Revamps 2023 Issue

Another cause of poor catalyst circulation is the loss of the fines (0-40 microns) in the circulating catalyst inventory. These fines act as a lubricant and maximise the deaeration time of the catalyst. Good catalysts have a deaeration time of 12-13 seconds. This number can drop to seven seconds when the fines are lost, so a standpipe longer than 45 ft may need aeration to prevent catalyst circulation problems. Catalyst properties can influence circulation. Higher den- sities and larger size particles make them behave more like a ‘type B’ instead of the ‘type A’ solid they are intended to be. These are the Geldart classifications used in the indus- try. If the catalyst gets too fine, it can act like a type ‘C solid,’ which stick together and bridge. The secondary cyclones see the finest catalyst and tend to have oversized diplegs, which can compound the plugging tendency. The metals on the equilibrium catalyst (e-cat) can affect cracker operation. Tramp iron can cause the catalyst in the secondary cyclones to bridge in the standpipe. This type of iron is not a dehydrogenation catalyst. Organic iron in the feed deposits on the catalyst surface and forms eutec- tics with alkali metals and vanadium, which are liquids at normal regenerator temperatures. The surface tension of these compounds is high, and when these materials solid- ify, the catalyst is covered with nodules that could reduce the ABD of the catalyst by 0.1 cc/gm. These particles can lock together and cause flow problems. Any irregular-shaped particles will tend to bridge due to the packing that occurs as the material flows down the standpipes. Original catalysts were ground and tended to have flow issues. When spray drying was used to form more spherical catalyst particles, flow issues were reduced, and losses decreased since there were no edges to break off. If the spray drying produces hollow particles, these can fracture, and the resulting shells can reduce fluidity. Most FCC units do not have slide valve pressure differ- ential or catalyst circulation problems, so this information is relegated to the files. However, when a problem does occur, it proves to be a valuable source of information. References 1 Zenz F A, Standpipe Flow, Bubbling Aeration and Catalyst Characterization, Katalistiks Symposium Vienna, 1984. 2 Lieberman N, Troubleshooting Regen Slide Valve Problems, HTI, Quarterly, Spring 1996. 3 Zenz F A, Othmer D, Fluidization and fluid-particle systems, Reinhold Chem. Engr. Series , 1960. 4 NPRA and AFPM Q & A Transcripts 1973, 1975, 1978, 1983, 1991, 2009, 2014. 5 Wilson J W, Fluid Catalytic Cracking Technology and Operation , Pennwell, 1997. 6 Deldart D, Types of fluidization, Powder Technology, 7, 285-292, 1973.

Warren Letzsch is President of Warren Letzsch Consulting PC. He has 53 years’ experience in refining catalysts, petroleum refining and engi- neering design, and technology development for a major technology licensor. Email: wletzsch@verizon.net

Revamps 2023