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By Dan Hebert, Senior Technical Editor
The final frontier in process instrumentation is moving complex measurements from the lab to online analyzers. Unlike the relatively simple measurements of physical properties such as level, flow, temperature and pressure, analytical measurements of the molecular composition of liquids and gases present a host of challenges.
We can see many of these challenges in the joint development between ABB and ConocoPhillips of an FTIR (Fourier transform infrared) analyzer for hydrofluoric acid alkylation unit (HFU) monitoring. The analyzer is used for online acid analysis in refinery HFUs.
Prior to development of the online analyzer, measurements required expensive, slow and potentially hazardous manual sampling and lab analysis of the recirculating hydrofluoric acid catalyst. In this case, analysis was necessary to determine acid strength and the level of critical contaminants, such as water and fluorination byproducts. These HF acid parameters are crucial for HFU control and optimization, provided they can be delivered quickly enough to detect process unit upsets, including transient shifts in acid strength and contamination events caused by upstream disturbances.
Delivering results quickly is another key reason to go online. In this and many other applications, analysis speed is directly related to overall performance of the control system. Unfortunately, companies often get these benefits through long, painstaking and expensive product development that frequently involves a partnership using the vendor’s analyzer expertise and the end customer’s operating experience.
For example, ABB began working with the ConocoPhillips’ R&D laboratory in Bartlesville, Okla., in 1996 to develop an online acid analysis system. Testing and development on HF alkylation pilot-scale units took two years. This included sample system design, metallurgy considerations and model development.
The analyzer was installed at the Phillips Petroleum refinery in Sweeny, Texas, in May 1998. After two more years of onsite testing, the company launched the analyzer for the HF alkylation market. The entire development process took five years—and that for an analyzer whose general principles of operation have been known for many years.
The development of an accurate, precalibrated chemometric model for the process variables of HF acid strength and percent of water and acid-soluble oil was the key breakthrough.Traditional laboratory reference techniques for these measurements are poor and do not provide a reliable basis for precise calibration.
Fortunately, the HF acid recycle stream is of relatively simple composition, a fact that allowed ConocoPhillips to develop the required universal calibration model on the basis of a gravimetrically prepared calibration standard. The model was run on a laboratory-scale pilot alkylation reactor under real-life process conditions, but without any olefin feed in order to maintain exact acid composition.
ABB developed a low-maintenance safety-engineered field sample panel that requires minimal field intervention inside the acid area. This is critical, as personnel entering an HFU require full personal protective equipment, including a self-contained breathing system.
The analyzer has a remote, field-based sample flow cell and associated safety and sample conditioning system mounted in the acid area—physically separate from the analyzer optics station, which is normally located in a control room, saving time and reducing hazards.
Reducing required maintenance and accounting for operational hazards are critical when moving analysis from the lab to an online, real-time application—an area where many vendors of lab analysis equipment fail. They understand chemical processes in lab environments, but often cannot adapt their equipment for plant use. Because of this shortcoming, traditional process industry firms such as ABB may be the ones to first cross the final frontier of online analysis.
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