This Month’s Puzzler:
The byproducts from our chlorination process are carbon tetrachloride and hydrochloric acid. We’re desperately trying to eliminate the CCl4 because we’re frequently tagged with air and water emission fines. During process upsets, which can last for up to a day, as well as startups and shutdowns, we send up to ten times our normal output of CCl4 and miscellaneous chlorinated alkanes to our treatment system. This is more than our activated carbon beds (ACBs) can tolerate. We use a group of ACBs for vapor removal and another set for liquid removal. We are following the EPA’s best engineering practices: “a granular activated carbon bed (ACB) followed by packed bed aeration.” The EPA allows a maximum contamination level of 5 ppb for water and 0.5 ppm for air. The “reportable quantity” for CCl4 is 10 lb/year. Our current process (see “existing” on the figure posted online) involves four steps: 1) caustic scrubbing; 2) decanting; 3) washing/filtering; and 4) condensing the vapor. We have limited space on site because we’re located near a highway but we own several acres within 200 yards of the plant. A contractor has come up with an alternative design to avoid some of the solid waste created by the ACBs: an anaerobic reactor and aerobic biofilter for the liquid and a bubble column followed by an anaerobic reactor for the vapor (see “proposed” on the figure). What do you think of this idea? Can you suggest any other alternative unit operations that could effectively eliminate our outflows to sewer and air? What can we do about the air emissions from our distillation column? Our plant manager says just pay the fines and ignore the problem.
Limit Bioreactor Use
Although effective, the EPA best practices with ACBs is wasteful and will generate a large amount of solid waste. Based on the figure, I will assume the miscellaneous chlorinated alkanes (MCAs) and the CCl4 have the same properties and can be treated effectively in the process. The total flow for the worst case, without the anaerobic reactor, is 65 lb/h. This is flow at the inlet of the ACBs. Using Figure 1.16 for CCl4 in the “Handbook of Environmental Engineering Calculations” by C. C. Lee and Shun Dar Lin, I calculated a CCl4 saturation of 92.2 lb of CCl4/100 lb of carbon at 77°F and a partial pressure of 1.86 psia, assuming 14.7 psia for the total pressure. I developed a simplified formula from the figure: CCl4 saturation = 86.323 × (Pyi)0.1067 where Pyi is partial pressure. Assuming operation to saturation, this means a worst-case event would result in consumption of 2,368 lb/d of activated carbon. Typically, ACB is discarded when it’s 50% saturated, so that’s 4,736 lb/d for the vapor alone.
If normal usage is 10% of that, then the CCl4 flow to the vapor ACB is only (39 + 26) × 0.1 = 6.5 lb/h with a total flow of 106.5 lb/h. Note that the ascorbic acid is part of the proposed construction; ascorbic acid kills bleach. This yields a CCl4 partial pressure of 0.17 psia and a saturation of 71.5 lb of CCl4/100 lb of carbon. Assuming 50% saturation, that’s 218 lb/d or about half of a 55-gal drum every day. Now, let’s consider the liquid side of the balance.
I found the worst usage rate for the liquid ACB to be 61.0 lb/d by making some general assumptions: none of the carbon in the bed is bypassed — this is known as full-bed contact; the solution has a pH of 5.3; a sharp wave front exists through the bed, allowing contact time; it is a fixed bed; and the final concentration of CCl4 (or equivalent MCAs) is 2 ppb at the discharge of the ACB. I used the Freundlich equation to calculate the ACB usage rate: mGAC/Q = (Co – Ci)/(KfCo1/n), where mGAC/Q is the granulated activated carbon (GAC) consumed per volume; Ci is inlet concentration of CCl4, Co is its outlet concentration, Kf is the Freundlich capacity factor, Kf = 11 for CCl4; and 1/n is the Freundlich intensity factor, which is 0.83 for CCl4. If the feed to the liquid ACB is 16 ppb instead of 97 ppm, GAC consumption is only 29.1 lb/d. (An example calculation is given on pp. 1,138–1,156 of the fourth edition of Metcalf and Eddy’s “Wastewater Engineering: Treatment and Reuse.” The units from Metcalf and Eddy are in milligrams, liters and minutes. Other useful information can be found at:
http://goo.gl/yc1j2c and www.carbtrol.com/voc.pdf.
Based on this analysis, I question whether the liquid stream requires bioreactors. Unless the EPA poses a harsher standard than 5 ppb, the existing process should meet emission limits. Of course, I am assuming that the operating data are correct. If the design were set to 0.5 ppb instead of 2 ppb because the EPA limit was only 1 ppb, then the liquid bioreactor would be warranted. Obviously, treating the vapor CCl4 stream offers a big benefit by cutting ACB carbon consumption by nearly a factor of 10.
An alternative to consider is regeneration of the carbon versus disposal. This might be economically feasible. The fly in the soup is how often and how many times the carbon can be regenerated.
The bioreactors also pose a few concerns. First, the assumption that each stage will have the same efficiency is questionable. Second, ensuring adequate distribution of nutrients to the bacteria working at each stage always is a problem. Third, any surging of chlorine into the beds from the preceding unit operations would kill the bugs. Lastly, preventing contamination of the ACBs from the materials upstream is important.
Of course, the key question is how much the moving of the vent gases to a new site 200 yards away will cost. It might be better to rebuild the process or part of it.
Another question is whether there is a better approach upstream of the ACBs. Pressure-swing distillation might be an effective way to overcome the water/CCl4 azeotrope resulting from scrubbing the Cl2 and HCl.
Dirk Willard, consultant
Wooster, Ohio
January’s Puzzler
An 18-head gear-driven piston pump with a worm gear is the heart of our soap-making process. A few years ago, it became noisy, so operations installed a soundproof barrier with a door to block the deafening thump-thump of the pump. For a plant expansion, we now want to increase the capacity of the pump by a modest 20%, which is well within its range. However, the new attention to the pump has prompted a question as to why it’s noisy. The maintenance department notes: 1) a mechanic changes the oil weekly because it’s clogged with oils and caustic; 2) stripping away insulation has exposed corroded copper steam tracing that leaks; 3) there are no seal pots to handle lubricating fluids during startup and shutdown; 4) some of the type-304 stainless steel pipe has been welded repeatedly to repair leaks; and 5) the concrete is corroded below the pump base. What is causing these problems? Can we repair the pump quickly, ideally in only three days? How should we expand the capacity?
Send us your comments, suggestions or solutions for this question by December 11, 2015. We’ll include as many of them as possible in the January 2016 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 1501 E. Woodfield Rd., Suite 400N, Schaumburg, IL 60173. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.
And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.
