Cracking with oxygen
Approaches to economic solutions for refineries

New regulations for “clean fuels” and changes in demand are some of the major challenges faced by refineries. An efficient response to this situation is the increase of the use of residues in the FCC (Fluid Catalytic Cracking) in order to enhance the production of middle distillate and reduce the fraction of lower-value residues. To quantify the effects of this procedure, test runs with a small-scale FCC unit have been performed. The additional capacity required for the use of more residues was supplied by oxygen enrichment in the FCC.

Refineries are confronted with a major challenge due to the worldwide, increasing demand for high quality fuels, such as diesel and kerosene, and shrinking markets for heating oil and
heavy fuel oil. Consequently, it is an important goal to increase the production of middle distillates and simultaneously reduce the fraction of lower value residues. In terms of technology, chemistry and economics, the admixture of residue oils from atmospheric or vacuum distillation to the crude oil used in FCC (Fluid Catalytic Cracking) plants is a suitable measure.
Refineries are facing a number of major challenges these days, some new and some older, in part enforced ones:
-Refinery operators have to comply with the new “clean fuels” regulations, while
-the pressure persists to improve economy and margins, and
-the demand for refinery products is shifting, e.g. towards more diesel and kerosene in Europe, while the demand for heating oil and heavy fuel oil is decreasing
The economic pressure resulted in a reduction of the number of refineries in Western Europe at slightly increased capacity. Both the capacity and the number of refineries decreased in the US. The remaining refineries are being operated at higher
load. Essentially that means less spare capacity is available to respond flexibly to shifting markets. The example of shifting diesel demand versus gasoline shows that there are substantial changes in the offing. Compounding the problem is the economic situation of the refineries. In the period 1993 through 1999, the return on capital employed (ROCE) of the refining operation was typically a low 4 to 6%. The
easiest solution to this economic pinch is the operation of the refineries at a higher load. Obviously, there are limits to this option. Once the max. operational capacity is reached, the load can be increased only by making changes in the existing equipment. This often involves major investment. However, for a few processes, solutions are available which allow the refiners to expand capacity substantially at a low cost. The most economic of these processes is the FCC, in which oxygen enrichment can be used to increase throughput by approx. 15% and conversion by up to 3% with only a small investment of capital.

-The use of oxygen is profitable.

Hardware needs for oxygen enrichment
Figure 1 shows a schematic diagram of an FCC plant. In the oxygen enrichment process, oxygen O2 (blue arrow) is admixed to the air for regenerating the catalyst. More than 30 FCC plants throughout the world apply oxygen in the regeneration process so that this technology can be considered “mature”. Experience has shown that the conversion to oxygen enrichment is not associated with any undue problems. The hardware required for conversion is straight-forward and simple (Figure 2): Oxygen from a liquid oxygen tank, a dedicated on-site 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 of the air blower in order for this unit to need no approval for operation with oxygen. The piping of the air duct is usually made of carbon steel and does not need to be changed for adding oxygen. However, certain restrictions apply in other areas, such as maximum allowable gas velocities in elbows. For safety it is advisable to have a block-and-bleed installation during the shut-down of the oxygen addition to ensure that there are no undetected creeping gas flows from the FCC back to the O2 source.
Testing at Vienna University of Technology
The technology of oxygen enrichment in FCC plants is straightforward and not spectacular. But since all reactions in an FCC riser are limited by kinetics, the results are difficult to calculate. Therefore, a test program was developed in cooperation with the Institute of Chemical Engineering, Fuel and Environmental Technology of Vienna University of Technology to quantify the effects of oxygen enrichment on throughput, conversion, and product composition in an FCC pilot plant.
The feeds to the FCC pilot plant were to be varied: not only the typical vacuum gas oil, but also atmospheric and vacuum residue was to be admixed. Since these feeds accelerate the heavy metal poisoning of the catalyst, the accumulation of heavy metals was measured also. The same equilibrium catalyst from an FCC plant in a nearby refinery was applied in all tests. The composition of the catalyst and especially the load of heavy metals was analyzed before and after the experiments.
