Juan Perez de Haro, Antonio J. Berlanga
Gonzalez Compania Espanola de Petroleos S.A.
San Roque, Spain
Nina Schroder
AGA Gas AB Sundbyberg, Sweden
Olle Stenberg
AGA Innovation
AB Lidingo, Sweden
At the Gibraltar refinery in San Roque, Spain, Compania Espanola de Petroleos S.A. (Cepsa) finds it easier to cope with continuously changing feedstocks by using oxygen enrichment of fluid catalytic cracking (FCC) regenerator combustion air.
Intermittent enrichment operation has enabled the fluid catalytic cracking unit (FCCU) to quickly adapt to changes in feedstock, to increase conversion, or alternatively, to increase throughput at maintained conversion.
In 1986, AGA AB installed an oxygen supply system to enrich the FCC regenerator combustion air at the Gibraltar refinery. Test runs were performed in an attempt to increase throughput and to try resid processing in the FCC. At that time, the unit was limited by the air blower capacity.
Cepsa continued to use oxygen enrichment after the trials until the air blower capacity was expanded from the design capacity of 100,000-112,000 normal cu m/hr (84.76-94.93 MMscfd). This, together with a change in the oil market, reduced the incentive for using oxygen on a regular basis.
In October 1990, the use of oxygen was resumed as the result of increased regenerator capacity demand.
OXYGEN ENRICHMENT
The use of oxygen in FCCUs has been reported in literature.1-4 Historically, the main incentives for trying this technique have been:
- Air blower limitations
- Velocity limitations
- Seasonal variations
- Feed variations.
In the following short description of the reasons for using FCC oxygen enrichment, the examples should be considered general guidelines.
For a specific case, it is necessary to do a thorough evaluation based on the local conditions.
BLOWER LIMITATIONS
When the air blower is the limiting factor for an FCCU operating at constant conditions, the production capacity is directly proportional to the amount of oxygen available for catalyst regeneration. This gives the following expression for FCCU capacity:
[SEE FORMULA]
If the air is enriched with pure oxygen, the new unit capacity will be related to the increase in oxygen input according to:
[SEE FORMULA]
The percentage capacity increase then becomes:
[SEE FORMULA]
The oxygen concentration can be calculated from:
[SEE FORMULA]
Combining Equations 3 and 4 gives the relationship between capacity increase and oxygen concentration in the -enriched air:
[SEE FORMULA]
This relationship is presented in Fig. 1.
VELOCITY LIMITATIONS
Superficial gas velocity is defined as the volumetric flow rate of the gas divided by the cross section area of the unit.
An FCCU may be operating at the maximum superficial gas velocity in the regenerator.
In this case, there is one additional restriction to the system when one wants to increase overall capacity while keeping gas flow constant.
This restriction is the fact that there is a maximum limit for the gas velocity in the unit, above which catalyst entrainment will occur.
This can be achieved by reducing the air flow and keeping the superficial gas velocity in the regenerator constant by adding oxygen. Because the total gas flow is constant, the FCCU capacity will be proportional to the ratio between the concentration of oxygen with and without enrichment:
[SEE FORMULA]
Rearranging Equation yields the relative capacity increase:
[SEE FORMULA]
This relationship is demonstrated in Fig. 1.
SEASONAL VARIATIONS
Depending on the geographical location of the refinery, the seasonal variations in air blower capacity can be extensive. There are actually two main variations to be considered.
During summertime, the demand for gasoline is generally higher than at other times of the year. This change in optimum product mix can in some cases be achieved by applying oxygen enrichment.
An increase in regenerator capacity can facilitate operating at a higher conversion and gasoline yield. If the unit is already optimized for gasoline production, the increase in gasoline demand can be satisfied by increasing the feed flow and boosting the regenerator capacity by oxygen addition.
Another seasonal variation is the change in air blower capacity due to changes in ambient air temperature and humidity. An indication of the order of magnitude of this effect will be given later. The decrease in air blower capacity and the increase in gasoline demand both occur in the summertime, and both can be compensated for by oxygen enrichment to the regenerator.
FEED VARIATIONS
Resids and very naphthenic feeds yield more coke on the catalyst than does the gas oil normally processed. This results in increased regenerator capacity demand. If air blower or velocity restrictions are present, the use of oxygen can be a convenient way to increase flexibility at constant processing capacity.
