Evaporation Process Improvement, Service, and Training. Let me help you
Angola, New York 14006, United States
Controlling Density
Typical deterioration of finished solids caused by progressive fouling and controlling it.
Typical deterioration of finished solids caused by progressive fouling and controlling it.
Typical deterioration of finished solids caused by progressive fouling and controlling it.
This graph illustrates how fouling affects finished product density over time and how it can be controlled by increasing the heat input. I call this the glide slope and natural progression of fouling over the course of a campaign. Note that this can only be possible if your process has steady state feed flow, feed density, condensate removal and non condensable removal as they should. This type of control is a simple cascade increasing adjustments to the steam setpoint with a steadily progressive change responding to the measured changes in density. This control loop can only be enable once density is at a steady state. Enabling it while the density is on the rise or fall during initial start up will result in oscillations. If your other process variables are not in control and at steady state they must be corrected for a density cascade control scheme to function properly
Typical causes of density oscillation
Typical deterioration of finished solids caused by progressive fouling and controlling it.
Typical deterioration of finished solids caused by progressive fouling and controlling it.
This illustrates the affect that oscillations or poor control of feed flow in blue or condensate removal in green will have on finished density. Notice that the cycle is identical to that of the density in red. The increase and decrease in flow would have this affect if the heat input was at a steady state. In order to control densities natural tendency to glide slope as fouling occurs, these oscillation conditions must be identified and corrected first. No density control scheme will correct for these problems. They will only make it worse.
Feed flow is easy to correct with loop tuning, proper placement of the flow elements, control valves, and a good feed pump seal.
Condensate removal that oscillates is generally caused by either a pump mechanical seal leak or a check valve leak.
Change is slow when controlling density
Typical deterioration of finished solids caused by progressive fouling and controlling it.
Change is slow when controlling density
As shown here in a three effect evaporator. It takes upwards of 15 minutes for the product to pass from the feed tank to the density meter at the end of the process.
Therefor, a slow reacting control loop is essential. The integral relationship must be adjusted to match this time element in the tuning parameters. This must also be understood when making manual adjustments by operators.