Modular solutions

Automated on-line process analyser systems provide a real-time view of the process and the capability to make rapid adjustments to ensure better process control. Analyser systems can be relatively simple, such as a system to monitor moisture in a single hydrocarbon gas stream, or complex, such as an extensive multi-stream system to monitor process fluid composition at a number of points in the process.

In either case, sampling systems extract, collect, and possibly precondition a sample, transport it to the analyser, condition it for introduction into the analyser and dispose of spent samples or return them to the process line. Such complex systems consist of valves and other fluid control devices: filters, gauges, and transducers, heaters and coolers, fittings, tubing and pipe.

In the past two decades, analytical devices have become more capable and reliable. However, there has been little change in the basic design of the sampling system. Operators estimate that these can represent up to 80 per cent of the problems associated with process analyser systems, and there is a consensus that they need to be improved. One important way to do this is by making sampling systems miniature, modular, and intelligent.

The smaller size of modular systems makes them easier to assemble and install, and allows operators to work on them on a bench, rather than a rack. Tools such as system configurator software reduce the development and layout time and costs. The reduced size, weight, and footprint also make it easier to couple the sample system to dedicated field-mounted analysers and place them at the sample point.

This reduces the need for long, heated sample transport lines. They also have less internal surface area than a traditional system, reducing the amount of possible adsorption of material from the fluid, and their smaller internal volume makes it easier to purge or flush the system and conserve expensive analyser fluids.

The Centre for Process Analytical Chemistry (CPAC), a joint industry-academic research consortium located at the University of Washington and including industrial affiliates, who are all end users or suppliers of process analytical chemistry instrumentation, has moved forward with a New Sampling/Sensor Initiative (NeSSI). The purpose of the Initiative is to 'facilitate the state-of-the-art evaluation and ongoing development of the next generation modular sampling system designs.'

Along with NeSSI, CPAC issued a request for proposals for design concepts for the new systems to all interested parties. The request included drawings of six systems actualy in use in process plants. They ranged from a relatively simple system to a complex eight-stream system. Potential suppliers of such systems responded with various proposals. Swagelok developed six concepts using modular technology originally developed for speciality gas handling systems used in the semiconductor industry.

One of the six systems, which measures ppm H2O and O2 in a high-purity hydrocarbon stream, was built to demonstrate that the modular concept was feasible and practical. It is a single sample stream system, incorporating an instrument air supply to purge the sensors and a nitrogen gas supply to calibrate individual sensors (zero gas). The complete system measures 24.6x75.2x19.0cm, which is considerably smaller than panels and enclosures of traditional design.

The system uses modular technology with small flow components positioned in a substrate channel. Ports located side-by-side in the channel represent the inlet and outlet ports of functional flow components, for example, valves, regulators or filters, that will be surface mounted on the substrate channel. Sequential flow components define the flow path through the system. A drop-down flow component allows different sample streams, purge gases, flushing solutions, or calibration and/or validation fluids to be introduced into the main flow path.

Real-time on-line analysis requires the sample systems to be automated and integrated into the overall plant operating system. The systems described above, known as NeSSI Generation I systems, resolve the physical issues of this using commercially available surface mount functional components and control solutions. Functional components, such as pressure transducers, thermocouples, and mass flow controllers, are connected through I/O modules to a controller, using standard 4-20mA signalling.

CPAC has developed a specification for NeSSI Generation II systems that would allow intelligent control of sample systems. Key features of these systems include compact and smart pressure, temperature, and flow sensors, smart valves with built-in electric-to-pneumatic actuators, multi-drop sensor bus communications, which may be wireless and a sensor actuator manager (SAM) to provide control and connectivity to the sample system.

While Generation I systems can use commercially available I/O-based control solutions, NeSSI Generation II systems will use a fieldbus-based control architecture. When integrated into a system, these devices will be able to talk to each other through the SAM. Many of the devices required to build NeSSI Generation II systems are not yet commercially available, but suppliers are working on their development. Moreover, CPAC is now beginning to define the concepts for Generation III sampling systems, which are expected to incorporate micro-analytical 'lab-on-a-chip' devices.

Dave Simko is manager for marketing resources at Swagelok in Solon, Ohio