In the field of Chemical Engineering, various chemical industries viewed as different industrial processes with different principles. Whereas American Chemist and Chemical Engineer, Arthur Dehon Little, postulated the concept of unit operations that would explain different industrial chemistry processes in 1916. In 1924, The Principles of Chemical Engineering was published which embodied the concept of unit operations and recognized as the standard textbook used by Chemical Engineers for decades.
A unit operation is one of the basic steps in a process which involves physical and chemical conversions.
Separation of solids, fluids, and gases are required in about all chemical forms. These forms regularly include a mass exchange between two stages, thus a few components from an effluent stream are specifically exchanged into incline mass separating agent (MSA). The separation process depends upon contrasts in physical properties, which leads to distinctive interface mass exchange rates for the components of the blend.
Unit operations and unit processing in the context of Chemical Engineering from the main principles for chemical industries and serve as the foundation of designs of chemical plants, factories, and equipment used.
As a whole, a unit operation is likely a multiple-step process which can be addressed to have a single function. In early process engineering, gas absorption is one of the very first Mass Transfers Unit Operations used in separating substances.
Gas Absorption is an operation where the transfer of feed from the gas phase to a liquid solvent. In the system, it involves no change in the chemical species present.
Absorption is used to separate gas mixtures, remove impurities, or recover valuable chemicals. The species which are being transferred to the liquid phase are referred to as solutes or absorbate. Stripping, the process of eliminating the absorbed solute from the solvent. Normally, absorbers are used with strippers to permit recovery and recycling of the absorbent.
For simplicity, only one component of the gas solute is being absorbed. The other components of the gas are assumed as non-soluble in the liquid, and the liquid is non-volatile.
Absorption can be classified into two processes: Chemical Absorption and Physical Absorption. Chemical Absorption follows an initial chemical reaction wherein there is a rapid and irreversible neutralization reaction in the liquid phase, Physical absorption is restricted by the formation of a solution of the gas in the liquid, and there is no significant chemical reaction that occurs. Absorption is functioned as equilibrium stage operations, in which liquid and vapor are in contact with each other.
Gas absorption processes are widely used in the industry, specifically for environmental regulations of emissions of gases. Thus, it plays a big role in reducing air pollution in many applications around the world. The biggest use for absorption technology is in power plants.
The process of gas absorption is that the removal of pollutants from a contaminated gas stream by permitting the gas to return into intimate contact with a liquid that allows the pollutants to be dissolved by the liquid. The solubility of the absorbed gas and rate of mass transfer is the fundamental physical principle of gas absorption.
Gas absorption is the principal design for controlling industrial air contamination, and for the most part goes for partition of acidic pollutant from the combined gas streams. Pollutions absorb are carbon dioxide, hydrogen sulfide, and natural sulfur compounds, the most significant being CO2. For both air contamination control and recuperation of procedure gases, stuffed towers are one of the most widely recognized mass exchange gadgets in current use.
They are utilized for control of dissolvable gases, for example, halide acids and to extract soluble organic compounds, for example, alcohols and aldehydes. At the point when the scrubbing solution is charged with an oxidant, for example, sodium hypochlorite, they are utilized to control sulfide scents from wastewater treatment offices and chemical plants.
There are different ways where absorption and stripping are being conducted, tray column (plate column), packed columns, spray towers (or absorbers), film absorbers, and ejector scrubbers (Schmidt, 2012). Generally, the pressure operating on the absorber should be high and inversely proportional to temperature, to reduce the volume required to hold the gas to flow.
Plate towers comprise of a vertical column with fluid streaming in the top and out the base. The vapor stage enters in the base of the segment and exits out of the top. The motivation behind the plate is to build the measure of contact region between the fluid and vapor stages. Primarily, plate columns are reasonable for both continuous and batch operations.
Pressure vessel that has a packed area is called packed column. The exhibition of a packed bed is profoundly dependent to the progression of material through it, which thus is reliant on the packing and how the stream is overseen.
It comprises of an empty round and hollow vessel made of steel or plastic, and spouts that splash fluid into the vessel. The inlet gas stream more often enters at the base of the tower and moves upward, while the fluid is splashed downwards from at least one level or more. It is likewise conceivable to place spray towers in co-current or cross-current set-up.
It is commonly utilized for the situation where the heat of absorption must be ejected. The film absorber works by sending the gas and solvent through a heat exchanger where the solvent makes a slim film on the dividers of the cylinders and the gas flows through the inside considering for solute transfer. The presence of good heat transfer in a film absorber makes it best for circumstances where low temperatures are required for a high restoration of the solute.
