Zeolite to matrix surface area ratio

All feeds require an optimized catalyst for the optimal conversion to lighter and more valuable products. This insight has always concerned FCC professionals. Even with vacuum gas oil as feed the optimization problem was evident. Wear and Mott used a MAT reactor to optimize the zeolite to matrix surface area ratio (ZSA/MSA) for a vacuum gas oil catalyst [4]. The naphtha yield increased with increasing ZSA/ MSA ratio, while the coke and dry gas yields decreased. This investigation showed that the optimization of the catalyst indeed was necessary and was very profitable even when vacuum gas oil was used as feed to the catalytic cracker. 

To be able to select an optimal residue catalyst, many parameters have been proposed such as the pore volume, the total surface area, and the zeolite to matrix surface area ratio (ZSA/MSA). But the only strong correlation we have found between the catalyst performance and physical properties when North Sea long residue has been used as feed, is between the ZSA/MSA ratio and the catalyst performance [13]. 

Pilot unit tests have indicated that there is an upper limit for the zeolite to matrix surface area ratio (ZSA/MSA) for a residue catalyst. This observation was in contrast to the optimization study, which indicated that the ZSA/MSA should be as high as possible for maximum naphtha yield. An increase in the zeolite surface area is, according to the optimization study, expected to increase both the activity of the catalyst and its naphtha yield. But for catalysts with a high ZSA/MSA ratio, close to four or even higher, the observed naphtha yields have been lower than expected in the pilot unit tests, which indicate that there might be an upper limit for the ZSA/ MSA ratio in a residue application. 

The zeolite to matrix surface area ratio can be used for optimization of catalysts for catalytic cracking of atmospheric residues. For North Sea long residues this ratio should be as large as possible, but the ratio should not exceed an upper limit. For the main catalyst type (A) used in this investigation the upper limit of the ZSA/ MSA ratio was around 3.5. There is also a lower limit for the matrix surface area. If the matrix surface area is lower than this limit, the catalyst will not be able to crack all the heavy components in the residue feed, and the coke on the matrix will increase dramatically. This will prevent the catalyst from working properly. Different type of catalysts must be optimized individually, as well as different type of long residues. 

The surface area of the catalyst as well as the pore size distribution can easily be measured, and the zeolite and matrix surface areas of the catalyst can be determined by the t-plot method. The different FCC yields can then be plotted as a function of the ZSA/MSA ratio, zeolite surface area or matrix surface area, and valuable information can be obtained [9], The original recommendation was that a residue catalyst should have a large active matrix surface area and a moderate zeolite surface area [10,11]. This recommendation should be compared with the corresponding recommendation for a VGO catalyst a VGO catalyst should have a low-matrix surface area in order to improve the coke selectivity and allow efficient stripping of the carbons from the catalyst [12], Besides precracking the large molecules in the feed, the matrix also must maintain the metal resistance of the catalyst. 

As can be seen in Figure 4.8, the activity of the catalysts increased when the zeolite content of the catalyst increased. Since the matrix surface area was kept the same, the ZSA/MSA surface area ratio also increased. When comparing catalyst C-1 and catalyst C-2, the zeolite surface area was increased with 31 m /g, and the ZSA/MSA ratio increased from 2.5 to 3.5. As expected from our optimization studies, the activity for catalyst C-2 was significantly improved compared with catalyst C-1. The increase in activity was however not by far so pronounced for catalyst C-3, where the zeolite surface area was further increased with 21 m /g compared to catalyst C-2, which increased the ZSA/MSA surface area from 3.5 to 3.9. 

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