Activation Energy Theory

2 Theory

2.1 Residence time

The time Sulfur and Nitrogen components spent inside the reactor to be hydro-treated is called the residence time. Since the catalystbed is a fixed length the only way to variate residence time is to variate the flow of oil through the reactor. This is expressed as the amount of oil per catalyst volume per hour, Liquid Hour Space Velocity (LHSV).

[4]

The amount of Hydrogen present in the reactor is expressed as a dimensionless relation between the oil and Hydrogen, H2 / oil.

[4]

2.2 Overall reaction rate

The overall process of the HDS/HDN catalyst reaction can be broken down into the following steps [5]:

1. Mass transfer (diffusion) of the reactants from the oil to the external surface of the catalyst

A catalyst lowers the activation energy but it does not change the energies of the original reactants or products. Rather, the reactant energy and the product energy remain the same and only the activation energy is altered (lowered) while the catalyst does not get consumed by the reaction (Image 5).

Image 5: The relationship between activation energy and enthalpy of formation (ΔH) with and without a catalyst [6]

The negative values of enthalpy for various desulfurization reactions (Table 3) show the reactions are exothermic and produce heat since a larger value (the energy released in the reaction) is subtracted from a smaller value (the energy used for the reaction).

Table 3: Heats of reaction for various desulfurization reactions [2]

Since the rate of a catalytic reaction is strongly dependent of the temperature (in liquid systems the pressure is also a contributing factor) the commonly known Arrhenius equation shows this temperature dependence of reaction rates. Therefor the activation energy is in fact a correlation between the reaction rate and the affecting temperature.

• A constant residence time and variations in temperature to determine the activation energy and effect of temperature on the reaction rate (Condition 3-5).

Condition 1 is known within Albemarle as a fast deactivation condition where a short residence time and a low temperature are used to get stable Sulfur and Nitrogen concentration within a short period of time.

Table 7: Test conditions

3.3 Performing an activity test

3.3.1 Unit configuration

The unit chosen for the activity test is a Multi Test Unit (MTU) with 10 reactors that can hold 10ml of catalyst bed and is configured for up-flow VGO Hydro-treating. Oil and hydrogen flows are being controlled by 10 separate flow controllers while pressure is controlled by a series of back pressure regulators linked from one header pressure. The temperature for all reactors is controlled by one round furnace that holds all reactors. Because VGO is solid at room temperature the unit has been divided into 4 heated cabinets. An overview of the unit configuration is given with Image 6.

Image 6: Overview of unit configuration