Most paper mills run 24/7 and the impact from a loss of function of continuous digester could be detrimental to the mill. Most places will take an annual or 18-month outage on the kraft recovery cycle to do maintenance. One of these types of annual routine maintenance is cleaning the digester. This article is from my perspective during my time as the Area Manager of a pulp mill, or fibreline, which made kraft softwood bleached pulp southern loblolly pine, in South Carolina, USA. Every year we took the pulp mill down to do maintenance. All the chests, towers and even the digester are pumped out and emptied. This allows for inspections and to improve our runnability in order to be able to run 24/7 for the next 18 months. The digester I am familiar with is a Kamyr Continuous Digester with the ability to run 1500 ADT. It is lo-solids down flow digester and wood chips are cooked to a Kappa of 30. This digester was started up in 2003 and is 225 foot high. It has several sections including an upper cook, lower cook, transitional zone, main cooking zones and a washing zone. There are two ways to clean a digester, chemically and/or manually. In my experience we do both within the same outage, first by chemically cleaning via acid cleaning and followed by mechanically cleaning via hydro blasting the screens.
Over time, digester cooking screens can get plugged up with scale, mainly with calcium carbonate and other non process elements such manganese and silica. As the main ingredient is wood chips and white liquor, these are the main contributor of calcium carbonate. In high temperatures, high pressures and alkaline pH, calcium carbonate will precipitate. Over time this will scale up your digester and your cooking heaters, thus reducing the cooking flows, throughput, and yields, leading to poor pulp quality. To help reduce this some plants feed an anti-scale chemistry to prolong the life of the digester.
Before we proceed, first a note on safety. A paper mill is a dangerous place and digester acid cleaning is extremely dangerous, which is why in my experience, it is done during the weekend when there are less people around. The surrounding areas should be roped off and announcements should be made to make people aware of the upcoming procedures. It is important to wear the right protective personal equipment (PPE). The mandatory PPE should be escape respirator, safety glasses, hydrogen sulfide monitor, rubber chemical boots, chemical suit, rubber chemical gloves, hardhat and the most important is a chemical face shield with a chin guard. I have had a safety incident that could have been much worse had the face shield not had a chin guard while performing the acid cleaning of a digester. This happened when, while handling the temporary pump, a hose with caustic under pressure came apart and sprayed upwards onto the face of the operator. The operator was safe because the face shield with the chin guard was integral in protecting the person. The contractor later thanked me for insisting on a face shield with a chin guard. Make sure all hoses; pumps and fittings are rated to be able to handle acid and caustic. It is also important that a working safety shower is located nearby and should be checked before starting this procedure. Water hoses should also be staged near personnel for safety. For lockout tag out procedures, the digester should be locked out to prevent chip feed and liquor from entering the vessel.
Strategy and Planning
In preparation for the digester acid cleaning, it is wise to use a company/vendor that has experience with acid cleaning due to dangerous nature of this procedure. The set up for the procedure included rentals of frac tanks with temporary containments, 32 per cent hydrochloric acid, 50 per cent sodium hydroxide (caustic), Rodine acid inhibitor. In addition, pumps, hoses and fittings that are rated for hydrochloric acid. You will need about two-thirds of amount caustic in liters or gallons for the amount of acid needed to fill the digester. Make sure area is properly barricaded and cleared of personnel, and announcements made due to hydrogen sulfide that could be generated. The hydrochloric acid is set up in frac tanks nearby and caustic is set up in truckloads. The setup also includes hoses and pumps to the suction of the white liquor pump. Consider a shut off Y valve so you can easily switch from one frac tank to another or to a caustic truck.
