Ultra-Dispersed Catalytic Hydroprocessing of Vacuum Gas Oil: Catalyst Recycle and Deactivation Study. A. S. Carss ascarss@ucalgary.ca and P. Pereira-Almao

<ppereira@ucalgary.ca>

1.0. Introduction

The world will face many challenges with regards to meeting global energy demand in the ensuing years. In the wake of fossil fuels production growth, an era marked by management of greenhouse gas emissions and increasingly stringent environmental regulations is being entered. For example, the state of California has implemented a Low Carbon Fuel Standard that aims at reducing the carbon intensity of its fuels by 10% by the year 2020 (Jacobs Consultancy, 2009).
The province of Alberta (Canada) ranks third, next to the countries of Saudi Arabia and Venezuela, for their amount of proven global crude oil reserves. In 2012, Alberta produced 556,000 barrels per day (BPD) of conventional crude oil (API > 20) and 1,900,000 BPD of unconventional crude oil in the form of oil sands bitumen (API=8). The proven reserves of unconventional oil in Alberta are 169 billion barrels, and they represent 99% of total reserves (AB. Gov., 2013).
A multifaceted energy conundrum exists today. Liquid petroleum fuels consumption is increasing at a rate of 1.5% per annum combined with a demand shift in the energy pool away from fuel oil and towards diesel and distillates, unconventional crude oils are becoming increasingly prevalent, and meanwhile carbon emissions need to be reduced (EIA, 2013).

2.0. Background

It is evident that conversion of the “bottom of the barrel” will need to increase as energy demands change. The key to achieving this objective prudently and in an environmentally responsible fashion will be through process design improvements, new process design and new technologies such as those that incorporate state-of-the-art catalysts.
Conventional catalysts used for hydrocracking (HCK) and hydrotreating (HDT) of extra heavy oil fractions are porous particles dispersed on a solid support. Such catalysts suffer from deactivation problems especially due to: formation of coke around the catalyst particle, which causes blockage of the catalyst pores, and deposition of metals (nickel and vanadium) in and on the catalyst particle. These problems are exacerbated when heavy and extra heavy feedstocks are processed, and a service period of the catalyst of one year or less can typically be expected (Ramirez, 2007).
Due to the deactivation problems peculiar of conventional HCK and HDT catalysts, interest in dispersed catalytic upgrading is increasing. Dispersed catalysts are regarded as well suited to upgrade extra heavy charges that contain high amounts of metals and asphaltenes. Said catalysts are transition metal, non-porous particles that are freely suspended in a hydrocarbon matrix and are tailored to be as small as possible. Ultra dispersed (UD) catalysts are sub-micronic or even nanometric particles. UD catalysts specific surface area and ability to deliver activated hydrogen to a reacting hydrocarbon matrix is therefore maximized.
Dispersed catalysts have been used and proven to work successfully for HCK of heavy feedstocks in a once through basis. One of the major challenges, and one that affects the economic feasibility of UD catalytic processes, is the recycling and reuse of the spent catalyst during continuous operation. Additionally, the deactivation of UD catalysts when recycled is not known and needs to be investigated. This work deals with studying the recycling and deactivation of UD catalyst.

3.0. Experimental

In this study molybdenum based UD catalysts were prepared in a catalyst preparation pilot plant. The plant emulsified an aqueous solution of ammonium heptamolybdate in VGO and VGO/n-C5 asphaltene solutions. The emulsion was subjected to a thermal treatment and UD particles were formed and suspended within the hydrocarbon matrix. (NH4)2S was incorporated into the emulsion in order to pre-sulfide
the catalyst.
The prepared UD catalyst feedstock was reacted in a separate pilot plant under low severity hydroprocessing (HDP) conditions. A range of processing temperatures, space-times and recycle ratio’s were explored. Data was collected to calculate kinetic constants and activation energies of HCK and hydrodesulfurization (HDS) reactions. A simple global kinetic model was utilized as well as a 5-lump, lumped parameter model. The calculated kinetic parameters were vetted against literature reported values.
A diagram of the ultra dispersed catalyst-recycling unit (UDCRU) is presented in Figure 1. It consists of a feed, reaction and product separation zone. Following the hot separator there is a recycle pump, which when functioning in the recycle mode of operation, may return hot separator bottoms material containing catalyst and unconverted feed to the reaction zone for an additional pass through the reactor.

