How to perform cascade control of alkali concentration

Since many factors can interfere with online density meters, cascade control is not recommended.

It can be controlled in series with the current, achieved by adjusting the alkali concentration at the outlet; the series control coefficient is adjusted based on the alkali concentration measured in the analysis chamber.

The alkali concentration is primarily controlled by the amount of water added, and this amount of water is determined through cascade control based on current and alkali density. Which is the primary variable: alkali solution density or current? When the current in the electrolyzer is increased or decreased, should the alkali concentration be controlled via ratio control or cascade control?

This post was last edited by swl on 2017-6-13 at 14:31. Question for the original poster: Is the alkali concentration in the ion-exchange membrane electrolysis process at your facility under automatic control? If self-control is implemented, there are the following questions I would like to ask further: 1. Is it simple adjustment? Still cascade control? 2. Is it cascade control? Which parameter is it associated with? 3. How is the quality of the adjustment? 4. Is there a large discrepancy between the adjustment results and the chemical analysis results? 5. At the control post, are assessments conducted for each small team? Thank you!

This post was last edited by swl on 2017-6-13 at 14:26. Hello, the original poster: Is automatic control implemented for the alkali concentration in the ion-exchange membrane electrolysis process at your facility? If self-control is implemented, there are the following questions I would like to ask further: 1. Is it simple adjustment? Still cascade control? 2. Is it cascade control? Which parameter is it associated with? 3. How is the quality of the adjustment? 4. Is there a large discrepancy between the adjustment results and the chemical analysis results? 5. At the control post, is an assessment conducted for each shift? What is the basis for the assessment? Thank you!

Our unit has implemented automatic control for ion-exchange membrane caustic soda production； 1. Use cascade control after the current stabilizes ； 2. The primary variable in the cascade control is the density of the alkaline solution, while the secondary variable is the current signal related to the slurry; the control is aimed at regulating the flow rate of pure water added ； 3. With all parameters of the electrolyzer remaining stable, cascade control is employed to maintain the concentration of the alkaline solution between 31.9% and 32.3% ； However, when the current is increased or decreased, cascade control is applied, resulting in large fluctuations in the alkali solution concentration, ranging between 31.5% and 32.5% ； 4. Based on the density of the alkali solution at the time of sampling, the corresponding concentration is obtained with little deviation in the analysis results ； 5. We require that the indicator value be kept between 31.9% and 32.2%. Samples are taken twice per shift, and the average alkali concentration is calculated. A reward of 50 yuan is given if the alkali concentration is 31.9%, 20 yuan if it is 32%, 10 yuan if it is 32.1%, and no reward is given if it is 32.2% ； A penalty of 10 yuan is imposed for every 0.1% below or above 31.9–32.2%. When the value is 0.5% below or above the specified target, an incident analysis meeting is held as a quality issue, and the responsible persons are subject to disciplinary action.

Thank you to the original poster for the detailed explanation. I have a few more questions I’d like to understand in more detail: 1. The control range you mentioned is between 31.9–32.3%, is this the online concentration? What is its final concentration (concentration at shipment)? Does it correspond to the online concentration? 2. Does the average alkali solution concentration obtained from sampling twice per shift correspond roughly to the online concentration at the same time period?

1. 31.9–32.3% is the analytical concentration; our online concentration is measured using an alkali density meter. We have prepared a reference table for alkali density at different temperatures, and the concentration at the time of shipment is close to the value indicated by the online density meter. 2. When the current remains stable, the average alkali concentration obtained by taking samples twice per shift corresponds roughly to the online concentration during the same period. During changes in the current level, the readings from the online alkali density meter fluctuate significantly; the operators on site analyze these trends and adjust the amount of water added in order to keep the alkali concentration within the desired range.