The present refining market is highly dynamic, which emphasizes the need for greater operating flexibility in the fluid catalytic cracking (FCC) unit. Continued development in FCC technology is a high priority. For example, a new technology allows refiners to optimize catalyst circulation rates in the riser, independent of the unit heat balance. This capability enables improved conversion, product selectivity and emissions control while simultaneously reducing operating costs. This article will discuss how an advanced FCC catalyst recycling process technology increases the flexibility of the FCC unit to shift between different processing objectives while lowering costs and maximizing product values to meet the challenges faced by the global refining industry.
ADVANCED FCC CATALYST RECYCLING TECHNOLOGY
The basic concept of the advanced FCC catalyst recycling process technology is to recycle catalyst from the FCC reactor stripper back to the inlet of the riser. Modern catalyst systems are inherently more coke tolerant than their older counterparts. Thus, they can accrue appreciable quantities of coke and still retain a substantial fraction of base activity. Recycling of catalyst from the reactor stripper in modern FCC units represents an additional activity component being added to the riser. A new term, carbonized, has been adopted to describe this catalyst. At present, six advanced FCC catalyst recycling process units are in operation, with an additional four units being engineered. Fig. 1 shows the layout of an FCC unit with the a catalyst recycling technology.
| Fig. 1. Equipment diagram of the advanced |
FCC catalyst recycling process.
Catalyst recycling and heat balance
In the traditional FCC process, the catalyst circulation rate is fixed by the heat balance. Under these conditions, the catalyst circulation only increases in response to a greater heat demand by the reactor. Therefore, in a conventional FCC system, the extent that regenerator catalyst to oil ratio (cat/oil) increases is expressed as:
Process changes that increase the regenerator catalyst circulation rate and raise the coke make required to satisfy the altered heat balance include:
- Increasing riser temperature
- Decreasing feed preheat
- Increasing regenerator catalyst cooler duty
- Injecting a heat load into the riser (steam, water, light cycle oil)
In contrast, carbonized catalyst recycle from the reactor stripper via the advanced FCC catalyst recycling process standpipe is not constrained by the heat balance as it does not significantly alter the total coke yield. Because catalyst is circulated from the riser outlet, down to the riser inlet, and back up to the riser outlet starting and ending at the same temperature, little enthalpy change occurs in the loop. Result: There is practically no impact upon the coke yield. Thus, the riser cat/oil can now be expressed as:
The advanced FCC catalyst recycling process impacts the heat balance by increasing ∆ coke on the catalyst circulating to the regenerator. The ∆ coke is defined as the difference in coke content between the regenerated catalyst and spent catalyst. As the advanced FCC catalyst recycling process circulation rate is increased, the ∆ coke increases due to the recycling catalyst particles completing additional passes through the riser prior to regeneration. Because regenerator temperature is a strong function of ∆ coke, the increase in ∆ coke from the advanced FCC catalyst recycling process increases the regenerator temperature and decreases the regenerator cat/oil ratio, as shown in Table 1.
Despite the decrease in regenerated cat/oil, this FCC catalyst recycling technology enables a refiner to increase the total riser cat/oil ratio to levels considerably higher than a traditional FCC unit while simultaneously increasing the regenerator temperature. Reducing feed contaminants through severe hydrotreating reduces the ∆ coke in the unit, which cools the regenerator. Such conditions present a significant challenge for refiners on maintaining the regenerator hot enough to control carbon monoxide (CO) and nitrogen oxide (NOx) emissions below acceptable levels. The advanced FCC catalyst recycling process can provide an alternative solution to traditional methods of maintaining high regenerator temperatures, and it simultaneously enhances unit performance through increased total riser cat/oil ratio. Table 2 shows an economic comparison between using the direct-fired air heater (DFAH) and the advanced FCC catalyst recycling technology to increase regenerator temperature by an equivalent amount.
While both approaches will achieve a higher regenerator temperature, firing the DFAH will result in a lower riser cat/oil ratio and, consequently, a loss of conversion and margin. Conversely, the advanced FCC catalyst recycling process increases the riser cat/oil ratio, resulting in a conversion increase and, ultimately, a gain in margin, all without consuming additional fuel gas. In addition, the higher regenerator temperatures improve burn kinetics and allow the FCC operator to lower the excess oxygen in the regenerator while simultaneously reducing CO and NOx emissions.
FCC catalyst recycling process and dry-gas yields
The advanced FCC catalyst recycling process can also improve product yields. The catalyst recirculating through the advanced FCC catalyst recycling process standpipe enters a proprietary mixing chamber at the base of the riser at temperatures several hundred degrees Fahrenheit below the regenerated catalyst temperature. When these two catalyst streams are properly blended, the resultant catalyst stream contacting the feed in the injection zone of the riser is at a significantly lower temperature, thus reducing thermal reactions that produce unwanted dry gas and coke. Fig. 2 illustrates how the reactor-feed-injection zone temperature decreases as a function of new FCC catalyst recycling process cat/oil ratio, while Fig. 3 graphs how the dry-gas make decreases in response to lowering the feed-injection-zone temperature, as measured in the circulating riser pilot plant.
| Fig. 2. Advanced FCC catalyst recycling |
process cat/oil vs. feed contact zone temperature.
| Fig. 3. Pilot-plant feed contact zone |
temperature vs. dry-gas yield, 1,000°F riser
Catalyst activity retention as a function of coke
While the advanced FCC catalyst recycling process technology increases riser cat/oil ratio, the impact of coke deposition on catalyst activity must be understood to predict the benefits of the higher catalyst circulation rate. To determine the relationship between coke deposition and catalyst activity, testing was conducted on several commercially available equilibrium catalysts (ECATs). Table 3 summarizes the physical properties of three test catalysts.
