Integrated system and method for providing a controlled atmosphere in a food storage facility

A method for providing a controlled atmosphere in a food storage facility, which comprises: PA0 a) feeding a portion of an atmosphere from a food storage facility, containing carbon dioxide, oxygen and nitrogen, to a feed side of a membrane having a higher permeability to carbon dioxide than to nitrogen, PA0 b) recycling the carbon dioxide, oxygen and nitrogen not permeating the membrane back to the storage facility, and venting the carbon dioxide-enriched gas permeating the membrane to the atmosphere, and PA0 c) using a membrane-based system to maintain the required atmospheric composition and pressure in the food storage facility.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an integrated system and method for 
providing a controlled atmosphere in a food storage facility. 
2. Description of the Background 
In the food industry, produce is stored under controlled atmospheric 
conditions of temperature, gas composition and humidity in order to 
greatly extend storage life and quality. Typically, the atmosphere in a 
storage facility is controlled by purging with an inert gas or by 
scrubbing with a physical or chemical means in order to maintain carbon 
dioxide and oxygen concentrations within acceptable ranges. 
At present, nitrogen gas used for purging the storage facility atmosphere 
is provided by on-site nitrogen generation systems or by vaporized 
cryogenically produced nitrogen gas. However, more recently, nitrogen 
generation from compressed air has been effected using membranes or 
pressure swing absorption (PSA) technology. 
Despite the recent application of these technologies to the generation of 
nitrogen for controlled atmosphere storage, it would be extremely 
desirable to be able to maintain all of the required atmospheric 
controlled conditions with a single system without the addition of an 
external purge gas. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
integrated system and method for providing a controlled atmosphere in a 
food storage facility. 
It is a further object of the present invention to provide an integrated 
system or method for providing a controlled atmosphere in a food storage 
facility, which utilizes membrane system(s) and a control system. 
The above objects and others which will become apparent are provided by a 
method for providing a controlled atmosphere in a food storage facility, 
which entails: 
a) feeding a portion of the atmosphere from a food storage facility, 
containing carbon dioxide, oxygen and nitrogen, to a feed side of a 
membrane having a higher permeability to carbon dioxide and oxygen than to 
nitrogen, 
b) recycling the carbon dioxide, oxygen and nitrogen not permeating said 
membrane back to the storage facility, and venting the carbon 
dioxide-enriched gas permeating the membrane to the atmosphere, and 
c) using a membrane-based system to maintain the required atmospheric 
composition and pressure in the food storage facility.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the present invention, it has been discovered that the 
concentrations of carbon dioxide and oxygen in a food storage facility 
atmosphere can be effectively maintained using membranes in a closed 
integrated system. Although membranes have been used for the external 
on-site generation of nitrogen for use as a purge gas and for simple 
carbon dioxide scrubbing in food storage systems, the present invention 
entails the use of membranes to separate carbon dioxide from nitrogen 
using the atmosphere of a food storage facility in an integrated closed 
system while substantially maintaining the oxygen content. 
In essence, the present invention reduces the carbon dioxide level in a 
food storage facility atmosphere by running the atmosphere through a 
membrane which is more permeable to carbon dioxide than to nitrogen. The 
carbon dioxide-enriched gas is vented to the atmosphere while 
substantially all of the nitrogen and oxygen are recycled to the food 
storage facility. Simultaneously, a second membrane system is used for the 
addition of oxygen and nitrogen to maintain the required atmosphere in the 
food storage facility. 
The present invention also provides an integrated system for practicing the 
above-described method. 
The present method and a system for practicing the same will now be 
described by reference to FIGS. 1 and 2. 
In FIG. 1, the atmosphere from a food storage facility, containing carbon 
dioxide, oxygen and nitrogen, is fed through a conduit (1) to the feed 
side of separation membrane bundles (3) on a carbon dioxide skid. The 
atmosphere is compressed (2) and filtered prior to being fed to the 
membrane bundles. Semi-permeable gas membranes are used which are 
preferably made of bundles of thin hollow fibers inserted into a pressure 
vessel. 
