Fritz A. Smith
Mobil Research & Development Corp.
Princeton, N.J.
Robert W. Bortz
Mobil Research & Development Corp.
Paulsboro, N.J.
Shape-selective catalytic technology to control cold-flow properties of middle distillates has been demonstrated during a 10-year period.
The Mobil Distillate Dewaxing (MDDW) process, a shape-selective process utilizing ZSM-5 as a catalyst, has been used commercially in 21 refineries of various types: topping/reforming, visbreaking, and reforming/catalytic cracking refineries.
Mobil Oil Corp. has brought a number of important shape-selective applications to full commercialization.
A descriptive list of the most common applications is shown in Table 1. Each of these processes utilizes a catalyst which contains the shape-selective zeolite, ZSM-5.
The MDDW process, which was commercialized in 1978, will be used in this article as an example of a shape-selective application.1 2
THE CATALYST, ZSM-5
Much has been published about ZSM-5, including its structure, shape selectivity, and activity. An artists rendition of the intersecting pore channels of ZSM-5 is shown in Fig. 1.3
The dimensions of the straight and sinusoidal channels vary from 5.1 to 5.6 A. Waxy molecules, which are predominantly normal paraffins and singly branched isoparaffins, can enter pores having such dimensions and be cracked into lower molecular weight hydrocarbons.
Fig. 2 shows how ZSM-5 works in the MDDW process. Long, narrow (waxy) molecules in the middle distillate boiling range enter the pores of this synthetic zeolite and are cracked to smaller molecules, mostly in the C3-C7 boiling range.
The catalyst is shape/size selective because those molecules with dimensions larger than about 5.5 A must bypass the ZSM-5 crystals. Such large molecules do not react. They are screened from the active cracking sites inside the ZSM-5 pores.
A more rigorous way to view the shape-selective nature of ZSM-5 is to consider the variability of diffusivities among molecules.4 5 This has been done for lower molecular weight paraffin and aromatic molecules and is shown in Fig. 3.
Diffusivities can vary more than eight orders of magnitude for hydrocarbons. From this type of data, and practical pilot plant results, it can be concluded that the long, narrow (waxy) molecules have relatively high diffusivities and the bulky (non-waxy) molecules have relatively low diffusivities.
Shape-selective catalysis used to produce middle distillates having improved cold flow properties, such as freeze point, pour point, cloud point, and cold-filter plugging point (CFPP), takes advantage of the differences in diffusivities.
For those molecules that can diffuse into ZSM-5, it is also necessary to consider catalytic activity.
The relative cracking activity of ZSM-5 has been shown to be a function of the aluminum content of the crystals. This is shown in the log-log plot of Fig. 4.
Here the relative cracking rate (alpha)6 of n-hexane is related to the aluminum content of ZSM_5.7 Temperature requirements and catalyst aging rates for the MDDW process are affected by the activity of the ZSM-5 used in the process.
Mobil's experience in commercially manufacturing ZSM-5 to take advantage of its unique diffusional and activity properties began in 1973. During the past 17 years, numerous changes have been made to Mobil's ZSM-5 manufacturing facility so that ZSM-5 catalysts can be improved and/or tailored for specific applications, including the MDDW process.
The reliability of ZSM-5 type catalysts in fixed-bed applications has been demonstrated in 21 MDDW units and in over 30 other fixed-bed units. These include units for the conversion of methanol into gasoline, lube oil dewaxing, toluene disproportionation, xylene isomerization, and ethylbenzene synthesis,
ZSM-5 has also been developed and demonstrated in fluidized form for FCC units for octane upgrading.
MDDW PROCESS
A simplified flow schematic of an MDDW unit is shown in Fig. 5.1 The flow scheme of the basic unit is very similar to that of a hydrodesulfurization (HDS) unit. Design concepts, materials of construction, and equipment arrangement are similar.
GENERAL MDDW ARRANGEMENTS
There are a number of ways that an MDDW unit can be utilized in a refinery. Four types of arrangements are shown on Fig. 6.
Case A shows a generalized situation for any waxy gas oil. In this case, the MDDW unit is a stand-alone unit; that is, it is not coupled with a hydrodesulfurization unit.
The schematic in Fig. 5 is an example of a stand-alone MDDW unit. Waxy oils are fed to it and the cold flow properties are adjusted as required.
Since MDDW catalyst is not designed for significant desulfurization, the feedstock for Case A should be low in sulfur content.
Case B provides for desulfurization upstream of the MDDW unit. However, HDS units convert some nitrogen compounds into NH3 while desulfurizing.
In some instances, the NH3 must be removed by appropriate separation means so as not to reduce the activity of the MDDW catalyst. In addition, the severity of the HDS unit may have to be properly adjusted to be compatible with the MDDW unit.
With the appropriate design, this configuration can be operated to produce low sulfur, low pour point distillate and a high-octane gasoline component.
