Data processing: structural design – modeling – simulation – and em – Simulating nonelectrical device or system – Chemical
Reexamination Certificate
1998-07-20
2001-04-03
Teska, Kevin J. (Department: 2123)
Data processing: structural design, modeling, simulation, and em
Simulating nonelectrical device or system
Chemical
C703S002000, C703S006000
Reexamination Certificate
active
06212488
ABSTRACT:
This invention relates to operations performed in a digital computer, and more specifically to a computer software program for simulating a chemical reaction involving breaking down of large molecules of hydrocarbons into smaller, lighter, more valuable molecules.
BACKGROUND OF THE INVENTION
The most important hydrocarbon refining process, which in the 1940's revolutionized the refining industry, is catalytic cracking of large hydrocarbon molecules. The catalytic cracking process is the largest catalytic process in the world, and is extensively used today for the production of gasoline from high boiling hydrocarbons such as residual and gas oil fractions. In many refineries heavy residual oil resulting from distillation of crude oil, or so called topped crude or simply resid, is pretreated in a hydrotreating process before sending the resid to a fluid catalytic cracking (FCC) process step. As used herein a “heavy oil” is taken to mean a hydrocarbon liquid boiling at atmospheric pressure in a range of from about 650° F. to as high as 1500° F., and which contains a variety of very complex chemical compounds.
A principal factor which affects the economic viability of a fluidic catalytic cracking unit is the amount of the feedstock that is converted to a desired product such as gasoline. In many refineries the feed to an FCC unit consists of a heavy oil containing an unlimited mixture of complex molecules of straight and branched paraffins, cycloparaffins, and aromatics ranging from monoaromatics to four or five rings with a vast variety of side chains. It is generally very difficult to maintain a desired conversion of this heavy oil feedstock in an FCC unit.
In recent years the use of computers has increased greatly as a means to examine complex chemical reactions by simulation techniques. Digital computer simulation of catalytic cracking reactions is particularly valuable in many areas relating to refining of heavy oil such as predicting what a cracking reaction will yield under different operating conditions, in optimizing operation of a riser reactor, in planning and scheduling operations, and perhaps most importantly in selecting optimum crude and catalyst purchases.
An effective kinetic model to describe riser reactor cracking of hydrocarbon oils includes two essential features: First is a reasonable physical description of the riser reactor dynamics, accounting for variations in temperature, space velocity, residence time, cracking rates and catalyst deactivation over a wide range of feed stock composition and process conditions. Second is reliable predictions of the variation of the rate constants for cracking and product selectivity as a function of oil and catalyst properties.
A highly effective method and apparatus for simulating a catalytic cracking reaction by relating conversion, selectivity, and product yields to feedstock properties is disclosed and claimed in U.S. Pat. No. 5, 774,381 issued Jun. 30, 1998, to Paul F. Meier. However, the method disclosed in that patent relates hydrocarbon conversion to process variables and to feedstock properties that are routinely measured at the refinery such as API gravity, measured impurities (sulfur, Ni, and V), basic nitrogen, carbon residue and viscosity. While the disclosure of this patent is regarded as representing a significant contribution to the simulation art in predicting conversion of heavy hydrocarbons to cracked products, it is insufficient to differentiate chemical differences between crude feed types, i.e., sweet or sour, or between pretreatment, i.e., virgin or hydrotreated, or between fresh or recycle streams. Accordingly, it would be highly desirable to have a kinetic model for a riser reactor in catalytic cracking that is independent of the feed source or pretreatment.
An object of this invention is to improve efficiency of commercial refining operations.
A more specific object is to predict how well a specific oil fraction would run in a riser reactor of an FCC Unit.
Another object of this invention is to obtain data that facilitates improved selection of catalyst and/or crude oil stocks purchased for processing in a refinery.
Yet another object is to obtain kinetic reaction data that can be integrated into a process model for optimized operation of a total FCC process.
Still, another object of this invention is to make the kinetic model independent of the feed source and pretreatment.
Another more specific object of this invention is to predict essentially continuous boiling point distribution curves of C
5
cracked hydrocarbon products.
SUMMARY OF THE INVENTION
According to this invention, the foregoing and other objects and advantages are attained in a computer implemented method for mathematical modeling of reactions associated with catalytic cracking of residual and gas oil feedstocks to lower molecular weight products as a result of contacting with catalyst in a riser reactor of an FCC unit. The computer program simulating this reaction relies on the kinetic model for the riser reactor cracking of residual and gas oil feeds. A lumping scheme for the kinetic model according to this invention incorporates a relatively large number of small fixed boiling-point range pseudo-components, referred to herein as product lumps, for describing product material. A smaller number of larger boiling-point range pseudo-components, referred to herein as basic lumps, are defined for describing feed conversion and selectivity. A reaction conversion network flowing from heavier to lighter components is defined for the basic lumps, and an equation which can be generalized to any number of pseudo-components predicts changes in concentration for both the basic and product lumps as the simulated reaction proceeds.
In a preferred embodiment of this invention, the lumping scheme defines thirty-four pseudo-components for the product lumps covering an overall boiling point temperature range beginning at about 40° F. and extending to about 1500° F. Preferably five pseudo-components are defined for basic lumps covering the same overall boiling point temperature range. Each of the pseudo-components are treated as pure components for constructing kinetic conversion equations. Pure component characteristics are used for describing compounds up to and including C
4
hydrocarbons. The description of the hydrocarbons for the kinetic model is preferably based on measurements that yield chemical analysis in terms of total hydrogen, aromatic carbon, and aromatic hydrogen present in the feed. Conversion reactions are considered to be first order, and the kinetic model includes: rate constants for individual pseudo-components that are experimentally determined as functions of oil and catalyst properties, temperature, and reactor configuration; activation energies that are determined using linear interpolation and extrapolation between known values of activation energies for gas-oil and gasoline; and a second order decay function to account for catalyst deactivation. The computer simulation sequentially calculates the reaction rate constants at riser temperature, the fractional conversion and selectivity of the feed, distribution of product lumps included in the basic lumps, and yields of lower molecular weight products. The results of the simulated reaction, which include hydrocarbon conversions achieved, and yields of cracked products can be presented as printed numerical outputs, or presented as computer generated graphic displays. Accordingly, the kinetic model in this invention allows prediction of a nearly continuous distribution of products in terms of boiling-point.
In another aspect, apparatus according to this invention includes a programmable computer for storing the kinetic model for simulating the riser reactor cracking reaction, along with the required input data including process conditions for the reaction to be simulated, such as: oil properties of the fresh feed, recycle throughput ratio, equilibrium catalyst properties, space velocity, catalyst residence time and reactor temperature.
In accordance with yet another asp
Ghonasgi Dhananjay B.
Meier Paul F.
Wardinsky Michael David
Anderson Jeffrey R.
Phillips Petroleum Company
Teska Kevin J.
Thomson William
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