The oxygen concentrations in the regenerator fluidization gas was varied. In each test run, the composition of the cracking products and the conversion were analyzed and the composition of the off-gas from the regenerator was measured. The feed load of the system was varied between 100 and 135%. The temperature in the riser and regenerator was kept as constant as possible.
The experiments showed that oxygen enrichment resulted in improved regeneration of the FCC catalyst leading to higher catalyst activity. As shown in Figure 3, oxygen enrichment facilitated an increase in plant load at constant conversion by approx. 10% (dotted blue line). Alternatively, it was possible to increase the conversion at a constant load by approximately 2 to 3% (continuous red line). When the increase in catalyst capacity was used to increase the conversion, less residue was generated: as shown in Figure 4 3.5 to 5 % by weight less residue was generated with oxygen enrichment as compared to air alone. This can help the refinery to reduce the low value residues to produce more gasoline and middle distillates.
The tests showed that oxygen enrichment in FCC units allows the refinery operator to:
-increase the FCC capacity at constant conversion, or
-increase conversion at constant throughput,
-utilize the improved conversion to admix heavier residues, such as atmospheric residue,
-get more flexibility in the choice of feedstocks,
-process heavier feedstocks, and
-reduce the quantity of residues produced by the FCC unit.
This test plant also allows to process feed oils from customers applying their equilibrium catalyst to predict the effects of oxygen enrichment. However, the small-scale pilot plant generates trends only,
rather than numbers that can be applied directly to large-size units.
Operating results of the CEPSA refinery
The CEPSA FCC in San Roque (Spain) is a UOP side-by-side design with complete combustion of CO in the regenerator. The original design capacity of 4,200 m3/d has since been increased to an actual capacity of 6,000 m3/d. Since the existing air blower was not designed for this increased demand it was decided to employ oxygen enrichment. CEPSA’s experience at the San Roque refinery was that oxygen enrichment increases the flexibility of the FCC with regard to varying feedstocks. This allows to increase conversion, or alternatively to increase throughput at constant conversion. This was of particular interest to CEPSA since the feedstock for the FCC plant changed daily. Because the feed changed on a day-to-day basis, the decision concerning the use of oxygen was also made daily on the basis of the production demand. The highest oxygen concentration used at the CEPSA plant was 22.4 vol %. The resulting potential was exploited in three ways:
-the unit was able to cope with rapid changes in feed composition;
-conversion could be increased for a
given feed;
-as another option, throughput could be increased at constant conversion.
A frequently asked question is whether oxygen enrichment does not raise the temperature in the regenerator beyond tolerable levels. A simple heat balance showed that oxygen enrichment resulted only in a negligible reduction in heat sink capacity due to the reduced amount of nitrogen. The main heat sink is the catalyst whose heat capacity is not changed by the addition of oxygen. Accordingly, there is basically no correlation between regenerator temperature and oxygen concentration of the regenerator air feed. In contrast, the
coke burn-off from the catalyst has a much more significant impact on the regenerator temperature. The operating results of the CEPSA FCC at San Roque can be summarized as follows:
-increasing the oxygen content by 1% resulted in an increase in regenerator capacity of 6%.
-increasing the oxygen content by 1% increased the regenerator temperature by less than 2 °C. Coke burn-off, rather than O2, defines the temperature rise.
-Oxygen increases the ability to handle high coke formation.
-This, in turn, allows the refiner to treat heavier feeds, especially by adding residues to the feed.
-Oxygen enrichment allows to increase conversion and/or throughput capacity of the FCC.
-Normal operating conditions were easily obtained with enriched air.
-Oxygen reduces the formation of residues in the FCC.
Costs of oxygen enrichment
The modification of an FCC unit for oxygen enrichment requires relatively small investments, typically of the order of $100000 to $ 300000. An oxygen source must also be provided, though often a simple liquid oxygen tank is sufficient. The costs of this depend primarily on the tank size, i.e. ultimately on oxygen needs and
size of the FCC. The length of the O2 duct between O2 source and FCC also contributes to the costs.
