Mineral oils: processes and products – Products and compositions – Fuels
Reexamination Certificate
1999-08-13
2001-04-03
Howard, Jacqueline V. (Department: 1764)
Mineral oils: processes and products
Products and compositions
Fuels
C585S014000, C585S734000, C208S027000
Reexamination Certificate
active
06210559
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
The present invention is a process for producing a distillate fuel heavier than gasoline. In particular, it is a process to optimize the production of a distillate from a hydrocarbon synthesis process. The use of Fischer-Tropsch (hydrocarbon synthesis) liquids as pure or as a component of distillate fuels is well known in the art. The products of the Fischer-Tropsch synthesis are predominantly normal paraffins. Economically it is desirable to operate Fischer-Tropsch catalysis at the highest possible Schulz-Flory alpha, in order to minimize undesirable light paraffins. High alpha operation over high activity cobalt catalysts, results in a high boiling, paraffinic wax product that is unsuitable for direct distillate blending. A high quality diesel blend stock is typically produced from the high Schulz-Flory product using hydroisomerization and or mild hydrocracking of the 700° F.
+
wax. Sie, S. T. [Catalysis Letters 1990, 7, 253-270], invokes the hydroconversion of the entire hydrocarbon synthesis product. This hydroconversion results in 100% paraffinic products, although the degree of branching may vary. One of the great advantages of Fischer-Tropsch derived diesel fuels is their high inherent cetane number. There is a great incentive to maximize the cetane of the fuel in order to increase its value as a diesel blend stock, however, the product diesel must also meet any appropriate cold flow specifications, such as diesel cloud point or cold filter plugging point (CFPP). High cetane number corresponds with high molecular weight and low levels of branching, while cold flow often requires lower molecular weights and high levels of branching. Optimization of these two properties, either in blending or in actual plant operation is unwieldy due to the time consuming nature of both engine cetane and CFPP determinations. The present invention uses
13
C NMR to rapidly determine both cetane and cold flow properties. These determinations are then used to optimize both product blending and unit operation. More detailed information about the molecular structure is also provided by the
13
C NMR analysis and can serve as a valuable process diagnostic.
SUMMARY OF THE PRESENT INVENTION
The present invention is a process for producing a distillate fuel heavier than gasoline. The process uses Fischer-Tropsch (hydrocarbon synthesis) products from which the distillate fuel is produced. The process includes hydroisomerization selectivity and conversion which are typically controlled by catalyst selection, and variation of process conditions such as temperature, pressure, space velocity or gas treat rate. Any of these parameters could result in a wide range of diesel properties, such as cetane and cold flow properties. Therefore, in its broadest aspect, the present invention comprises obtaining the
13
C NMR spectrum of the distillate product and determining numbers representative of the engine cetane number and a cold flow property in a process for producing a distillate fuel heavier than gasoline, wherein the distillate fuel is produced from a Fischer-Tropsch product that is hydroisomerized, blended and fractionated. In a preferred embodiment, cetane number and a cold flow property are determined by
13
C NMR in order to optimize the distillate fuel production process. Cold flow properties include cold filter plugging point, cloud point, pour point and low temperature flow test. Cetane number and cold flow properties of all paraffinic diesel fuels are essentially inversely correlative properties. High cetane number corresponds with high molecular weight and low levels of branching, while cold flow often requires lower molecular weights and/or higher levels of branching. The position of the branches along the molecular backbone can also significantly influence both engine cetane and CFPP. Optimization of these two properties, either in blending or in actual plant operation is unwieldy due to the time consuming nature of both engine cetane and CFPP determinations. The present invention uses
13
C NMR to rapidly determine both cetane and cold flow properties and the use of said determinations to optimize both product blending and unit operation.
A preferred embodiment of the present invention includes the steps of separating and selectively treating the product of a Fischer-Tropsch process. The initial separation is a heavier fraction (a) and a lighter fraction (b). The lighter fraction (b) is further separated using a temperature separator having an adjustable temperature into at least two fractions: (i) at least one fraction including light normal paraffins, and (ii) at least one fraction including heavy normal paraffins wherein the separation between light and heavy paraffin fractions is determined by the temperature. At least a portion of the heavier fractions (a) and at least a portion of the (b)(ii) fractions are then hydroisomerized and then blended with at least a portion of the fraction of (b)(i) to produce a blended stream. This blended stream is then distilled and the
13
C NMR spectrum of the distillate product is obtained, determining numbers representative of the engine cetane number and a cold flow property. The cold flow property may be cold filter plugging point, cloud point or pour point. The temperature of the separator is adjusted in response to the
13
C NMR data to optimize cetane number and the cold flow property.
REFERENCES:
patent: 5378348 (1995-01-01), Davis et al.
patent: 5689031 (1997-11-01), Berlowitz et al.
patent: 5807413 (1998-09-01), Wittenbrink et al.
patent: 5814109 (1998-09-01), Cook et al.
patent: 5895506 (1999-04-01), Cook et al.
Berlowitz Paul J.
Cook Bruce R.
Silbernagel Bernard G.
Sysyn Debra A.
Exxon Research and Engineering Company
Hantman Ronald D.
Howard Jacqueline V.
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