Pretreatment of resid FCC feedstocks
How a refinery optimised hydrogen consumption to raise API “shift”, enhancing FCC economics. The effect of heavy feedstock reactivity on the operation of refinery units, and its impact on current and future operations, is discussed
Byron G Johnson, ConocoPhillips Company
Brian M Moyse, Haldor Topsoe
Viewed : 11129
Hydroprocessing of heavy bottom of the barrel residues has been carried out since the mid-1960s. Initially, these fixed-bed atmospheric resid desulphurisation (ARDS) units, typically configured with two trains and four reactors per train, and with total catalyst inventory of ~2 million lb, were utilised to manufacture low sulphur fuel oil (LSFO). In most cases, the lead or guard reactor is smaller in size than the succeeding main vessels and can often be bypassed if delta P and/or plugging of this guard reactor occurs during the normal operating runs. Should this occur, feed is then passed directly to the first main reactor.
Today the focus has shifted to the point that almost half of the residual fuel hydroprocessed is as a pretreatment for fluid catalytic cracking (RFCC). This trend has not only influenced the actual operation of resid FCC pretreaters, but has also affected the selection of catalyst type to be used. For the refiner, it is desirable to have an operating run on the pretreater, which is consistent with the FCC unit turnaround schedule. In addition, it is desirable to have some flexibility to be able to process heavier or opportunity crude feedstocks during the run.
In addition, since the primary focus is no longer hydrodesulphurisation (HDS) but more towards carbon removal (HDCCR), metals removal (HDM), and hydrogen uptake, the type and functionality of the catalysts to be considered should be reviewed and optimised, in the light of the current and/or expected feed slate, together with the refinery objectives.
Since the FCC pretreater catalyst can accommodate 30 times more Ni+V than the equilibrium FCC catalyst, it is important to select catalyst types with a high metal capacity. At the same time, high HDN and high hydrogenation activity needs to be maintained in order to realise the full economic advantage from enhanced FCC operation, due to better feed pretreatment. In most cases, the refinery operating parameters are more or less fixed, the only real flexibility or opportunity to influence the operation that the refiner has available is selection of catalyst function or type and/or feed slate.
Because of the lead times required to produce the ~2 million lb of catalyst required to fill an ARDS unit, it must be ordered well in advance of its intended installation. Also, since crude slate or availability can be unpredictable as well as having a significant effect on unit performance, catalyst and crude selection become extremely important in planning the plant turnaround schedule, and in estimating overall plant economics.
ConocoPhillips currently owns and operates 14 refineries, one of which is in Borger, north of Amarillo in the panhandle of Texas. This location comprises a 140000bpd refinery, together with and an 80000bpd natural gas liquids complex. A simplified flow scheme for this refinery is shown in Figure 1. As can be seen, Unit 42 is an integral part of this complex. This is the ARDS unit, which pretreats feed for both of the heavy oil cracker (HOC) FCC units.
The Borger ARDS unit, which was built in 1982, was designed by Gulf and is now a ChevronTexaco-licensed plant. It was originally designed to process some 50000bpd of atmospheric crude tower bottoms derived from domestic US crude sources.
This resid hydroprocessing unit is a conventional design consisting of two trains with four reactors in each train. The unit capacity has been increased over the years, and it now processes in excess of 75000bpd, which is 50% above nameplate capacity. Currently, the unit is operating on Run 14, but the main focus of this article will be Run 13, which was terminated in October, 2003.
During 2003, when Run 13 was already underway, a previously planned refinery project, which included a crude units revamp, was implemented. One of the desired outcomes of this project was an improved distillate/gas oil draw. However, as expected, this decreased the API gravity of the crude tower bottoms fraction, making the ARDS feed a heavier cut. As a result, from January 2003, the feed to Unit 42 was more difficult to process. In addition, the amount of Arab Medium in the crude slate processed in range was increased from 20 to 30 vol%, the balance being domestic West Texas Sour (WTS). Also, during the same run, some other opportunity crude types were processed in the unit to a limited extent.
All of these changes resulted in a heavier ARDS unit feedstock (Table 1) with a sulphur of ~3.1 wt%, Ni+V ~45ppm. This represents an increase in the sulphur and metals (Ni+V) levels of 13% and ~30%, respectively, above that experienced in all of the previous ARDS cycles.
Prior to the merger with Conoco, Phillips had always maintained a significant R&D effort for all areas of hydroprocessing in Bartlesville, Oklahoma, USA. This included feedstock evaluation and catalyst testing in both pilot plants and canisters in various reactors in actual units. In the hydroprocessing of resid, it is imperative to understand the influence of the feedstock properties on the unit operation. Even though refiners normally operate to meet certain product specification like sulphur or metals, there are other reactions taking place, which can have a significant impact on the operation.
For this type of hydroprocessing, the feedstock reactivity plays an important role and will depend upon crude origin, composition, cut, previous history, etc. These factors will, in turn, affect the important parameters like sulphur content, Ni+V content, Conradson carbon residue (CCR), iron content, nitrogen and aromatics. Added to this is the refiner’s particular desire to meet specific needs in terms of product properties and cycle length. As a result, it becomes a complicated matter to find the optimum operating solution to meet as many processing goals as possible. On the positive side is the fact that most resid hydrotreaters operate at high pressure and low liquid hourly space velocity and have high treat gas rates, with high purity hydrogen as well.
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