Apr-2005

Catalyst addition in ebullated-bed units

A simple theoretical approach is available for calculating the catalyst metals content, catalyst physical properties and the effect on catalyst activity. Different catalyst addition rates are reviewed and compared

Scott Sayles and Jim Bailor, KBC Advanced Technologies

Article Summary

Conversion of the bottom of the barrel to lighter fuels allows the refiner to process heavy crudes and take advantage of the relative value of heavy-to-light crude margins. The use of hydroconversion processes to make lighter fuels from the bottom of the barrel depends largely on the ability of the unit to handle the contaminants, especially metals, in the heavy oil. Fixed-bed units are limited to a run length determined by the capacity of the catalyst to adsorb metals.

However, two commercial hydrocracking processes using catalyst addition are available to convert residua into lighter products. These are the proprietary H-Oil and LC-Fining processes.1,2 Both of these are based upon ebullated-bed reactors, which allow catalyst addition/withdrawal and isothermal operation. Catalyst addition and withdrawal gives the H-Oil and LC-Fining processes the ability to control the metals and carbon content on the catalyst. This allows the ebullated-bed reactor to operate for longer periods of time at higher levels of conversion than a fixed-bed design.

Refiners operating these processes spend thousands of dollars a day on catalyst costs. Optimisation of the catalyst addition rate (CAR) is a trade-off between improved yields, product qualities, operability and cost. A key tool in optimising the CAR is calculation of the catalyst properties as a function of the CAR, the unit feed rate and the feed qualities. This article will discuss a simple theoretical approach for calculation of the catalyst metals content, catalyst physical properties and the effect on catalyst activity. Different CARs are also reviewed and compared.

System description
The ebullated-bed hydrocracker has been described in the References.3 The reactors operate at high pressure, up to 3000psig, and temperatures up to 850ºF. Residua (1000ºF+) conversion varies, depending on feedstock, catalyst, space velocity, temperature and pressure. 

Commercially, the catalyst is added in a batch process, typically once per day. Fortunately, over time, these additions can be represented by a continuous function. The catalyst used in the process is typically 1/16 in extrudes. As expected, the catalyst activity decays as metals and carbon are added to the catalyst particle. Catalyst removal and addition allow for replenishing of the reactor with fresh catalyst while in operation. The reactors are analogous to a CSTR and can be treated as a well-mixed system. This means that the catalyst removed will have a composition equal to that of the reactor catalyst inventory. As a result, the withdrawn catalyst has the full array of catalyst activity. The added catalyst is considered as 100% fresh and is mixed instantaneously with the inventory in the reactors.

Basic assumptions
The CAR is assumed constant and equals the withdrawal rate on a fresh catalyst basis. The addition and withdrawal being equal based upon fresh catalyst implies that the catalyst inventory remains constant. Catalyst addition and withdrawal quantities are assumed to be relatively small compared to the catalyst inventory and they occur at a continuous rate. This is the largest deviation from the actual addition method used; however, the error is believed to be small. To further aid in deriving an equation to describe the effects of catalyst addition, an assumption is made of constant per cent demetalisation (%HDM) as the catalyst metals level varies.

Derivation
Prior catalyst activity prediction methods are typical to those in Reference 8 and do not work well with computer calculation. A more simplified approach offered in this discussion allows the operating process engineer to make quick checks on the catalyst performance. 

The mass balance for adding metals onto the catalyst is a function of the feedstock and product metals concentration, plus the amount of catalyst added and withdrawn (nomenclature follows text). The incremental change in catalyst metals concentration in the ebullated bed is:
Cm = [ (metals addition rate from feed) – (metals withdrawal rate with catalyst) ]/ I

The metals addition rate to the catalyst in the reactor from the feed is:
Metals addition rate from feed = FF * M
Where:
M = [ MF – MP*(LR/100) ] = MF * (1 - %HDM/100)   

The metals withdrawn with the catalyst are defined by the mass balance:
CAR = CWR (constant inventory)
Total mass rate withdrawn with catalyst = M + CAR
Metals withdrawal rate with catalyst  = FF * CAR * Cm * (M + CAR)

Therefore, the incremental change in catalyst metals concentration can be described as:
dCm/dFF = [ M - Cm*(M+CAR) ] / I

Integrating this derivative gives:
Cm  =  {  M  -  (M-(M+CAR)*Cmo) * Exp[ -FF*(M+CAR)/I ]  }  /  { M+CAR }

This equation describes the catalyst metals at equilibrium for any CAR or feed metals level. Application of this equation can be made directly to both commercial and pilot operations.

However, many pilot unit studies are conducted in ebullated beds without catalyst addition. This is an economical way of developing data on new feedstock and catalysts. A method is needed to understand how the pilot unit data without catalyst addition can be used to predict commercial operation with catalyst addition. The previous equation can be used to evaluate pilot data by setting the CAR to zero, then: