Revamps 2022 Issue

As shown in Figure 1 , a sample probe is put into the line, and the sample is taken after enough time passes to purge the sample line. A condenser cools the sample so that most of the vapours are condensed. The vapour sampling uses a gas meter and sample tube that can facilitate the use of a chromatograph to get the wet gas analyses. Samples are taken in duplicate. A typical sample probe is shown in Figure 2 . If a sample is taken in the stripper, a wire mesh might be used to prevent plugging. It is important to have all the needed equipment ready, including spare bags, so the sampling is not delayed. Some refiners do this sampling themselves, while others have a third party perform the test work. The appropriate data needs to be taken to complete the weight balance. This includes the air rate and flue gas analyses for the coke make, as well as the feed rate and gravity. Radioactive tracers Elements created in nuclear reactors, radioactive tracers, can be traced using detectors that measure their radiation. Various gases are used as tracers depending on the appli- cation. The half-life of the tracer is very important since a long time would pose safety issues, while the half-life time needs to be long enough for the tracer to be useful after it arrives at the refinery before it decays after testing. Argon 41 is frequently used in FCC units. The test needs to be carefully planned to capture the necessary information. Detailed cracking unit drawings are required to locate the detectors and tracer feed locations properly. The detectors need to be shielded from extrane- ous signals passing through the unit. Gas injected with the feed can measure the vapour resi- dence time and mixing in the feed riser, the distribution of gas to the riser termination device and second-stage cyclones. If a close-coupled termination device is not used, the dilute phase residence time can be determined to justify a technology revamp. Samples taken at the ends of the stripper can help mea- sure the distribution and residence time of the steam and how much of the gas is going to the regenerator. Adding a tracer to the air stream going to the air grid in the regenera- tor provides the gas residence time, distribution of the gas in the bed, and the amount of gas going to each cyclone. Irradiated catalyst can be added to each of the three ves- sels, providing much of the same data for the catalyst as the gas. As a result, the slip factor for the riser can be cal- culated, in addition to the residence times for each vessel and the mixing efficiencies. Short-circuiting in the bed of the spent catalyst entering the regenerator can be studied and compared to the colour of the individual catalyst parti- cles in the regenerated catalyst standpipe. Black and white particles are the result of spent catalyst by-passing the bed and going to the regenerated standpipe. Uneven catalyst flow to the cyclones due to a malfunctioning air distributor can be pinpointed. Gamma scans Gamma scans are used to measure catalyst densities since the strength of the signal is inversely proportional

Sample collection

Sample probe

Cooling coil

Sample bag

3 way valve

Slop

Vapour sampling

Gas sample

Vacuum source

Sample bag (liquid previously drained)

Gas meter

Figure 1 Bag method flow diagram

to the density of the medium through which it flows. The most common use of these scans is to determine the bed levels in the reactor-stripper and regenerator. Standpipe scans can be used to locate aeration and/or circulation problems. The density profile of the catalyst in the feed riser can be determined by shooting various positions in a given plane. Core-annular flow can be examined at different feed rates. Main column scans will locate flooding loca - tions and show the impact of salt deposits at the top por - tion of the tower. Leak detection Radioactive tracers can be used to find a leaking heat exchanger without opening the exchanger. A quantitative amount of leakage can be calculated using this method. Helium can also be used to look for leaks by taking samples across both sides of the exchanger. Helium is inert, will not contaminate the streams, and is easily measured. Most of the time the amount of leakage is not the issue because the exchanger needs to be taken out of service due to the contamination of a stream and repaired. There is usually a spare for critical exchangers.

1/4” SCH pipe probe Length to reach centre line of euent line End may be open or welded shut 1/8” holes in side

1” ange to match port ange

Packing gland

1/4” valve

Safety chain

1/2” SCH 80 pipe handle

Figure 2 Typical sample probe assembly

Revamps 2022