Monitoring FCCU Feed Vaporization
by Ronald G. McClung
Complete FCCU feed vaporization is important for a number of reasons including assuring thorough catalyst to oil contact and minimizing coke deposition in parts of the reactor after the riser. Feed vaporization becomes all the more important when processing increasing amounts of residuum and converting from a bed cracker to a riser cracker.
This article provides an empirical approach by which to estimate dewpoint and feed vaporization. Even though the correlation results provide a tool specific to one commercial location, the approach should be applicable for any gas, oil or residuum containing feed.
Inadequate feed vaporization leads to heavy liquid droplets being entrained to other parts of the FCC reactor. In the case of a 'bed' cracker, this lack of vaporization is not as critical from an operational view. The liquid droplets have an opportunity after the reactor riser in both the dense and dilute phases to absorb or react on the catalyst surface.
However, in units which have been converted to 'riser' crackers, additional opportunities to contact catalyst are minimized by design. Therefore, any liquid droplets formed as a result of inadequate feed vaporization will condense in various downstream parts of the reactor or fractionator systems. Condensation to coke on sufficiently cool spots in the transfer line (uninsulated hangers) or in the plenum (metal surfaces cooled by wet steam) are not uncommon.
There are various factors which effect Feed vaporization:
2. Reactor catalyst mix temperature.
3. Steam partial pressure or other inert media partial pressure.
These three factors are really thermodynamic and physical property related. As a practical matter thorough mixing of catalyst and oil is essential and there are a number of effective designs of feed nozzles available from licensors that can provide this through mixing. So, mechanical considerations aside, obtaining good vaporization of FCCU feed is a matter that can be dealt with using the variables of steam partial pressure and riser mix temperature.
The following empirical approach has been used to quantify the effects of steam partial pressure and estimate feedstock dewpoint, assuming that thermodynamic equilibrium calculations give the best estimate of dewpoint.
Operating Data Required
2. Riser mix temperature (or cat/oil ratio, with catalyst and feed temperature so mix temperature can be estimated).
3. Riser pressure (nearest to mix zone as possible).
4. Atomizing steam and any other steam source or inert gas injected into the riser in advance of the catalyst/oil mix zone.
2. A simple linear regression tool.
2. Correlate the dewpoint of #1 with selected TBP distillation points and corresponding steam, total inerts or hydrocarbon partial pressure calculated above. Note: For the example shown later in this article, the 90% point temperature correlated well with feedstock dewpoint at a given steam partial pressure.
Once this correlation is established, operating deliberately above the dewpoint assures that the thermodynamic requirements for complete feed vaporization are met.
If for troubleshooting reasons or looking at historical data, the amount of feed vaporized needs to be estimated, the following additional steps are required.
2. Correlate the percent vaporized vs. the difference in reciprocal absolute temperatures, i.e.
The correlation should give a smooth monotonic increasing curve, asymptotic to 100% vaporized at
Correlations for a commercial application of this previously described approach are given in this section.
This particular refinery had occasions for processing very heavy feedstocks largely due to the presence of varying percents of deasphalted oil. A sampling of several of the 30 points used in developing of these correlations is given in Figure 1 to illustrate the feed variability and heaviness.
The correlation of dewpoint with distillation 90% point is illustrated in Figure 2. Various other distillation points were used in this correlation, but none gave as good a result as the 90% point.
Note also that steam partial pressure is a significant variable in determining feedstock dewpoint.
The correlation for estimating riser vaporization is illustrated in Figure 3. The line shown on the plot is the best fit through the data with the corresponding predicted point shown on the curve. The remaining points (actual) shown in Figure 3 are the feed vaporization estimates from the equilibrium flash previously described.
An emprirical approach to estimating feedstock dewpoint and the amount of feed vaporized has been provided. Correlations can be developed specific to each FCC unit and its feedstock in order to monitor feed vaporization.
For recommendations regarding software, contact the author by letter, phone or fax:
Manager of Cracking Technology
Petroleum Catalysts Group
101 Wood Avenue
P.O. Box 770
Iselin, New Jersey 08830-0770
Fax: (908) 205-6703