*3.4. Deactivation Model*

In general, the five intrinsic mechanisms of catalyst decay are poisoning, fouling, thermal degradation, chemical degradation, and mechanical failure [27]. Poisoning and thermal degradation are generally slow and irreversible, while fouling by coke and carbon is rapid and reversible. Generally, one of the main challenges in the acetylene hydrogenation process is catalyst deactivation, by coking and increasing acetylene concentration in the product stream. Typically butene and butadiene, as side products in acetylene hydrogenation, has led to oligomer and green oil formation on the catalyst surface [28]. The adsorbed acetylene and produced 1,3-butadiene react on the catalyst surface and green oil is produced. The deposited oligomers and green oil on the catalyst gradually reduce the catalyst activation during the process run-time [29]. The proposed correlations in the literature, that predict catalyst activity lack accuracy, so applying these activity equations in the model results in a notable error between simulation results and plant data. In this research, a power low decay model, modified by feed concentration, to account for coke formation, is proposed to calculate the catalyst activity. The considered deactivation model is as follows:

$$\frac{da}{dt} = k\_d e^{-(\frac{E\_d}{RT})} \times a^n \times \mathbb{C}^m \tag{22}$$

The proposed decay model is applied in the dynamic model and the available plant data are used to calculate the activity parameters, considering the absolute difference between plant data and simulation results as the objective function.
