Chemical treatment to solve FCCU problems
From fouling and catalyst carryover to high feed nickel levels and varying feed qualities, FCCU problems can be solved quickly and cost effectively, says the author, through a well-designed and properly applied chemical programme
Nalco Energy Services
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A wide range of fluid catalytic cracking unit (FCCU) problems confront refinery engineers as they continuously look at ways to maxi-mise their run lengths and minimise maintenance expenses even as feed qualities are changing. This wide array of FCCU problems are exemplified in the following five case studies of programmes that have been undertaken in the past few years at six refineries, each with its own, distinctly different, processing issues. In all five cases, Nalco was brought in by the refiner to evaluate and solve the FCCU difficulties they were experiencing.
Each chemical programme resulted in a resolution of the problem and considerable cost savings to the refiner. These solutions have allowed the refiner to deal with the ongoing change in feed qualities while maximising throughput.
The five cases were:
— The application of two chemicals – an inorganic dispersant/coke suppressant and an organic dispersant – eliminated FCCU slurry/feed exchanger fouling, reduced exchanger cleaning costs and improved the heat balance in the FCCU main fractionator of one US refiner
— The implementation of Nalco’s proprietary Nickel Passivation Plus programme, to reduce excess nickel in the FCCU feedstock, helped another refiner to reduce coke production and increase unit throughput
— In the three months immediately following introduction of a chemical programme and simultaneous mechanical modifications in its residue FCCU (RFCCU), one refiner eliminated fouling and a recurring gasket failure problem in the slurry steam generator, allowing the processing of an additional 1200bpd through the RFCCU
— FCCU catalyst carryover at a northern European refinery was boosting ash levels in the FCCU slurry oil, which the refiner used as part of the fuel for its on-site power plant. As a result, particulate emissions exceeded permissible levels until a chemical programme enabled the refiner to resume using this low-value stream to generate high-value electric power and steam
— To deal with widely varying FCCU feed stream qualities, a US East Coast refiner implemented a chemical additive programme that enabled it to meet FCCU feed desalter goals for consistent salt removal and brine clarity
— The application of a salt dispersant allowed a customer to continue operating its FCC unit at full rates. It had been forced to reduce throughput in the past due to NH3Cl fouling in the overhead, which was caused by operation of the overhead at lower temperatures.
Slurry exchanger pressure drop
A 34000bpd US refiner was experiencing severe fouling in its FCC slurry/feed exchangers. Because of the resulting high exchanger pressure drop (âˆ†P ) on the slurry side, the exchangers required cleaning every 90 days, each time resulting in a reduction in unit throughput of 4000–5000bpd for four to five days. In addition, the inability to remove heat from the slurry created heat balance problems in the main fractionator and debutaniser (Figure 1). Because of this problem, Nalco was asked to provide a solution.
A thorough survey of the system was done to understand the design limitations on the exchangers. In order to characterise the type of fouling in the system, an analysis was done on the deposits and the FCC slurry. The deposits were located primarily at the inlets to the exchangers, resulting in a high âˆ†P across the exchangers. The high-pressure drop eventually affected removal of heat from the bottom of the main fractionator and required cutbacks in unit throughput.
Deposits taken from the fouled exchangers were analysed at the Nalco research laboratories in Sugar Land, Texas, USA. The fouling was determined to have two primary causes: coking in the bottom of the fractionator and deposition of inorganic catalyst fines. An analysis of one of the fouling deposit samples is shown in Table 1. The high carbon-to-hydrogen ratio measured in the sample indicated coke formation, most likely in the bottom of the tower itself. The inorganic components of the deposit served as evidence that catalyst fines were being depositing along with the coke.
In addition to a deposit analyses, a proprietary test method was used to analyse samples of the FCC slurry. Nalco was interested in looking at the molecular nature of fouling precursors to assist in the determination of the fouling mechanism. The aim of this approach is to measure the total aromatic and total aliphatic character, average chain length and the aliphatic groups attached to the aromatic nuclei. The aromatic character can then be further divided into unsubstituted and substituted/fused aromatics (Table 2). The interpretation of this technique measures the fouling tendency of slurry oil streams. This analysis supported the information derived from the deposit analysis.
To determine the most effective dispersant to prevent agglomeration and deposition of polynuclear aromatic compounds (PNA), each of the available products was tested at a dosage of 20ppm (Figure 2). Product EC3051A was clearly found to be the best product for this particular situation. EC3051A was injected upstream of the feed/slurry exchangers.
In order to address the coke formation, it was recommended that the refiner inject EC3045A, an additive that functions as a coke suppressant and inorganic dispersant, upstream of the main fractionator slurry return inlet (Figure 1). The recommended injection rate was 10 to 12ppm. The temperature of the fractionator bottoms ranged from 680°F to 690°F (360–365°C).
The antifoulant programme was monitored by tracking the pressure drop through each exchanger, and through modelling of the exchanger U-values and fouling factors, using the proprietary Monitor heat transfer simulator. Since the start of the programme, the exchanger âˆ†P has remained low, and exchanger run-lengths have increased from 90 days to more than 15 months. No reductions in FCC throughput have occurred since the start of the antifoulant programme.
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