Pulp & Paper

 

A Method for Finding Black Liquor Viscosity based on Droplet Break-up

Black Liquor is the spent liquor from the process of pulping wood. It contains dissolved wood components with a mixture of water and inorganic salts. Each component plays a significant role in determining the properties of black liquor. Knowledge of these properties is necessary  to accurately assess the black liquor flow, heat transfer, spray, and droplet combustion characteristics.

Simulent Consulting was asked to provide a methodology to determine the black liquor properties.  The methodology proposed was based on a jet/droplet breakup model which combines the pendent droplet technique with numerical simulation.

Black Liquor splash plate nozzle

Spray characterization of a Splash plate nozzle

Black liquor is the by-product of the pulping process containing water, organic matter separated during cooking and inorganic cooking chemicals.  Black liquor is burned in recovery boilers and is usually sprayed by splash plate nozzles.  Because the drop size and the size distribution are both of great importance for the control of the combustion process in the furnace, it is important to understand the affecting processes in liquor spraying.

Simulent Consulting was asked to develop analytic models based on the sheet instability analysis to obtain the initial droplet size of the black liquor in a splash plate nozzle, the mean droplet size for the black liquor spray generated by a splash plate nozzle and the spray characteristics such as mean droplet size and spray cone angle of a pressurized swirl nozzle. The nozzle used was a commercial nozzle to spray black liquor into a pressurized gasifier.

A two-phase free surface flow algorithm was used to obtain the liquid sheet thickness and velocity at the exit of the nozzle and validated with experiments under the same conditions. The liquid flow inside the nozzle was first modeled using a single fluid model.  A modified instability model was developed to obtain the initial mean droplet size and velocity of the droplets at the exit.

The most important parameters affecting black liquor spraying and drop formation are the operating temperature, pressure, liquor viscosity and density.

Further development enhancements of the above models include:

  • Airflow simulation in the vicinity of the splash plate nozzle;
  • Turbulence effects;
  • Heat Transfer inside the nozzle, and between liquor flow and air;
  • Evaporation of black liquor;
  • Flashing effects;
  • Parallel processing

Modeling Spray Characteristics and Industrial Spray Nozzle

Simulent Inc. develops models for simulating the spray and atomization process for different nozzles.  We have also developed software for calculating the spray characterization in splash plate nozzles, swirl nozzles, and air assisted nozzles.

Simulent Consulting was asked to obtain the characteristics of the spray at the exit of a swirl nozzle. To do this Simulent used one of our developed models for simulating the spray characteristics in a pressurized swirl nozzle.

Applying the model through four stages of simulation and with a series of experimental tests performed under the same conditions for water the numerical results of the simulation showed no significant differences when compared to the experimental results.  This showed that our models are correct and reliable.

Optimization of a Black Liquor Nozzle Design

Simulent Consulting Inc was asked to propose a new orifice in the optimization of a Black Liquor Nozzle design.  By simulating several different profiles for the core region and using optimization analysis the best profile was found for the core region. Studies showed that the  design of the existing nozzle was a source of pulsation inside the nozzle causing the production of a very wide range of droplet sizes which made the nozzle’s behaviour unpredictable. Also depending on the amount of flow pulsation the spray angle was usually very large.

The new orifice proposed by Simulent not only reduced the pulsation effects observed in the existing nozzle but also reduced the non-uniformity effect and the spray angle of the nozzle. This created a novel design as was demonstrated by several simulations examining the Black Liquor flow from its inlet to the orifice.