Increasing Motor Octane in the FCC Unit, Part 1
During the recent past refiners have been forced to increase the octane of the gasoline pool to meet the impacts of:
These needs were met without major capital investment by increasing severities in existing catalytic cracking units and judiciously choosing proper catalysts to maximize octanes.
Projecting into the near future, the need for additional premium and mid-grade gasoline grades is expected to increase. This demand stems directly from the public's dissatisfaction with the "knocking" or self-ignition tendency of the regular unleaded grade products. To meet the demand for higher octane gasoline, refiners are reviewing options on possible investments in incremental isomerization, alkylation, etherification, polymerization, reforming, catalytic cracking capacity and the use of high octane blend stocks.
Significance of MON.
Minimum Motor Octane Numbers (MON) for pooled gasoline are also beginning to be set to ensure good fuel performance. These octane values are higher than those of past years and have forced the refiners to review, in the short term, means by which octane can be improved from existing capacity.
Since the FCC gasoline constitutes the highest percentage of the gasoline pool in most refineries, increase in octanes of this blending stream has a significant effect on total refinery octane production.
There are numerous options available to refiners for enhancing octanes from the catalytic cracking unit. These involve operational and catalyst changes. On the operational side, changes in reactor temperature, conversion level, gasoline end point, recycle rate and feed quality have impacts on Research Octane Number (RON) and MON. Operational changes can result in octane gains of up to 3 RON and 1.0 MON. Catalyst selection can also enhance octane up to 3 RON and 1.5 MON depending on the base catalyst and octane level.
The changes in octanes noted are directly attributable to chemical composition shift in the gasoline. These in turn are related to changes in catalytic cracking operating variables and catalyst types. These parameters and their affect on cracking mechanisms for improving gasoline MON are reviewed in this two part series. In Part 1, the non-catalytic techniques to increase MON will be reviewed while Part 2 will discuss the catalytic design variables that affect MON.
An octane number is a relative measure of a fuel's knocking or self-ignition property in an internal combustion engine. Octane numbers are measured with a standard single cylinder variable compression ratio engine. To measure the fuel's MON, the engine is operated at 900 revolutions per minute and the compression ratio is increased until the onset of knocking. Compare this compression ratio relative to that for a known ratio of iso-octane/normal heptane fuel mixture to determine MON. Directionally, as the compression ratio of the engine increases so does the required octane number of the gasoline if engine knocking is to be avoided.
RON is measured in a similar manner but at a lower engine severity to simulate low load conditions.
The octane properties of pure hydrocarbons (Ref. 1) and their blending values are illustrated in Table 1. The data indicate that MON blending values are highest for branched olefins, aromatics isoparaffins and light naphthenes. This implies that operating variables and catalyst properties that enhance these hydrocarbons, in gasoline boiling range, would produce high MON. To increase the concentration of these hydrocarbons, operating and catalyst changes allow shifting the relative rates of the basic reactions (Ref. 2, 3, 4).
a. PARAFFINS (1) ----> PARAFFINS & OLEFINS
b. ALKYL NAPHTHENES (1) ----> NAPHTHENES & OLEFINS
c. NAPHTHENES (1) ----> OLEFINS
d. NAPHTHENES (2) ----> AROMATICS
e. ALKYL AROMATICS (1) ----> AROMATICS & OLEFINS
f. OLEFINS (3) ----> ISO-OLEFINS
g. OLEFINS (4) ----> PARAFFINS
Most of the time the refiner will be changing the relative rates of cracking and hydrogen transfer reactions. The latter reactions will be minimized in order to preserve the olefins generated during the primary cracking.
There are several operating variables that affect MON such as feed quality, reactor temperature, conversion, gasoline end point, gasoline Reid vapor pressure (RVP) and cycle oil recycle. Their effects are as follows:
Hydrocarbon type distribution in
the gas oil will determine its crackability as well as the
chemical composition of the gasoline. The hydrocarbons in the
feed will influence octane as follows
To characterize the gas oils and provide a guide to their crackability, many sophisticated methods (Ref. 5,6,7) have been developed. However, use of these methods is time consuming. To provide the quick turnaround needed for operating personnel, feed gravity measurement can provide adequate guidelines. As long as the distillation range of the gas oil is constant, changes in gravity would represent shifts in its chemical composition. Figure 1 shows the relationship of MON vs. feedstock gravity for a full boiling range gasoline. Octane can also be readily correlated with aniline point and UOP "K" factor.
