August 31, 2005
a trainer again
Let me tell you how a sulfuric acid plant operates. Very simply, and I'm not going into much detail.
You start with a giant blower that pulls in outside air and runs it through a Drying Tower, where 1,800 gallons per minute of 93% sulfuric acid cascades through a maximum contact ABSORBTION system that removes any moisture from the air. The dry air is then fed into a Sulfur Furnace, where it provides oxygen to combust molten sulfur, carefully melted and heated to 380 degrees F before it is atomized in the furnace.
It burns and produces SO2 gas.
The SO2 gas is passed through a converter filled with vanadium pentoxide catalyst that converts the SO2 to SO3 gas as rapidly as possible. Maintaining temperature control is essential to make this reaction happen. Therefore, the gas passes through a series of waste-heat boilers and gas-to-gas heat exchangers before it hits the Interpass Tower, where the gas is ADSORBED (not ABSORBED) in a steam of 98% sulfuric acid, pumped at a rate of up to 3,600 gallons per minute.
The temperature of the recirc acid is just as important as the temperature of the gas. Interpass acid MUST be between 170 and 190 degrees F or it won't ADSORB, and your fumes go right out the stack.
As your 93% acid keeps ABSORBING moisture from the air, it gets weaker. As your 98% acid ADSORBS more SO3, it gets stronger. So, you cross-breed the 98% with the 93% and add water through dip legs that extend almost all the way to the bottom of a pump tank. That's how you ADD WATER TO ACID without causing an explosion. You do it from the bottom in a brick-lined tank.
The gas leaving the Interpass tower makes one more pass through the converter, where any remaining SO2 is converted to SO3 and ADSORBED in another acid-bath. Ideally, at the end, you have nothing but nitrogen leaving the stack with a trace amount of unconverted SO2 gas (less than 3 pounds per ton of acid, according to our operating permit, and we ALWAYS beat that standard if things were working right).
If you don't believe me about this shit, just ask catfish. He operated the acid plant for a long, long time. I was his boss for several good years.
It's been a while since I trained anybody on how to do this (it's a complicated job), but I still remember how.
Some things you just don't forget.
Damn, that must've been a fun process to operate.
Was the control room all electronic? Back when I started in the chemical biz, all of the process variables were tracked on paper strip charts, and every few hours the lead operator would write down, on a huge "horse-blanket" spreadsheet, all of the important process measurements.
When the switched to electronic controls in the 1980's, it was a different world. You had touch screens that showed diagrams of the whole process, and you could see how all of the process variables were related.
Some of the stuff we handled was pyrophoric. Diethylaluminum, for example. If you exposed it to air, it would catch fire.
But we didn't have to monkey with sulfuric acid, thank Gawd.
That reminds me of the basic hazmat training we got at Intel. "AAA" stood for "Always Add Acid", as in, if you want to dilute acid, add the acid to the water, not the other way around. The acid in question, by the way, was HF. That's nasty, nasty stuff; it soaks through your skin and just keeps burning. People have died from as little as 2.5% body surface area exposure. They used it for Wet Etch.
I worked in Ion Implantation, which was pretty hazardous (pyrophoric source gases, high voltage, mechanical hazards, vaccuum systems, and cryogenics). But it didn't involve acid. I doubt they even have Wet Etch stations in chip fabs anymore. Wet Etch removes material under the resist (lateral underetching), which limits how small of a feature you can create.
I've wondered about the contol system myself. Was it a SCADA or a full blown PLC- HMI system? Allen Bradley or some othe manufactuerer? Maybe Foxboro?
You should have been a chemistry teacher. I goddam hated chemistry in high school and college. I was always good with physics and electrical things. I'm a hands on guy. None of that theoretical hogshit for me. Talk computers, networks, telecom or anything electrical to me and I can stick with the best engineers. Chemistry just gripes my ass and always has.
I really enjoy reading posts like this one, its like hearing Latin at Catholic mass. I know something important is going on, but, I haven't a clue what its all about....lucky you.
Thanks for the primer, Rob. Interesting.
The difference between a "Trainer" and an "Educator" is that a trainer will show you how to DO a job, an educator will tell you everything else about the job.
ya know.....sounds a little more complicated than working in a lead foundry, but I get the idea. although our permit is a bit higher for SO2 emissions(max of 530 lbs/hr). but at the same time we have to worry about the sodium nitrate plumes going up the stack. the neighbors don't appreciate waking up and seeing a yellow cloud around their homes, but they shouldn't bitch....it's basically just laughing gas after it mixes with air. just fucking inhale!!!!! you'll like it!!!!
DipNut, yeah a lot of the chemicals used in the manufacture of Semiconductors are nasty nasty nasty. I remember when I first started to work for Xicor (I don't now, left before they merged with Intersil) and was given the 'right to know' lecture. Some of the chemicals would shut down the plant and cause an evacuation if 1 part per 1 or 2 billion were detected in the air.
Remember a friend who did some work for LAM research and in their 'RtK' safety lecture, the lecturer came in with a quartz box that had a little powder in it. My friend was basically told that if that powder were tossed up into the air it would kill everyone down wind from the building to San Francisco about 10 miles away. Nasty Nasty Nasty stuff.
Yeah, chip fab uses a lot of nasty stuff that NEVER becomes non-toxic until you dilute it down to zero. I live in Orange County, CA about 40 miles from the San Onofre nuclear station. Everybody worries about it, but not about the Motorola chip fab that is running in Irvine......
Compare some of those chemicals to what comes out of a nuclear power plant and you will see just how batshit-crazy the anti-nuke crowd is...
So called 'High Level' radioactive waste drops down to be less radioactive than the parent ore in about 1,000 years......
Damn, Rob; no wonder you can make that "homemade wine'" so easy. Piece o' cake after the brewin you did.
Incidentally, Rick T is right about the envirowackos going off on the demon chem plants (there's a fuckin BRICK plant they bitch about here because of the niox emissions, -it's a college town) . They never say squat
about some of the chrome polish outfits that supply a truck plant right under their noses. Some of that shit will take the hide off anything.
Rob, I'm curious about something. Is there another reaction step before you get to sulfuric acid? SO3 is just sulfur trioxide, but sulfuric acid is H2SO4. I am guessing the adsorption process also includes a reaction with the water, where it picks up the two H's and the last O. Is this the case?
SO3+H2O --> H2SO4
Nevermind. I found the rest of the reaction process here:
Manufacture of sulfuric acid
Things like that bug me until I know...
DC, you pretty much nailed the formula.