In accordance with the present invention, any membrane may be used which 
allows the preferential permeation of carbon dioxide or carbon dioxide and 
oxygen through the membrane fibers. Such membranes generally allow greater 
than about 50% of the carbon dioxide present to permeate through the 
membrane fiber. It is even more preferred to use membranes which allow 
greater than about 90% of the carbon dioxide present to permeate through 
the membrane, while substantially minimizing the loss of oxygen and 
nitrogen through the membrane so that the non-permeated gas mixture can be 
recycled. Examples of such membranes which are more permeable to carbon 
dioxide and oxygen than to nitrogen are polyimide, polysulfone, silicone 
rubber, natural rubber, butyl rubber, low density polyethylene, 
polycarbonate, poly(phenylene) oxide, nylon-6,6, polystyrene and cellulose 
acetate. See Encyclopedia of Chemical Technology, vol. 15, page 118 
(Kirk-Othmer). 
In the system of FIG. 1, the system is sized such that the carbon dioxide 
removal capacity is greater than the expected carbon dioxide generation 
rate from the fruit. The oxygen, nitrogen and the remaining carbon dioxide 
exit the feed channel of the membrane at high pressure and are recycled 
back to the storage facility through conduit (4). The carbon dioxide 
content of the facility can be controlled at a given level using this 
technique. 
FIG. 1 also illustrates the use of a second membrane system known as a 
make-up skid, which is used to produce an atmosphere of the proper 
composition to balance the pressure and to maintain the oxygen and 
nitrogen concentrations in an airtight facility. The make-up air stream, 
which is used to replace the oxygen and nitrogen consumed by the food 
stored in the storage facility or lost in the separation process, may be 
mixed with the recycle stream from the carbon dioxide skid and reinjected 
into the storage facility. The design of the make-up skid is similar to 
that of the carbon dioxide removal skid. 
The carbon dioxide content, oxygen content and storage facility pressure 
are maintained by controlling the operation of the carbon dioxide skid and 
make-up skid. This may be seen by reference to FIG. 2, wherein a sample 
pump, carbon dioxide and oxygen analyzers, carbon dioxide and oxygen 
controllers and pressure switches or similar apparati are used to effect 
the operation of the system. 
The carbon dioxide skid and make-up skid are conveniently cycled on and off 
depending upon the carbon dioxide content of the atmosphere relative to 
the carbon dioxide setpoint. In accordance with the present invention, a 
level of carbon dioxide greater than the carbon dioxide setpoint will 
result in both skids being turned on to remove carbon dioxide and make-up 
oxygen and nitrogen. By contrast, a carbon dioxide level which is less 
than the carbon dioxide setpoint will result in both skids being shut off. 
The oxygen content is also measured and a controller affects the 
performance of the make-up skid based upon the oxygen level relative to 
the oxygen setpoint. If the oxygen content of the facility is low, a 
bypass around the membranes opens up to increase the concentration of the 
make-up atmosphere. When the oxygen content is high, additional membranes 
are brought on line to decrease the atmosphere oxygen concentration. In 
this manner, both carbon dioxide and oxygen levels are continuously driven 
towards their respective setpoints. 
The storage facility pressure relative to the outside atmosphere is sensed 
by one or more differential pressure switches which are used to effect the 
operation of the skids. As an example of pressure differential settings 
which may be typically used in a food storage facility, the following may 
be noted. However, these examplary values are provided solely for purpose 
of illustration and are not intended to be limitative. 
If the differential pressure is low, for example, such as less than -0.25" 
W.C., or low-low, for example, such as less than -0.5" W.C., the switch 
activates the make-up skid and shuts down the carbon dioxide skid in order 
to elevate the building pressure. When the differential pressure is high, 
for example, such as greater than 0.25" W.C., or high-high, for example, 
such as greater than 0.5" W.C., the carbon dioxide skid is activated and 
the make-up skid is shut down to lower the facility pressure. However, the 
important point is that as long as the differential pressure of the 
storage facility is within the acceptable range of -0.25 to +0.25" W.C., 
the control of the skids will be based on the carbon dioxide and oxygen 
concentrations. Thus, within this acceptable pressure range, both skids 
will operate automatically depending upon the carbon dioxide and oxygen 
levels relative to the respective setpoints. 
The membrane system is designed to maintain the nitrogen, oxygen and carbon 
dioxide concentrations in a storage facility atmosphere. However, it is 
also possible, in accordance with the present invention, to use the carbon 
dioxide removal system in conjunction with a nitrogen and/or air injection 
system in place of the make-up skid to control oxygen, carbon dioxide and 
nitrogen concentrations as well as building pressure. 
Furthermore, in accordance with the present invention, conventional 
analyzers and sensors are used in order to ascertain when the system has 
reached optimal concentrations or setpoints of carbon dioxide and oxygen. 
