*2.9. Model Specification*

For simulating the packed bed absorber, the RateFrac model is adopted. The model flowsheet generated in ASPEN PLUS is shown in Figure 6. In the RateFrac model, the NRTL method is used. The flow model is countercurrent. The packed column height is divided into ten stages. The mass transfer coefficients and the wetted interfacial area for mass and heat transfer were calculated according to the empirical correlation of Billet and Schultes [30] with the constants CL = 2.4 and CV = 0.8 [36]. The heat transfer coefficient is estimated by Chilton–Colburn method [40]. Other relevant parameters are obtained by the default correlations of RateFrac [41] (see Tables 2–4). The soybean oil used as a solvent is a blend of acids (see Table 5). The tar was considered as a mixer of benzene, toluene, and ethylbenzene. The air participates in the process as tar holder—the water is used as the cooling liquid to adjust the temperature of the gas inlet to fit in the experimental data.

**Figure 6.** Flowsheet of process simulation in ASPEN PLUS.


**Table 2.** Binary diffusion coefficients (cm2/s) used in Aspen PLUS at conditions of an experiment of 30 ◦C, flow rates 53 mL/min, and at a bed height of 0.5 m.


*Appl. Sci.* **2020**, *10*, 2362 **Table 4.** Heat transfer coefficients (Watt/m<sup>2</sup> K) used in Aspen PLUS at conditions of an experiment of 30 ◦C, flow rates 53 mL/min, and at a bed height of 0.5 m.


#### **3. Results and Discussion**

## *3.1. Model Validation*

In this phase of simulation, the experimental data of the first minute reported by Bhoi [19] is used. The tar model used in this study was a mixture of compounds benzene, toluene, and ethylbenzene with mass fractions: 50% benzene, 30% toluene, and 20% ethylbenzene. The mass fraction values of these materials were selected because they are almost the mass fraction values of the tar compounds collected and measured from a fluidized bed gasifier [42]. The studied solvent temperatures for this experiment are 30, 40, and 50 ◦C. The studied solvent flow rates are 53, 63 and 73 mL/min. The studied heights of the packed bed are 0.5, 0.8, and 1.1 m. The pressure of both solvent and gas stream is 20 psig. The simulation results are presented in terms of removal efficiencies of tar compounds and reported as a function of operating parameters, i.e., the solvent temperature, the solvent mass flow rate, and the packed bed height.

Tar removal efficiency (η) was calculated using the following equation [19]:

$$\eta = \frac{\mathbb{C}\_{\rm in} - \mathbb{C}\_{\rm out}}{\mathbb{C}\_{\rm in}} \tag{43}$$

where *Cin* is cona centration of tar compounds (benzene, toluene, and ethylbenzene) at the inlet of the column [*ppmv*], *Cout* is concentration of the tar compounds (benzene, toluene and ethylbenzene) at the outlet of the column [*ppmv*].

From the simulation, Figures 7–9 show a comparison between the experimental data and the results predicted by both rate-based and equilibrium-stage models. Profiles in these figures show how the removal efficiencies of tar components change by changing the critical target parameters: solvent temperature, the flow rate of solvent (soybean oil), and the packing bed height. As shown in the figures, the prediction results of the removal efficiencies of tar components for both rate-based model and the equilibrium model are high compared to the experimental data, but the results obtained from the rate-based model have a better prediction for experimental data in comparison with the equilibrium-stage model. For assessing the accuracy of the models, MAPE (the mean absolute percentage error) is calculated between the values calculated using the models and those obtained from empirical measurements. The values of MAPE are shown in the Tables 6–8.


**Table 6.** MAPE between the values calculated using the models (rate-based (RB) and equilibrium-stage (EQ)) and those obtained from empirical measurement at a bed height of 0.5 m.

**Table 7.** MAPE between the values calculated using the models (RB and EQ) and those obtained from empirical measurement at a bed height of 0.8 m.



**Table 8.** MAPE between the values calculated using the models (RB and EQ) and those obtained from empirical measurement at a bed height of 1.1 m.

**Figure 7.** Effect of solvent temperature on the removal efficiency of tar components at a bed height of 0.5 m and different solvent volumetric flow rates of 53 mL/min (**above**), 63 mL/min (**middle**), 73 mL/min (**below**).

**Figure 8.** Effect of solvent temperature on the removal efficiency of tar components at a bed height of 0.8 m and different solvent volumetric flow rates 53 mL/min (**above**), 63 mL/min (**middle**), 73 mL/min (**below**).

**Figure 9.** Effect of solvent temperature on the removal efficiency of tar components at a bed height of 1.1 m and different solvent volumetric flow rates of 53 mL/min (**above**), 63 mL/min (**middle**), 73 mL/min (**below**).

The deviation of results between the equilibrium model and the experimental data is because the equilibrium model assumes that the vapor and liquid left on the plates are in thermodynamic equilibrium [19]. In actual operation, the equilibrium between the gas and liquid phases is rare [29]. Although the rate-based model shows a good agreemen<sup>t</sup> with experimental data, there is a deviation. It may be explained because of the rate-based model based on the empirical correlations used for calculating the mass and heat transfer parameters [26]. It is clear that the deviation of results between the rate-based model and the experimental data increased by increasing the bed height. The explanation for this trend is that by increasing the packing bed height, the conditions will be close to the equilibrium state; this explains why the excellent agreemen<sup>t</sup> between the rate-based model and the equilibrium model increased by increasing the bed height. As a whole, the simulation results predicted by rate-based model are in the range of the experimental results.