Preparation is key to the success of good digester clean, especially an acid clean. My experiences have been performing the acid cleaning at the start of the outage. This can hold up the whole outage if not planned correctly. It is important to meet as a team with all parties involved and at a minimum includes those of the waste water plant, operations, maintenance, environmental and safety. It is important to come up with a plan to dump contents of the digester safely and be able to handle the flow without incurring any environmental excursions. From a process standpoint, as there are many valving arrangements, it is important to plan which valves need to be opened or closed during the acid cleaning. Using engineering drawings and P&ID’s to trace out where the acid needs to flow through are important in planning. Field tracing and marking of valves beforehand can speed up the process. I have had many issues with being unable to figure out why we are not seeing acid at certain places and it was always because there was a valve that was in the wrong position. Again, this can adversely affect the timeline and delay other dependent activities and thus delaying the whole timeline for the rest of the outage. Besides acid cleaning the digester, this is the time you could acid clean the heat exchangers, coolers that are part of digester towards the end of the process.
While acid cleaning the digester, my experience is the rest of the pulp mill operating as normal processes until there is no more pulp left. Then begins the task of emptying and draining tanks and vessels and locking out the rest of the process. It is important to make sure those processes do not overwhelm the sewers and cause Hydrogen Sulfide while acid cleaning. Make sure all personnel carry a Hydrogen Sulfide monitor and additionally, coordinate dumping of anything else with the dumping of the digester.
The first step is to ensure that the continuous Kamyr digester is completely drained and to blow out all pulp contents. The digester is then filled with water, circulated and dumped. Prior to dumping, it is important to plan and communicate with the wastewater treatment plant personnel. They should be prepared to handle high pH excursions. Then, it is filled a second time to the top most screens such as upper cooking screens. To verify the digester level, check the digester pressure and leave the drain open on the upper circulation pump to check if water is running out. This is an indicator that water has reached that level. Check to make sure the pH is lower than 11. If it is higher than 11, dump and fill the digester again. Make sure to keep the outlet device running. The process of acid cleaning can take several hours.
Hydrochloric acid inhibited with rodine is then pumped from the frac tank to the suction of the white liquor pump that feeds the digester. The acid is then pumped through until various exchangers such as the economizer until it reaches the discharge of the upper cook pump. The upper cook pump is used to recirculate the acid in the top of the digester. Then begin recirculation of the lower cook zone after it has reached the upper cook for 10 minutes. Feed 50gpm of acid for the first half hour. This will then be increased to 150gpm for 60 minutes, while increasing the circulation until reaching max flow. Once the acid is recirculating in the upper and lower cook zones through the center pipe of the upper cook, valve in each of the cooking heaters. Circulate acid through each of the heater to help clean these out. Acid will sink in the digester and the acid solution will move to each section. Recirculate each section and use the acid solution to clean any other heat exchangers such as the reboilers or economizer. Pull samples every 30 minutes from the suction of each zone’s pump, such as the cook pumps and main recirculation. For each sample taken, do a titration and record at each zone or level. Continue to circulate through all zones until concentration levels off and becomes stable per titration testing. Once the concentration is less than 1.0 per cent, and all sections have been circulated, and pumped, the cleaning will be considered complete.
It is now time to begin neutralizing with caustic; this is done so acid is not dumped into the sewers and cause health and safety issues. Once the acid concentration is found to be less than 1 per cent, the digester is considered clean and the acid needs to be neutralized with 50 per cent caustic. Just like the acid addition, caustic is also fed to the suction of the white liquor pump. Caustic is pumped from a truck to the suction of the white liquor pump. This is where having a “Y” valve would make it easier to switch from acid to caustic. The valving will now change as the caustic is then fed to the bottom of the digester through the counter wash and side dilution nozzles. Unlike acid cleaning where the acid sinks from the top to the bottom, neutralizing comes from the bottom to the top, essentially creating a pad at the bottom. The cook pumps should be pumping at its max as the cook flows are set to maximum flows during this time.