ACE pilot-plant runs were then conducted for the three catalysts to determine activity retention as a function of carbon content on the catalyst surface. Results are shown in Fig. 4. These data highlight a similar activity decline for catalysts A and B and a more pronounced decline for catalyst C. Significant differences in activity retention as a function of coke are not always obvious from the catalyst physical properties. For instance, while ECATs B and C have similar MAT activities, they do not have similar activity retention properties. Therefore, it is important to conduct activity retention tests when optimizing a catalyst for FCC catalyst recycling process. This testing enables the determination of the effective cat/oil response in the new FCC catalyst recycling system, which can be defined as:
where ARC is the slope of the catalyst activity retention as a function of coke determined in the ACE unit and K is a constant.
| Fig. 4. Relative activity vs. coke for three ecats.|
To determine constant, K, the three example catalysts were tested in the circulating riser pilot plant by increasing the advanced FCC catalyst recycling process cat/oil and maintaining a stable regenerator cat/oil. Fig. 5 shows the testing results. These data illustrate that the conversion response to an increase in the advanced FCC catalyst recycling process cat/oil is best achieved by ECATs A and B, which have better ARC properties relative to ECAT C.
| Fig. 5. CRU conversion response to new FCC |
catalyst recycling process ratio for three ECATs.
FCC CATALYST RECYCLING PROCESS YIELDS BENEFITS
Once the activity retention properties of the catalyst and the operating severity of the unit are understood for the advanced FCC catalyst recycling process, the remainder of the product yields can be estimated from the calculated increase in effective cat/oil ratio. For ECAT A, the full product yield shift in response to a change in advanced FCC catalyst recycling process cat/oil is represented in Table 4. This table highlights that while riser cat/oil increases purely as a function of the new FCC catalyst recycling process valve output and the unit heat balance, the effective cat/oil is the primary driver of yield shifts on the FCC unit applying the advanced catalyst recycling method.
To confirm the theoretical pilot-plant data, it is important to observe the conversion shifts in the commercial advanced FCC catalyst recycling process units that have optimized the catalyst formulation to take advantage of this new catalyst recycling technology. Fig. 6 shows the conversion response for an increase in the advanced FCC catalyst recycling process for three separate commercial units. These data were filtered for constant feed quality and riser outlet temperature to isolate the impact of the new catalyst recycling process ratio on conversion. The commercial results are consistent with the pilot plant yields, thus demonstrating an approximate 2 vol% to 3.5 vol% yield improvement over the range of FCC catalyst recycling process ratio.
| Fig. 6. Commercial unit conversion response |
to advanced FCC catalyst recycling process cat/oil.
Improved riser design for revamps
In the traditional FCC catalyst recycling process design, regenerated catalyst from the regenerator is combined with the carbonized catalyst from the reactor stripper at the base of the riser, using a proprietary mixing chamber, as shown in Fig. 7. For new units, the mixing chamber can be easily incorporated into the FCC design. However, incorporating the advanced mixing chamber in a revamp can prove challenging with regard to physically fitting the new chamber within the existing configuration without major structural modifications. To install the advanced FCC catalyst recycling technology as part of a revamp scenarios, the redesign can also include a proprietary mixing chamber and riser, as shown in Fig. 8. While the new mixing chamber would extend below grade, the redesigned mixing riser design fits within the existing configuration without major structural changes or modifications to the feed injector elevations.
| Fig. 7. Side view of advanced mixing chamber |
on an FCC unit.
| Fig. 8. Revamp comparison of an advanced |
mixing chamber vs. a proprietary mixing riser design.
Need for flexibility in FCC operations. An advanced FCC catalyst recycling technology has demonstrated, in both pilot-plant and commercial testing, a unique ability to manipulate overall riser cat/oil ratio to increase the effective riser catalyst activity gain outside the traditional limitations of the unit heat balance. This added flexibility enables FCC operators to gain a competitive advantage by offering improved yield selectivities, enhanced operational controls, and reduced operating costs. In addition, a new mixing riser design allows many refiners to revamp their existing units to gain the full benefits of improved FCC catalyst recycling process technology. HP
An upgraded and revised presentation from the AFPM Annual Meeting, March 45, 2012, San Diego California.
|The authors |
Matthew Lippmann is a group leader in Honeywells UOP FCC alkylation and treating development group in Des Plaines, Illinois. His responsibilities include advancing the FCC technology platform and managing the FCC pilot-plant areas. Prior to working for UOP, he was a technical services group leader at the HOVENSA LLC refinery in St. Croix and was the process engineering supervisor for the FCC, alkylation and delayed coking areas. Mr. Lippmann earned a BE degree in chemical engineering from Drexel University.
Lisa Wolschlag is the senior manager of Honeywells UOP FCC, alkylation and treating development department located in Des Plaines, Illinois. In this role, she is accountable for improving and advancing UOPs FCC, alkylation and treating technology portfolios. Ms. Wolschlag has 20 years of experience with UOP that has included research and development, field operating service, technical service and process development. She holds a BS degree in chemical engineering from University of Illinois and an MBA degree from the University of Chicago.