Catalyst deactivation during LCO upgrading to high quality diesel
Advancing the chemical engineering fundamentals
Catalysis (T2-13P)
Keywords: LCO upgrade, catalyst deactivation, low emission diesel
The upgrading of LCO and SR Diesel fractions to produce a low sulfur high cetane number diesel is a growing area of interest. We had analyzed different routes to revamp the available HDT commercial units to produce a high quality diesel [1] from a low value streams as LCO and LKGO. The cycle length depends on the deactivation of the hydrogenation capabilities, for a particular type of catalyst and hydrogen partial pressure used in the hydrotreating unit. This paper is focuses in studying the relative deactivation of the hydrogenation ring opening, and cracking functions along the cycle length when LCO was upgrade to low emission diesel [2].
The performance of a commercial unit had been followed up by analysis of the product at the de beginning middle and end of run during the one year operation to produce a 50 ppm diesel component. That included the testing of the product in a Mercedes Benz EURO IV diesel engine to measure NOx and PM emissions. In parallel two long term test were performed in a microplant using a new generation WNiPd/TiO2Al2O3 mesoporous catalyst. There the activity, selectivity, and stability were measured using 100% of LCO fraction (Feed I) to produce a constant 50 ppm sulfur product. The two set of experiments were performed at 50 and 100 ppm sulfur in the product during three months, taking samples along the cycle length. Two different deactivated catalyst samples were obtained at the end of the tests (Spent 1 and Spent 2).
Fresh and Spent catalysts were characterized using different techniques. Acid centers on surface were analyzed by using pyridine thermo-desorption (TPD), 13C- and 29Al- solid NMR and FTIR spectroscopy. Metal sites dispersion was measured by XPS technique and NO adsorption. The relative roles of the active sites were measured by testing the catalysts with “probe” molecules (methyl tetralin, dibenzothiophene, and hexadecane - Feed II).
In commercial operation it was observed relative changes in the hydrogenation, hydrogenolysis, isomerization, and cracking reactions as a function of time on stream; there, the temperature was increased along the cycle to have constant sulfur in the product [2]. Both, the metal and acid sites were deactivated at different rate, but the most important was the deactivation of the acid sites function on the catalyst surface. They were responsible for the naphthenic rings opening, paraffins cracking, and dealkylation reactions that produced an increase in aromatics content and a reduction in cetane number as a function of time on stream. This behavior was confirmed in the pilot plant test where pure LCO was tested in a three months cycle length.
The hydrocracking of aromatics into paraffins occurred thought a complex path of reactions but, in spite of that, the test of the active sites using probe molecules and the physicochemical characterization of the catalyst allowed us to determine that the acid sites are more affected than metal sites. The total number of acid sites was reduced, as well as the ratio of week to strong sites (ammonia TPD and FTIR of pyridine). These modifications affected the relative rate of isomerization, ring opening and cracking reactions that reduce the alkyl mono-ring aromatics and alkyl-cycloparaffins, and increased the light paraffin compounds production. The concentration of metal sites on surface - measured by XPS and NO adsorption - were slightly reduced, while the amount of carbonaceous deposit was slightly increased (13CNMR, and chemical analysis) during the three months on stream. These other modifications of surface slightly reduced the hydro- genation of aromatic at constant sulfur removal. The paper will discussed in detail the effect of deactivation of acid sites in the ring opening and cracking reactions
1] Galiasso R; Diesel options to meet future US and European specification, Melbourne Paper 13 Sep (2001); 2] Galiasso T. R “Diesel Upgrading into a low emission fuel”, Fuel processing Technology 87. 9, - (2006), 759-767.
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 13:30 to 15:00, in session Catalysis (T2-13P).