If large amounts of O2, i.e. in excess of 1000 m3/h, are consumed continuously, an air separation unit may become economically feasible. Many industrial gas suppliers offer lease options for such units or over-the-fence delivery sparing the refiner the investment.
Economic results of the CEPSA refinery
Two test runs with oxygen enrichment were carried out at the San Roque refinery within a short period of time keeping the quality of the feed consistent. Oxygen enrichment was tested at two different levels. The main test parameters are listed in Table 1.
In the first test run, the temperature of the feed oil was kept constant at 235°C. Oxygen was added to the regenerator air. The feed inlet was raised until the output of the wet gas compressor became limiting. The temperature of the riser was allowed to increase. After seven hours, samples
were taken and then the second test run was initiated. In the second run, the oxygen concentration was increased to 22.4%, the feed preheat was lowered by 11°C to 224°C, and the riser temperature was kept almost constant except for an increase by 1°C. The feed amount was increased until the wet gas compressor capacity again became limiting. After four hours, samples were taken and the addition of oxygen was discontinued.
As planned, the quality of the feed was basically constant during the tests. Only the aniline point increased by 2.2°C. The 90% distillation end point for gasoline was 158°C before the tests, 156°C during the first test run, 157°C during the second test run, and 162°C after the tests. The octane indices were not measured.
The economic data shown in Table 2
were obtained in the analysis. CEPSA drew the following conclusions from these test runs:
-It allows to use HCO (heavy cycle oil) as
a normal feed component.
-The limitation posed by the air blower capacity can be eliminated.
Subsequently, the oxygen enrichment test installation was converted to a permanent supply and the FCC was successfully operated in this mode for several years.
Economic results of a refinery in Brazil
A refinery operator in Brazil permitted the use of his proprietary valuation numbers to calculate the effects of oxygen enrichment. This operation was a fairly typical case in that the production of gasoline was the primary goal of this FCC. All other products are less valuable, though the value of LPG comes very close to that of gasoline. The data included the change in conversion and ensuing change in product spectrum that resulted from the increased load and oxygen enrichment. Based on this these data, we calculated the respective turn-over using this refineries’ internal rating numbers. The results for oxygen enrichment to 22.9% are summarized in Table 3. Oxygen enrichment increases the capacity of the plant. Due to the influence of oxygen there is a slight shift in product composition in favor of LCO/Diesel and decanted oil. This was paralleled by a shift in costs and revenues. Altogether, the refinery increased its profits by 5.93% by oxygen enrichment as compared to the operation with air.
However, these data are dependent on the oxygen price. Oxygen enrichment necessitates a relatively small investment only, but increases operating costs, which is equivalent to low fixed costs, but high variable costs. Consequently, this option becomes less attractive with increasing oxygen prices. The O2 price is very site-specific and different between individual FCCs. By comparison, the installation of a new air blower is associated with higher investment costs that need to be depreciated over several years. This is equivalent to high fixed costs, but low variable costs.
Economic results of the Holborn refinery
Various refiners confirmed that the payback of the oxygen enrichment installation in their FCCs also was on the order of a few weeks only. A more detailed insight was granted by Holborn refinery in Hamburg. Holborn is especially interesting because they use their FCC primarily to produce middle distillates rather than gasoline. Therefore, the FCC is operated at a low cat/oil ratio of less than five and the riser operating temperature is a mild 500°C.
Holborn has two main incentives to consider oxygen enrichment:
1. The air blower of the regenerator often reaches its limits leading to an insufficient amount of air being available to burn-off all the coke on the catalyst. This limits flexibility in the choice of feed oils with higher ConCarbon content.
2. As the FCC riser is operated for middle distillates at low severity it generates barely enough coke to keep the temperature of the system up. Oxygen enrichment can improve this situation by burning more coke off the catalyst and at the same time reducing the amount of inert nitrogen needing to be heated up without use.
The profit for the Holborn case were calculated based on the internal valuation numbers using the procedure described above. The result evidenced an increase of the net profit by approx. 10% with oxygen accounting for only 0.22% of the combined feed cost.
