Environmental Engineering Reference
In-Depth Information
7.1.4.3 Metals Leaching Process
Either fixed bed reactor or ebullated bed reactor require careful evaluation before being
selected for the design of leaching process. The design of the process shown in
Fig. 7.18 [462]
incorporates two ebullated bed reactors operating in a semi continuous mode. Thus, while one
of the reactors is in an operating mode, the other is on standby. Leaching reaction proceeds
while catalyst particles are ebullated. After leaching is complete, catalyst is transported to the
leached catalyst holding tank by fluidization using leaching solution as the fluidizing media.
The leaching solution continues to be sent to the leaching agent holding tank until all of the
catalyst was unloaded. The regeneration of the spent leaching solution takes place in one of the
three regenerators. The leached catalyst is washed to remove any traces of the leaching
solution, while contaminated water from washing is sent to the water treatment plant.
7.1.4.4 Decoking of Leached Catalyst
The oxidative regeneration used for decoking spent catalysts has been practised commercially
for decades. It involves controlled burn-off of coke using air and/or diluted air. Details of
commercial process were given in
Chapter 6
dealing with catalyst regeneration. It is believed
that the same processes can be adapted to suit decoking of the leached catalysts.
7.1.4.5 Other Auxiliary Processes
Additional processing units are necessary, if the rejuvenation process is not integrated with a
refinery operation. Details of the additional process requirements for stand-alone rejuvenation
process were given by Marafi et al.
[462]
. They include fuel system, general water system,
power and steam generation and distribution, gas treatment plant, liquid effluent treatment
plant, and water treatment plant. Chemical analysis laboratory and process control room may
be necessary as well. Obviously, the requirement for these processes is minimized in the case
that the operation is conducted on or near the site of refinery. Because of their impact on
capital and operating costs, the need for the additional processes has to be carefully evaluated
during the design of rejuvenation plant.
7.1.4.6 Design Basis
For preliminary process design Marafi et al.
[462]
assumed a plant processing 6000 tons of
spent hydroprocessing catalyst annually.
Table 7.9
shows details of mass balance. In Option I,
3171 tons of the heavy and medium portions of de-oiled catalysts are processed to yield
318 tons of metals and 2853 tons of leached catalysts. However, in Option II, only 987 tons of
medium fouled catalysts are processed to produce 60 and 927 tons of metals and catalyst,
respectively. In this case, the leaching solution contained 6% oxalic acid and 8% Fe(NO
3
)
3
in
water. The optimal leaching temperature has been found to be 313 K. In Option III, only the
portion of lightly fouled catalyst was decoked without being subjected to leaching.
