Ask Greg McMillan
We ask Greg:
What role do you see dynamic simulation playing in the future of best vessel design for pH control?
Greg's Response:
pH systems are extremely sensitive to the vessel design. A poor design can make good pH control impossible in nearly all systems despite the use of the most advanced control techniques.
In vessels, there are many different internal flow patterns from agitation and many different parameters to quantify the amount of agitation. For pH control, the flow pattern should be axial, where the fluid is pulled down from the top near the shaft, circulated along the bottom to the sidewalls, and pulled back up to the top near the sidewalls. If the agitation breaks the surface without froth, its intensity level is in the ballpark for good pH control. The pattern is called axial because of the vertical up-and-down flow pattern parallel to the axis of the shaft. Baffles that are 90 degrees apart, extend vertically along the entire length of the sidewall, and are one-sixtieth of the diameter in width are recommended because they help establish the vertical flow currents to reduce vertexing, swirling, and air induction from the surface, plus they increase the uniformity of the flow pattern. Propeller and pitched blade turbines provide an axial flow pattern. A double-spiral blade and a tangential jet nozzle cause an undesirable corkscrew axial pattern because the concentration change has a long and slow corkscrew flow path.
For a vessel with axial agitation, baffles, and a liquid height that is about the same as the diameter, the equipment dead time is approximately the turnover time that can be estimated as the liquid mass divided by the summation of the influent, reagent, and recirculation flows plus the agitator pumping rate. If the ratio of the equipment dead time to the time constant is equal to or less than 0.05, the vessel is classified as a vertical well-mixed tank. Horizontal tanks have a length much greater than the height. No matter how many agitators are installed, the complete volume cannot be considered as axially mixed. There will be regions of stagnation, short-circuiting, and plug flow.
Dynamic simulations with mixing nonuniformity and dead time can show the importance of a vertical well-mixed tank.
For much more knowledge, see the ISA book Advanced pH Measurement and Control Fourth Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).