The linked article said it was a “digester” which turns wood chips into paper pulp.
I am just curious for more information on what that is. This is not the sort of thing I have heard about (digesters exploding at paper mills). Maybe I just have not paid attention but this seems an unusual occurrence.
What is a “digester” in a paper mill and why might one explode?
(I know no one knows, yet, why this one exploded, just asking how they work…like a pressure vessel or something.)
Wikipedia sez: “Chemical pulp is produced by combining wood chips and chemicals in large vessels called digesters. There, heat and chemicals break down lignin, which binds cellulose fibres together, without seriously degrading the cellulose fibres.”
I think that all of us can name our top favorite international companies of chemical pulp production line equipment, but let’s not turn this into a fight.
I wondered, in the back of my mind, if this was related to the pandemic. Either the workers are under pressure to do things differently to social distance and/or the plant was running as fast as possible to make TP.
I have absolutely no evidence of any of that. I just wondered if it was a factor.
The paper mill in tho small town I went to university at used hot sulfuric acid under pressure to break down the lignin. A patch on one of the digester vessels was made out of metal 6 inches thick, held on by bolts over an inch in diameter. The mill was right downtown, of course.
Basically, the digester is where the wood chips, water, & acids are cooked under pressure to break down the wood. So the digester is effectively a pressure cooker, on a gigantic scale. And we hear about occasional pressure cooker explosions in home kitchens. (Now, most cookers have safety measures built in. But explosions were more common pre-1970.)
Typical home pressure cookers are about 10 pounds; large family-sized ones can go up to 20-25 pounds. These digesters are often 1000 tons (2.2 million pounds). up to 3500 tons (17 million pounds). Given the damage to a kitchen from a home pressure cooker explosion, one of these digester ones exploding must be tremendous.
In terms of large scale industrial processes - so not anything related to human/animal biology or medical science - Digestion is a term used in two major heavy industries. The first being paper as you see in this article (primarily the Kraft process) and the second is Alumina production (primarily the Bayer process). The core concept is using heat, pressure and introduced chemicals inside a vessel to dissolve the raw material into a different format for further processing. The high level core concept of pressure-heat-chemicals is common to thousands of other industrial processes but is not generally called digestion except in Paper and Alumina production. One common element between paper and Alumina is the use of caustic soda as the primary chemical in the digestion vessel, and the desired product is dissolved into the caustic soda liquid which is then transferred out.
So I think you could accurately refer to lots of other industrial processes as digestion, equally you can rename the digestion process in Paper and Alumina as something else. But it seems that the common industrial nomenclature has been established that the use of caustic soda as a dissolving agent under pressure is called Digestion and the vessels used to perform digestion are called Digestors!
Makes me wonder if somehow the use of “digester” for alumina production is its application in “paper”-ish products, e.g., sandpaper and other paper coatings. There’s also “paper” made from alumina. I.e., some crossover tech, so crossover name.
Paper mills are a lot like oil refineries; it’s too expensive and financially risky to build new ones (at least in the US). That means the equipment is really old and prone to calamitous structural failures.
Consider a tree. It’s essentially a fiber reinforced polymer. The polymer is lignin, an amorphous phenolic. The fiber is primarily cellulose, a straight chain polymer composed of glucose molecules. The digester is used to treat wood chips with a mixture of sodium hydroxide and sodium sulfide in an aqueous solution at high temperatures and pressures to free the cellulose fiber from the lignin matrix. The fiber once freed from the matrix is called wood pulp. It is then further treated before being turned into paper.
This is oversimplified, there are also hemicelluloses in the wood, arabinose rhamnose, xylose and mannose, but of those only xylose and mannose are present in any significant quantities in the wood pulp produced, and there are extractives present before pulping that are usually dissolved as well.
Digesters can be fed with chips continuously. These are imaginatively termed continuous digesters. There are two continuous digesters at the Jay, ME mill but both are more than 150 feet tall and based on the news report the digester that ruptured was only 70 feet tall so not one of those continuous digesters.
There are also digesters that are fed chips and liquor, then sealed and cooked in a batch process. You can probably guess what these are called, pulp and paper makers aren’t very creative. These are usually smaller and a mill will have several of these to make sufficient pulp to run the process.
Based on the news reports I’m assuming it was one of these batch digesters. I’m only peripherally involved with batch digesters, but continuous digesters run at up to 165 psig and 320 degrees Fahrenheit, so you can imagine what a catastrophic failure of one of these vessels with say 15 tons of wood chips and thousands of gallons of cooking liquid at those temperatures and pressures would look like.
Both continuous and batch digesters are extremely durable, they can last for more than 50 years. They will wear from friction, but it is possible to weld overlay to restore the vessel thickness to meet the boiler code. I’ve had no concerns about working on operating continuous digesters that were older than I was.
I do not believe so.
I think the only common link between the processes is the use of highly basic liquor (straight caustic soda for alumina, a blend of caustic soda and sodium sulfide for paper) to preferentially dissolve components from raw materials into a liquor.
Just an educated guess, but perhaps the research science, engineering design and equipment manufacturers were largely common between the industries. Leading to shared terminology
Bio-gas production also uses digesters. I guess you could qualify that as not a “major industrial process”, but the big ones would at least qualify as a minor industrial process
Methane digesters utilize all kinds of waste inputs, including wood scraps.
Very true. Anaerobic Digestors are common to most cities in the world for sewage and industrial effluent treatment and their use for biogas production from alternative feedstocks seems to have increases by an order of magnitude in the last decade. There are even some big ones that are local landmarks (anyone who has been on a boat in the Boston Harbour will recognise the dozen large eggs at the Deer Island WWTP). Those eggs are anaerobic digesters.
A biological process seems far fitting to the term Digestion than a chemical industrial process, especially one that is inorganic like the Bayer Process!
