Question

A pulp mill utilizing the saw dust from nearby lumber mills to produce a market pulp for paper production. A recent development had the company thinking of increasing the pulp production by 50% for entering the nonwoven market. A close examination at the pulp mill has the engineers to conclude that the digesters were the bottleneck. Currently, the woodchips were steamed, mixed with a pulping liquor and underwent delignification at 150°C for 3 hours. For each load of woodchips, a minimum of 2 hours is needed for loading, heating the digester up, explosive decompression unloading, and cleaning up the digester for the next load. Your task is to find a way to ease the bottleneck quickly at a low cost. State your assumptions clearly and your solution to the problem as stated.

Answer 1

Assuming that the installed digester is a traditional type. To increase the production, modifications are required in the traditional one. To produce the pulp, chip column must be moving which is the most important task of an operator. The chip column movement in a two- vessel hydraulic digester can be improved by converting the digester to two vessel steam/ liquor phase digester.

In this modified design, the impregnation is done in impregnation vessel which is liquor filled. Cooking temperature is reached by direct heating in steam filled digester top. Maintain the liquor level below the top separator and the chip level is controlled to 3-4 feet above the liquor level. The chip column is pushed downwards due to the weight of chips above liquor level. Use the lower part of digester for washing.

Separate column for impregnation will definitely help to reduce time in digester thus increasing production rate.