In Case C, the MDDW unit is upstream of the HDS unit. Here, separation takes place after the MDDW unit.
As in Case B, a low sulfur, low pour point distillate is produced along with a high octane gasoline blending component. However, in this case the mercaptan content of the gasoline will be higher because the feed to the MDDW unit will have a higher sulfur content.
Even though the MDDW catalyst is not a good desulfurizer, it will make enough H2S to influence the mercaptan content of the MDDW gasoline. The mercaptans are made via a reaction between olefins and H2S, both of which are made by the MDDW catalyst. However, the mercaptans can be removed by Merox treating.
In Cases A, B, and C, the research clear octane of the MDDW gasoline will be in the range of 88-93. This octane is due to the high content (e.g., 60%) Of C5-C7 olefins. These olefins are similar to those found in light FCC gasolines.
The clear motor octane is about 12-15 numbers lower than the research octane number. A cascade MDDW/HDS situation is shown in Case D.
In this scenario, the olefinic MDDW gasoline is saturated, thereby reducing the research octane level by 10-20 numbers. However, this naphtha is good reforming feedstock, so the octane can be easily raised to significantly higher levels.
The cascade processing scheme of Case D also provides the opportunity of introducing a low pour point, high sulfur feed between the two reactors. This allows a refiner to minimize the size of the dewaxing reactor.
The Case D processing scheme of Fig. 6 can be applied effectively to making low freeze point, high-quality jet fuel as well as to low pour point diesel and home heating oils.
With respect to jet fuels, yields on crude can be increased from 7 to 11 vol %, depending upon the nature of the crude oil.
Whether making jet fuel or No. 2 oils, MDDW frees a refiner from the uneconomic practice of cutting end point to meet cold flow properties.
REFINERY APPLICATIONS
Following are some examples of use of the MDDW process in different types of refineries.
TOPPING/REFORMING REFINERIES
Until rationalization caused Mobil's Wilhelmshaven refinery to be shutdown in Germany in 1985, Mobil operated a 12,000-b/cd MDDW unit in a manner similar to Case C of Fig. 6.9
The refinery was of the topping/reforming type.
The addition of MDDW in 1979 allowed the refinery to upgrade significant amounts of heavy fuel oil to diesel fuel. Comparisons were made for both winter and summer operations for Arabian Light and North Sea crudes.
The winter comparisons are for a CFPP specification of -15 C., maximum. The summer comparisons are for -2 C., maximum.
Table 2 shows the winter diesel fuel situation for the pre-MDDW operation, with and without flow improver, and for operation after the MDDW unit was added to the refinery processing scheme. Without either flow improver or MDDW the refinery had to make a light diesel cut, as shown by the 85% boiling point, in order to meet the CFPP specifications.
By using flow improver, the diesel fuel production was increased by 10-11% at the expense of heavy fuel oil. However, with the addition of an MDDW unit, the refinery increased diesel fuel production by 40-50%.
Also, the MDDW technology allowed the refinery to take advantage of the maximum allowable cut point, namely 350 C. at the 85% boiling point.
It is worth noting that even more diesel fuel could be made if the 85% boiling point specification could be relaxed. In the summer the yield advantage was not as great because the CFPP specification could be relaxed to -2 C. (Table 3).
Flow improver provided for a 3-5% yield increase while MDDW provided for a 7-9% increase. In all cases, MDDW resulted in more uniformity in diesel fuel physical properties year-round.
This can be seen in Tables 2 and 3 by comparing density, viscosity, and 85% points.
Although the cetane index was also quite constant, MDDW did result in a 1-2 number decline.
This is to be expected because the highest cetane compounds are the waxy ones that get cracked to lower molecular weight compounds. The C5+ gasoline fractions made at Wilhelmshaven had clear research octane numbers as high as 93, motor numbers of 78, and a front-end octane number of 94.
However, due to high mercaptan contents, this material, which represented about 5% of the refinery gasoline pool, had to be pretreated and reformed. In 1983, a Merox unit was built and streamed so that the refinery could use this high-octane component more effectively and economically by blending it directly into finished gasolines.
Wilhelmshaven also did blending studies with MDDW and conventional diesel fuels. The results of one of these studies is shown in Fig. 7.
Here, blending results are shown for cloud point, pour point, and CFPP. The observed nonlinearity of such blending is expected. Also, no adverse effects were found as a result of blending the MDDW product with the conventional product.
VISBREAKING REFINERIES
Three of the 21 MDDW units that have been put on stream were integrated into visbreaking refineries. One of these, the Agip Petroli refinery in Venice, Italy, reported on its refinery-upgrading project at a conference in Singapore.10
Because this refinery was small (65,000 b/d) and had high operating costs and low conversion capability, it was a shutdown candidate.
The refinery took a number of actions simultaneously to increase its overall productivity.
