Hi — my name’s Dave Anderson. This week I’m blogging about pain points in pH and, from an analytical perspective, there are a number of them within a biopharmaceutical plant where effective analytical monitoring can improve operations. First and foremost, the Process Analytical Technology (PAT) Initiative has opened the door to take traditional off-line, laboratory measurements and put those methodologies in the process or at the process. The customers benefit through real time measurements of key parameters. By automating the process, customers can capture the data as well as document the calibration logs electronically.

One particular pain point is with pH control in bioreactors. Customers are asking for tighter pH control, some as tight as +/- .01 pH units. One customer mentioned if they can improve pH tolerances by 10%, they estimate savings of $250,000 per batch.  The problem with maintaining tight pH measurement is how the sensors are used in the process. Prior to a batch, the pH sensors are calibrated with pH buffers to ensure the sensors will accurately respond to real process changes. After the sensors are calibrated, those chemicals must be cleaned from the sensor. The typical cleaning process is to steam sterilize the vessel at a minimum of 121º C for 30 minutes.  Some processes require steam cleaning at 145º C for 60 minutes.  The pH-sensitive glass will become stressed from the exposure to high temperatures and subsequent cooling. This stress can result in significant offsets in the pH readings or critical failures in the form of cracked glass. Because of these concerns, redundant sensors are often utilized.

To optimize plant productivity, there are new pH sensors with Accuglass® technology that can withstand the thermal stresses from the cleaning process. For example, comparison testing with competitive sensors proved the Model 3800 steam sterilizable pH sensor will withstand more steam-in-place cycles than any sensor on the market.

Additionally, today’s transmitters can deliver real-time diagnostics at the operator’s work station. Monitoring one pH loop locally at a transmitter is simple, but when operators have hundreds to thousands of I/O points with different measuring technologies to manage, it can be overwhelming. Diagnostic information such as “glass impedance” and “zero offset” will update the operators whether a sensor has a critical failure or is beginning to drift. For those operators with less experience, there can be Embedded Help Menus that explain in detail what the conditions mean and appropriate actions to take. This is easy to access in AMS® Suite by clicking and dragging the question mark over the field in question.

These two technologies allow plants to confidently automate pH measurement, greatly minimizing operator interface. When an operator needs to get involved, these tools allow them to make informed decisions with the assurance of accuracy and stability in the process.

9 Jun, 2010  |  Written by  |  under Combustion

Hello, my name is Doug Simmers, and I’m worldwide product manager for combustion flue gas analyzers at Rosemount Analytical. Although I’ve been involved with process instrumentation and control for 25 years, my initial experience came from marketing distributed control systems. This gave me a rich exposure to many different applications, from power plants to refineries, to pulp and paper mills to water and wastewater plants. To me, an analyzer was just another bubble on the piping and instrument diagram. An engineer would establish a control loop around this bubble, and –“done!” On to the next loop. On startups, however, I noticed that the analytical loops were frequently left in manual and no one could provide a clear answer as to why. Process engineers appeared to consider the measurements critical for the good control of the process. The control logic didn’t appear to be particularly challenging. Final control elements (usually fans and dampers) didn’t appear to be a problem.

Closer investigation uncovered an inherent mistrust of the analyzers themselves. As I moved away from the DCS world and got more deeply involved in analyzers, I came to understand that this mistrust was not always undeserved. Over the years, I’ve made up a list of the most common negative concerns made about process analyzers:

  • “Analyzers drift too much”
  • “Analyzers require too much maintenance”
  • “Speed of response is too slow”
  • “We put in redundant analyzers, and they never agree”

Every operator had his or her own horror story about a process upset caused by an analyzer that wasn’t working right. The stakes are high when controlling most processes, and instruments that cause upsets are not endured for long. As a class of instrument, analyzers are found to be abandoned in place more frequently than instruments taking physical measurements, like pressure, temperature, flow and level.

As I learned more about analyzer technology, I came to understand the challenges and principles that cause analytical measurements to be mistrusted:

  • Analytical measurements are inherently more difficult than physical measurements, though very important to process optimization.
  • Analyzers measure the actual composition of a process gas or liquid — not just its temperature or pressure.
  • Analyzers drift because they are frequently in direct contact with the process. While physical measurements are often protected from the process (thermowells for thermocouples and isolation diaphragms for pressure transducers), analyzer sensing cells have to endure the temperature and acidity in process fluids.
  • Maintenance is often high because some analyzing technologies such as infrared spectroscopy require sample conditioning systems that are more costly and maintenance intensive than the analyzers themselves. Sample conditioning systems also reduce the speed of response.
  • There can be complex interferences — one component of a gas stream can affect the measurement of another component.
  • Analyzers are more difficult to understand than most common process instruments

So it’s clear that analytical instruments are often mistrusted, but there are reasons for this mistrust that can be overcome by better understanding of how they work, and what to expect from them. For example, unlike many other measurements, most gas analyzers can be calibrated automatically on-line with the process running, with calibration gases sequenced to the sensors and adjustments made via microprocessors.

This will be an underlying theme of this blog — improving the understanding and the resulting confidence in combustion flue gas measurements.

Despite all of the problems (real or perceived), one thing is clear — a good analytical measurement provides direct indication of the process composition, which reduces process variability, improves efficiency,  maximizes throughput, and provides a great diagnostic of process equipment degradation failure.

Nothing optimizes a process better than a reliable analyzer!

Future Blog Subjects

  • History of combustion flue gas analysis
  • Operators are the real customer
  • Rethinking hazardous areas
  • Furnace diagnostics
  • Deep dives into how analyzers work