Robust Valves to Handle Black Liquor Efficiently and Effectively

Pulp mills that use a kraft process to convert wood into wood pulp must invariably deal with black liquor. As this by-product of the digester has several challenging properties, processing it can have a significant impact on valves.

By Nilou Baniassadi, Pulp & Paper Market Lead – Armour Valve

Evaporation Plant Efficiency

For black liquor to be combustible, it must reach at least 60% dryness. As the liquor leaves the washing station, it is made up of about 15% dry substances, so it must be pumped into an evaporation plant to be concentrated.

Ideally, operators want to evaporate black liquor in the most cost-effective way. Connecting several evaporators in a series and running the liquor through them in sequence has several advantages, as each evaporator handles a portion of the temperature change. About 0.2 tonne live steam is sufficient to evaporate 1 tonne of water from the black liquor; the steam from one evaporator can be used in the next to reduce how much steam needs to be produced.

What is Black Liquor

Black Liquor is a combination of lignin, hemicellulose, spent chemicals, and water and is comprised of 15% solids, 40-45% soaps, 35-35% lignin, and 10-15% other organics. Approximately 7 tonnes of black liquor are produced in the manufacture of one tonne of pulp. Once produced, black liquor must be pumped to an evaporation plant where is it is concentrated. It is then burnt to produce energy and recover the cooking chemicals. The strenuous nature of this process puts a significant amount of demand on the valves involved. As black liquor is thick, sticky, and has a high solid content, robust valves are necessary to avoid failure.

Valves for Evaporation of Black Liquor

So, which control valve should be used during the evaporation process? In the early stages of the process, butterfly valves, such as the Somas eccentric, are a wise choice because they can handle the black liquor until the substance exceeds 45-50% solids. Once the liquor is evaporated beyond this point, ball segment valves are a better choice to handle the higher solid percentage and additional physical demands of the partly evaporated substance than the butterfly valve disc design.

Valves for Black Liquor After Evaporation

Which valves can handle the stresses of bringing the concentrated black liquor to the recovery boiler? At this point, the solution is heavy and may have up to 70% solids, combined with salt cake and sulfur. This is an exceptionally clingy and thick substance that presents challenges for any valve type. Common failures include build-up around shafts and seats which cause valves to stick or leak.

Somas offers a ball segment valve, configured specifically for black liquor applications that is ideal at this stage of the process. In this configuration, a PTFE shaft seal prevents the black liquor from entering the bearing area. A shaft bearing ensures smooth movement of the shaft during operation, and further protects the shaft in the event that any liquor does enter this area of the valve.

There is also a risk that the media will enter behind the seats and block the operation of the valve. To account for this, locked in seats are designed into the valve. Further, valves that are in contact with black liquor should be equipped with heat tracing. Keeping the valve body warm will prevent the black liquor from solidifying and making the valve inoperable during the startup of an unplanned shutdown.

Importance of the Recovery Boiler

After it has been evaporated, black liquor is burned in a recovery boiler, which serves three key functions:

• Eliminates black liquor: The recovery boiler destroys the organic matter in black liquor and is an environmentally-sound method of dealing with this by-product.
• Generates steam: The recovery boiler creates all of the steam needed in a kraft pulp mill. The steam turbine produces electrical power for the mill.
• Creates chemical reactions: The boiler also produces sodium carbonate and sodium sulphide (Na-2CO3 and Na2S).

In the recovery boiler’s lower furnace, the final sulphide compound is created from the various organic and inorganic compounds in the black liquor. This compound is an active kraft pulping agent. The measure of success of the chemical recovery process is the reduction efficiency.