Micro Activity Testing of FCC Catalysts
The Micro Activity Test (MAT) is widely used to characterize performance of FCC catalysts due to its relative simplicity and low cost. As strictly defined by ASTM Procedure D-3907, the MAT provides a relative activity for conversion of a standard feedstock. However, MAT testing has been expanded to provide additional information such as product selectivities, operating variable and feedstock effects. As a result, a variety of baseline conditions for the M AT unit are used within the petroleum industry and MAT unit results should not be directly compared without taking into account these differences.
BASF's MAT laboratory runs approximately 20,000 tests annually for a variety of internal and customer-related purposes. This article discusses the MAT and some of its uses as currently applied by BASF.
As catalytic cracking technology was developed, several laboratory tests were also developed to characterize catalyst activity. Early versions required much larger quantities of catalyst and feedstock than the current MAT, and used longer reaction times characteristic of the lower activity amorphous catalysts. With the introduction of zeolitic catalysts, new tests were implemented with shorter reaction times more closely approximating those experienced by commercial cataIysts. Also, the development of chromatographic procedures for product analysis greatly reduced the amount of product (and, hence, catalyst) required in a small laboratory test. The initial MAT test developed in the 1960's used a fixed bed of pelleted catalyst. Later the procedure was modified to use a fixed bed of powdered catalyst, of the same particle size as used in commercial fluid units.
The MAT has been defined as ASTM Procedure D-3907, most recently updated in 1986. While a specific ASTM test procedure does exist, few (if any) laboratories routinely practice strict adherence to it. Rather, modifications have been implemented in both apparatus and run conditions to more adequately address the specific interests of the testing organizations. For example, a particular oil company may choose to test using one of its own refinery feedstocks at conditions applicable to an individual FCC unit.
BASF MAT Procedures
Figure 1(a) and Figure 1(b) show a schematic of the MAT unit. The MAT uses a fixed bed of catalyst contained in a Pyrex glass reactor which is supported in a temperature controlled furnace. Properties of the standard gas oil feedstock used for BASF's MAT are listed in Figure 2 and standard MAT operating conditions in Figure 3. The catalyst is charged to the reactor and preheated under nitrogen flow. The catalyst can be either a commercial equilibrium sample, usually pretreated to remove residual coke deposits, or a fresh sample which has been laboratory steam-deactivated.
The oil feed is then injected at a controlled rate using a syringe pump. Following the feed injection cycle, remaining hydrocarbons are stripped from the catalyst bed and reactor by continuing nitrogen purge. Liquid "syncrude" product is collected in a cold receiver, and total gas volume is determined by water displacement. Gas and syncrude products are analyzed by gas chromatography, and the spent catalyst is analyzed to determine coke deposition. The results are material balanced to generate a full slate of yields, with liquid product boiling range determined from the GC simulated distillation.
Procedural Differences Affect MAT Unit Results
Differences between the ASTM test procedures and BASF's standard test conditions are noted in Figure 3. This nonconformity to ASTM test procedures is by no means unique. Many other laboratories use a wide range of other "baseline" conditions which also vary from the ASTM method. A partial listing of the variety of baseline or "standard" conditions used by other companies is shown in Figure 4. Any of the noted parameters can also be examined as process variables.
For fresh catalyst evaluations, differences in steaming conditions prior to MAT testing also exist. As noted, in addition to operating condition differences, BASF has incorporated modifications in the equipment design and test procedures which have improved the reliability of the results and allowed additional data to be derived from the tests. It is apparent that comparison of MAT results from different laboratories may lead to different conclusions due to the widely different "standard" test conditions employed.
Figure 5 lists a summary of the features of the MAT as it is currently employed. It is noteworthy that as instrumental analytical procedures have improved, the amount of information which can be generated from such a simple test has been greatly increased, so that not only activities but also detailed selectivities and properties, such as gasoline octane by GCRON, can be included in the product workup procedure. At the same time, it is important to realize the limitations of the MAT, particularly its inability to fully duplicate the conditions of a continuous FCC unit in all respects. For example, the MAT uses a relatively low space velocity (15 WHSV) to prevent excessive pressure drop through the catalyst fixed bed. Also the catalyst contact time is much higher (48 seconds) than in a commercial unit, resulting in a low overall catalyst/oil ratio. Alternatively, MAT's can be run at much higher space velocities and shorter contact times, but the high pressure drop which results tends to accentuate coke yields. Thus, data from MAT tests should normally be interpreted on a relative or comparative basis only. Typically, data on a new catalyst is compared with a base case for which commercial operating data is available, with differences in MAT results used in BASF's FCC Projection Model to predict performance.
MAT Unit Results Predict Relative Commercial Performance
Figures 6 and 7 compare MAT selectivity results and commercial results on two catalysts: BASF's US-260 and competitive catalyst AC. In both the commercial and the laboratory comparison, the US-260 makes more LPG, gasoline, and LCO at the expense of heavy cycle oil yields. This improvement in bottoms cracking is a catalyst characteristic that BASF's MAT test is especially reliable in measuring. Both commercial and laboratory comparisons show that a 10 percent relative increase in dry gas make was an undesirable byproduct of the heavy oil cracking. The laboratory results also correctly predict that LPG olefin content will increase and LPG paraffin content will decrease. This increase in LPG olefinicity usually accompanies an increase in bottoms cracking. When the relative percent changes in the two tables are compared, it can be seen that the BASF MAT test accurately indicated the percent reduction in bottoms and the percent increase in gasoline, LPG, and dry gas of the commercial comparison.