In-line Analytical Measurements Provide Intelligent pH Management

Real-time measurements aid quality assurance and batch traceability.
By Stefan Bardeck | February 15, 2011

In bioethanol production, more and more parameters are being measured directly in the process. Advantages of these real-time measurements include better plant control and the fact that in-line measurement data facilitates quality assurance and batch traceability. A new, intelligent concept for predictive maintenance increases system availability and considerably simplifies operation of measurement equipment.

The adjustment of pH value plays an important role in process steps involved in bioethanol production. If enzymatical processes are involved in converting starch from different feedstock crops into sugars, for example liquefaction and saccharification, tight control of the pH value results in higher yields. The same is true for yeast fermentation processes in which sugar is converted to alcohol as well as for other alcohol-producing fermentations. In yeast recovery processes, pH is lowered to a defined level keeping the recovered yeast in a healthy condition by killing unwanted bacteria.

Requirements for an in-line measurement device can be divided into two categories; those for measurement performance, and those for usability and maintenance. For in-line pH measurement, the first category includes accuracy, repeatability and integrity of the measurement value, electrode lifetime and the least possible variation in these quantities when the electrode is subjected to cleaning and sterilization-in-place cycles. The second category includes simple installation and commissioning, and ease and safety of calibration and maintenance. With modern technology, this category can also include device diagnostic functions which provide information on the current operational status, and can be used to predict the remaining electrode lifetime based on the electrode load history. These functions enable operators to plan and carry out need-based maintenance, which also leads to lower maintenance costs and a reduced risk of plant downtime due to a failed measurement device.

Designs for Harsh Conditions

Intelligent sensor management (ISM) combined with an electrode designed for the demanding applications in the bioethanol industry provide functionality while meeting the extremes in pH, pressure and temperature. Desirable design elements include high-alkali-resistant membrane glass to safeguard precise measurement regardless of the pH level and pre-pressurized liquid electrolyte to minimize the effects of diaphragm clogging and ensure long service life.

ISM is a new platform based on sensors with embedded digital technology that set a new standard in pH management. The key to ISM is a microprocessor integrated within the sensor head, which is powered by and read through the connected transmitter Critical electrode information such as sensor identity, calibration data, time in operation and process environment exposure are all recorded by the microprocessor. The data is utilized to continuously monitor the condition of the sensor and recommend appropriate actions based on actual sensor exposure to process conditions. The adaptive diagnostic information directs maintenance, thereby reducing downtime and plant operating costs. Further, the output signal of the electrode is digital, which secures integrity of the measurement value, even under the toughest conditions.

Plug and Measure

Installation of a new pH electrode typically requires calibration as the first step which may involve bringing pH buffer solutions out to the field and performing a two-point calibration. In addition to it being time consuming, this critical step may need to be conducted outdoors under adverse weather conditions, or when exposed to an uncomfortable or hazardous process environment. One particularly useful ISM function is that electrodes can be calibrated offline using iSense Asset Suite software via a simple USB connection to a laptop or desktop computer This means electrodes can be precalibrated in the laboratory, and stored until they are required. Such functionality eliminates the need to carry cleaning agents and buffer solutions through the plant to do tedious on-site calibrations. Precalibrated sensors carry all configuration data on the integrated microprocessor and share it with the transmitter immediately upon connection. This avoids configuration errors and ensures the electrodes are immediately ready for measuring.

Enhanced Diagnostics

Advanced diagnostics provide a continuous flow of status and maintenance information to the transmitter. Along with monitoring glass and reference impedance, ISM bases its diagnostics on actual process exposure history. By constantly keeping track of process pH value, temperature and operating hours, ISM calculates when sensor calibration, cleaning or replacement will be needed. Any maintenance requirement is therefore recognized at an early stage. All data is easily accessible through the transmitter’s user interface. The same data is also available via HART protocol on Foundation fieldbus or Profibus networks.

Exposure to the aggressive process conditions found in bioethanol production directly impacts the lifetime of pH electrodes. Since all ISM sensors contain a temperature sensor for pH compensation purposes, historical temperature exposure, pH values and other process conditions can be used to forecast remaining useful life. Mettler Toledo’s Dynamic Lifetime Indicator uses a proprietary algorithm to do just this, and is displayed on ISM-compatible transmitters. The ability to accurately forecast a sensor’s remaining lifetime, decreases maintenance costs associated with unnecessary sensor replacement and minimizes unexpected process shutdown due to sensor failure. By continuously monitoring reference impedance, pH glass impedance and temperature exposure, the adaptive calibration timer displayed on the transmitter, predicts when calibration is next required in order to maintain measurement reliability.

Automated Maintenance

Another desireable feature is an automatic cleaning and calibration system, particularly in cases of extreme electrode fouling. Using it in combination with a retractable housing, a well-designed system automatically extracts the pH probe without interrupting the process and without the need for a slipstream. Inside the housing, the probe is automatically cleaned and subsequently calibrated, after which it is re-inserted into the process.

Thanks to advanced electrode diagnostics and plug and measure capability, intelligent pH measuring systems facilitate maintenance and calibration procedures when compared with conventional pH system. At the same time, the reliability of the measuring instrument is increased as well. These features lead to substantial benefits when pH in-line control is implemented for optimizing processes in bioethanol production. In addition to the pH measuring systems described here, the Intelligent Sensor Management concept has already been expanded to other parameters including dissolved oxygen, conductivity and turbidity.

Author: Stefan Bardeck
Market specialist, Mettler Toledo Process Analytics,
Switzerland +41 44 7296 211