Sr. Carlos Flores is plant manager of the Infra, SA facility in Tlalnepantla, a suburb of Mexico City. The three-year-old plant manufactures ethylene oxide (EtO) blends, mainly for sterilization applications. Infra, SA employs 200 workers, however, just two of them are involved in the operation of the plant's Croll Reynolds EtO scrubber, supplied by Croll Reynolds Co. Inc. of Westfield, NJ.
The CR scrubber was recommended and specified by Air Products & Chemicals Inc., a major supplier to the Infra plant. Air Products and Croll Reynolds application engineers cooperated together to achieve the exact capacity required to meet the plant's needs.
EtO emissions are banned in Mexico, just as they are in the U.S., so Infra could not operate the plant without an effective scrubbing system. The scrubber, in use since the end of 1998, is designed for a maximum flow rate of 10 scfm and operates at an efficiency of better than 99%. Since it needs to run for as much as eight hours at a time, it is designed for continuous operation.
EtO scrubbing is similar to conventional gas scrubbing, but a longer residence time in the scrubbing unit is required because of mass-transfer and reactivity constraints. The system is based on the absorption of EtO in solution, and its subsequent hydrolysis to inert ethylene glycol.
The Croll Reynolds equipment includes two packed towers in series, each five ft long and 8 in. in diameter, and a reactor tank and solution tank combined in a single 4,300 gallon tank, 8 ft diameter by 13.5 ft high. The two components of the tank are separated by an internal baffle. Other system components include a circulating pump and a heat exchanger for cooling the "hot ambient" recirculating liquid during the hot Mexican summers.
While EtO scrubbing is basically similar to conventional gas contaminant scrubbing, there is an important difference. Unlike conventional acid gas scrubbing, where conversion is virtually instantaneous, EtO scrubbing requires a definite, predictable, and relatively long time to convert to ethylene glycol. Analysis of conversion speed and provision for adequate dwell time are critical parameters in the design of a successful system - an area where there is no substitute for actual field experience!
EtO scrubbers are available in various configurations to suit specific conditions of service. In applications requiring continuous treatment of sizable quantities of gas, a complex system may include alternate storage tanks, chemical feed and neutralization tanks, forced or induced draft fans, and associated pumps, piping and controls. In many cases, however, when gas is discharged in small amounts on an intermittent basis, a simple system of single-stage tower, sump, piping loop and pump, as shown at the right, may be completely adequate.
Absorption and dwell time:
keys to EtO scrubber operation
The EtO scrubbing system is based on absorption of ethylene oxide and its subsequent hydration to ethylene glycol. Reaction rate is a function of temperature in the presence of a catalyst, typically sulfuric acid. Energy consumption is minimal. The final product of a system is an ethylene glycol solution containing a small amount of acid. In cases where no discharge can be tolerated, calcium hydroxide can be added to the solution to precipitate the acid as calcium sulfate, leaving a solution of 30 weight percent neutral ethylene glycol.
The heart of the system is a counter flow packed column where the initial absorption takes place. As shown in the diagram above, EtO gas enters at the bottom of the column, and flows upward through the packing while the scrubbing liquid flows downward by gravity. In the process, intimate mixing and absorption of the gas takes place. The liquid carries the absorbed gas into the reaction vessel or sump, which is designed to allow sufficient residence time for complete conversion of EtO to ethylene glycol.
In most installations, the size of the column is based on total gas rate to be handled and column height may vary with required efficiency. Most columns are quite high – 25 feet or more – with diameters from 4 to 12". Height is seldom a problem, however, because outdoor installation (as shown above) or installation where the column projects through the roof, is perfectly feasible. Since the end product of the cycle is an ethylene glycol solution – common antifreeze – only minimal insulation will be required for a successful outdoor or semi-outdoor installation.
Plant manager Flores reports that the Croll Reynolds system has operated exactly as specified for the past three years.
Croll-Reynolds Clean Air Technologies/Croll-Reynolds Company, Inc., 751 Central Avenue, Westfield, NJ 07091-0668. Tel: 908-232-4200; Fax: 908-232-2146.