This included the addition of a cascade MDDW/HDS unit of the type described as Case D in Fig. 6 to increase distillate yields. This application was developed by Agip Petroli for its Venice refinery and includes the addition of specific low pour/high sulfur oils between the reactors.
Major actions taken by the refinery are summarized in Table 4.
Other actions included upgrading of a high-pressure HDS unit, improved instrumentation, and a reduction in the operating staff.
The results of all the process improvements at the Venice refinery on the refinery yield structure are shown in Table 5.
The most important yield shift is that heavy fuel, bitumen, fuel, and losses have been reduced by 11.6 wt % and kerosine and middle distillates have been increased by 9.7 wt %.
The refinery found the total results from its process, utilities, staffing, and marketing changes to have significant economic advantages. These are reported in the referenced paper.10
REFORMING/FCC REFINERIES
Nine of the 21 MDDW units have been utilized in the more complex reforming/FCC types of refineries. Applications in these refineries usually result in a significant interaction between the FCC and the MDDW units.
For instance, some of the shifts that occur are:
- Heavy atmospheric and/or light vacuum gas oils become feed for both MDDW and FCC units.
- FCC LCO and ICO become potential MDDW feeds rather than FCC recycle.
Results of these stock shifts can be some or all of the following:
- Improved FCC feed stock
- Less FCC decant oil
- Higher yields of jet or diesel fuel
- Higher quality middle distillates.
RELIABILITY
Refinery processes have to be reliable. With fixed-bed catalytic processes, this usually means long cycles, long catalyst life, minimum selectivity shifts as the catalyst ages, and the ability of the catalyst to withstand a reasonable amount of physical handling.
These reliability factors have been demonstrated in commercial operations. In the Wilhelmshaven application discussed in the preceding, 46,400 lb of MDDW catalyst were loaded into the reactor initially.
This was supplemented by an additional 21,740 lb after 20 months of operation when throughput was increased. An aging plot is shown in Fig. 8 covering the first 44 months of operation.
During that time, only five decokings were necessary. This demonstrates the long cycles and long life of MDDW catalyst.
Neither Wilhelmshaven nor any other well-operated MDDW unit has shown any significant shift in yield as the catalyst ages.
The yield/pour point curve is a constant for any given feedstock, irrespective of catalyst age.
In light of the trend to offsite regeneration, fixed-bed catalysts must be able to stand up to the rigors of multiple loadings and dumpings. There is one commercial MDDW unit that uses MDDW only in the winter because of the very low temperatures encountered in the area of that refinery.
After two winters of operation, the refinery began the practice of dumping MDDW catalyst and replacing it with HDS catalyst to hydrotreat FCC feedstocks during summer operation. After summer operation, the MDDW catalyst is replaced for winter operation.
This has been going on for 6 years and demonstrates the ability of the catalyst to be handled.
ACKNOWLEDGMENT
This article could not have been written without the contributions of many Mobil R&D scientists and engineers over more than a decade. However, special acknowledgment should go to Dr. N. Y. Chen for his process research work and to H. R. Ireland for his early process development work and the successful start-up of the first commercial MDDW unit. The other Mobil authors shown in the references are also acknowledged.
REFERENCES
- Chen, N. Y., Gorning, R. L., Ireland, H. R., and Stein, T. R., "New process cuts pour point of distillates," OGJ, June 6, 1977, p. 165.
- Ireland, H. R., Redini, C., Raff, A. S., and Fava, L., "Distillate Dewaxing in Operation," Hydrocarbon Processing, Vol. 58, 1979, p. 119.
- Meisel, S. L., McCullough, J. P., Lechthaler, C. H., and Weisz, P. B., Chemtech, Vol. 6, 1976, p. 86.
- Kokotailo, G. T., Lawton, S. L., Olson, D. H., and Meier, W. M., Nature, Vol. 272, 1978, p. 437.
- Olson, D. H., Kokotailo, G. T., Lawton, S. L., and Meier, W. M., Journal of Physics and Chemistry, Vol. 85, 1981, p. 2238.
- Miale, J. N.. Chen, N. Y., and Weisz, P. B., Journal of Catalysis, Vol. 6, 1966, p. 278.
- Olson, D. H., Haag, W. O., and Lago, R. M., Journal of Catalysis, Vol. 61, 1980, p. 390.
- Donnelly, S. P., "MDDW Commercial Experience," presented at the Japanese Petroleum Institute Symposium, Tokyo, Nov. 15-16, 1982.
- Schleyerbach, C., and Schuller, O., "Experience with Catalytically Dewaxed Diesel Fuel and Heating Oil," presented at the German Society for Mineral Oil Research and Coal Chemistry," Aachen, Oct. 6-8, 1982.
- Callera, G., and D'Azberton, A., "Productivity Recovery in a Small Thermal Conversion Refinery," presented at the Institute for International Research, Singapore, Feb. 27-28, 1989.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
