(31e) Using Radioisotope Techniques for Process Investigations

In all chemical plants and refineries there generally is not enough instrumentation in the right places in
our processes. During the design phase, decisions were made between knowing more about the process,
which means buying more instruments, and saving capital funds. When a problem appears with the
operation of a vessel, there are always gaps in the knowledge of what is happening due to a lack of
instruments in the places where knowledge is not routinely required. Process engineers and operations
personnel need to know what is happening so they can take corrective action. When knowledge is
missing, they often take actions that make matters worse.

For distillation and separation processes Chemical Engineers have historically relied upon plant process
measurements such as flow rates, temperatures, and pressures and model or simulation results for data
with which to solve troubleshooting or re-design problems. An array of on-line diagnostic services are
available that can provide additional data offering real-time information on how pieces of process
equipment are actually operating. The most common application is Gamma Scanning. This test is
primarily applied, but not necessarily limited to, distillation or separation columns. Gamma scans
provide a density profile of the internal process of operating distillation columns and other process
vessels. The density profile can be used to diagnose the hydraulic operating conditions of mass transfer
devices such as damage to internals, flooding, degree of entrainment or weeping, liquid levels on trays
and distributors, liquid distribution through packed beds, etc. The presenter will show case studies
where scanning revealed vital process information that helped solve an operating problem, or helped
make a revamp/re-design successful. Additional derivatives from gamma scanning of detecting flow
distribution patterns will be demonstrated. The ThruVision Scan technique is a specialized horizontal
gamma scan used to generate a topographic profile of the internal cross-sectional density of process
equipment. This profile is useful for the detailed study of liquid flow distribution through packed
columns as well as having applications beyond distillation columns. A hybrid application involving tracers
and scanning on fixed bed catalytic reactors will also be discussed.

Another technique involving radioactive materials is the Tracer Test, in which a radioactive tracer is
introduced into the process for investigating the process flows. External radiation detectors are placed
at strategically chosen locations outside vessels and piping to monitor the tracer flow through the
process. Either liquid, gas or a solid tracer can be utilized depending on process conditions and the fluid
to be studied. This tracer technique can identify liquid or gas maldistribution in fixed or fluidized bed
reactors. The residence time through a tank or reactor can be measured and compared with the
calculated residence time. Observed differences can relate directly to the degree of mixing or channeling
inside the reactor. These tracer techniques have also been used in identifying leakage of heat
exchangers and valves.