*1.1. Tar Treatments*

The methods of tar removal can be categorized into primary and secondary measures based on the place where tar is removed [11]. With the primary methods, the tar is removed by applying processes such as thermal or catalytic cracking in the gasifier itself, while the secondary methods, the tar is separated outside the gasifier [2]. Although the primary methods have some disadvantages such as the complex construction of the gasifier and the limited flexibility of feedstock, it promises high tar removal efficiency by promoting this technology with time [2]. However, Figure 1 shows that a combination of both methods can only achieve high tar removal efficiency. Currently, secondary methods are fitting for tar separation from the produced syngas because of their low cost and simple measures [12]. Wet scrubbing process is one of the secondary methods, which applied an absorber to exclude the tar. The absorber can be a plate or packed column. It is recommended to use the packed absorber because of its high capacity [13].

**Figure 1.** Illustration of the need for primary and secondary measures [2].

Furthermore, the packed absorber can operate with lower overall pressure drops than the tray columns [14]. The packed absorber materials are categorized into random or structured packing. The modern random packings had a different wide range of geometries and shapes and are made from ceramic, metal, or plastics. The structured packings are ideal for lower pressures (i.e., less than 2 bar) and lower liquid rates (i.e., less than 50 <sup>m</sup><sup>3</sup>/m2·h) [14].

A suitable solvent must be appropriately selected for the absorption process since the solvent type has a significant influence on equipment sizing and operating costs [15]. Phuphuakrat et al. [16] summarized that the absorption process should concentrate on separating the components of the tar that cause the fouling problem. These components as per tar classification of Bergman et al. [17] are heterocyclic compounds, light polycyclic aromatic hydrocarbons such as naphthalene, and the heavier hydrocarbons that condensate easily. According to Phuphuakrat et al. [16], light aromatic hydrocarbon tares (one ring aromatic hydrocarbon) are not the reason for blocking and fouling problems, they studied some scrubbing liquids such as diesel fuel, vegetable oil, engine oil, and water as a solvent to remove the tar. The removal e fficiencies by using these solvents are shown in Table 1.


**Table 1.** Absorption e fficiencies of tar components by di fferent solvents (%) [16].

From Table 1, it can be observed that diesel fuel is the most e ffective solvent used to remove naphthalene. However, diesel is considered an uneconomic solvent because of its simple evaporation, which raises the losses of the solvent [16]. The vegetable oil has proven to be e fficient to separate naphthalene [12]. The water has comparatively high removal e fficiency for phenol because the phenol is a hydrophilic component and it can lose H+ (ion) from a hydroxyl group, whereas the other components are nonpolar substances [12]. Applying water as a solvent to remove the tar achieves removal e fficiency of about 31.8%, however water is not as e ffective solvent since the tar has a low solubility in water and the separation of the tar from the water is di fficult and expensive [12]. Phuphuakrat et al. [16] placed the e fficiency of the solvent as: vegetable oil > engine oil > water > diesel fuel. Paethanom et al. [18] published the tar removal e fficiency of vegetable oil as 89.8% and cooking oil as 81.4%. Bhoi [19] investigated the e ffect of two kinds of vegetable oils, namely soybean and canola oil to separate the tar. The author summarized that there is no significant di fference between the soybean and canola oils for all the conditions of absorbent like temperatures and volumetric flow rates. Ozturk et al. [20] analyzed the relationship between the operating time and removal e fficiency of some oily solvents like benzene and toluene. They concluded that the removal e fficiency declines with time because of increasing the tar concentration in the absorbent.

