Analytical instruments for the process industries
31 March 2008
When there is a need to monitor a process variable that extends beyond pressure, flow, temperature, and level, some type of process analyser is involved. There are four categories of analytical instrumentation
Figure 4: Swagelok concentration monitor
Electrochemical—Measure specific ion concentration, most commonly hydrogen (pH);
Composition—Detect and measure specific chemical components in the process stream;
Spectro-photometric—Use light absorbing characteristics to detect and measure specific components; and,
Physical property—Measure specific gravity, density, viscosity, etc.
Since manufacturing processes are generally designed for specific functions, the composition or characteristics of a given product should fall into relatively narrow bands. For example, while a broad spectrum analyser that can break down any unknown substance into its component parts may exist in a laboratory, such are not generally practical in a real-life production environment. Gas chromatograph and mass spectrometer devices can quantify a wide range of substances, but their cost and relative complexity make them a choice when simpler technologies can’t do the job.
‘Customers ask us about reliability more than accuracy or precision,’ says Gary Brewer, product manager for ABB’s process automation division. ‘Manufacturers don’t usually have large support staffs, so if the analyser goes down, the whole process can go down. So the rule of thumb is to apply the simplest technology possible that will get the needed measurement.’
CHOOSING AN ANALYSER
When trying to quantify a component or contaminant in a product, it is critical to know what substance you’re looking for. In situations where more than one test has to be performed, more than one analyser or analyser technology will likely be involved.
Consider an example where the task is to analyse effluents in flue gas from a boiler, determining quantities of sulphur dioxide (SO2), nitrous oxides (NOx), acid gasses, and mercury. SO2 can be measured by infrared absorption, non-dispersive infrared, or ultraviolet. For NOx, use chemiluminescence or ultraviolet. Mercury calls for ultraviolet. Acid gases may require a gas chromatograph or mass spectrometer. Even though ultraviolet may work for three of the four, the situation will likely require more than one sensor, perhaps even a separate sensor for each substance. Multi-component sensors are available, but there are trade-offs with cost and complexity.
Figure 1: Yokogawa
Some relatively simple tests, such as pH or dissolved oxygen, can be handled by a probe inserted into a process stream. However, few analysis technologies are this simple. Most involve moving a sample of the product to a device where it can carry out a more complex analytical operation. Analysis becomes, in effect, a batch operation where a sample is extracted from the process stream and checked, just as if it were carried to a lab manually. This process can be automated and happen at appropriate intervals. Methods for designing piping to move samples to the analyser is another issue. In some cases to save cost, one device can serve more than one process line, as long as it can handle the span of variables required. Technologies have their own cycle time for analysis. Near infrared is fast enough that it can be essentially continuous, while a gas chromatograph can take 30 minutes to complete a test.
The key to selecting an analyser, like any piece of instrumentation, is to understand your process and what information is critical to the larger control strategy.
NEW IN PH
Yokogawa’s new unbreakable sanitary pH3A sensor (Figure 1) is designed for use in food and pharmaceutical industries where conventional pH sensors cannot be used because of the risk of product contamination from broken glass.
It combines the robustness of steel with the chemical resistance of Pfaudler PharmaGlass (PPG) coated enamel, specifically developed to increase the surface smoothness to eliminate product build-up, thus reducing the need for cleaning.
The high chemical resistance of the sensor makes it suitable for CIP (cleaning in place) and SIP (sterilisation in place) cleaning regimes. Usually, pH sensors have to be removed during these procedures.
Unlike conventional glass electrodes, the pH sensitive enamel only makes contact with the fluids on the outside of the probe, making aging of the probe impossible. As no air bubble can be present on the inside, the probe can be mounted easily in any direction (even vertically) in most pipeline and vessel applications.
The diaphragm in the new sensor consists of a ground ceramic disk shaped like a hockey puck, which is shrink-fitted into the end of the probe tube. The contact area between the ceramic puck and the ground glass surface forms the diaphragm gap, allowing electrolyte penetration to the product. The pressurised reference junction prevents the product and the electrolyte from mixing, making clogging and poisoning practically impossible.
