QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING:                  www.wissenschaftler-avh.in

B.CATALYST/ADDITIVES(Contd.)

Q-35:  How do you  predict  FCCU  Performance with Laboratory Testing?

A-35:

  Predict  FCCU Performance with Laboratory Testing

Introduction

 Conducting testing before commercial implementation reduces risk for a refiner

 Examples of questions that pilot testing can answer include:

• What will be the effect of a potential feedstock change on yields?
• How will a new catalyst technology perform?
• Which catalyst technology is best for my operating goals?
• What effect will an additive have on my yield structure?

 On a lab scale, the goal is to match the complex processes occurring in a commercial FCC unit. In the unit, catalyst deactivates over a period of many weeks due to temperature, steam, and contaminant metals. Commercial deactivation conditions are too slow to be practically copied in the lab, so an accelerated lab deactivation is done to generate a simulated Ecat to match the chemical and physical properties of the commercial Ecat. Bench scale (ACE or MAT) or pilot-scale (DCR) test equipment is then used to simulate the reaction conditions in the FCC unit  and react catalyst and feed to produce products.

Laboratory Deactivation

Approaches

When studies are being done for feedstock selection or process development, commercial Ecat is usually used and no lab deactivation is needed. However, catalyst selection studies and catalyst R&D start with fresh catalyst. Fresh FCC catalysts need to be deactivated before testing because fresh catalysts are too active and the selectivities seen in fresh catalysts do not represent  Ecats in the FCC unit. Temperature, metals and steam are therefore used to turn fresh catalyst into simulated Ecat.

 Commercial Ecat properties that we want to match with simulated Ecat include: surface area, unit cell size (UCS), metals concentration, metals oxidation state, and metals distribution.
Accelerated conditions to simulate hydrothermal deactivation of zeolite typically involve times of 2 to 50 hours, temperatures between 1400°F and 1525°F, and steam concentrations between 50 and 100%

. At temperatures below 1400°F, it may be impossible to match the equilibrium unit cell size, and at temperatures above 1525°F unrealistic zeolite sintering can be encountered.Contaminant metals such as nickel and vanadium accelerate catalyst deactivation and have dehydrogenation activity that increasescoke and hydrogen. It is important to test catalysts with contaminant metals in order to realistically assess the performance of catalysts with metals trapping and passivation technologies. The best way to simulate the contaminant metals is to apply the same metals level to the fresh catalyst that is present on the Ecat, and then deactivate all the fresh catalyst samples in a study under the same conditions.

Deactivation methods that simulate poisoning by contaminant metals include the Mitchell method (MM), cyclic metals impregnation(CMI) and cyclic propylene steaming (CPS)

 The Mitchell method involves impregnation of fresh catalyst with organic Ni and V naphthenates followed by steam deactivation for 4 to 20 hours.

 The CMI method involves multiple cycles of cracking with metals spiked feedstock and regeneration, resulting in a deactivation time of more than 50 hours.

 The CPS method involves impregnation of fresh catalyst with organic Ni and V compounds, followed by aging in a cyclic redox environment for ~20 hours. The reducing atmosphere (which simulates the riser) is a blend of steam, nitrogen and propylene, and the oxidizing atmosphere (which simulates the regenerator) is a blend of steam, air and SO2.. Grace developed the CPS deactivation method

 The CPS method provides a good match to Ecat properties. The CPS method has been adopted by many labs around the world and can be easily fine-tuned to match the severity and specific deactivation conditions of different commercial units by adjusting the temperature. In deactivating catalyst.with contaminant metals, it is important to include the effect of sulfur competition by using SO2 as part of the simulated regenerator conditions. Under commercial regenerator conditions, calcium oxide and barium based metals traps are rapidly poisoned by sulfur and lose their vanadium trapping ability. This sulfur poisoning does not happen with rare earth based vanadium traps. Testing of vanadium traps in the laboratory without simulating the SO2.present in a commercial regenerator can give a false prediction of catalyst metals trapping ability.

Commercial FCC units differ in their catalyst turnover rates. When it is desired to very closely match lab simulated Ecat to Ecat from a specific refinery, age distribution deactivation can be used. Commercial Ecat consists of catalyst particles with varying age, surface area,unit cell size (UCS), metals level, activity, and selectivity. Sink/Float experiments that separated refinery Ecat into age fractions have determined that activity and selectivity are dominated by the youngest fraction of the catalyst. Typically, the youngest 20% of the inventory contains less than 10% of the contaminant metals and contributes about 50% of the overall activity. For a specific unit, the metals and activity distribution will depend on catalyst addition rate, deactivation rate and catalyst activity.

In summary
 fresh FCC catalysts need to be deactivated before testing. Cyclic propylene steaming (CPS) is a rapid method to match Ecat properties and yields. To better match a specific refinery’s Ecat, CPS with age distribution can be used, but it is more time consuming.

FCC Catalyst Testing

Performance testing of FCC catalysts can be done by either bench scale testing or pilot plant scale testing. Examples of bench scale testing equipment include fixed bed microactivity  testing (MAT) and fixed  fluidized bed testing, one example of which is the ACE ® (Advanced Cracking Evaluation) instrument marketed by Kayser Technology. Several pilot plant designs are in operation throughout the world and include both once through and circulating designs. The most common is the Grace developed Davison Circulating Riser(DCR) .MAT and ACE testing have the advantages that they are easy to set up and require small amounts of material. The DCR pilot plant has the advantage that it mimics all the processes present in commercial operation and it can operate at the same hydrocarbon partial pressure.. Both the regenerator and the stripper are equipped with slide values for control of catalyst circulation rate.The DCR is typically operated in adiabatic mode, where changing feed preheat or regenerator  temperature will result in a change in catalyst circulation to maintain reactor outlet temperature, the same process control strategy used in many commercial FCC units. Due to the similarity between the DCR riser and the commercial FCCU riser, yields obtained from the DCR simulate commercial FCCU. The DCR is a highly flexible pilot plant and has been used to successfully evaluate many different feedstocks including resids, naphthas, gases, and feeds from non-petroleum sources such as vegetable oils and pyrolysis oils.