**4. Conclusions**

A rate-based model and equilibrium model have been built by applied Aspen Plus software for simulation of tar absorption using soybean oil as a solvent. Both models has been validated against experimental data at different operation points. The experimental data published by Bhoi [19] was used for validation of the models. Comparison between the results predicted by two mathematical models (rate-based and equilibrium stage) and the experimental data shows that the rate-based model has a higher accuracy than the equilibrium model. The deviation of results between the rate-based model and the experimental data increases by increasing the bed height where the conditions will be close to the equilibrium state. An analytical study of tar absorption process by using soybean oil has been presented which reveals the following points:

The removal efficiencies are different between the tar compounds (benzene, toluene, and ethylbenzene). The ethylbenzene has the highest removal efficiency. The removal efficiencies η of these components can be ranked as follows: η-value of ethylbenzene > η-value of toluene > η-value of benzene. The difference of removal efficiencies can be explained because of the different K values for these components: the lower th K value, the higher the removal efficiency, i.e., K-value of ethylbenzene < K-value of toluene < K-value of benzene at specific pressure and temperature [19].

The slope curves of removal efficiency for benzene, toluene, and ethylbenzene between 30 and 50 ◦C were approximately 0.4, 0.14, and 0.08, respectively which means that decreasing the temperature by 1 ◦C will enhance the removal efficiency for benzene, toluene, and ethylbenzene approximately by 0.4%, 0.14%, and 0.08%, respectively.

Increasing the solvent volumetric flow rate enhances the removal e fficiency. The slopes curve of removal e fficiency for benzene, toluene, and ethylbenzene between 53 and 73 mL/min are approximately 0.17, 0.084, and 0.071, respectively. It is essential to consider the energy consumed by increasing the volumetric flow rate of the solvent.

Increasing the height of the packed bed has a significant e ffect in enhancing the removal e fficiency.

A methodology for selecting the optimum (most economical) operation conditions has been presented which it is vital in case scaling lab-scale experiment for the pilot plant.

Finally, it should be mentioned that the built and validated model is an essential tool for studying tar absorption process because of its ability to predict the process performance with changing parameters and hence saving the cost and time.

**Author Contributions:** A.A. is responsible for preparing the original draft and developing the applied methodology. F.A. supported the writing process with his reviews and edits. The conceptualization was conducted by C.H. B.E. supervised the research progress and the presented work. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors received no specific funding for this work. The corresponding author would like to thank the Technical University of Darmstadt, enabling the open-access publication of this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
