tnriched 



Oxygen 



Outlet 



Heater' 



Compressed 



Air 



intake 



w>«w»»wi»y»y»»wwN<»<ww»wi»y^wwy» jM >»w««w*ff 



i^phj 



Figure 2. A schematic representation of the hollow-fiber system used at the HRC. 



livery of liquid Nj. Also, cost of liquid N2 to different 

 operators can vary considerably. 



We are interested, therefore, in the development of 

 new types of atmosphere generators which remove the 

 O2 from air in ways other than by using propane. Two 

 types of these "air separator" generators now are avail- 

 able commercially: "pressure-swing" units that re- 

 move O2 by adsorption, and "hollow fiber" systems 

 that remove it by diffusion. 



Pressure-swing adsorption binds O^, water, and 

 some other gases onto a Carbon Molecular Sieve 

 (CMS), leaving nearly-pure N^ that then enters the CA 

 room. It is called "pressure-swing" because (a) the air 

 entering the unit must be under high pressure, and (b) 

 it consists of two columns of CMS, and alternates 

 ("swings") between them . . . the column not adsorbing 

 gases is being regenerated so that it can be used again. 



The hollow-fiber membrane system is a unit filled 

 with tiny plastic tubes, through which air is forced 

 under pressure. The plastic is much more permeable 

 to some gases, such as O^, CO2, water, and ethylene, 

 than to others such as N2- Thus, as the air passes 

 through the long tubes, the more permeable gases pass 

 out through the plastic, and at the exit port the remain- 



ing gas is mostly N2, which then enters the CA storage. 



Through a lease agreement with Permea, Inc., a 

 subsidiaiy of Monsanto Company and manufacturer of 

 a hollow fiber system (Prism Alpha''""), we obtained a 

 unit in September, 1987, for use at the University of 

 Massachusetts Horticultural Research Center (HRC), 

 Belchertown. This unit was designed to generate an 

 atmosphere of 2 to 8% O2 and 98 to 92% N2 at a flow rate 

 of 125 to 535 standard cu. ft. per hour, at a pressure of 

 150 PSl. 



We immediately encountered a major problem. 

 We had been assured that the 5 horsepower (HP) air 

 compressor at the HRC would adequately power the 

 unit, but it did not. Through aging it was inadequate, 

 ran constantly, and overheated. As a result, we were 

 unable to use the N2 generator to pull down our CA 

 rooms. 



In January, 1988, we rented a 15 HP air compres- 

 sor (which was oversized but available) so that we 

 could use the Nj generator to regenerate a CA atmos- 

 phere in a room that had been opened, partially emp- 

 tied, and re-sealed. We were pleased with its operation, 

 so in August, 1988, we purchased a new 5 HP genera- 

 tor. Later, it was attached to operate in tandem with 



Fmil Notes, Summer, 1989 