#### *1.2. Modelling Approaches for the Packed Column Used for Absorption Processes*

Mathematical models contribute to a better understanding of the process and play an essential role in enhancing plant e fficiency. A recent literature review tells that there are several studies concerning modelling the absorption process. These studies are based on two standard models: the equilibrium model and the rate-based model. The equilibrium-stage model is developed by Mofarahi et al. [21], whereas the rate-based model depends on the early work published by Pandya [22], who presented a model for rate-based CO2 absorption. Recently the rate-based model and equilibrium model of CO2 capture with Amin solvent in a packed column have been investigated by several authors. Many studies [23–25] applied the rate-based model for studying CO2 absorption. Afkhamipour et al. [26] compared the rate-based and equilibrium models for CO2 capturing with AMP solution in a packed column. Bhoi [19] employed equilibrium model to explain the experimental data for absorption tar by vegetable oil. The author tested two vegetable oils as absolvents namely soybean oil and canola oil.

To the best of our knowledge, few studies have been published regarding the modelling of tar absorption with vegetable oil in a packed column. Most of these studies have concentrated on modelling absorption process for CO2 capture, while the modelling of the tar absorption process is rarely presented. The objectives of this study are as follows:

Assembling a property package for tar-soybean oil and build a rate-based model as well as equilibrium model by applied Aspen Plus software for simulation of tar absorption using soybean oil as a solvent. Aspen Plus software is used because of its extensive property databanks and rigorous equation solvers.

Validation of both the mathematical models (rate-based and equilibrium stage) against experimental data is carried out at di fferent operation points. The experimental data reported by Bhoi [19] is used for validation of the models.

The accuracy of the results predicted by the two mathematical models (rate-based and equilibrium stage) is compared with the experimental data.

Analysis of tar absorption process is essential to study the process parameters on tar removal efficiencies such as the solvent temperature, flow rate of solvent, and height packed bed.

This study presents a methodology for selecting the optimum (most economical) operating conditions.

This work is a contribution to the knowledge available for modelling studies for tar absorption using vegetable oil as a solvent in the wet packed column.

#### **2. Description of the Experiment**

The experimental research accomplished by Bhoi [19] was used as a tar removal unit, illustrated in Figure 2. From Figure 2 the pilot plant comprises two major sections, namely the gas mixing section and an absorption column, the gas mixing section consists of a sequence of instruments, which have di fferent functions. The gas mixing section prepares a simulated gas from air and tar with a specific temperature, pressure, and volumetric flow rate [19]. The air as tar holder is heated to 350 ◦C to make sure that when the tar injected into the heated air, the liquid tar components are evaporated immediately and carried by the air stream [19]. The wet packed bed scrubbing system consists of a stainless steel column, water bath heater to heat the solvent soybean oil or canola to a specific temperature, and a peristaltic pump to recycle the solvent to the absorption column [19]. The designed internal diameter of the column is 50 mm, and the height of the column is 150 cm [19]. The selected packing materials were from kind metal Raschig rings with size 6-mm, a metal material to provide better strength and wettability compared to ceramic and plastic packings [19]. Raschig rings used in the experiment are of size (diameter × length × thickness) 6 × 6 × 0.3 mm respectively, density is 900 kg/m3, the specific surface area is 900 m<sup>2</sup>/m3, packing factor is 2297 1/m, and the void fraction is 89% [19].

**Figure 2.** Schematic diagram of a bench-scale wet scrubbing set-up [27].

#### *2.1. Mathematical Models of the Packed Column Used for Tar Absorption Processes*

Modelling packed column aims to predict the overall performance of the packed column. The gas with a temperature *TV*,*in*, flow rate . *NV* and mole fraction *yi*,*in* for the component *i* enters the bottom of the packed column, and it exit with temperature *TV*,*out*, mass flow rate . *NV*, and mole fraction *yi*, *out* for the component *i*. The liquid with temperature *TL*,*in*, flow rate . *NL* mole fraction *xi*,*in* for the component *i* enters the top of the packed column, (countercurrent flow), and it exit with temperature *TL*,*out*, flow rate . *NL*, and mole fraction *xi*, *out* for the component *i*. Basically, two common models are used for calculating the absorption process parameters: the equilibrium model and the rate-based model [26].
