How pump control using a mass flow controller can optimize our detergent color

June 16, 2020 Ron Tietge

Using pump control to dose undiluted dye

The major household companies around the world use liquid colourants (dye) for adding color to their detergents, laundry care products and cleaning products. The colorants are very concentrated and often only a small amount is needed to give the right color to the end-product.

The two most important requirements here are:

  1. Repeatability; a consistent color of the end-product is of major importance, as every bottle has to be the same color. Therefore, color dosing has to be very consistent, always.
  2. Accuracy; as colorants do not tend to be cheap, an accurate dosing of the colorant is very much desired.

What would be an appropriate way to dose these liquid colorants?

The first solution which comes in mind is using a pump to dose the colorant. However, this is not a very accurate way of dosing, at least not for this application. Combining a pump with a Coriolis mass flow controller will give you the opportunity to control the pump and therefore a more accurate way of dosing is possible.

mini CORI-FLOW with Tuthill pump
Combination of mini CORI-FLOW with Tuthill pump

How does pump control work?

Flow generated by a pump is volumetric flow with all characteristics that come with this. The accuracy will be heavily depending on the fluid properties, making the flow temperature and pressure dependent. Furthermore, most pumps cannot handle a fluctuating back pressure as this creates slip, instability and a high deviation in flow rate.

A method to improve the accuracy is by measuring the flow of the pump with a Coriolis mass flow meter, since these instruments measure true mass flow (such as kg/h, g/h etc.). The output of the meter is often connected to a PID controller which in turn corrects the frequency of rotation (RPM) of the pump.

A disadvantage of this method is that the flow of a pump is not stable which makes filtering of the Coriolis output signal necessary, causing a slow output signal. Because this signal is used as control signal as well, the result will be a slow response and slow control.

​Using a flow controller for pump control

To avoid this, you can use a Bronkhorst Coriolis mass flow meter or Ultrasonic volume flow meter with integrated PID controller with a separate control signal controlling the speed of the pump. Because this control signal is, unlike the output signal, not interfered by the filter, the combination of flow meter and pump can reach unmatched control speed and stability. The flow will be immediately corrected if there is a change in back pressure by making the pump run faster or slower almost instantly, keeping up the required accuracy.

This method of flow measurement is independent of fluid properties and therefore there is no need for recalibration or working with conversion factors in case of a fluid change.

Ultrasonic instrument with pump control

Low flow pump control

In our example of dosing liquid colorants, an (ultra) low flow Coriolis mass flow controller will give the benefit of dosing undiluted dye. It makes it superfluous to mix the colorant with water to create a high enough flow. Not only is undiluted dye much more concentrated and therefore higher in quality than mixtures with water, also savings on water consumption can be achieved and algae growth within the equipment can be avoided.

Video explaining the Coriolis mass flow measuring principle

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