A storage tank is purged continuously with a stream of nitrogen. The purge stream leaving the tank is saturated with the product stored in the tank. A major part of the product lost in the purge could be recovered by installing a scrubbing tower to absorb the product in a solvent. The solution from the tower could be fed to a stage in the production process, and the product and solvent recovered without significant additional cost. A preliminary design of the purge recovery system has been made. It would consist of the following:
1. A small tower 0.5 m diameter, 4.0 m high, packed with 25 mm ceramic saddles, packed height 3.0 m 2. A small storage tank for the solution, 5 m3 capacity
3. The necessary pipe work, pump, and instrumentation
All the equipment can be constructed from carbon steel. Using the following data, evaluate how long it would take for the cost savings to pay for the capital cost of installing the recovery system:
1. Cost of product $5 per lb
2. Cost of solvent $0.5 per lb
3. Additional solvent make-up 10 kg/d
4. Current loss of product 0.7 kg/h
5. Anticipated recovery of product 80%
6. Additional utility costs, negligible
Other operating costs will be insignificant.