Figure. 1. Ultra-Dispersed Catalytic Recycling Unit Schematic

4.0. Achievements

A novel methodology for studying deactivation of UD catalyst under recycling mode of operation was developed and utilized in this work. A catalyst cutoff model was developed. In this model, at steady state in the recycling mode of operation, UD catalyst to the system was cutoff and replaced with virgin feedstock. In this way, the concentration of catalyst in the hot separator diminishes with time. An unsteady state component balance was solved for the hot separator and reactor concentration profile with time. The catalyst concentration evolution after catalyst cutoff for a recycle ratio of
1 is shown in Figure 2.

Figure. 2. Catalyst Concentration vs. Time for Catalyst Cutoff Experiments

The product quality of the hot separator bottoms material, such as the micro carbon residue can be related to the catalyst concentration present in the reacting media. Referring to figure 2, the following scenarios may occur: 1. The product quality may
decline proportionally to the catalyst concentration, 2. The product quality may decline disproportionally faster than the catalyst concentration, or, 3. The product quality may decline disproportionally slower than the catalyst concentration. The three scenarios represent non-deactivating catalyst, deactivating catalyst, and catalyst present in higher than optimal concentration respectively.
It was found in this work that the theoretical unsteady state molybdenum component balance matched the experimental concentration data very well. Utilizing the catalyst deactivation model described, it was found in this work that the catalyst, after 3.3 passes through the reactor did not suffer from deactivation problems. Using fresh catalyst tie-in points, product quality data from the catalyst cutoff model was validated.
Dynamic light scattering (DLS) and Inductively Coupled Plasma Spectroscopy (ICP) were used to determine catalyst particle size and concentration in feedstock and hot separator bottoms products from reaction. As part of this work, a catalyst agglomeration study was carried out and the size of catalyst particles in the hot separator bottoms products was investigated. The experimental data indicated that the HDP reaction temperature was the variable that most strongly affected the hot separator bottoms
particle size. Recycle ratio and space-time had a negligible effect on the catalyst particle size.

5.0. Conclusions and Recommendations

In this research UD catalyst was continuously recycled in a pilot plant, a catalyst cutoff deactivation model was developed and it was shown that UD catalyst was resistant to deactivation after several passes through the pilot plant reactor. The data obtained was used to solve for kinetic data and a catalyst agglomeration study was carried out.
Recommendations for future work were developed and include: experimenting with different feedstocks including vacuum residue, experimenting with different catalysts including bi and tri metallic catalysts, and changing the operating conditions of the plant. These recommendations will be presented and discussed.

6.0. References

AB. Gov. Alberta Oil Sands Industry Quarterly Update Summer 2013, obtained from http://albertacanada.com/files/albertacanada/AOSID_Quarterly_Update_Summer

2013.pdf. (accessed March 1, 2014), 2013.

EIA (U.S. Energy Information Administration), International Energy Outlook 2013,
obtained from www.eia.gov/forecasts/ieo/more_highlights.cfm (accessed March
27th, 2014), 2013.
Jacobs Consultancy, Life Cycle Assessment Comparison of North American and Imported Crudes, obtained from http://www.eipa.alberta.ca/media/39640/life%20cycle%20analysis%20jacobs%20 final%20report.pdf (accessed April 21, 2014), 2009.
Ramirez, J., Rana, M.S., and Ancheyta, J. Characteristics of Heavy Oil Hydroprocessing Catalysts. In: Ancheyta, J., and Speight, J.G., editors. Hydroprocessing of Heavy Oils and Residuas. Boca Raton (FL): Taylor and Francis Group, p. 121-190, 2007.