Boosting diesel production with varying feedstocks
Evaluation of technology, chemistry and economy for increasing middle distillate output and producing less residue, using oxygen enrichment in fluid catalytic crackers with constrained capacity
Christer Morén and Michael Heisel, Linde Gas & Engineering
Alexander Reichhold, Vienna University
Andreas Krause, Holborn Europe Refinery
A J Berlanga-Gonzalez, Málaga University
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Demand for more diesel and kerosene, in shrinking markets for heating oil and heavy fuel oil, increases interest in treating more FCC residues while increasing middle distillate production. Though refiners may acknowledge this in principle, they often have no reliable data available on the quantitative effects of this approach. Experimenting with operating plants is prohibitive and thus potentially positive effects remain untapped.
To quantify the effects of “admixing”, residue tests were carried out at the Vienna University of Technology in a small FCC pilot plant. To process more feed through this small pilot plant, additional capacity was provided via oxygen enrichment in the regenerator. Feed oils and equilibrium catalyst (E-Cat) were supplied by OMV refinery in Schwechat, in Austria. Various levels of adding residues were tested. Results were generated with respect to options for maximising middle distillates, conversion to other products (as coke and dry gas) and FCC temperature changes.
Test results were then evaluated with respect to their economic impact on several FCC units. Conditions are determined under which oxygen enrichment is economical, as well as which options can be derived with respect to flexibility in feed composition and product slate, based on product and intermediates’ values in a European and a Brazilian refinery. These results are compared with the experience of a few years’ operation in a Spanish refinery.
Economic pressures have resulted in a reduced number of refineries in western Europe at slightly increased capacity, while the capacity and number of refineries in the USA has decreased.
The remaining refineries are operated at higher load. That means less spare capacity available for flexibility towards shifting markets. The well-observed trend towards shifting diesel demand versus gasoline points to substantial changes. Compounding the problem is the economic situation of the refineries. Based on a recent study by WEFA, the return on capital employed (ROCE) of refining operations in western Europe during the period between 1993 and 1999 was a low 4 to 6%, on average. The simplest solution to this economic pinch, of course, is to try and operate the refineries at higher load. Which is what many refiners have already done.
However, refiners in many areas, from Germany to the USA, are finding limits to this option. This is because once operable capacity is met, any further load (capacity) increases are viable only if changes are made to existing equipment, usually necessitating a major investment.
Nonetheless, for a few of the most important refinery processes, there are low-cost solutions available for substantial capacity increases. Among these processes, the economically most important is the FCC, where oxygen (O2) enrichment can increase throughput by approximately 15%, and conversion by approximately 3%, at very low capital expense. But is this an economical option?
There are already more than 30 FCC plants throughout the world applying O2 enrichment in the regenerator, which is why this technology can be considered mature. The experience is that no problems result from the conversion to oxygen enrichment. The hardware required is straightforward and simple, as shown in Figure 1.
Oxygen from a liquid O2 supply tank, a dedicated onsite air separation unit or from a pipeline, is metered via a control unit into the air duct leading to the FCC regeneration. Preferably, O2 is added downstream from the air blower, which circumvents having to qualify the air blower for operation in an oxygen-enriched environment.
Air duct piping is usually of carbon steel. Normally, no changes to this piping is required if O2 is added downstream from the air blower. Due to the O2 addition, some restrictions have to be observed, like maximum allowable gas velocities in elbows. That usually is no problem, but it has to be checked. In the interest of safety during shutdown of the oxygen addition system, a block-and-bleed installation is advisable, ensuring that there are no undetectable creeping gas flows from FCC backwards to the O2 source. Altogether, the installation of oxygen enrichment for FCC regeneration is a low investment option, typically in the range of US$100000 to US$300000.
The technology of oxygen enrichment in FCC is straightforward. However, the reactions in an FCC riser are non-equilibrium based, making the results difficult to predict. Therefore, a test programme was elaborated with the Institute of Chemical Engineering, Fuel and Environmental Technology of the Vienna University of Technology, to quantify the effects of O2 enrichment on throughput, conversion and product composition in an FCC pilot plant.
The feeds to the FCC pilot plant were to be varied. They should not only contain the typical vacuum gasoil (VGO). Atmospheric residue was also to be admixed. Of course, this step increases the risk of heavy metal poisoning of the catalyst, which is why the accumulation of heavy metals was to be measured. In addition, the plant load was varied between 100% and 135%. For all the tests, the same E-Cat was applied, which was withdrawn from a full-size FCC plant in a nearby refinery.
Composition of the E-Cat was analy-sed before and after the experiments, especially with regard to heavy metals content. The test parameters and feed composition are shown in Table 1.
The table shows how oxygen concentrations in the regenerator fluidisation gas were varied. For each test run, the composition of the crack products and conversion were analysed, and the composition of the regenerator offgas was measured. The system’s feed load was varied between 100 and 135%. The characteristics of the feed’s hydrogenated VGO and atmospheric residue are shown in Table 2 and Figure 2.
The temperatures in the riser and in the regenerator were kept as constant as possible. The riser temperature was 523°C (±1°C) and the regenerator temperature was maintained at 626°C (±1°C). Note the distillation features of the feed oils shown in Figure 2.
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