Kinetics and Reaction Pathways of Naphthenic Acids Model Compound Reaction with Subcritical Methanol for Total Acid Number Reduction

Naphthenic acids are the primary contributor to the total acid number of the oil and it
can be expressed in terms of total acid number (TAN). This paper studies on the
reaction mechanism of naphthenic acids model compound upon reacting with
subcritical methanol. It also discusses further on the kinetics and reaction pathways of
the model compounds of naphthenic acids after reacting with subcritical methanol.
Crude oil containing these acids which are corrosive can cause many problems such
as equipment failures, high maintenance cost, reduce product quality and
environmental disposal problems. The model compound; cyclopentane carboxylic
acid with a molecular structure of C6H10O2 and z = -2 will be tested with subcritical
methanol under temperature range from 180℃ to 220℃ with a temperature interval of
20℃ in 25 ml hydrothermal autoclave reactor. The reaction time for the mixture is
from 10 min to 30 min with a time interval of 10 min. The well-known Peng-Robinson
equation of state will be used to compute the amount of methanol by fixing the
temperature and methanol partial pressure. From this experiment, the highest TAN
reduction is achieved at 220℃ and 30 minutes where the reduction is about 17.56%.
Upon the reaction, the components present in the sample were identified through
GC/MS analysis. Reaction pathways for each sample is identified and there are
production of ester, alcohol and alkene. From the results obtained, it is proved that the
reaction is first order kinetics and independent with concentration. The value of
activation energy obtained through the Arrhenius plot is 59.8966 J/mole-K and the
value of pre-exponential factor, A is 260.05 s-1. This shows that subcritical methanol
able to lower the reaction activation energy and also increases the speed of reaction.
To sum up, it is proven that the subcritical methanol is an effective way to reduce the
total acid number in crude oil.