Start of run

It is most likely that in designing a new FCC unit the expander will drive the air blower and produee enough horsepower in the end-of-bladelife eondition to supply the horsepower required by the air blower at the expander s end-of-run effieieney. There would also be an allowanee for deviations from expeeted expander performanee and air blower performanee. Thus, the expander ean be expeeted to have available, at start of run, a eonsiderable amount of exeess horsepower. This exeess horsepower must be used in some eeonomie manner without jeopardizing the eontinued safe operation of the FCC unit over its normal on-stream run time. 

But suppose we are operating a heat exchanger subject to rapid rates of initial fouling. The start-of-run heat-transfer coefficient U is 120 Btu/[(h)(ft2(°F)]. Four months later, the U value has lined out at 38. The calculated clean tube-side velocity is lV2 ft/s. This is too low, but what can be done ... 

Mobil s Low Pressure Isomerization Process (MLPI) was developed in the late 1970s (123,124). Two unique features of this process are that it is operated at low pressures and no hydrogen is used. In this process, EB is converted to benzene and diethylbenzene via disproportionation. The patent believed to be the basis for the MLPI process (123) discusses the use of H-ZSM-5 zeolite with an alumina binder. The reaction conditions described are start-of-run temperatures of 290—380°C, a pressure of 273 kPa and WHSV of 5—8.5/h. The EB conversion is about 25—40% depending on reaction conditions, with xylene losses of 2.5—4%. The PX approach to equilibrium is about 99 ndash 101%. The first commercial unit was licensed in 1978. A total of four commercial plants have been built. 

An Arrhenius plot for the apparent kinetic constant at the start of run, ka, for catalyst A, is shown in Rgure 4. From this kind of graph the apparent kinetic expressions [equation (1)] were obtained ... 

The plot of the apparent kinetic constants, ka, as a function of Wvr the vanadium deposited on catalyst, at different temperatures levels, for catalyst C, is shown in Figure 5. A linear relationship can be observed (interrupted lines in Figure 5) between both variables, for same temperature level, however the interception, at the start of run, differs from the experimental values for ka. The dot-and-segment lines in Figure 5 correspond to the interpolation between the experimental values for ka, obtained during the long deactivation test, and the ka data for the fresh catalyst. 

At the begining of the screening test an Arrhenius plot could be made for each catalyst then, begining from the temperature level taken for "start-of-run" the screening test could be continued, raising temperature, until about Wv= 0.10/... 

The reactor inlet temperature ranges from 50°C at start-of-run to about 65°C at end-of-run conditions. One important feature of the two-stage system is that the catalyst can be replaced in each reactor separately, without shutting down the ETBE unit. 

The catalytic functions of the platinum is the most important one for reforming reactions because it can not be restored after start-of-run damage unless the run is stopped and the catalyst is regenerated. Therefore this work stressed the effect of the gas composition during the dehydration and the reduction of catalysts on the catalytic functions of the platinum. 

The composition of the gas during dehydration and reduction at start-of-run affects the practical activities of reforming catalysts. 

Catalyst performance is determinedby activity, selectivity andstability. Whereas activity is indispensable, selectivity is often of prime importance (e.g. lube base oil yield in catalytic dewaxing), particularly if an improved selectivity can break a bottleneck in a unit (e.g. by lower gas makes which break up the gas train bottleneck a in a hydrocracking unit). Catalyst life is determined both by the start of run activity and deactivation rate. With high activity catalysts in low severity duty (e.g. naphtha hydrotreating), catalyst life can be very long (e.g. 5-10 years), and in some cases the... 

After 120 days of operation, a test was conducted to demonstrate catalyst addition and withdrawal. Approximately half of the 120-day-old catalyst slurry was withdrawn from the reactor and replaced with an identical amount of "fresh catalyst slurry. The "fresh slurry had been prepared and activated at the start of Run E-7 and had been held In a separate vessel for 120 days under N2 at ambient temperature. The result of this catalyst exchange is shown in Fig. 7. The average rate constant for the catalyst in the reactor increased by about 60%, close to the theoretical Increase. 

LHSV and catalytic bed volume are interrelated parameters that control both the level of sulfur reduction and the process throughput. Increase in catalyst bed volume can enhance desulfurization. UOP projects that doubling reactor volume would reduce sulfur from 120 to 30 ppmw.64 Haldor Topsoe reports that doubling the catalyst volume results in a 20 °C decrease in average temperature if all other operating conditions are unchanged, and there is a double effect of the increased catalyst volume.79 The deactivation rate decreases because the start-of-run temperature decreases, and the lower LHSV by itself reduces deactivation rate even at the same temperature. 

Hydrodemetallation tests were conducted with Kuwait atmospheric residue feed at operating conditions selected to simulate the guard reactor in a typical residue processing service. In one such test, the temperature was initially held constant at a typical residue start-of-run (SOR) temperature condition for nearly three and one half months and then raised to a typical middle-of-run (MOR) temperature condition and maintained at that level for an additional equivalent length of time. In an another similar experimental run, SOR temperature was maintained for nearly two months and then the temperature was raised to a typical end-of-... 

Figure 4 is an example of a long-term bench plant test for a catalyst combination system. Several ten days after the start-of-run, the catalyst system showed stable deactivation. During a stable deactivation period, the catalyst deactivation rate is constant. If the operation mode was a constant product sulfur mode, the temperature-increase-rate of reaction (TIR) was constant and small. Then, after the stable deactivation period, the catalyst system showed a higher deactivation period, in which the TIR became constantly larger than that during the stable deactivation period. The point at which the deactivation rate changes is called a breakpoint. 

To combat the inevitable loss in desulfurizing activity of the catalyst that must be presumed to occur with time under any predetermined set of reaction conditions, the bed inlet temperature may be increased slowly, thereby increasing the overall temperature of the catalyst bed and so maintaining constant catalyst activity. Thus, depending on the nature of the feedstock, there may be a considerable difference between the start-of-run temperature and the end-of-run temperature. 

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