The art of catalytic cracking reactions has evolved considerably over the past fifteen to twenty years. It was common to employ a fluidized bed of catalyst particles in the catalytic cracking of petroleum feed stock to form desired light oils, gasolines, solvents and the like. Although it is possible with existing testing equipment to predict how a particular catalyst will behave with a given feedstock, the advances in the field of catalytic cracking has led to reactor designs which cannot be predicted by existing test units. There is a considerable lack of suitable reaction data for modelling and simulating the more advanced industrial scale riser catalytic cracking reactor which has typical contact times in the range of two to twenty seconds. There is significant uncertainty as to how to predict performance of industrial scale riser reactors. Hence the use of this technology in the petrochemical industry is severely hindered by the limited data and understanding of fast catalytic cracking reactions of different feedstocks in combination with various catalysts. It is this very data that the technical staff of a refinery needs to make crucial decisions about possible changes in operating conditions, modification of existing units, scaling up, processing of different feedstocks depending upon the source of supply, change of the catalysts, adaptation of the process to new conditions of the ever-changing gasoline market and other like considerations.
The same lack of relevant data applies to the regeneration of cracking catalysts under the conditions of riser regenerators. This is also a crucial matter, because the combustion of coke has a significant influence on the overall thermal balance of an industrial scale refinery. The endothermic heat consumed by the cracking reaction is normally supplied by the heat generated by the coke combustion.
Data about the fast regeneration of cracking catalysts is required to develop new cracker-regenerator configurations where both the regenerator and the cracker are transport line reactors. Several technical advantages can be claimed for transport line regenerators--uniform in control in coke levels in the catalyst at the regenerator exit, improved catalyst performance and selectivity and higher zeolite structure stability.
As mentioned, there are a variety of laboratory scale testing units available to determine the activity of selected catalysts and their effect on catalytic cracking of various feedstocks. An example of such a testing unit is disclosed in U.S. Pat. No. 4,419,328. This patent discloses a conventional fluidized bed controlled by a computer. A continuous flow of hydrocarbons is fed to the unit. In this unit, there is only a similarity between the reactant residence time (few seconds) whereas the catalyst time on stream is 300 seconds to 10,000 seconds. This is a major problem for a true modelling of riser reactors. The patent discloses that the fluidized bed of the reactor is fed with a continuous flow of hydrocarbons that produce fluidization. If the flow is stopped, the bed is defluidized without any continued contacting of the catalysts with the introduced hydrocarbons. Moreover even during the continuous operation of the reactor, no uniform residence time can be secured for the hydrocarbon molecules in the fluidized bed. There is significant dissimilarities existing between the time the reactant molecules contact the catalyst and the time the catalyst is exposed to the reacting hydrocarbon environment. As a result, this system could not in any way adequately simulate the conditions of a riser reactor.
Refiners commonly employ a microactivity test unit to establish the activity of catalysts for particular feedstocks. In conventional fluidized bed processes and the like, such units can be very valuable in saving the refiner millions of dollars per year in product value by predicting the effectiveness of the catalyst used in the cracking unit. The microactivity test unit (MAT) is based on the concept of continuously contacting a hydrocarbon feedstock with a catalyst sample of approximately one gram during a 75 to 100 second residence time. The procedure is defined in ASTM (D3907-80). In the MAT test, the catalyst/oil ratio is defined on a cumulative basis which means that the C/O ratio is obtained after a mass of catalyst contacts a hydrocarbon flow for about 75 to 100 seconds. Then in the MAT apparatus, the C/O ratio depends on the catalyst time-on-stream. This results in a significant difference with the conventional riser reactor units, where the catalyst flow and hydrocarbon flow are set for a given operating condition and the catalyst/oil ratio is not a function of a catalyst time-on-stream.
Another significant difference between the MAT and the riser reactor is with respect to contact times. In a conventional riser reactor, the catalyst and the hydrocarbon stay in intimate contact for about two to twenty seconds before being separated in cyclones. In the MAT unit, however, the catalyst reacts with hydrocarbons for about 75 to 100 seconds.
Additional differences can be found between the riser and MAT unit in the way coke is laid down on the catalyst. While in the riser, the coke concentration is only the function of catalyst residence time, in the MAT the coke concentration depends on both the bed axial position and catalyst time-on-stream. Consequently, in the MAT the interpretation of coke deactivation effects and catalytic cracking data is very complex.
This information demonstrates that the MAT technique only allows one to establish relative performance of catalytic materials and is of questionable application or extrapolation to catalytic riser reactors. The kinetic models derived from the data obtained using the MAT are of little use for effectively simulating riser reactors and scaling up thereof.
In accordance with this invention, a testing unit and method is provided which simulates the reaction conditions in a catalytic riser reactor. The system may be used to accurately predict the activity of a catalyst for a given feedstock as well as the conditions of regenerating catalysts.
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