Influence of Operating Parameters on the Efficiency of a Multi-Components Flash Distillation Operation

1. Introduction

This research focuses on the study of a multi-components flash distillation because it is very used in the petroleum industrial practice as a unit feeding a binary or multi-component distillation column for the production of various and important chemical products [1]. The flash distillation is a single –stage separation technique in which a feed mixture is partially vaporized in order to get a vapor enriched with more volatile component and a liquid enriched in the less volatile components. The vapor and liquid streams leaving the tank or drum are in equilibrium. The flash distillation tank provides the distillation column by both a liquid and gas stream. Generally, there are two kinds of flash distillation operations namely: isotherm flash and adiabatic flash [1,2] The run of flash distillation unit presents many challenges, such as a strong variation of products concentrations at the exit of the flash unit versus the operating parameters. This variation of concentrations is due mainly to the complex phenomenon of vaporization occurring inside the flash tank.[2]. These considerations motivate the need for investigate the effect of the operating parameters on the efficiency of a flash operation because the parametric sensivity can give important information on the run of a given process [3,4,5]. For this purpose, the mathematical model of a multi-components flash distillation was set and solved using the data related to a nominal operating point. Afterwards, the influence on the vaporized fraction and outlet concentrations of the main operative variables was evidenced.

2. The Flash Distillation Unit

A flowsheet of flash distillation operation is presented in Figure 1 [8]. This unit consists of a tank supplying a distillation column. The feed entering the flash tank is entirely liquid and it consist generally of a hydrocarbon mixture called stabilized naphtha which will be partially vaporized . At the exit of the flash tank, the vapor and liquid streams will be headed towards the inlet of a multi-components distillation column (debutanizer). Table 1 gives additional parameter values related to the studied process.

3. Model Assumptions

The main model assumptions are listed below:

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isothermal operation (isothermal flash);

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steady state mathematical model;

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ideal behavior of the mixture.

4. Mathematical Model and Method of Resolution

The mathematical model of flash distillation operation can be resumed essentially to the following steps:

1.

Compute the saturated pressure P_i^S for each component i (i=1,.. Nc), by using Lee-Kesler equation :

2.

Compute the liquid-vapor equilibrium coefficient k_Fi = P_i^S/P ( i=1,.. Nc)

3.

Assume a starting value for δ (δ = 0.1 for example)

4.

Compute V_F according to the following equation V_F = δ.F

5.

Compute L_F according to the following equation L_F = F - V_F

6.

Compute molar fractions in liquid phase x_Fi = z_Fi/(1- δ.(1-k_Fi)) (i=1,.. Nc)

7.

Compute molar fractions in vapor phase y_Fci =-k_Fi. x_Fi (i=1,.. Nc)

8.

If | (Σ y_Fci) -1 | ≤ tolerance then y_Fi = y_Fci and stop the computing.

Otherwise compute normalized molar fractions in vapor phase

y_Fi = y_Fci/Σy_Fci (i=1,.. Nc) and go to the next step

9.

Compute k_Fi = y_Fi/x_Fi for all components (in our case, i=1,..8) according to

x_Fi = z_Fi/(1- δ.(1-k_Fi)) (i=1,..Nc) (from step 6)

y_Fi = y_Fci/Σy_Fci (i=1,..Nc) (from step 8)

Update δ by using the Newton-Raphson convergence technique:

δ_new = δ_{old ̶} F(δ)/F’(δ)

F(δ) = Σ z_Fi/(1- δ.(1-k_Fi)) (i=1,..Nc)

F’(δ) = Σ z_Fi (1-k_Fi)/(1- δ.(1-k_Fi))² (i=1,..Nc)

Go to step 4

5. Results and Discussion

5.1. Influence of Temperature on the Vaporized Fraction

Figure 2 show the effect of varying the temperature inside flash tank on the vaporized fraction for different values of the pressure. It can be seen that the vaporized fraction is very sensitive to the operating temperature; this effect can be explained by the fact that the increase of the temperature accelerate the vaporization. For a fixed value of the pressure inside the flash drum, the vaporized fraction increases as the operating temperatures becomes higher. This last effect is very pronounced at low pressure relatively to high pressures values because the curves have tendency to be separated from one another. Indeed, the vaporized fraction value becomes approximately constant especially at high values of the pressure. Furthermore, for a fixed value of the operating temperature, the vaporized fraction becomes higher if the operating pressure decreases, and this effect is pronounced for low pressure values.

5.2. Influence of Feed Composition on the Vaporized Fraction

Table 1 shows the effect of varying the feed composition on the vaporized fraction. It can be seen that the vaporized fraction is very sensitive to the feed composition. This last effect can be explained by the difference in volatility from a component to another.

Table 1. influence of feed composition on the vaporized fraction (δ).

x(1)	0.001	0.101	0.301	0.101	0.501	0.001
x(2)	0.006	0.101	0.006	0.006	0.306	0.006
x(3)	0.240	0.040	0.140	0.140	0.040	0.040
x(4)	0.315	0.015	0.215	0.015	0.015	0.015
x(5)	0.121	0.221	0.021	0.221	0.021	0.021
x(6)	0.102	0.201	0.002	0.202	0.002	0.002
x(7)	0.131	0.231	0.031	0.231	0.031	0.431
x(8)	0.080	0.080	0.280	0.080	0.080	0.480
δ	0.491	0.428	0.611	0.396	0.911	0.188

Table 1. influence of feed composition on the vaporized fraction (δ).

x(1)	0.001	0.101	0.301	0.101	0.501	0.001
x(2)	0.006	0.101	0.006	0.006	0.306	0.006
x(3)	0.240	0.040	0.140	0.140	0.040	0.040
x(4)	0.315	0.015	0.215	0.015	0.015	0.015
x(5)	0.121	0.221	0.021	0.221	0.021	0.021
x(6)	0.102	0.201	0.002	0.202	0.002	0.002
x(7)	0.131	0.231	0.031	0.231	0.031	0.431
x(8)	0.080	0.080	0.280	0.080	0.080	0.480
δ	0.491	0.428	0.611	0.396	0.911	0.188

5.3. Influence of Feed Flowrate on the Vaporized Fraction

Table 2 show the influence of feed flowrate on the vaporized fraction (δ) for different values of the pressure (T=180 °F). It can be seen that the vaporized fraction is non sensitive to the feed composition value. Table 3 show the influence of feed flowrate on the vaporized fraction (δ) for different values of the temperature (P=110 psi). As noticed previously for the pressure, it can also be seen that the vaporized fraction is non sensitive to the feed composition value.

6. Conclusions

This research highlighted the effects of operating parameters on the vaporized fraction value of a flash distillation operation. Four operating parameters were studied, namely: temperature pressure, feed flowrate and the feed composition The obtained results showed that for a given pressure in the flash drum, the vaporized fraction is very sensitive to the operating temperature, and it increase as the temperature increases.. The vaporized fraction is very sensitive to the feed composition. This last effect can be explained by the difference of the volatility from a component to another in the mixture feed. Furthermore, it was found that the vaporized fraction is non sensitive to the feed composition value. Finally, in a future study, the effects of operating parameters on the vaporized fraction value for a non ideal mixture will be investigated.