Samples should be taken at the all the zones such as the main recirculation, lower cook recirculation, and upper cook recirculation and tested for pH. When the pH of the upper cook is greater than 7, the digester is considered neutralized and the digester is ready to be dumped. The wash pump is used to recirculate the remaining caustic to neutralize any acid vapors at the very top. In addition, water is then valved to the cold blow pump and make up liquor pump to help rinse the digester while it is being dumped. Now is the time to coordinate with the team to make sure nothing else would be dumping and flush the sewers with water. Notify waste water plant and mill personnel that the digester is ready to dump and there is potential of hydrogen sulfide. Continue to recirculate all zones. Open the drain valve and open the vent valve at the top of the digester. Slowly open the drain valve and be careful to not overflow the sewers. A tote of defoamer should be staged by the sewers and ought to be opened to the sewers if necessary to reduce foam. As the level drops past each of the screen, stop its respective recirculation pump. Continue to drain digester until it is empty.
The whole process can take a day to complete if all goes well. This is not something that can be easily completed without teamwork. In my experiences, we try to time the end of the acid cleaning so the digester can be locked out by Monday morning first shift so the contractors can do their jobs. This can be challenging as the whole pulp mill needed to be drained and locked out too by this time. As conflicting priorities occur, it is best to have everything drained before the digester begins to dump.
Hydroblasting and Inspections
After all the acid cleaning is complete, and the digester is completely locked out, it is time to inspect the digester. Once everything is locked out and drained, the digester contractor removes the top of the digester top separator. This allows the contractors to begin to rig an elevator inside it, which consists of ring shaped a platform that surrounds the centre pipe in the middle. Steel cables and motors are attached to the platform that allows it to go up and down the digester. After completing a confined space entry and making sure there is enough air circulation in the digester, the inspection can begin. It is now time to ride the digester for a full inspection. It is important to wear a full body harness and be tied off. While riding the digester, it is important that everyone synchronizes pushing the motor button to allow the platform to go up and down to make sure the platform is levels. Initial inspection will show fibre and some scale, which should be mainly calcium carbonate on the digester screens. You will see hard white precipitated calcium carbonate clinging on to the diagonals or the horizontal slots of the screens. It is a good idea to take samples of the scale for further analysis just to verify. If there is an area with more scale, it should be noted and consider increasing more anti-scale chemistry to those sections to prolong opening to get better circulation and flows through those screens. The screens that have “windows” should be removed to allow ease of mechanical cleaning and removing the scale out from inside the screens. Those screens without “windows”, I would highly suggest installing them. Mechanical cleaning consists of using 10k (10,000psi) to hydroblast or water jet off the scale. The hydroblasters will ride the digester from top to bottom, placing their hoses in the center. It will take some time for the hydroblasters inside the fully clean each screen from top to bottom. The windows will allow all the crud to easily be removed and washed down to a drain at the bottom of the digester.
Once all the scale is removed, it is important to ride the digester for the final time and inspect all the screens. It is amazing how many items I am able to find inside the screens, including welding rods and rags. It is important to remove all trash as it will clog up the system or hurt the system downstream. By this time, the digester is squeaky clean and it is time to put together and begin start up. The “windows” on the digester screens should be bolted back and up and for extra safety measures should be tack welded in place. This will eliminate windows accidentally being open up and bypassing the screens causing channeling or short-circuiting the recirculation and plugging up lines with chips. In addition, metals such as windows and bolts can cause havoc downstream and could easily obstruct or cause failures in pumps, or finally eat away and cause busted pipe. Hopefully you will not have to open this up until the next outage.
The whole process from start to finish is usually a weeklong affair. In which other preventive maintenance and sometimes-capital improvements are also completed, as it is such a busy time, it is important to make sure it is put back together properly. The digester is the heartbeat of the pulp mill or Fibreline needs to be well maintained, as taking it down to do any repairs on the inside is a long and arduous task. Digester anti-scalant may be added to help prolong the life of the digester, this may be costly but it is an insurance that can prevent high shutdowns cost and have significant impact on production of a mill as a whole. However, nothing can remove scale like an acid cleaning or manual cleaning and if done correctly can be safe and yield many benefits.