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Jul-2010

Developments in cyclone technology improve FCC unit reliability

Developments in FCC cyclone technology reduce problems of abrasion that cause unscheduled unit shutdowns

Ye-Mon Chen and Mart Nieskens, Shell Global Solutions
S B Reddy Karri and Ted M Knowlton, PSRI
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Article Summary
US refineries are facing the difficult challenges of reduced demand and low refining margins. Projection for future gasoline demand in the US is relatively flat at best, while regulatory ethanol requirements will cut into refining supply to the gasoline market. This has a direct impact on fluid catalytic cracking (FCC) unit operation.

In this challenging environment, improving unit reliability and reducing both operational and maintenance costs are the key focus for refiners. This article introduces a FCC cyclone technology with a proven track record of helping refineries to improve unit reliability while reducing operational and maintenance costs

Conventional cyclones: a weak link in FCC operation
Two recent industry surveys reveal the pervasive problems of cyclones used in FCC operation. Table 1 summarises the survey results by Grace Davison as presented at its 2002 Dublin FCC conference. The results indicate that catalyst losses from cyclones were the number one problem in FCC operation. The 2008 NPRA survey again revealed that FCC cyclone reliability is a major concern for member companies.

Cyclones are commonly considered a mature technology in the industry. As a result, most FCC technology licensors rely on vendors to provide cyclones that are categorised as conventional cyclones in this article. Recent surveys indicate that these conventional cyclones have not met refineries’ needs, both in terms of performance (catalyst loss) and reliability. Shell Global Solutions has developed an innovative cyclone technology, which is categorised as improved cyclone technology here.

PSRI cyclone study
Particulate Solid Research Inc (PSRI) is an independent industrial consortium serving 28 member companies, which include:
• Oil companies, such as Shell, ExxonMobil, BP, Total, Chevron, ConocoPhillips, Marathon, Sunoco, Syncrude, Petrobras
• Chemical companies, such as Dow, BASF, SABIC, Sasol
• Technology providers, such as Shell Global Solutions, UOP, Shaw Stone Webster, KBR, CPFD, Solutia, IFP, INEOS, Cristal Global.

PSRI conducts research and testing programmes that address the common concerns of member companies. These programmes are guided by elected technical advisors from member companies. One recent PSRI programme studied and benchmarked different FCC cyclone technologies, since cyclone erosion and reliability are highlighted as the major concerns of FCC operation for member companies in recent surveys. The most pervasive problem is erosion in the secondary cyclone, particularly in the lower cone and in the transition to the dipleg, which is the focus of the study.

There is a fundamental difference between first- and second-stage FCC cyclones in their erosion patterns. Highly loaded first-stage cyclones normally experience little to no cone erosion, whereas lightly loaded second-stage cyclones can exhibit severe cone erosion. This seems to be counter-intuitive at first. However, the key difference in erosion pattern lies in the differences in the solids flow patterns and vortex formation, as shown in Figure 1.

As a result of high solids loading and low gas velocity in a typical FCC primary cyclone, gravitational force plays a key role; the solids appear to fall rapidly down into the cyclone cone and dipleg, as shown in Figure 1, taking only one to two full turns to exit the bottom of the cyclone. The vortex length in the highly loaded primary cyclone is much shorter because the high solids loading dampens the formation of a robust vortex. Therefore, the vortex does not whip the solids at a high velocity around the cone in the primary cyclone.

In a typical FCC second-stage cyclone, the solids loading is approximately 1/1000 to 1/10 000 of the loading in the first-stage cyclone. Due to light solids loading and high gas velocity, the vortex is relatively long, energetic and, more importantly, unstable, moving asymmetrically about its axis. As the swirling solids in the outer vortex approach the cone in a second-stage cyclone, the long, rapidly rotating vortex accelerates the solids stream and causes it to intensify its rotation. (The solids spin faster, similar to the motion of a figure skater pulling inwards.) The outer vortex in a second-stage cyclone typically takes from four to six turns before exiting the bottom cone (see Figure 1), and the spinning continues into the top portion of the dipleg below the cone. The concentrated solids stream rotates at a high velocity, and the unstable, continuous movement of the vortex causes the significant erosion observed in the cone and the top of the dipleg of second-stage cyclones.

The PSRI cyclone test programme was structured to benchmark three different solutions to mitigate the damaging erosion occurring in FCC second-stage cyclones:
•    Increasing the cyclone length (L/D) of a conventional cyclone
•    Increasing the angle of the cone of a conventional cyclone
•    Adding a vortex stabiliser to a conventional cyclone simulating the improved cyclone technology.
The cyclone test facility is shown in Figure 2. It consists of:
•    A 3ft-diameter fluidised bed
•    An 8in-diameter standpipe, approximately 55ft in length
•    A slide valve to control the solids flow rate around the unit
•    An 8in-diameter riser approximately 70ft tall
•    A 19in-diameter first-stage cyclone
•    A 17in-diameter second-stage cyclone.      

Air was used as the conveying gas in the test unit. The solids used were equilibrium FCC catalyst with a median (Dp50) particle size of approximately 75 microns. The fines (material <44 microns) concentration in the catalyst was approximately 8 wt%. The particle density of the catalyst was 93 lb/ft3. Loadings to the second-stage cyclone varied between 0.5 and 
90 g/ft3 (0.00007–0.013 lb/ft3).

The secondary cyclone was constructed in modules for easy change of dimensions. Figure 3 shows the cyclone with several different barrel lengths. Multiple coatings of drywall joint compound were added to the inside of the cyclone before each test. The amount of erosion occurring in the cyclone was measured by weight loss in the drywall compound over a specified period of time.
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