The contract welder threw his helmet 30 feet against the wall and let out a string of insults against my company. He was furious. He’d been burned by caustic soda several times because our operations group thought that blowing a line out was cleaning it. I sided with him on this one. It was late. We were sunburned and the temperature exceeded 90°F. We’d been trying to get the final weld done for an hour and the sun was hitting the tree line. We took a break and talked about beer.
We worked out a solution. We tilted the long length of pipe upward and shot cold water in; we did this repeatedly until the pipe was full, mostly with water. Water sputters but doesn’t spit hot caustic. The weld wasn’t pretty but it held. It was dusk when the welder drove away and I trudged back to my office. I thought of all the cleaning situations that went wrong.
Cleaning pipe and equipment is as much a part of process engineering as equipment design and system controls but nobody really covers it. Let’s get started.
As I said in last month’s column (“Make Your Outage a Success”), you need plans in place for cleaning. First, it’s foolish to clean an entire system from a feed tank to a product tank. So, build in a system of blocks that allow you to clean only what actually requires attention. Blowing a product back to storage is a good idea — it’s only a first step. The next step is neutralizing the pipe, gently. Generally, do this with water; it’s important to avoid conditions that affect corrosion, like low pH, or cause phase changes, like foaming or sedimentation. Evaluate the impact of temperature: if a fluid reacts violently, chill the purge liquid; or if the fluid clouds or waxes up, think about heating, either indirectly or more efficiently by steaming. Consider doing some lab tests.
Adding caustic to water can make sense; food and pharmaceutical processes often rely on caustic to break down cell walls. The trouble is that high pH causes trace sulfates and nitrates as well as more common carbonates, those typical in well water, to precipitate on pipe walls.
A course of inhibited hydrochloric acid (HCl) at a warm (not hot!) temperature might alleviate this problem. Citric acid has been used as a chelating compound to help with descaling. Keep the temperature below 115°F for straight-grade stainless steel pipe; I recommend using low-carbon (L-grade) because it resists weld pitting.
Plan for where to dump the contaminated liquids from cleaning. I suggest flowing water in through a hose connection — it’s best to install a back-flow preventer — and pumping it to a rental tank for disposal using an air diaphragm pump connected to the pipe being cleaned. With caustic and acids, a pH strip will tell you when the water’s pH matches that of the plant water; I prefer on-the-spot pH measurements to analyzers because they require maintenance (which usually is done badly).
Reactive, or toxic fluids pose a significant problem. When I worked with titanium tetrachloride, we neutralized (killed) the tank by slowly adding water after putting in a thick blanket of defoamer. The water tube was inserted into the foam and we watched the tank temperature to avoid eruptions of toxic HCl and titanium oxides; a vent system scrubbed the vapors. The treatment was complete when the temperature reached that of the plant water. As with most toxic chemicals, the liquid was slowly evaporated away with the remaining solids going into drums for disposal.
Certain processes, such as those in paper and dye making, pose a particular problem with color in their discharge. Dilution isn’t necessarily the answer. Water restrictions only allow so much dilution. The best solution seems to be a treated sand filter if you can identify the reactivity of the chemicals in your waste stream or cleaning solution; this analysis can be pretty extensive and even frustrating if you fail to pinpoint all the potential bad actors.
A possible alternative is passing a side-stream through a bed of activated carbon to remove color. The idea is to clean enough of the colored stream to comply with regulatory requirements.
Bio-remediation might be another option although I’m not aware of its use to remove color from wastewater. However, elevated levels of chlorides and high or low pH undermine the effectiveness of bio-remediation.
DIRK WILLARD is a Chemical Processing Contributing Editor. You can email him at firstname.lastname@example.org.