WATER/WASTE TREATMENT

Switch from manual dosing to automated system contributes to precise microbiological control for mill"s alkaline fine paper production process

Simpson Reduces Machine Downtime with Automated Water Disinfection

By Garry Bohren and Jerry Weisenfelder



As increasing numbers of paper mills convert from acid-based papermaking systems to alkaline production, microbiological control of process water has become a greater priority. In traditional processes, the acidic environment acts as a natural disinfectant, preventing many types of microbiological contamination. However, due to economic factors, paper mills are moving away from this process and toward alkaline production.

While there is a clear cost advantage to alkaline systems, there is a far greater potential for microbiological contamination of water used in the production processes. The results of poor microbiological control are deposits, corrosion, sheet defects, increased downtime, and rejected product.

Such problems were evident at Simpson Paper's alkaline fine paper mill in Anderson, Calif. The mill's two paper machines suffered downtime due to microbiological contamination and occasional sheet breaks, a result of the mill's unsuccessful attempts to manually feed chlorine to the shower water for microbiological control.

However, a switch to an automated chlorine control system has significantly decreased the frequency of equipment downtime. With the new system in place, the mill has reduced downtime, as well as saved on man-hours for cleaning and monitoring. Based on the results of the first unit installed, two more automated chlorine controllers have been added at the mill. All three units have resulted in quick return on investment.

MICROBIOLOGICAL CONTROL CRITICAL. The Anderson facility, constructed in 1964 and acquired by Simpson from Kimberly-Clark in 1972, currently produces a variety of coated and uncoated alkaline fine paper, including drawing, bond, magazine style, record covers, and wine labels. The mill operates two paper machines that combined produce 470 tpd of paper.

Like all paper operations, the Simpson mill uses large amounts of water, and in Simpson's case, the water is obtained from its onsite well. This freshwater is used as shower water, makeup water in the recirculated systems, and for many additional process applications at the mill. Apart from freshwater, microbiological control is also necessary for the paper stock prior to the headbox because warm conditions make this recirculated water ideal for microbial growth.

Prior to the adoption of its current chlorine control system, the mill practiced manual control of gas chlorination of the influent water for microbiological control. Chlorine dosage was based on the results of chlorine residual tests conducted once daily and on flow pacing. Mill operators used the daily residual test to determine a baseline level for chlorine feed, and this level was increased or decreased based on the current water flow into the mill. The mill's goal was to hold a consistent 0.3 ppm chlorine residual level in the water at all times.

Manual chlorine dosage control proved to be imprecise, however, and chlorine residual levels were most often erratic. One of the problems was that increased hydraulic loads did not necessarily result in a corresponding increase in chlorine demand. Furthermore, residual testing does not measure the oxidant demand in the system, which constantly changes, regardless of flows. The final result was periods of severe chlorine overfeed and underfeed.

Chlorine overfeed is not just wasteful, it degrades the paper machine clothing fabric and other crucial equipment. Through the long term, it can be extremely costly. On the other hand, chlorine underfeed leads to microbiological contamination of the paper. This contaminated paper is considered broke and is generally sent back for repulping.

At the Simpson mill, high microbiological levels meant that the machines had to be shut down and cleaned frequently and, on a few occasions, had to be shut down completely for a mini-boilout of the system, which resulted in the loss of production for several hours.

REDUCING DOWNTIME, PAPER LOSS. The mill finally gained accurate control of the chlorination process by adopting a High Resolution Redox (HRR) monitoring and control system that measures the chlorine demand and efficacy in the headbox influent water and feeds chlorine at dosage levels to precisely meet that demand (HRR readings are measured in millivolts [mV]). Unlike residual measurement or flow pacing techniques, this monitoring and control system measures the ever-changing oxidant demand in the water, so accurate chlorination can be achieved by treating the system to maintain specific, predetermined levels.

In August 1994, the Simpson mill installed an automated chlorine control system designed by Stranco Inc. (Bradley Ill.) to monitor and control influent water going to the headbox on its No. 1 paper machine. Shortly afterward, the mill installed identical systems for the headbox on its No. 2 paper machine and for the millÕs influent well water.

After determining the current chlorine demand and efficacy in the water, the system sends a 4-20 mA signal to a diaphragm pump controlling the chlorine pump rate. The pump adjusts the chlorine feed based on this signal. To hold a 0.3 ppm residual level, the mill keeps the two controllers on the paper machines set at 310 mV, and the freshwater controller at 650 mV. Through the use of the automatic systems, precise microbiological control levels are maintained according to the chlorineÕs actual effectiveness in the water.

PROJECT RESULTS. Since the installation of the new system, the Simpson mill has had significantly less unscheduled downtime due to microbiological contamination. The reduced downtime has also relieved mill operators of the time-consuming burden of cleaning the paper machine, thereby allowing them to concentrate on other important duties.

The Simpson mill continues to conduct daily chlorine residual tests, and results show that the system holds chlorine levels right at 0.3 ppm. The need to manually adjust the chemical feed rate has been eliminated.

Since the installation of the original HRR chlorine control system, the mill has switched from gaseous chlorine to a sodium bromide/bleach program. This was done for safety reasons. The mill has found that the chlorine controller is equally effective regardless of what type of oxidant is used and required little adjustment when the type of oxidant was changed.

New technologies are helping paper mills reduce the risks of product contamination brought on by microorganisms. Although operators at the Simpson mill frequently check to make sure the chlorine controllers' HRR millivolt readings are within range, the mill plans to automatically log data provided by the controller. This will give the mill a permanent record of the influent microbiological levels and will provide even more information with which to precisely control the production process.

GARRY BOHREN is a process technician, Simpson Paper Co., Anderson, Calif., and JERRY WEISENFELDER is a process specialist, Betz Dearborn Paper Process Group, Long Beach, Calif.