Comparing the gasoline-FCC in Brazil and the Holborn middle distillate-FCC, substantial differences in the effects of oxygen addition are apparent. These differences are due to the difference in cracking severity: more severe cracking in the gasoline-FCC is associated with more extensive coke formation and an ensuing higher air demand for regeneration. The much higher air demand in the gasoline-FCC reduces the economic effect of a given amount of oxygen added.
Equipment needs for test runs
Typically, refinery operators want to see the effects of oxygen enrichment first hand and, therefore, tests are desired. The test runs usually require only little investment. The majority of the equipment required for the tests is available for rent. The tests generate reliable data for the decision on whether or not to convert to permanent oxygen use. A test installation requires:
-an O2 dosing station, a so-called Flowtrain,
-a trailer tank for supply of liquid O2,
-piping or pressure hoses connecting the O2 tank and the FCC unit.
Flowtrain and liquid oxygen supply tanks can be leased from Linde. Only the installation of the piping between tank and FCC is associated with investment costs, though often pressure hoses can be used for connecting tank and FCC. These hoses can also be obtained on a rental basis.
The subsequent test runs usually take between four and six weeks depending on whether or not the effects of strongly varying compositions of feedstocks and/or the addition of residue oil is to be tested. Usually, oxygen is added in the test runs to the predetermined level.
Alternatively, the oxygen content of the regenerator air can be slowly increased while monitoring the temperatures in the plant. The oxygen flow can be interrupted at any time without difficulties. However, since it takes the equilibrium catalyst several hours to adjust to the new conditions it is not advisable to interrupt the tests. The interruption of oxygen addition does not adversely affect plant safety.
Switching to permanent use
The rented Flowtrain can be converted from a lease to a buy option. Often, a tailor-made device may better serve the purpose at hand. For the supply of oxygen,
either a tank installation, an on-site air separator or over-the-fence deliveries from a pipeline may be considered. Which of
these supply options is best suited depends on the amount of oxygen required, the fraction of time in the supply is needed, and the range of fluctuation anticipated.
The control of the Flowtrain has to be integrated into the FCC unit’s control system including safety interlocks, etc. While alarms may be adequate during the test period, an automatic switch-off may be required in the permanent installation. The details have to be discussed for each case and adapted to the refinery’s overall control and safety system.
The FCC unit is guarded by Flowtrain against unplanned admission of O2 by a number of safety installations.
1. Surplus oxygen: The Flowtrain contains a high alarm in the flow controller which reacts when the target value for oxygen is exceeded by 0.2 % (v/v). This alarm allows the operator to re-adjust the oxygen content by appropriate means. If the oxygen content continues to rise
there is a high-high alarm plus safety switch at 0.35 % (v/v) of excess oxygen and the O2 flow is stopped completely.
2. Failure of oxygen injection: A failure of the oxygen supply is irrelevant for plant safety, means that coke accumulates on the catalyst over time leading to reversible deactivation of the catalyst. If the oxygen supply cannot be re-established
within 1/2 hour, the amount of feed oil must be adapted to operating conditions with air only.
3. Failure of instrument air: upon failure of the instrument air, the block-and-bleed valves automatically switch into
safe position, i.e. the block valves are
closed and the bleed valve is opened.
4. Low temperature switch for oxygen: If the oxygen temperature drops below –5 °C (23 °F) an alarm rings. This type of failure can happen when feeding the plant from an liquid oxygen tank. If the temperature drops below –20 °C (-2 °F), the oxygen flow is stopped to ensure that no liquid oxygen enters the air duct of the regenerator where it might cause thermal stress. This measure also effectively prevents the instrument air in any of the actuators from freezing.
Safety controls and switches of the DCS system of the FCC: If the FCC is switched off by the DCS of the refinery, the air flow is switched off also. Since the oxygen flow is coupled to the air flow, switching off the air necessarily also switches off the O2. That means that the O2 control is indirectly connected to the safety switches of the FCC.