Ejector Scrubber is a modern contamination control gadget, for the most part equipped on the exhaust flue gas stacks of big furnaces, however may likewise be used on any number of other air exhaust systems. This sort of innovation is a part of the group of air pollution controls collectively referred to as wet scrubbers. Greater amount of mass transfer enabling better physical absorption (Schmidt, 2012).
Gas absorption includes the re-distribution of solute between the gas stage and the liquid stage when the 2 stages come into close contact and accomplishes equilibrium condition.
Phase equilibrium exists at the gas liquid interface during gas absorption is usually assumed in the analysis and design of absorption equipment, but the validity of this assumption has been in doubt since Higbie’s pioneering gas absorption studies.
In the figure below, the connection between the concentration of solute in the gas stage and in the liquid stage at steady temperature and weight is known as the equilibrium distribution curve.
Most cases of feasible concern in the food production, either the time is adequate to appear at equilibrium, or else the computation can be completed on the suspicion that it is and a phase effectiveness term, a partial accomplishment of equilibrium, acquainted with take into account the conditions really achieved. Suitable productivity esteems can be found from distributed data, or sought experimentally.
After the streams in a contact stage have reached equilibrium, they are isolated and after that go in inverse ways to the adjoining stages. The division of the gas and the liquid does not commonly exhibit extreme complication and some type of cyclone separator is regularly utilized.
Two methods of contacting the gas and liquid are possible: counter-current operation and co-current operation. We will focus principally on the counter-current gas absorption, as it was widely used in the industry. The main differences between the two configurations will be highlighted. Note that for counter-current operation, the gas enters the column or tower from below as leaves at the top, while liquid enters from the top and flows in opposite direction and exits from the bottom.
Notations : In terms of mole fraction and total flowrates
y : mole fraction of solute A in the gas phasex : mole fraction of solute A in the liquid phaseG : total molar flowrate of the gas stream (gas flux), kg-moles/m2.sL : total molar flowrate of the liquid stream, kg-moles/m2.s
Gy and Lx are the molar flowrates of A in the gas and liquid respectively (kg-moles A/m2.s) at any point inside the column.
Inside the column, mass transfer takes place as the solute (component A) is absorbed by the liquid. The quantities of L and x (for the liquid side) and G and y (for the gas side) varies continuously: as we gradually move up the column, component A is continuously being transferred from the gas phase to the liquid phase. Thus, in going up the column, there is a decrease in the total gas flowrate, and a decrease in the concentration of A in the gas phase. At the same time, in going down the column, there is an increase in the total liquid flowrate, and an increase in the concentration of A in the liquid phase. Thus,
For dilute systems, the solute content is small relative to the non-soluble inerts and non-volatile liquid. Thus, we can assume:
G1 = G2 = G = constant
L1 = L2 = L = constant
The relationship between these variables L, x, G and y is the operating line equation. The operating line equation is obtained by material balance around the column (as shown in Envelope 1 of the Figure above).
At steady-state: IN = OUT
Thus, G y + L 1 x1 = L x + G1 y1
Using the dilute system assumptions, we simply the equation and obtain:
G y = L x + G y1 – L x1
Since L and G are assumed to be approximately constant, the operating line is a straight line of the form y = mx + c, with the gradient of L / G, the liquid-to-gas ratio.
The operating line connects the 2 end points – point 1 (x1 , y1) that represents conditions at the bottom of the column, and point 2 (x2 , y2) that represents conditions at the top of the column.
For dilute solution, the equilibrium solubility line is also straight, as represented by Henry’s Law, y = mx, where m is the Henry’s Law constant which is also the gradient of the line.
When these 2 lines are plotted on mole fraction coordinates, we have the following Figure
Any point P (x, y) on the operating line represents gas-liquid contact for which the analysis can be carried out using the 2-film theory covered in earlier section. The larger the distance between the operating line and equilibrium line, the larger the concentration difference for mass transfer, and thus, the easier the separation.
Note: Operating line for gas absorption lies above the equilibrium line.
Also, in the analysis of gas absorption, we will need to know the minimum liquid rate that can be used for a given separation, i.e. to remove a specified amount of solute from the gas. This is known as the minimum liquid-to-gas ratio. The analysis is applicable to both tray and packed column.
This mode of operation is seldom used in practice. See the Figure below. The main points to note about this operation are as follow:
It is less efficient than counter-current operation