Measuring pH in liquids with many dissolved solids is a constant challenge for all types of industrial plants. For Kyokuto Petroleum Industries (KPI) Japan, continuous failure of their pH measurement in their desulfurization scrubber was costing significant time and money.
The KPI scrubber is a magnesium hydroxide system. The pH measurement is installed after neutralization and the wastewater includes many magnesium sulfate solids. The flow chamber was sometimes clogged with solids.
KPI had been using a pH sensor with a water jet cleaning system. The cleaning system, however, was ineffective against solids that coated and plugged the glass and liquid junction of the sensor. After a few days of operation KPI could no longer trust the pH values being received as the readings would fluctuate significantly. Even when the sensor was cleaned manually, a time-consuming and expensive process, the pH sensor had to be replaced every month.
As a result, KPI often had to control the dosing of magnesium hydroxide manually based upon the experience and intuition of the operators. In addition to the cost, personnel time and extreme inconvenience, the plant was faced with significant local regulations regarding emission of SOx. If the manual control led to low magnesium oxide dosing, a SOx emission could occur with subsequent fines. If the dosing was too high, the costs of the expensive magnesium hydroxide soared and increased costs.
On top of all this, the pH sensor KPI had been using was connected to an analog device which made it impossible to get sensor status from the instrument automatically. The company could not predict sensor failure or receive other data without manual intervention.
With good reason, KPI was looking for a method of applying automatic control to its set points based on reliable pH measurement.
KPI switched their pH analysis to the Rosemount Analytical Model 1056 using the PERpH-X 3300HT-10-30 with SMART pre-amp as a sensor. The PERpH-X sensors are designed with an enhanced double junction reference that is specific to extreme applications. Reference flow into the process stream is controlled using a porous Teflon® junction that can be replaced in the event of fouling or plugging. The specially designed junction is chemically resistant and has a large surface area to maintain a steady reference signal in dirty or oily applications. This reference also resists poisoning which can occur with other sensors as a result of the diluted hydrochloric acid used to manually clean the sensor during preventive maintenance.
The 3300 has been able to operate accurately for two to three weeks before manual cleaning and the sensors are able to be used more than nine months in the process as KPI also takes advantage of the rebuildable reference feature of the 3300. The 3300 has lasted nine times longer than the previously used sensor – a dramatic improvement saving time and money.
The HART-enabled pH measurement by the 3300 allows KPI to monitor other valuable information such as reference impedance, glass impedance, temperature, etc. By monitoring these sensor parameters, KPI could detect a sudden large deposit of the reference impedance. Figure 1 shows the increasing reference impedance trend as well as the result of a sudden deposit. This information is used by KPI to improve the timing of maintenance. They are also able to use temperature data to determine clogging of the flow chamber which saves both maintenance costs and prevents shutdowns. Using the Emerson 1056P-HT with a 3300HT sensor with the SMART pre-amp and jet sprayer, KPI has been able to control the dosing volume of magnesium hydroxide automatically saving time and improving efficiency. While savings in actual product costs measures in the thousands of dollars, the value of the accurate measurement is incalculable.