ORTVIKEN OPTIMIZES FOR ON-LINE INSTEAD OF LAB RESIDUAL PEROXIDE ANALYSIS
Hydrogen peroxide is the bleaching chemical the SCA Ortviken Paper Mill uses to bleach the TMP for the LWC it produces on two paper machines, LWC 1 and LWC 4. Although peroxide is an environment-friendly and effective bleaching agent, it is relatively expensive. Therefore there is money to be saved and uniform quality to be gained if its use is strictly controlled. Thats why Ortviken opted for the patented RPA-5000 Residual Peroxide Analyzer instead of laboratory analysis to help control the two-stage peroxide bleaching process that feeds pulp to LWC 1.
An easy choice
"The choice between on-line and lab analysis was easy,"says
Anders hlund, the Pulp mill Supervisor at Ortviken. "On-line measurement gives us four minute intervals between measurements. "We really cant operate our peroxide bleaching efficiently without the RPA. Without this system, Id estimate that wed use between 5 and 8% more fresh hydrogen peroxide."
Daniel Nordin, the Electrical and Instrument Department Supervisor at the mill "Its accurate and reliable and practically maintenance and calibration-free. Since we installed the RPA, weve checked it when it needs to be filled with catalyst, or about once a month. "In other words, its not labor intensive, which means a saving in itself apart from that we gain from using only as much peroxide as we need to reach our brightness target."
Optimizing peroxide bleaching
Between 250 and 400 tons of pulp at a brightness level between ISO 76 and 77 are fed to LWC 1 daily for the production of several grades of LWC. The RPA is positioned between an intermediate mixing tank and the first bleach tower. Filtrate from a wash press positioned after the two peroxide bleaching stages flows into the mixing tank where fresh make-up peroxide and other chemicals are added.
The job of the RPA is to keep the amount of peroxide into the first bleach tower at a constant level to optimize pre-bleaching. Additional fresh hydrogen peroxide and other chemicals are added between the two bleach towers. The instrument is on a four minute measuring cycle and feeds information continuously to a DCS. Optimized peroxide dosages are the sum of residual peroxide in the filtrate and the fresh make-up peroxide added to the bleaching process to obtain certain brightness in the first bleaching stage.
Brightness transmitters on LWC 4
Two transmitters from BTG monitor brightness in the 250 and 400 tpd, single-stage peroxide bleaching process that feeds LWC 4. Unlike the line for LWC 1, several pulp qualities are produced on this line. Brightness levels at ISO 69 to 74 are lower than on LWC 1. The first brightness transmitter is positioned before the twin-wire press at the beginning of the line. It monitors the brightness of the incoming unbleached pulp coming to identify the kinds of problems that can result, for instance, from seasonal differences in pulpwood.
After the twin-wire press, the high consistency pulp is mixed with hydrogen peroxide and additional chemicals before moving on to the high-consistency bleach tower. After the bleach tower, the pulp is diluted to medium consistency, after which, the second transmitter monitors brightness. "An alarm goes off if brightness deviates too far from the set point," says Mr. hlund. Both he and Mr. Nordin agree that the transmitters supply reliable and accurate results. "The instruments are checked against lab tests about three times a week, and it has been a long time since theyve needed any adjustment," says Mr. Nordin, adding, "They contribute to stable brightness in this process."