#### *3.2. Analysis of Tar Absorption Process*

#### 3.2.1. Effect of Solvent Temperature

Figures 7–9, show the effect of the temperature of soybean oil solvent on the removal efficiency of tar components at bed heights 0.5, 0.8, and 1.1 m, and solvent flow rate of 53 mL/min (above), 63 mL/min (middle), 73 mL/min (below). It is clear from the figures that the increase in solvent temperature has a significant effect on the removal efficiencies. The removal efficiency decreases with increasing the temperature from 30 to 50 ◦C. The principal reason for this effect is that by increasing the solvent temperature, the solubility of tar compounds decreased, hence increasing the equilibrium ratio (K-value) [19].

On the other hand, increasing the solvent temperature leads to increasing the wettability of the solvent due to the decreased viscosity, as a result, the mass transfer and removal efficiency increase [19]. The effect of decreasing the solubility is more significant than increasing the wettability on decreasing the removal efficiency, so the removal efficiency is reduced by increasing the solvent temperature. The trend of this effect is similar for all the tar components, but the change rates (decreasing rates) of removal efficiency are different from component to component. The change rate for benzene is higher than toluene and ethylbenzene, for example at operating conditions of the volumetric flow rate of 53mL/min and a bed height of 0.5 m. Here, the removal efficiency decreased for benzene by about 8% by increasing the temperature from 30 to 50 ◦C, i.e., the change rate of removal efficiency for benzene is 0.4%/◦C. Whereas the change rate value is 0.14%/◦C for toluene and 0.08%/◦C for ethylbenzene. It is also observed that the effect of increasing the soybean oil solvent temperature on the change rates of removal efficiency is influenced by the increase of the solvent volumetric flow rate. Increasing the volumetric flow rate of the soybean oil leads to a change rate decrease of the removal efficiency. For benzene at bed height of 0.5 as an example, the change rate of the removal efficiency at volumetric flow rate of 53 mL/min is 0.4%/◦C and decreases to value 0.33%/◦C at volumetric flow rate of 63 mL/min and it continues decreasing to a value of 0.27%/◦C at 73 mL/min.

Furthermore, increasing the bed height influences the change rate of the removal efficiency. By increasing the temperature, the change rate of the removal efficiency decreases by increasing the bed height. This trend is shown in toluene and ethylbenzene for example at a volumetric flow rate of 53 mL/min. Here, the change rate of removal efficiency for toluene at a bed height of 0.5 m is 0.14%/◦C, and it decreases to value 0.077%/◦C at a bed height of 0.8 m, and it continues decreasing to a value 0.034%/◦C at a bed height of 1.1 m.

#### 3.2.2. Effect of Bed Height and Solvent Volumetric Flow Rate

Figure 10 shows the effect of solvent volumetric flow on the removal tar efficiency. It is evident that an increase in the solvent volumetric flow rate has a significant effect on the removal efficiency. The removal efficiency is dramatically enhanced when the solvent volumetric flow rate is increased. The reason is that by increasing the solvent volumetric flow rate, the mass transfer rates of tar compounds increase, resulting in higher tar removal efficiencies [19]. The trend of this effect is similar for all the tar components, but the change rates (increasing rates) of removal efficiencies by increasing the solvent volumetric flow rate are different from component to component. The change rates for benzene are higher than toluene and ethylbenzene.

**Figure 10.** Effect of bed height on the removal efficiency of tar components at solvent volumetric flow rates of 53.63 and 73mL/min, the temperatures are 30 ◦C (**above**), 40 ◦C (**middle**), and 50 ◦C (**below**).

Furthermore, increasing the bed height enhances the removal efficiency because of the mass transfer increasing between gas and liquid [19]. This trend of effect is similar for all tar components, but the change rates (increasing rates) of removal efficiency are different from components to another. Here, the change rate for benzene is higher than toluene and ethylbenzene.

#### 3.2.3. Optimum Operation Conditions

Selecting the optimum (most economical) operation conditions should consider the requirements of the process as well as the operation cost and annualized charges on equipment. The requirement for tar concertation depends on the intended end use of the produced gas (gas application) [3]. Several researchers reported that the tar concentration should be up to 50–100 mg/Nm<sup>3</sup> for ICE and less than 5 mg/Nm<sup>3</sup> for gas turbines [2]. According to Hlina et al. [43], the tar concentration should be less than 0.1 mg/Nm<sup>3</sup> for Fischer-Tropsch synthesis.

As previously mentioned, the removal efficiency is affected by three parameters: the temperature, the solvent volumetric flow rate, and the height of the bed. It is clear that increasing the bed height enhances the removal efficiency of tar components, but on the other hand, this leads to an increase in the amount of packing material needed to fill in a packed column which means more cost will be added to the total cost of the plant. Therefore, the bed height depends mainly on the requirement for tar concentration. It should meet these requirements considering the solvent volumetric flow rate to be set at a minimum value. Increasing the volumetric flow rate of the soybean oil solvent enhances the removal efficiency of tar components, but on the other hand, this leads to an increase in the

operations cost. This operation cost results from the electrical energy consumed by the recycling pump. The optimum volumetric flow rate is linked to annualized charges on equipment. The total annualized cost should be calculated, which is equal to the sum of operation cost and annualized charges on equipment. The optimum flow rate of the solvent meets the minimum of the total annualized costs.

The temperature of the solvent has to be selected carefully. It has been observed that a decrease in the solvent temperature will increase the removal efficiency. However, the heat duty that should be added to the process to cool the solvent to the setup temperature (the inlet solvent temperature to absorber) should be considered. Therefore it is necessary to estimate the heat duty used to cool down the solvent temperature. The heat duty was calculated by ASPEN PLUS software. The results are illustrated in Figure 11; it clears that the heat duty to cool-down the solvent decreases by increasing the temperature of the solvent. From this curve, one can conclude that the solvent with high temperature is preferred for the deliberate process, but in a condition that this temperature achieves the requirement of tar concentration.

**Figure 11.** Heat duty of the heat exchanger.
