Question 5.23: What is the sound engineering solution to a described proces...
What is the sound engineering solution to a described process situation? A chemical company in southern Louisiana manufactures 100,000 tons per day of herbicide and was faced with a potentially costly dilemma. The plant needed to treat the plant’s output but required a Cl_{2} stripper with a capacity of 75 gpm to do so. This represented a 50% increase in the capacity of the existing Cl_{2} stripper or the need to build (1) a second stripper, (2) a new stripper, or (3) find a packing that would allow for the 50\% increase in capacity in the existing stripper. If possible, solution 3 is the most economical choice. This means that the plant would need to find a packing that had a substantially lower pressure drop compared to the current packing (to allow for the increased throughput) and the new packing would also have to have an increased transfer efficiency to be able to meet the effluent specifications at the higher flow rate.
In addition to the capacity issue, the stripper (or packed tower, shown in Fig.1b), having a diameter of 18 in. and packing height of 28.5 ft, performs the function of stripping elemental chlorine (Cl_{2}) from hydrochloric (muriatic) acid (HCl). The existing packing (media) in the stripper tower is 2-in. Pall rings. Originally developed in the 1920s, Pall rings have traditionally been used for scrubbing applications. However, as needs dictated over the years, the Pall rings (and other similar packings) have found their way into stripping process applications.
For the plant in Louisiana, in this process situation the target contaminated liquid stream is hydrochloric acid from which elemental chlorine must be stripped. The gas stream is simply the air driven by an air blower (fan). The new packing material, in addition to the requirements stated above, also needed to have adequate acid and chlorine resistances to ensure a service lifetime of longer than 10 yr.
When approached by the Louisiana plant with this problem, the environmental engineer in charge considered several possible packing materials. Previous discussion has indicated that Q-PAC (supplied by Lantec Products) can be used to optimize a scrubber process system. However, in this instance, being a stripping process system, a different packing was found to be the solution to the needs of the Louisiana chemical plant.
The packing material recommended by the environmental engineer was #2 NUPAC in polyethylene. Although slightly more expensive than polypropylene, polyethylene offers better resistance to oxidative attack than polyethylene. This packing material also offers both improved mass transfer properties as well as reduced pressure drop compared to Pall rings.
In February 1999, the 28.5-ft bed of the packed tower was packed with #2 NUPAC. The performance of the tower after repack was excellent, so plant personnel were relieved that no new capital project would be required. Stripping of elemental chlorine remained at 99\% efficiency in the hydrochloric acid liquid system at the increased flow of 75 gpm in the existing air stripping system. The upgraded stripping process system is summarized as follows:
Reactor design = packed tower (Fig. 1b)
Packing material = #2 NUPAC (Lantec Products)
Tower diameter = 18 in.
Packing height = 28.5 ft.
Target pollutant liquid stream = hydrochloric acid containing elemental chlorine
Liquid design flow = 75 gpm
Liquid maximum flow = 85 gpm
Gas stream = clean air
Service life of packing = 10+ yr
Design stripping efficiency = 99+\%
Flow pattern = liquid flows downward, air flows upward in a countercurrent flow pattern