The Process of Pulping
Wood pulping is the extraction of cellulose from wood, plant or a wide variety of other fibrous sources by dissolving the lignin that binds the cellulose fibers together. The 4 primary processes used in pulping are kraft, sulfite, neutral sulfite semi-chemical (NSSC), and soda. The selection of pulping process is largely determined by the end product, fibrous materials readily available, and economics. Kraft pulping is the most common form of chemical pulping, at 80% of the total chemical pulping industry. This equates to nearly 130 million tons of kraft pulp generated annually. Most kraft pulping is performed in batch digesters, although as of recent years, continuous digesters have been installed more frequently. Wood chips, cooking liquor and steam are combined in a pressure vessel for each batch. The length of the cooking process depends on the desired yield and hardness of pulp but generally lasts 5-6 hours with temperatures maintained between 320 and 355°F. At the end of process, the pressure of the steam in digester pushes the cooked pulp out of the digester. The pulp proceeds through various stages of sorting, washing and other treatments before it is pressed and dried into finished products.
Batch Digester Materials of Construction
Batch digesters have historically been constructed of carbon steel with a thickness that includes a corrosion allowance to account for elevated erosion, corrosion mechanisms. In some cases a very thin 304 stainless steel cladding was applied at construction for nominal corrosion resistance. Batch cooking systems have changed over recent years seeking lower energy costs and liquor consumptions. Although deemed overall successful for site operations the more complex control systems have accelerated deterioration of the drums and other equipment making material selection more important than ever. When repairing or upgrading the alloy selections from construction many owners selected 309 stainless steel because of the chromium content known to be the most important alloying element in resisting corrosion attack by cooking liquors. Field 309 stainless steel overlay was frequently used for corrosion protection of both continuous and batch digesters made of carbon steel. More recently, an increasing number of batch digesters with 309 stainless steel overlays have been experiencing much higher corrosion rates making them less desirable and subject to replacement by superior alloying materials.
ER312ss Applied with AZZ’s Patented Field Machine Welding Process
Due to the high chromium content in ER312 stainless steel filler materials, the weld overlay application was susceptible to weld solidification cracking when applied to a carbon steel substrate. AZZ weld engineering developed a patented method to eliminate the weld solidification cracking as well as minimize the weld overlay’s thermal expansion mismatch with the base material. The weld deposit exhibits duplex microstructure which consists of both ferrite and austenite. The weld deposit solidifies as primary ferrite grains followed by precipitating austenitic grains. The resulting weld overlay with ER312ss achieves greater than 25% chromium in the overlay deposit and is considered to exceed the needs of most aggressively operated batch digesters corrosive environments. New digester vessels are often manufactured using duplex stainless steels similar to ER312ss alloy. The ER312ss weld overlay continues to be an efficient solution to significantly extend the life of these vessels and postpone replacement as well as a viable repairing alloy application to duplex lined vessels.
Machine Welding Approach to ER312ss or Other Duplex Stainless Steels
AZZ Specialty Welding is the world leader in pressure vessel life extension services. With the most extensive portfolio of life extension repairs, AZZ can develop an ASME section VIII weld metal repair strategy with machine applied deposits to restore the wall to original thickness and extend the life of your asset with alloying materials. AZZ’s proprietary GMAW machine welding process is ideal for ID applications, particularly digesters, because of the attractive economics proven versus other methods. Constructed by AZZ engineers with sensor enabled parameter monitoring and digital control systems, the machine welding process has the ability to be easily field modified to handle any geometry and difficult welding position including conical sections of the digesters.
AZZ Specialty Welding
Whether the digester needs to be repaired emergently or during a planned turnaround, AZZ has unparalleled innovative leadership in the pulp & paper, nuclear, refining, power generation and petrochemical industries. AZZ has over 2000 ASME qualified procedures and over 1000 active welder certifications. Find out more about our engineered solutions and welding technologies at azz.com/WSI or follow us on LinkedIn.
 Nicholas P. Cheremisinoff, Paul E. Rosenfeld, in Handbook of Pollution Prevention and Cleaner Production, 2010