Ball Valves Used for Black Liquor in Pulping Kraft Process

Many toxic, corrosive, and reactive chemicals are used in pulp and paper mills in chemical pulping. This incredibly hostile environment requires well-engineered automatic valve systems and durable valve actuators, spring units, and positioners. 

Suppose you understand how paper is made and what chemicals are usually used in the atmosphere. In that case, you can instantly understand why special consideration is required for valve and actuator selection.

In the early stage of paper production, digesters combine high heat and a heavy alkaline liquid called "white liquor" to convert wood chips into a pulp during separation from wood chips. White liquor (mainly sodium hydroxide and sodium sulfide), process water, heat, and pressure are used to separate wood chips into lignin and cellulose fibers. The white liquor breaks down the lignin (the substance that binds the wood fiber together) in wood chips. The resulting combination of water and spent white liquor produces "black liquor." This liquid pulp residue mixture made up of lignin and hemicellulose, and inorganic compounds such as sodium hydroxide and sodium sulfide is toxic. Another by-product of this process is Hydrogen Sulfide, a toxic, corrosive, and flammable gas. 

Effects on Ball Valves

In piping and ball valves, managing these process chemicals and by-products is difficult. Pipe diameters range from 2"-10", but the most common sizes are 4", 6" and 8." Usually, these processes are ANSI / ASME Class 150 and ANSI / ASME Class 300, the most common end connection being 150 # and 300 #. Process temperatures are usually high, between 250 ° F and 350 ° F. The valve body and end connection materials are generally 316 stainless steel, but you'll also find carbon steel requested. Applications can include manual lever, gear operators, pneumatic actuators, and electrical actuators in valve assemblies.

The most concerning issue is the sticky molasses-like liquor builds upon the ball valve face and contaminates the back seat spaces, inducing strong internal clearance forces, resulting in valve lock-up over long static periods. A superior ball valve seat design from A-T Controls addresses the viscous material problem.  

A-T Controls developed a solution for this extreme pulp and paper service and black liquor service with notable improvements to their FMS series. This valve incorporates a "scraper seat" feature that eliminates the molasses-like residue baked on the ball's face and eliminates valve lock-up. The addition of Aflas ® o-rings seal the seat cavity from media build-up, and a Venturi hole in the ball is also provided.

Typical black liquor ball valve from A-T Controls:

• Series: A-T Controls FMS
• Sizes: ½" -8"
• Rating: ANSI/ASME Class 150/Class 300 with 150# and 300# flanges, API 607.
• Ball and Seats: Tungsten Carbide coated metal scraper seats (Aflas® o-rings and Venturi Ball for black liquor service.) 

Please consult a Piping Specialties Applications Engineer for material selection for your black liquor application. This information is a guideline, and customers are solely responsible for construction materials, preparation of the valves for service, and lubricants being compatible with their black liquor application.

For a free consultation, call Piping Specialties at 800-223-1468 or visit their website at https://psi-team.com.

Refractometer Application in Kraft (Sulphate) Pulp Process: Digester Washing Zone and Blowline

Introduction

The first operation in the Kraft pulping process involves the extraction of cellulose from wood by dissolving the lignin that binds the fibers together. This is done in a strongly alkaline solution.

This process is known as cooking. After the wood pulp is obtained, it is washed and bleached to obtain the fibrous product.

To optimize the pulp chemical consumption and water usage, the black liquor concentrations have to be measured before and after washing.

Application

Incoming wood is debarked and chipped to an optimal size to minimize fibre damage, and to maximize the impregnation with the cooking liquor. The chips and the cooking liquor are fed into a large vessel known as digester. The pulping reaction takes place under pressure and at a high temperature.

After cooking, the pulp passes through a blow line to the blow tank and then to a washing section. The diffuser washers separate the black liquor from the fibers by washing them with a washing liquor or water. The products from the fiber line are a clean pulp, and a diluted black liquor known as weak liquor.

The washed pulp is then screened before it is sent to the bleaching plant, and the weak black liquor passes from the washing section to the chemical recovery process.

Instrumentation and installation

The K-Patents SAFE-DRIVE Refractometer PR-23-SD measures in real-time the Total Dissolved Solids (TDS) content in black liquor.

The K-Patents refractometer is installed in-line in different points after the digester. TDS measurement

in the blow pulp suspension after the digester enables monitoring of the diffuser operation. Together with other measurements (e.g. filtrate and flush liquor) this provides the mill with the ability to control the performance of the washing zone in the digester. In addition, TDS measurement in the blow line allows the performance of the digester to be monitored, ensuring that it yields the correct concentration. The combination of these measurements facilitates continuous calculation of the mass balances of the digester.

The refractometer’s measurement is unaffected by bubbles, particles, consistency, flow, ion changes, pH, temperature, pressure, color or turbulent flow. The measurement surface is periodically cleaned using an integrated and automatic prism cleaning system.

The K-Patents PR-23-SD 

Black liquor concentration measurement with the SAFE-DRIVE process refractometer helps to increase washing efficiency, obtain a consistent pulp quality, reduce bleaching chemical consumption and environmental load, and increase evaporation efficiency.

For more information, contact PSI Controls (Piping Specialties, Inc.). Call them at 800-223-1468 or visit their website at https://psi-team.com.

Process Refractometers Used in Black Liquor Recovery Boilers

The recovery boiler plays a central role in the chemical cycle of a modern pulp mill. The recovery boiler is a chemical reactor, which is used for recovering chemicals from spent kraft liquor and generating energy at the same time.

In the recovery boiler, the organic matter is burned. The dry solids liquor content required for firing is at least 60 %, but preferably more than 65 %. Black liquor is concentrated by evaporating water from the liquor. When the concentration of black liquor is maximized, so is the energy production. Before entering the burners, sodium sulfate decahydrate, or glauber salt, is added to cover chemical losses.

Application

Chemical curve: R.I. per Black
liquor Conc% at Ref. Temp. of 20 ̊C

The liquor should have a high content of combustible dry solids in order to minimize flue gas emissions and maximize boiler efficiency.

Too low concentration of dry solids fed to the burners may result in a steam explosion with consequent damage or destruction to the boiler. Therefore, it is essential to utilize a refractometer to monitor the black liquor feed to the recovery boiler to ensure a safe operation.

Instrumentation and installation

The K-Patents Digital Divert Control System DD-23 complies strictly with all recommendations of the Black Liquor Recovery Boiler Advisory Committee (BLRBAC).

The DD-23 system includes two SAFE-DRIVE Process Refractometer sensors in the main black liquor line, two indicating transmitters and a divert control unit in an integrated panel.

The sensors are installed using K-Patents patented SAFE-DRIVE Isolation valve. This allows for safe and easy insertion and retraction of the refractometers under full operating pressure, without having to valve off the liquor piping or having to shut down the process. The SAFE-DRIVE Isolation valve contains a steam wash system for automatic prism cleaning. The system contains a SAFE-DRIVE Retractor Tool SDR-23 for safe sensor insertion and retraction.

For more information about refractometers used on blacl liquor recovery boilers, contact Piping Specialties by calling 800-223-1468 or visit https://psi-team.com.