soda combines with the carbon dioxide in the storage atmosphere forming an 

 insoluble precipitate. The reaction is rapid and effective, particularly when a 

 unit is freshly charged. The water-type carbon dioxide scrubber operates on an 

 entirely different principle. Water is used to absorb the carbon dioxide from the 

 air in the storage room. This carbon dioxide rich water is then exposed to out- 

 side air (low CO2 content) so that it loses the dissolved carbon dioxide. It is 

 then reused to absorb more carbon dioxide from the storage atmosphere. 



Because of the extreme effectiveness of the caustic soda carbon dioxide re- 

 action, considerable variation in design and specifications could exist in the 

 caustic soda type carbon dioxide scrubbers and yet provide adequate capacity. 

 With water-type carbon dioxide scrubbers, proper design is much more important as 

 capacity is quite sensitive to such things as the dimensions of the absorption and 

 desorption sections (towers), the amount of water circulated, the way air and 

 water are mixed in the tower, and the amount of air circulated. 



In the absorption section, it is important that there be imimate contact of 

 all air flowing from the CA room with a large quantity of water for sufficient 

 time that carbon dioxide is absorbed. This means that the diameter of the tower 

 must be restricted, the water flow be rapid, and the height of tower must be 

 greater. In the desorber section where carbon dioxide is removed from the water 

 circulated, there must be adequate contact of the water with outside air for 

 sufficient time :;hat as much CO2 as possible is discharged. This means having a 

 reasonable amount of outside air circulated through the desorber, considerable 

 quantity of water well distributed, good contact between water and air for a long 

 enough period to permit the carbon dioxide to be discharged. 



Since the caustic soda type and water-type carbon dioxide scrubbers differ in 

 operating principle, the caustic soda type cannot be easily converted to the water- 

 type. In particular, the depth of packing is often inadequate to be used for 

 either the absorption or desorption tower of the water-tyne. The diameter of the 

 tower is usually more than is desirable, particularly for the absorber section. 

 The pumps are generally of inadequate canncity. Observations of water-type 

 scrubbers in Massachusetts have indicated the desirability of increased water 

 flow rates, of adequate contact between K'ater and air in the desorption tower, and 

 the use of outside air of low carbon dioxide content. Work in Michigan has lead 

 to recommendations of a 4-1/2 foot d(!pth of bed in pack'^d absorption and desorp- 

 tion towers, and a water flow rate of 10 gallons per minute per 1,000 bushels of 

 apples. Tower diameters were designed to 45 gallons per minute per square foot 

 size and used with blowers of 50 cfm per square foot of tower at 4 inches water 

 gauge pressure capacity. The design variables in water-type scrubbers are inter- 

 related so that recommendations are somewhat arbitrary. However, the general 

 principles stated above apply. In making conversions of caustic soda type to 

 water-type carbon dioxide scrubber systems, they may be used as guides for 

 achieving most satisfactory performance. Depending on the circumstances and the 

 capacity of the water-type c.nrbon r'ioxids s-rrbber, it may be necessary to use 

 caustic soda during the initial period of CA storage when carbon dioxide production 

 is unusuallv high. This can normally be dons with lit'le inconvenience or alter- 

 ation to the water-type carbon dioxide scrubber construction. 



John R. Perry 



Agricultural Engineering 