The use of enamel glass, in combination with the pressurised reference junction, results in an extremely reliable and accurate measurement over the sensor's whole range of 0-10 pH.
PH IN CHEMISTRY
Endress+Hauser has secured the attention of the chemical industry following the successful installation of its Cleanfit CPA472D retractable assembly for heavy-duty pH processes (Figure 2) in the Rhine-based chemical plants of Bayer and Lanxess, and will now offer the assembly as a standardised heavy-duty range. The unit meets not only the special requirements of the chemical industry with its wide-ranging resistance to chemicals, but also provides a good level of pressure stability at pressures up to 16 bar and at temperatures up to 140ºC.
Figure 2: Endress+Hauser
Analogue standard or digital Memosens electrodes can be used, reaching effective immersion depths of 125 or 260 mm.
The process assembly is constructed in modular form with interchangeable component materials, so chemical factories can order from six different materials suited to their specific applications. An optional sight glass gives operating staff the possibility of visually monitoring the installed sensor.
An interesting safety feature of the CPA472D is a patented locking device which blocks the automatic entrance into the process if a sensor is missing. This assembly is therefore suitable as a universal solution in safety-related areas such as in production plants for titanium dioxide, plant protectives or biodiesel.
FOUR ELECTRODES FOR CONDUCTIVITY
Rosemount Analytical PUR-Sense™ Model 410VP conductivity sensor is a four-electrode unit designed for sanitary applications (Figure 3). Emerson says it offers the industry’s widest conductivity measurement range, providing good linearity between 0.1 µSiemen/cm to 600 mSiemen/cm.
Typical applications include monitoring the concentration of high conductivity fluids such as CIP solvents and low conductivity rinses. It can also be used for monitoring eluents (substances used as a solvents in separating materials) in chromatographic separations, and for the detection of liquid interfaces—all with the same sensor.
To make it work, an alternating current is injected through the outer electrodes and an analyser connected to the sensor measures the voltage across the inner electrodes. During operation, the current is continuously altered to keep the voltage constant. Thus, the injected current is directly proportional to the conductance of the solution. Because the voltage measuring circuit draws almost no current, series capacitance and cable resistance—which cause considerable error in two-electrode measurements at high conductivity—are essentially eliminated.
Swagelok’s CR-288 (Figure 4) is said to be the first compact device in the industry to provide in-line analysis of liquid chemical concentration with results that are repeatable within 0.01%. It was originally designed for the semiconductor industry. The software allows calibration for different chemical mixtures and also provides a graphical readout that can be customised for data logging. Response time is about one second.
Figure 3: Rosemount
To work, the instrument uses an LED light source, a mirror, a sapphire and a photodiode-ar5ray detector to measure refraction. Light is beamed at a sapphire window that is in contact with the process liquid. A reading is taken as the light bounces back, enabling software to determine the index of refraction for the liquid in question. That index of refraction may then be compared to the index of refraction for the desired blend or concentration of chemicals. The only components that come into contact with the liquid are the sapphire window and the body of the sensor, which is made of ultrahigh purity modified PTFE. A thermocouple inside the optical fluidic cell takes a temperature reading. A liquid’s index of refraction is a function of temperature.
A MODULAR PLATFORM FOR SAMPLING SYSTEMS
Swagelok® modular platform components, together with configurator software, are used to lay out and assemble complete process analyser and sample-handling systems in miniature modular design. The approach allows system designers to reduce the size, weight, and flow path volume. The components consist of substrates, manifolds, and various surface mounted valves such as shut-off, needle, metering, toggle, and check valves, as well as pressure regulators and filters. Configuration software allows the user to place, define, and connect surface mount components on a computerised layout grid. It identifies all of the additional flow connectors and even generates a bill of materials and assembly diagram to simplify ordering and final assembly.
—Peter Welander, Control Engineering, contributed to this article
Figure 1: Yokogawa’s unbreakable, enamel-coated pH sensor.
Figure 2: E+H’s retractable assembly for heavy duty pH measurement in the chemical industry
Figure 3: Four electrodes are better than two: Rosemount’s conductivity sensor
Figure 4: Swagelok’s compact composition monitor.
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