This incentive has been of major importance at Cepsa, where the FCCU frequently must process varying feeds.
OTHER EFFECTS
Besides the previously mentioned incentives for oxygen use, there are also other positive side effects. If the oxygen is supplied in liquid form, the evaporation of oxygen can add cooling capacity to the cooling towers.
In addition, the increased flue gas flow from the regenerator is used for power or heat generation. (This shall also be taken into consideration when the total economic evaluation is made.)
An increase in oxygen concentration of 1.0% corresponds to a 1.73 wt % increase in flue gas flow. This increment is proportional to the energy available to recovery via steam production and/or electricity savings.
GIBRALTAR FCCU
The FCCU at Cepsa (Fig. 2) is a UOP side-by-side design with a high-efficiency regenerator, i.e., complete CO combustion. The design capacity is 4,200 cu m/day (26,400 b/d). The actual capacity today is 6,000 cu m/day (37,760 b/d).
Six streams from the refinery feed the FCCU. Conversion normally varies between 55 and 75%, depending on feed quality.
The stream of heavy cycle oil is usually not recirculated. The catalyst inventory is 125 tons. Normal operating temperatures for Cepsa's FCCU are as follows:
- Oil feed, 220-240 C. (428-464 F.)
- Riser, 500-520 C. (932-968 F.)
- Regenerator dense phase, 735 C. (1,355 F.)
- Regenerator dilute phase, 740 C. (1,364 F.)
- Regenerator flue gas, 735 C. (1,355 F.).
The cyclones and strippers in the FCC reactor have been replaced in order to handle the increased catalyst load. The maximum velocity in the cyclones, to avoid excess catalyst entrainment, is 75 fps.
The air blower capacity varies depending on the outdoor conditions. As a demonstration of some typical temperature and humidity values, air blower capacity at the Gibraltar refinery is as follows:
- Winter (12 C., 50% humidity), 2.8 million normal cu m/day (98.88 MMscfd)
- Summer (30 C., 80% humidity), 2.6 million normal cu m/day (91.82 MMscfd).
Occasionally, the capacity further decreases because of rain or sand in the air. Oxygen addition can compensate for this loss in capacity.
OXYGEN SUPPLY/CONTROL
The oxygen supply system is outlined in Fig. 3. The heat of vaporization of the liquid oxygen (LOX) is used in cooling the water for the cooling towers.
The oxygen supply is shut off when one of the following conditions is reached:
- High temperature in the regenerator
- High oxygen concentration in the air pipe
- High pressure in the re-generator
- Low temperature after the oxygen vaporizer
- Low pressure in the air pipe
- Air blower shut off
- Manual emergency shut off.
Many of the controls are redundant (e.g., 2 and 7). The oxygen supply pressure is 8 bar.
At start-up of oxygen use, the flow is slowly increased. The flow is manually controlled by the operator to maintain flue gas oxygen concentration at 0.35%.
The maximum operating riser temperature is 525 C. (977 F.). The highest oxygen concentration used at Cepsa is 22.4%.
ENRICHMENT EXPERIENCE
Experiences from a 9-month period of intermittent oxygen enrichment are shown in Figs. 4-12. During this period, the normal operation variables were studied. The main incentive for oxygen enrichment was to increase the flexibility of the unit.
Because the feed changed on a day-to-day basis, the decision whether to use oxygen was made daily, on the basis of the production demands (Fig. 4).
The largest oxygen addition used at Cepsa was 60,000 normal cu m LOX/day, and the maximum oxygen concentration in the regenerator gas feed was 22.4 Vol %.
During the monitored period, the operating conditions changed on a day-today basis. Important changing conditions that influence FCC performance include feed characteristics, output composition demand, and capacity demand.
The results from the period consist of 295 datapoints, each representing 1 day of operation. Of these 295 points, 100 represent operation with oxygen enrichment.
The amount of oxygen added to the air stream was governed by the need for extra regenerator capacity for a specific feed/output demand combination.
REGENERATOR CAPACITY
Because of the fluctuating operating conditions, the variation in overall performance of the FCCU makes an evaluation according to Equation 5 irrelevant.
The oxygen concentration was not varied systematically for one specific feed. The use of oxygen was determined by the required overall performance of the FCCU.
The regenerator capacity demand is reflected in the total amount of produced coke. In accordance with Equation 1, the capacity, and therefore the produced coke, should be proportional to the amount of available oxygen.
