressive with storage at 1.55 oxygen and 1.85 carbon dioxide at 38 degrees for 173 days averaging 92% retention of firmness. The claimed apparatus also eliminates carbon dioxide which in excessive amounts kills the produce allowing spoilage. Finally the apparatus lowers ethylene concentration to less than 50 parts per million on a continuing basis. This is useful because conventional apparatus generates ethylene as part of the combustion process. It is well known that ethylene causes premature ripening in stored fruit and produce. Similarly, lift trucks cannot be used in controlled atmosphere storage because of the ethylene generated during combustion. Figure 2 is a flow chart of the method of the invention. As stated above first 51 the gases are removed from the area where the controlled atmosphere is desired. The gases are then conpressed 52 into an elevated pressure for efficient operation. The nitrogen is then separated from the oxygen, carbon dioxide, water vapor and ethylene by diffusion 53. In some cases it is desirable to hydrate 54 the nitrogen separated for storage of some produce. In other cases this step may be bypassed 55. Finally, the nitrogen is reintroduced 56 to the storage area. The above embodiment is illustrative only the invention being defined solely by the subjoined claims.

Claims: A method for producing a controlled atmosphere in an area for the storage of vegetable material (1) comprising removing air from the area, compressing the air to a pressure within the range from 130 kPa to 2608 kPa and separating the air using a semi-permeable membrane consisting of a plurality of semi permeable cylindrical membranes in a pressure shell to reduce the quantity of carbon dioxide, oxygen, water vapour and ethylene in the air to produce a nitrogen-rich gaseous mixture comprising less than 2% oxygen and less than 50 parts per million ethylene and re-introducing the nitrogen-rich gaseous mixture into the area. A method according to claim 1 in which the nitrogen-rich gaseous mixture is hydrated before reintroduction to the area (1). A method according to claim 1 or 2 in which the air is separated by allowing oxygen, carbon dioxide, water vapour and ethylene to diffuse through a membrane into an area of low relative pressure while retaining nitrogen. Apparatus for carrying out the method according to any preceding claim for producing a controlled atmosphere in an area for the storage of vegetable material (1), the apparatus comprising a gas separator (23) consisting of a plurality of semi-permeable cylindrical membranes in a pressure shell connected to receive air from the area (1) and to separate the air and means to return the so separated air in the form of a nitrogen-rich gaseous mixture in which the quantity of carbon dioxide, oxygen, water vapour and ethylene has been reduced by the gas separator (23) to the area (1), and a compressor (15) is connected in series with the gas separator (23) in order to increase the pressure of the air supplied to the gas separator (23) to a pressure within the range from 130 kPa to 2068 kPa, and cooling means (17,19) arranged to cool the air supplied by compressor (15) to the gas separator (23). Apparatus according to claim 5 comprising hydration means (54) connected between the gas separator (23) and the area (1) for increasing the relative humidity in the area (1). Apparatus according to any one of claims 5 to 6 characterised in that it further comprises pressure controlling means (13, 10, 4, 9, 6, 11) connected to the area (1) for preventing the pressure in the area (1) from exceeding a predetermined pressure range. Apparatus according to claim 6, characterised in that the pressure controlling means comprises a first valve (4) connected between the area (1) and the compressor (15), a second valve (6) connected between the ambient atmosphere (8) and the compressor (15), pressure sensing means (1) connected to the area (1) for sensing the pressure therein and activation means (9,11) connected to the first valve (4) and the second valve (6) controlled by the sensing means (10) for activating the valves in order to maintain the pressure within the predetermined range. Apparatus according to claim 7, characterised in that it comprises an air filter connected to the second valve (6). Apparatus according to any one of claims 4 to 8 characterised in that it comprises a by-pass valve (14) for permitting air to be fed to the area (1) without passing through the separator (23) in order to control the amount of oxygen in the area (1), and means for adjusting the by-pass valve (14). Apparatus according to any one of claims 4 to 9 characterised in that it further comprises a filter (22) connected between the compressor (15) and the gas separator (23) for removing oil and particulate matter from the gas output via the compressor (15). Apparatus according to any one of claims 4 to 10 characterised in that it comprises an outlet (24) from the separator (23) for allowing waste gases to exit, and a moisture separator (27) connected to the outlet (24) for removing water. Apparatus according to any one of claims 4 to 11 characterised in that the area (1) is adapted for the storage of fruit. Apparatus according to claim 12 characterised in that the area (1) is adapted for the storage of apples. Apparatus according to any one of claims 4 to 13 characterised in that the semi-permeable membrane is so constructed that the removed gases diffuse through the semi permeable cylindrical membranes faster than nitrogen. Apparatus according to any one of claims 4 to 14 characterised in that the apparatus is constructed so as to avoid the introduction of contaminants into the gas stream.

Inventors: ASH WILLIAM R, CAMPBELL TERRY, FORNEY STEVE, ROE ELMAN, ASH, WILLIAM R., CAMPBELL, TERRY, FORNEY, STEVE, ROE, ELMAN
Applicants: AQUILO GAS SEPARATION BV, AQUILO GAS SEPARATION B.V.
Application year: 1988
Main classification: A23B7
IPC codes: B01D53, A01F25, A23B7, B65D88, A23L3, C01B21