Absorption Column Synthesis Essay

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Absorption column is an industrial column that being used to separate solute in the gas and to be absorbed by a liquid. Absorption column is frequently being used in processing of materials, purification and others activities. To ensure smooth and efficient operation, absorption column need to be designed for specific activities (McMahon & Osborne, 2013). A major application of absorption technology is by absorption in solutions of amine or alkaline salts for the removal of CO2 and H2S from natural gas or synthesis gas. Other than that, absorption column can be used to wash ammonia from a mixture of ammonia and air by using water.

McCabe et al. (2005) define absorption process occur when the

Disadvantage of this column is liquid hydrostatic head caused high pressure drop.

Gas and Liquid flow rate

As stated by McCabe et al. (2005), the first task in designing absorption control system is to determine the flow rates and composition of each stream entering the column. From the law of conservation of mass, any material entering a process must either accumulate or exit, or in simple word, whatever comes in must go out. By using material balance, the flow rates and its composition can be determined.

Material balance for the control surface section of the column as shown by the dashed line in Figure 5, are as follows:

Total material balance: L_a+V=L+V_a (2.1)

Component A balance: L_a x_a+Vy=Lx+V_a y_a (2.2)

Where; V – Molar flow rate of gas phase L – Molar flow rate at liquid phase x – Liquid composition y – Gas composition

The overall material balance equations for the column are

Total material balance: L_a+V_b=L_b+V_a (2.3)

Component A balance: L_a x_a+V_b y_b=L_b x_b+V_a y_a

The equilibrium lines are straight when solute concentrations are very dilute. Some of the restrictions of using Henry’s law are that it does not true for gases that react or dissociate upon dissolution.

Rate of Absorption and Mass Transfer Coefficients

The rate of absorption can be expressed by using individual or overall coefficients based on the liquid and gas phase. The rate of absorption can be expressed as r=k_y a(y-y_i ) (2.9) r=k_x a (x_i-x) (2.10) r=K_y a(y-y^*) (2.11) r=K_x a(x^*-x) (2.12)

The partial pressure difference (p – pi) can be used as driving force for gas phase as it is proportional to (y – yi). Diagrams based on the mole ratios Y and X are sometimes used as the operating line is straight. However, it is not recommended because ΔY and ΔX are not valid for measuring drive