Bottoms upgrading of FCC resid feedstock

A refiner chose a new catalyst to improve bottoms upgrading and maximise fuels production from resid feeds.

STEVE CHALLIS, Chalcat Consulting

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Article Summary

The fluid catalytic cracking (FCC)  unit is the main conversion process of many refineries, and the refinery featured here has two FCC units (FCC#1 and FCC#2) with a combined feed rate of around 20000 b/d. The configuration for the two FCC units is shown in Figure 1. The refinery’s objective is to improve FCC#2 bottoms upgrading and increase overall fuels production (diesel and gasoline) and minimise fuel oil production. Slurry is recycled from FCC#1 to FCC#2, and FCC#2 is operated to maximise bottoms upgrading (see Figure 1). This configuration makes FCC#2 the most challenging from a catalyst selection point of view. By meeting the desired objectives, the catalyst plays a key role in maximising the profitability of the FCC.

The refinery initiated a tender to select a new catalyst for FCC#2. Catalyst samples from all major suppliers were evaluated by the refinery R&D centre, and the refinery selected BASF’s bottoms upgrading catalyst for a commercial FCC trial. BASF helped the refinery prepare for this trial. During the trial of the new catalyst, the refinery increased fuels production. Pleased with the unit service and catalyst performance delivered, the refinery then selected the BASF bottoms upgrading catalyst for long term use at FCC#2.

In this article, the preparation and commercial trial for the bottoms upgrading catalyst is discussed. Technical terms employed are explained in a glossary at the end of the article.

Catalyst selection
Laboratory evaluation was the major factor in the catalyst selection process. Fresh FCC catalyst samples from all major suppliers were received for consideration. The samples were deactivated using a cyclic metals deactivation unit (CMDU), which is used for mimicking equilibrium catalyst (Ecat) that contains high nickel and vanadium levels. The deactivated samples were evaluated using a circulating riser unit (CRU). This laboratory pilot plant simulates commercial FCC operation. BASF’s bottoms upgrading catalyst performed well in testing, and the refinery’s R&D centre selected this catalyst for a commercial FCC trial.

BASF bottoms upgrading technology
This catalyst was an ideal fit for FCC#2 because it has an optimised porosity to improve diffusion of heavy feed components, and a coke selective matrix to improve bottoms upgrading. Furthermore, for this unit BASF experts optimised the zeolite to matrix ratio and rare earth on zeolite to deliver the desired yield pattern, with minimum coke and gas penalty. These key features enabled the BASF catalyst to out-perform competitors’ catalyst samples.

Trial set-up
BASF technical service worked closely with the refinery to set up the trial. FCC#2 processes a heavy feedstock (see Table 1). The refinery’s objective for the new catalyst was to improve FCC#2 bottoms upgrading and minimise fuel oil production. In addition to desiring a higher catalyst performance, several constraints affecting FCC#2 were preventing the refinery from achieving its objective:
• Operating guidelines for minimum feed temperature and minimum regenerator temperature prevented FCC#2 increasing cat-to-oil ratio and improving yields.
• Poor fresh catalyst loader performance prevented the refinery from increasing fresh catalyst addition rate to increase Ecat activity and improve yields.
• A blocked Ecat sample point prevented the operations team from collecting samples for analysis. This made unit performance evaluation more difficult.

To overcome these constraints, BASF provided continuous technical service to ensure that the new catalyst performed and delivered value. As a first step, the company’s technical specialists visited the refinery to complete BASF Cold Eyes and Catalyst Management of Change Reviews. These reviews considered expected feedstock quality, operating constraints and trial objectives and helped create a detailed trial plan and trial protocols. To overcome the constrained fresh catalyst addition, technical service designed the bottoms upgrading catalyst with a suitably high activity. To help the refinery optimise the fresh catalyst addition rate, BASF developed a catalyst activity management tool for the refinery to use and agreed to monitor the trial and provide recommendations to improve unit performance.

Catalyst changeover
Change to the new catalyst progressed quickly and smoothly; the rate of change is a function of the addition rate, unit inventory, and catalyst retention factor. The catalyst changeover was monitored using limited Ecat data and daily unit operating data, and there were no unit upsets or problems during the change. At 30% changeover, the catalyst circulation was stable and catalyst losses were in the normal range. At 60% changeover, unit yields and performance were improving, and the refinery was impressed with the performance of the new catalyst and BASF’s technical service.

Catalyst performance
The bottoms upgrading catalyst improved coke and gas selectivity, which enabled the refinery to increase riser severity and improve bottoms upgrading. In addition, the refinery was able to increase the slurry recycle from FCC#1 (worsening FCC#2 feed quality) while maintaining good product yields.

For maximum conversion, BASF uses coke and gas selective matrix and zeolite technologies in its catalyst design. For this trial, the matrix surface area was like the previous catalyst, but the zeolite surface area was increased. As Figure 2 shows, by optimising the zeolite to matrix ratio, the catalyst improved coke selectivity compared to the base case, competitor catalyst.

Because of improved coke selectivity, FCC#2 regenerator bed temperature decreased by around 5°C (9°F) (see Figure 3). Cat-to-oil, for similar riser severity, on average increased. Higher cat-to-oil favours improved bottoms upgrading and performance.

The lower regenerator bed temperature reduced the riser mix zone temperature and associated thermal cracking. This, coupled with reduced catalyst induced over-cracking, led to a significant reduction in FCC#2 dry gas yield (see Figure 4).

Improved coke and gas selectivity unlocked the potential for the refinery to increase FCC#2 riser outlet temperature (ROT) and cat-to-oil to improve bottoms upgrading and conversion. This provided scope for the refinery to increase FCC#1 slurry recycle to FCC#2. As Figure 5 shows, the FCC#2 slurry yield is on the lower side for higher FCC#1 slurry recycle. Despite worsening feed quality, the refinery maintained good FCC#2 product yields with the coke and gas selective BASF bottoms upgrading catalyst.
Continuing support
Pleased with its overall performance, the refinery selected the bottoms upgrading catalyst for long term use. Since the trial, BASF and the refinery have continued to improve the unit performance using BASF’s benchmarking tools. In addition, the company’s technical service has helped the refinery tackle several issues. For example:
• A long-standing problem with combustion afterburning has been reduced by optimising operating conditions. In addition, BASF can provide CO combustion promoter either pre-blended with fresh catalyst or as a separate additive.
• BASF helped the refinery investigate and reduce slurry exchanger and debutaniser reboiler fouling
• To help manage the ongoing issues with catalyst loader reliability, BASF continues to supply high activity fresh catalyst.

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