SCRUBBER IN CHEMICAL INDUSTRY

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Absorption refers to the transfer of a component from a gas phase to a liquid phase in which it is soluble. Stripping is exactly the reverse, the transfer of a component from a liquid phase in which it is dissolved to a gas phase. Absorption is undoubtedly the single most important process of gas purification processes.
The principle gas phase impurities commonly encountered in the process industry are hydrogen sulfide, carbon dioxide, water vapors, sulfur dioxide, nitrogen dioxide, hydrogen cyanide etc. In this particular project, the objective is to remove H2S from the gaseous stream coming from the high pressure separator.

 Design Steps
1.        Selection of absorbent & system loading.
2.        Material balance.
3.        Selection of column.
4.        Selection of trays.
5.        No of trays.
6.        Diameter of column.
7.        Design of plates.
8.        Plate efficiency.
9.        Height of the column.
10.   Material selection.

Selection of Absorbent & System Loading
                        The choice of the process solution is determined by the pressure and temperature conditions at which the gas to be treated is available, its composition with respect to major and minor constituents, and the purity requirements of the treated gas. In addition, consideration must, of course, be given to whether simultaneous H2S and CO2 removal or selective H2S absorption is desired. Although no ideal solution is available to give optimum operating conditions for each case, sufficient data and operating experience with several alkanolamines are on hand to permit a judicious selection of the treating solution for a wide range of conditions. In many cases, process requirements can be met by a number of different amines and an economic analysis is required.

An ideal absorbent should meet the following criteria:

1.      Have a high degree of solubility for the solute (minimizes absorbent required).
2.      Have low volatility (increases solute recovery and reduces absorbent loss).
3.      Be stable (reduces need to replace absorbent).
4.      Be non-corrosive (reduces need for corrosion resistant equipment).
5.      Have low viscosity (reduces pressure drop and pump requirements, increase mass flow).
6.      Be non-foaming when in gas contact (reduces size of equipment).
7.      Be nontoxic and nonflammable (safety).
8.      Be available from the process (reduces cost, reduces need for external source).

                        The most commonly used solvents for the removal of hydrogen sulfide from the gaseous streams are amines, especially alkanolamines. A number of different compounds belonging to this category can be used for this purpose. Some of the most commonly used amines are compared in the following pages.

     Mono-Ethanol Amine:
·          Preferred for low concentrations without contaminants COS & CS2.
·          Easy regeneration.
·          Very corrosive in nature.
·          High heat of reaction with H2S.
·          Relatively high vapor pressure.
·          Relatively low cost.

     Di-Ethanol Amine:
·        Used for the purification of refinery gases even with contaminants   COS & CS2.
·        Low vapor pressure.
·        Very less corrosive than MEA.
·        Gives excellent loading (0.7-1 mol of gas/mol of DEA ) at high concentrated aqueous  solution from 25-30%.
·        Easy recovery.
·        Relatively low cost.

    Di-Glycol Amine:
·          Low vapor pressure.
·          Relatively less corrosive.
·          Very good loading.
·          Relatively difficult recovery.
·          High relative cost.

    Methyl Ethyl Di-Ethanol Amine:
·          Very low vapor pressure, so concentrated solution can be used up to 60%.
·          Very less corrosive.
·          Very good loading.
·          Relatively difficult recovery.
·          Very high relative cost.
·          Sparingly miscible with hydrocarbons.

 Based on the above comparison and the data obtained from PARCO, Muzaffar Garh, DEA (Di-ethanol amine) is selected as absorbent. A 20% solution of the amine in water is used.
Our inlet conditions for the gas are 60 °C and partial pressure of the H2S is 10 psi. At these conditions, the equilibrium loading of the absorbent is 0.66 moles of H2S per mole of DEA.
Since the equilibrium loading cannot be achieved, so referring to the data provided by PARCO related to their Diesel-Max Unit (Mild Hydrocracking Unit), the system loading is taken as 0.33 moles of H2S per mol of DEA.

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