It was a fast build and there is some danger, so read and use precautions where indicated.
I've sterilized 25 one gallon bags of media so far, but it could easily fit 60. You will also need a low pressure boiler in the 300,000 BTU range.
One could also heat the bottom with some kind of flame I suppose, couple of propane crab cookers? Around 100,00 BTU each.
Step 1: Cutting the Tank
I decided to buy a new propane tank, so I blew my budget right at the start. It was $ 1600 Canadian plus they wouldn't let me put it my van. Add $50 delivery. It did look kinda heavy, so whatever. There were lots of larger ones in stock too. 250 gallon US is a reasonable size for my plan.
If you start with a used propane tank there is some danger of exploding and bad smells. I would at least steam it out before cutting.
To cut, wrap some stiff tape near an end to act as a guide ( not on the weld) and cut the end off. I left enough room for welding on a straight area not on the dish.
Step 2: Leg Mounting System
I used 1/8" by 2" square tubing. The casters are 4" heavy duty solid polyurethane. The frame was TIG welded by myself. I'm only an ok welder so most people could do it and it won't explode. I shaped and reshaped the angle(curve) to attach the tank to the legs.
Step 3: Attaching the Flanges
A friend water cut the flanges out of 3/8 steel plate for $200. Originally I was going to get a piece of angle iron curled to make flanges, but they were $300 each. I was a bit nervous to weld the flanges, as they have to hold 11 tons of force, so another friend that is a certified pressure welder came over to help. I had to squeeze the tank a bit to get the flange in place, but when it was on there was a 1/16 " gap around it to allow a deep weld. Just kidding, that's just how it went.
I left the tank 1/4" proud to make the inside channel for the O-ring. Grind and sand the weld 'till smooth where it is against the O-ring
The tank side also had a piece of 1/16"X1/4" that I hand rolled round and tacked on to hold the O-ring from the outside.
For the amount of force on the O-ring and the door I used eight 5/8" high grade bolts. Totally overkill, but they are really easy to use over and over and don't dent from the door hitting them.
I bolted on the lid side flange and used a chain block to force the lid into position then welded it from the outside.
At this time I built the door hinge. It has to be a three point pivot to allow the door to move flat against the seal. I used some scrap heavy pipe and the lathe to make them. The door swings all the way out of the way for unloading the racks.
Step 4: Hydro Test
Put it all together. Pressure release, bottom drain(weld one in), air vent, steam inlet, temperature probe bulkhead, pressure gauge.
This is the only safe way to test a pressure tank;
Hook up a garden hose and fill it with water making sure to leave the air vent open. When it is full, shut everything and then SLOWLY add small shots of water from the hose.
I got to 12 psi before water started leaking from a badly ground section. I tightened a little bit and it stopped. I then brought the pressure up and let it leak. Noting the problem areas until it got to at least 32 psi (buried the gauge).
It runs at 15 psi so I figure it's ok.
This photo is from afterward. It really doesn't leak that much.
I did a trial run with steam and there were no leaks. I guess the heat let the Bunan O-ring form to shape.
Step 5: Operation
Bags of media contain air and so a few venting and vacuum cycles get all the air out and really let the steam transfer heat fast. Use steam tables to tell when the media temperature is high enough to vaporize water inside the bag at the current vacuum. I pulled it down to -12" Mercury, which corresponds to 85 Celcius.
I have to build rolling racks and a water spray( to create a vacuum inside) and an air filter to keep everything sterile during cool down.
I've installed 5 Teflon coated thermocouples for monitoring temperatures inside the bags.
I intend on sterilizing 500kg (1/2 ton) of media based on brewery waste each week.
Total cost is a bit over $2500 plus the thermocouple wire and reader $300 which took me plus some help around a week to put together.