The feedstock composition has significant effect on MON. It would be highly desirable if the operator had total control of the types of feeds processed in the catalytic cracking unit. In practice, that is not the case and in most refineries operators have limited capability in controlling gas oil quality.
Aside from octane considerations, the refiner should consider other gas oil properties such as crackability and product selectivities. These properties in combination with catalytic cracking unit constraints may offset the octane benefits of the feedstock. Care should also be taken to exclude naphtha from feeds to the cracker. A small amount (1-2 vol%) of this low octane component in the feed can depress MON of FCC gasoline by 0.5 number. Naphtha behaves as a diluent for the cracked gasoline off the unit.
Reactor temperature is the easiest parameter for the operator to control and, compared with other variables, has the greatest impact on the Research Octane Number. Reactor temperature has substantially less impact on MON. Motor octane changes as a function of reactor temperature were obtained from commercial units and illustrated in Figure 2. Based on the pilot plant and commercial data the MON/ RON ratio for the reactor temperature effect is in the region of 0.3.This suggests that this parameter has a significant effect on gasoline octane sensitivity.
Conversion has a strong influence on the gasoline composition and consequently on its MON. Increasing conversion at constant reactor temperature by 10 vol. %, increases MON by approximately 0.8 numbers in the 50-80 vol% conversion region as shown in Figure 3. The octane response is reduced at lower conversion. Note that the RON response to conversion is approximately the same as MON. The ratio of MON/ RON is approximately 1 in the regions measured.
The octane gains observed are directly related to increase in aromaticity of the gasoline with increasing conversion. Since aromatics have low sensitivity, the increases in MON approaches that of RON.
Gasoline End Point
Refiners frequently find it necessary to undercut full range gasoline for special situations such as for maximization of fuel oil production during the winter season.
Reducing the end point results in increased gasoline octane. This is a consequence of fractionating out the low octane components normally found in the tail end of the gasoline.
A typical distribution of octane numbers with boiling range are illustrated in Figure 4. The data for the curve was generated from commercially produced gasoline. It was split into various fractions and the points plotted represented the average octane number of those fractions.
As noted on the curve, the last 10% of the gasoline has very low MON. This is because this higher boiling range contains heavy polycyclics which depress MON. Removing this fraction would increase the octane of the remainder. In this instance, the removal of the last 10% (~ 60°F or 33°C undercutting) of the gasoline would improve the octane of the remainder by 1 MON. The ratio of MON/ RON for this sample is approximately 1.
The octane distribution curve changes with feed type, unit severity and distillation range of the gasoline. The refiner should generate this type of information for his own unit to provide him with a guide as to the possible octane penalty versus gasoline end point adjustment. The refiners' own octane distribution curves might show an improvement in RON but a debit in MON when undercutting the gasoline. The curve is feed / unit specific (Ref. 8).
RVP control of the blended gasoline is normally accomplished by the use of C4 hydrocarbons. These compounds have good blending octane value and raise the octanes of the blended gasoline by 0.3 MON/2 psi change in the vapor pressure. The response may vary and depends on whether the refinery uses a mixture of C4's or pure individual hydrocarbons (n butanes). The ratio of MON/ RON derived from the use of C4's is approximately 0.5.
Cycle Oil Recycle
The recycle of light or heavy cycle oil improves the gasoline octane by approximately 0.3 MON for 10-20% increase in the combined feed ratio. This method is not practiced often since it reduces fresh feed capacity of the unit. The MON/ RON ratio for this variable is approximately 0.6.
1. American Petroleum Institute-Project 45 ASTM Publication 225, (1978).
2. L.F. Hatch, Hydrocarbon Process, Vol. 48, 77-88 (1969).
3. J.J. McKetta, ed., "Advances In Petroleum Chemistry And Refining", Vol. 5, pp 759-810.
4. W.A. Gruse and D.R. Stevens, "Chemical Technology of Petroleum", 3rd Edition, pp 375-387.
5. Reif, H.E., Kress, R.F. and Smith, J.F., Petroleum Refiner, Vol. 40, No. 5, pp 237-241 (1961).
6. Watson, K.M., Nelson, W.L., Murphy, G.B., Ind. Eng. Chem., Vol. 27 (1935).
7. White, P.J., Oil & Gas Journal, Vol. 66, pp 110-112, May 20 (1968).
8. A.D. Reichle, W.L. Shuetle, L.A. Pine and T.E. Smith, paper AM-81-42, NPRA, presented at NPRA Annual Meeting in San Antonio (1981).