Generally, it is desirable if the oxygen content is maintained at from 
about 1 to 21%, and the carbon dioxide content at about 0 to 20%. It is 
known, for example, for the preservation of apples that concentrations of 
about 1 to 3% oxygen and about 1 to 5% carbon dioxide are most preferred. 
Preferred values for many different types of fruits and vegetables are 
known to those skilled in the art. Moreover, one skilled in the art may 
experimentally ascertain the expected carbon dioxide generation rate from 
fruit, for example, in order to ascertain what carbon dioxide removal 
capacity is needed. 
Finally, in accordance with the present invention, any polymeric membrane 
may be used provided that it is capable of permeating carbon dioxide at a 
greater rate than oxygen which in turn is greater than the rate of 
permeability of nitrogen. For example, polyimide polymer membranes may be 
used, particularly those in accordance with U.S. Pat. Nos. 3,657,632; 
3,822,202; Re 30,351; 4,113,628; 4,705,540; 4,717,393; 4,717,393 and 
4,717,394, all of which patents are incorporated herein in the entirety. 
The present process for controlling the atmosphere of a food storage 
facility may be further understood and illustrated by reference to the 
following Table of events and the corresponding responses presented as a 
process control hierarchy with the attendant priorities. 
______________________________________ 
PROCESS CONTROL HIERARCHY 
FOR ATMOSPHERE CONTROL SYSTEM 
PRIORITY EVENT RESPONSE 
______________________________________ 
1 Critically High Building 
Exhaust air through 
Pressure mechanical vent. 
1 Critically High Building 
Intake air through 
Vacuum mechanical vent. 
2 High-High Building 
High-High pressure 
Pressure switch activates to turn 
(.DELTA.P &gt; 0.5" W.C.) 
on CO.sub.2 skid & shut off 
make-up; activate 
panel lite & horn. 
2 Low-Low Building 
Low-Low pressure 
Pressure switch activates to turn 
(.DELTA.P &lt; -0.5" W.C.) 
on make-up skid & shut 
off CO.sub.2 skid; activate 
panel lite & horn. 
3 High Building High pressure switch 
Pressure activates to turn on CO.sub.2 
(.DELTA.P &gt; 0.25" W.C.) 
skid & shut off make-up 
skid. 
3 Low Building Pressure 
Low Pressure switch 
(.DELTA.P &lt; -0.25" W.C.) 
activates to turn on 
make-up skid & shut off 
CO.sub.2 skid. 
If #3 is satisfied: 
4 High CO.sub.2 Concentration 
Turn on CO.sub.2 & make-up 
(CO.sub.2 &gt; 2%) skids. 
4 Low CO.sub.2 Concentration 
Shut off CO.sub.2 & make-up 
(CO.sub.2 &lt; 2%) skids. 
If #4 is satisfied: 
5 Low O.sub.2 Concentration 
Open make-up by pass. 
(O.sub.2 &lt; 0.9%) 
5 High O.sub.2 Concentration 
Activate Additional 
(O.sub.2 &gt; 1.1%) 
membrane modules on 
make-up skid. 
______________________________________ 
The specific pressure levels recited above are the levels which may be 
typically used in a food storage facility. However, other pressure values 
and ranges may be used depending upon the specific requirements of the 
food storage facility. Thus, the pressure levels recited herein are not 
required for operation of the present invention but only act as typical 
control set points. 
The above description is related to steady state operation of the membrane 
system while fruit is being stored. Pull down of the oxygen during start 
up of the facility can also be achieved using the membrane system and 
manifolding both compressors and feeding air to all of the modules to 
produce the greatest quantity of nitrogen-enriched air. This stream is 
returned to the storage facility as a purge stream to remove the bulk of 
the oxygen. The final adjustment of oxygen concentration in the facility 
can be achieved using vaporized liquid nitrogen or any other convenient 
source of inert gas. 
Membranes have been conventionally used to scrub carbon dioxide from air. 
However, the present invention is far more than merely a membrane system 
to separate carbon dioxide. Rather, the present invention provides an 
integrated system for maintaining a controlled atmosphere in a food 
storage facility using membrane system(s) and a control system. 
Finally, the terminology "W.C." used above, refers to the inches of water 
in a column as a unit of pressure and is a standard unit of pressure. 
Having described the present invention, it will be apparent to one skilled 
in the art that many changes and modifications can be made to the above, 
while remaining within the spirit and scope of the present invention.