The amount of produced coke vs. consumed oxygen in the regenerator is plotted in Fig. 5. The straight line corresponds to a linear regression including all values,, with and without oxygen enrichment.
Figs. 6 and 7 show the correlation of data from air and enriched air separately. In both cases the data fit the line based on all data, indicating no systematic difference between the two groups of data.
From the data in Figs. 5-7, it is clear that oxygen enrichment contributed to the performance of the unit when high amounts of coke were produced.
The oxygen addition has made it possible to operate at high levels of coke formation. This potential increase in capacity has been used in three ways:
- The unit is able to cope with quick changes in feed composition.
- For a given feed it is possible to increase converSion.
- It is possible to increase throughput and maintain conversion.
On a day-to-day basis, combinations of the different wan,s of using the capacity increase were utilized. Because , of the quick changes in operating conditions, it was not possible to make a meaningful analysis for one specific feed, keeping the operational conditions constant.
REGENERATOR TEMP.
The question of regenerator temperature level is often raised when discussing the possibility of increasing regenerator capacity b), oxygen enrichment. It is fairly easy to show, in a simple energy balance for the unit, that the increase in regenerator temperature will be negligible for moderate enrichment levels.
The net heat balance effect of the oxygen enrichment will result in a negligible reduction in heat sink capacity caused by the relative reduction in the nitrogen flow in the regenerator. This is because the main heat sink capacity comes from the catalyst.
The regenerator dense phase temperature is shown in Fig. 8. From this, it is obvious that there is no correlation between regenerator temperature and oxygen concentration in the regenerator air feed.
CONVERSION
For a specific feed composition and input, a strict relationship between the conversion and oxygen level can be expected. In other words, increasing the oxygen concentration to the regenerator should enable a deeper cut in the feed, resulting in higher conversion.
But as feed composition continuously changes, the conversion will not be strongly correlated to the oxygen concentration (Fig. 9). The degree of conversion is mainly influenced by other variables such as feed composition and unit throughput.
COKE FORMATION
Coke formation vs. oxygen concentration is depicted in Fig. 10. The results from the monitored operation period are not obvious.
A slight increase in coke formation, along with higher Oxygen concentration, can be observed. The trend is very weak because the coke formation is influenced by several other fluctuating variables.
The amount of oxygen fed to the regenerator is controlled by maintaining the effluent oxygen concentration at 0.35%. If the amount of feed to the FCCU is increased above the level corresponding to the maximum regenerator capacity with air, then oxygen enrichment is started.
For a specific feed, the ratio between the total amount of oxygen fed to the regenerator and the feed flow to the unit shall be constant, in order to maintain operating conditions.
If there is varying feed quality with fluctuating coke formation tendencies, the oxygen-to-feed flow ratio will vary along with the coke burning capacity demand.
In Fig. 11, the coke formation vs. the oxygen-to-feed ratio is presented.
The data are clearly segmented into two categories: operation with air and air enriched with oxygen. Oxygen enrichment enables the unit to cope with higher coke formation levels.
Fig. 12 shows the riser temperature attained at different oxygen-to-feed ratios.
The difference in performance with and without oxygen is quite clear.
Oxygen enrichment makes it possible to maintain a relatively high riser temperature, even at low oxygen-to-feed ratios. The high riser temperatures will generate more coke, in line with the results shown in Fig. 7.
The experience of oxygen enrichment from the monitored period shows no operational problems related to the oxygen use. The normal operating conditions could be maintained when the regenerator capacity was increased.
The increase in regenerator capacity results in:
- Capability to meet changes in feel composition
- Possibility of increasing conversion for a specific feed
- Possibility of increasing throughput at maintained conversion
- Increase in flue gas flow, reducing power consumption
- Increase in cooling effect from evaporation of LOX.
REFERENCES
- "How does adding oxygen to cat regenerator air affect unit operation," OGJ, Oct. 19, 1959, p. 134.
- Macerato, Frank, and Anderson, Sidney, "O2 enrichment can step up FCC output," OGJ, Mar. 2, 1981, p. 101.
- Hansen, T.S., "Study shows oxygen/resid relationships in FCC operations," OGJ, Aug. 15, 1983, p. 47.
- Bhasin, Liebelson, Chapman, "Oxygen increases FCC thru-put," Hydrocarbon processing, September 1983.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
