Significant Energy Cost Savings for Pulp Mills with Advanced Brownstock Washing Optimatization Solutions

Esko Kamrat and Keijo Pyörälä hosted the Experts Seminar at Hilton Helsinki-Vantaa Airport in October, where K-Patents Oy gathered pulp mill brownstock washing process experts under one roof. Andritz, Kemira StoraEnso and Metsä-Botnia pulp mills sent their representatives to share their expertise with the audience.. The seminar also sparked interest in the academic world and Aalto University as well as Mikkeli University of Applied Sciences sent a professor and researchers with in-depth knowledge about brownstock washing.

At the seminar, the latest research results, field experiences, and views on washing efficiency and higher level wash optimization were shared. The main focus of the seminar was on the following two topics; wash results should be measured and realtime measurement techniques should be used in order to understand how the washers work and to utilize the search results to improve wash efficiency. Until now, improving the wash efficiency has not been a priority and it has not been measured, apart from the amount of water and filtrates’ conductivity. These factors have not been optimized, but the focus in wash control has been merely on adjusting the filtrate tanks’ surfaces.

The topic is current as the prices for wood feedstock, chemicals and energy will climb higher, and the need for renewable energy production will rise from its present level. By optimizing the brown stock wash and raising the black liquor solids content, factories gain immediate profits. Savings are significant as cleaner and higher quality stock for bleaching, as well as optimized need for water and chemicals, will be achieved.

According to Aalto University professor Olli Dahl, factories should pay more attention to material efficiency, minimizing water usage and improving energy efficiency in the future. He also presented a serious challenge to raising black liquor solids content to achieve the sustainable energy production targets. There is a huge potential in the paper mills’ energy sales to the community and new business can be developed from it. In Finland, the quantity of black liquor produced renewable energy is substantial, about one-third of the sustainable energy production. By raising the black solids content in factories, sustainable energy production would increase by a percent. This would have a significant impact on the economy as according to Confederation of Finnish Industries’ estimates, increasing wind power by one percent would cost 2,5 – 3,0 billion Euros, Dahl pointed out.

Therefore, paper mill wash suppliers would be well advised to invest in utilizing real-time measurements in wash efficiency. Allowances towards paper mill energy efficiency research and increasing their renewable energy production quotas should be made in order to achieve sustainable energy production targets.

Background on Measuring the Brown Stock Wash Efficiency

Sulfate pulp production consists of series of water-based chemical processes. In pulp wash, stock is separated from black liquor, which contains the dissolved lignin and chemicals. The matter is capitalized in energy production and the cooking chemicals are recycled.

The quantities that describe the washer functions are the amount of used water, the quantity of impurities removed by the wash, as well as the efficiency of standardized in-feed and outlet consistency washer. These are gauged with wash-loss and dilution factor. The former indicates the alkali-loss, which is the amount of alkali leaving with the washed mass per pulp ton. Dilution factor tells the dilution of the evaporator-bound black liquor caused by the added water during the wash.

Oxygen delignification has become a common step between the cooking and bleaching, which has complicated the notion of wash-loss. In oxygen delignification either oxidized white liquor or pure sodium hydroxide is used to regulate the pH. Because part of the alkali is from cooking and the other part is from the oxidation stage, measuring the total amount of the cooking alkali will not tell the whole truth about the wash-loss. That is why chemical oxygen demand (COD) as a measurement tool for wash-loss has gained popularity. However, research results on the actual wash-loss components of bleaching1 have shown that lignin is the component that most effects the wash result, not COD. The amount of methanol, which is insignificant from the wash-loss perspective, significantly affects the COD. Therefore, the amount of lignin during the wash should be measured in order to optimize the wash result.

Marko Harinen from StoraEnso demonstrated why real-time indicator is needed to research the wash and why using traditional hand samples for measuring is impossible. Contrary to earlier beliefs, process changes are rapid and therefore connecting on-line information to the samples is not an option.

K-Patents has collaborated with different partners to create a refractometer, which is a measurement device based on Refractive Index (R.I.), which can handle the wash process’ challenges, such as air bubbles, particles, fibers or soap. With a process refractometer, the flow’s dissolved dry solids content coming to the washer can be measured straight from stock line.

The device has been used in several master’s thesis covering washer functions and optimatization in Veitsiluoto, Varkaus and Uimaharju. It is also being used in the current thesis and publication series by Mikkeli Technical University Center researcher Riku Kopra, titled Pulp Wash-loss and its Reliable Measuring in Factory Scale.

Measuring wash-loss with different methods

Dahl emphasized the significance of experimental research in measuring wash success. Kopra’s factory tests at Enocell’s Uimaharju factory to examine washer efficiency included Andritz’s DD-Washer and seven process refractometers, which were installed straight on to pulp and filtrate lines. The results showed that by regulating washer functions (sealant distances, drum rotation speed, moment, etc.) and with the help of process’ condition changes (temperature, pH, consistency, washing liquid quantity), wash line effeciency can be enhanced considerably. Monitoring requires real-time measurements.

Picture: DD-Washer factory test and measuring the dry solids content with a process refractometer (numbered).

Results from comparing different measuring devices designed to gauge wash-loss prove that the COD’s weaknesses are poor measurement repeatability and long delays. Also, COD is not an effective tool in wash-loss measuring as some of the components causing wash-loss even have a positive effect (formiat) and certain substances, like methanol, cause a lot of COD without effecting the bleaching reaction. Conductivity meter’s problems are too high a dirt accumulation and drift. In dry solids content of over five %, conductivity does not correlate anymore. With a process refractometer washer entering flows’ (liquid and pulp suspension) dissolved dry solids content can be measured in real-time and the results are reliable.

By using a refractometer and data handling tools, it is possible to detect wash line’s problem areas as well as to estimate wash efficiency in real-time. This enables wash optimatization and water usage reduction, which has direct consequences on financial aspects of the other processes, such as evaporation, bleaching and waste water processing. Real-time measurement also enables higher level adjustment developments for the whole brown wash process.