(257b) A New Adsorption Model for Describing Asphaltene Adsorption in Dynamic Condition at a High Temperature and Pressure

A new adsorption model for
describing asphaltene adsorption in dynamic condition at a high temperature and
pressure

Tatiana
Montoya¹, Camilo A. Franco², Nashaat N. Nassar^1,*,
Farid B. Cortés2

¹Chemical and Petroleum
Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta,
Canada.

²Grupo
de Investigación en Yacimientos de Hidrocarburos, Facultad de Minas,
Universidad Nacional de Colombia Sede Medellín, Kra 80 No. 65-223, Medellín,
Colombia, ²Department
of

*Corresponding
author e-mail: nassar@ucalgary.ca

Abstract

Nowadays, due to the
decrease in conventional resources of fossil fuel like light and medium crude
oils and to the increase in worldwide energy demand, special attention has been
paid to unconventional resources like heavy and extra heavy crude oils, in
order to meet the global energy demand. However, heavy and extra heavy crude
oils have high viscosities and low API gravities caused by their large content
of heavy hydrocarbons, like asphaltenes, affecting the production,
transportation and refinery processes. The pressure and temperature are
considered as key variables that affect the stability and aggregation behavior
of asphaltenes in the reservoir. Normally, asphaltenes are present in crude oil
as colloidal suspension surrounded by resins in micelle form[1]. However, many
authors[2,3] just have described adsorption isotherms of asphaltenes onto
different solid surface at atmospheric pressure without taking into account the
aggregation of asphaltenes nor the pressure effect. Therefore, we employed, for
the first time, a new model based on the ?Chemical Theory?[4] for describing
the adsorption behavior of the auto-associative asphaltene molecules at high
pressure/temperature onto solid surface. The model is related to the thermodynamic
equilibrium of sorption of asphaltenes onto solid surfaces taking into account
the i-merization of the asphaltenes and its interaction with the surface at
different temperatures and pressures. The model is based on the chemical
equilibria, equation of state and phase equilibrium. The first two terms
describe the behavior of the surface phase, and the phase equilibrium links the
surface phase properties to bulk phase properties. These assumptions were used
in a previous study to develop the solid?liquid equilibrium (SLE) model to
describe the adsorption behavior of asphaltenes onto porous and non?porous
solid surfaces. However, the SLE model neglects the effect of pressure on the
interactions of asphaltene?asphaltene and asphaltene-aggregate?solid surfaces
of the reservoir rock primarily at reservoir conditions (RC). Therefore, in
this study, a novel and original model called the SLE?RC model of adsorption
has been proposed to describe the adsorption mechanism mainly at reservoir
conditions, for which the pressure and temperature effect has been evaluated.
This model describes the temperature-pressure?dependent adsorption isotherms
with five parameters, namely: the maximum amount adsorbed, the constant of the
i?mer reactions, Henryxs law constant, the molar volume and the solubility
parameter of the asphaltenes. The proposed model has been validated with
adsorption tests on porous media under flow conditions at different pressures
and temperatures. The dynamic adsorption experiments were performed at
different asphaltene concentrations, pressures and temperatures from 100 to
2000 mg/L, 6.89 to 17.24 MPa, and 313 to 353 K, respectively. The SLE?RC model
was successfully validated using more than five experimental data describing
the adsorption isotherms of the asphaltene onto a packed bed of silica sand at
a high pressure and temperature and following a type III behavior with root
mean?square errors (RMSE%) below 2%. Keywords: Adsorption, Asphaltene, High Pressure, Self?Association,
SLE?RC model.

The SLE-RC model

The isotherm equation of the
new SLE?RC model that considers the effects of the temperature and pressure is
expressed as follows:

where the definitions of K and ψ are given by:

where ξ is a constant
defined as ξ =N_m/(
N_m-N); N (g/g) is the amount adsorbed, N_m (g/g) is the maximum adsorption capacity, K_T  is the reaction constant for dimer formation, SA (cm²/g) is the specific
surface area of the adsorbent, ν_as
(cm³/mol) is the asphaltene molar volume, d_as (MPa^1/2) is the solubility parameter
for asphaltenes, and d_T (MPa^1/2) is the solubility parameter for
the dissolvent.

Figure 1 and Table 1 show some results for test
pressures of 6.89, 10.34 and 17.24 MPa and test temperature of 313 K. As shown
in Table 1, there is excellent agreement between the SLE?RC model and the
experimental results with RMSE%
values < 2%. The solubility parameter d_as
increased as the pressure increased; this is not surprising because the
asphaltene solubility is a function of the pressure and is highly dependent on the
bubble point of the solvent. The decrease in H indicates that as pressure increases, the number of active sites
on the adsorbent would be easily accessible by the asphaltenes, thus increasing
the preference of asphaltenes for attaching to the adsorbent surface rather
than being present in the bulk phase. Conversely, the increase in K
as the pressure increases suggests that the pressure has a significant
influence on the self?associative behavior of the asphaltenes on the
adsorbent solid surface, indicating that the degree of asphaltene self?association
on the adsorbent sites increases as the pressure increases. However, the self-association of asphaltenes is
strongly dependent on their molecular structure and the interaction that occurs
between them and its strength.[5-8] Additionally, the increasing trend of N_m shown as the pressure
increases agrees with the experimental results.

Figure
1. Adsorption
isotherm of Colombian asphaltenes using
a dynamic method

and  the results of 5 parameter SLE model for different
test pressure and 313 K.

Table 1. Parameters
of SLE-RC model for different test pressures at 313 K.

Conclusion

A new five?parameter SLE?RC model
based on ?chemical theory? has been
introduced for describing asphaltene adsorption at a high temperature and
pressure and flow conditions. The model describes well the adsorption isotherms
at high pressure/temperature using five parameters found RMS% lower than 10. The
proposed new model was used for the first time in this work based on classic
thermodynamic concepts to improve the understanding of interactions
asphaltene-asphaltene and asphaltene?solid surface on the adsorption-equilibrium
process at reservoir conditions. This preliminary results based on experimental
data of several systems are very encouraging to further pursue the development
of the association theory with other solid surfaces as consolidated and other
unconsolidated media at reservoir conditions.

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