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Different Unit Operations and Unit Processes

December 21, 2008

Unit Operations

  • Originally coined as ” unit actions” by Arthur D. Little in 1915, included 12 actions such as pulverizing, mixing, heating, etc.
  • A part of a process unit that can be analyzed independently
  • Based on the philosophy that the widely varying sequences of steps can be reduced to simple operations or reactions that are identical in fundamentals regardless of the materials being processed

Fundamental Transport Processes

  • Momentum Transfer – deals with momentum changes that occur in moving media
  • Heat Transfer – deals with transfer of heat
  • Mass Transfer – deals with transfer of mass from one phase to another distinct phase

Rate of Transport – is the ratio of driving force and resistance

Driving force

  • When two substances or phases not at equilibrium are brought into contact, there is a tendency for a change to take place that will result in an approach toward the equilibrium condition
  • It is the difference between the existing condition and the equilibrium condition
  • Prime driver in causing change in transport processes

Resistance – impedes the transport operation

Unit Processes

  • Also called unit changes or conversion processes
  • Commercialization of chemical reactions
  • Primarily concerns/involves chemical reactions in the conversion of raw materials to products

Alcoholysis

  • In the broadest sense, the addition of hydroxyl group
  • Reactions like esterification, formation of ether, etc.

H2C        CH2 +  C2H5OH  →  C2H5OCH2CH3OH

O

Alkylene            Ethanol              Hydroxy Ether

Oxide

Alkylation

  • Introduction of alkyl group
  • In petroleum industries, low molecular weight fractions can be reacted to form higher molecular weight compounds
  • Used in the production of anesthetics, antipyretics, dyes, explosives, solvents, plastics etc.

Calcination

  • Heating material below melting point resulting to loss of moisture and other volatile compounds
  • Materials undergoing such process will also be oxidized or reduced
  • Used in treating ores or concentrates, clays etc.

CaCO3 heat CaO  +  CO2

Combustion

  • Burning of any gaseous, liquid or solid substance referred as fuels
  • Oxygen source can be either free form as in O2 or in combination with other elements or compounds like HNO3, H2O2 etc.
  • Process often evolves heat and light
  • Heat of reaction in combustion is a common source of energy for chemical process industries

Dehydration

  • Refers to removal of water of chemical combination from a compound
  • Most common way of preparing olefins from alcohols.  Alcohol is passed through alumina (Al2O3) catalyst at elevated temperature to remove water.

CH3CH2OH      alumina at 673 K CH2=CH2 +  H2O

Double Decomposition

  • Also known as exchange reaction

Electrolysis

  • Reactions are carried out in solutions of electrolyte or molten salts by passage of electricity
  • Electrodes are immersed in the solution and direct current is passed through the solution
  • Oxidation takes places at the positive electrode or anode while reduction takes place in the negative electrode or cathode
  • Example, electrolysis of salt solution results to the formation of chlorine gas at the anode while sodium hydroxide and hydrogen gas is formed at the cathode

At anode         Cl2(g)

NaCl (aq)     electricity

At cathode      2 NaOH  + H2 (g)

Fermentation

  • Deals with conversion of one substance to another with the use of microorganisms like yeasts and bacteria
  • Used for production of alcohols, acetone, antibiotics, acids like lactic and citric acids, etc.
  • Involves complex processes like oxidation, reduction, hydrolysis, esterification, etc.
  • Parameters being monitored are temperature, concentration, pH and to a certain extent pressure

Halogenation

  • Introduction of halogens
  • Most widely used is chlorination due to low cost and usefulness while iodination is seldom used to high cost
  • Used for production of insecticides, dye intermediates, industrial chemicals
  • Example, reaction of chlorine on methane in the presence of light.  Reaction can not be controlled perfectly resulting to production of several products.

CH4 +  Cl2 →  CH3Cl        +  HCl

CH3Cl +  Cl2 →  CH2Cl2 +  HCl  and so on

Hydrolysis

  • Reaction with water done in the presence of a catalyst
  • If reactants are not miscible, emulsifying agents are added
  • Examples are saponification of oils and fats, starch conversion, sugar inversion and breaking down of proteins

Ion Exchange

  • Refers to the interchange of ions that take place when an ionic solid is contacted with an electrolytic solutions
  • Can be used for the
    • Transformation of electrolytes like in water softening where calcium and magnesium ions are exchanged for sodium
    • Removal of ionic constituents like deionization of water
    • Separation of ionic substances like amino acids
    • Concentration of ionic solutions

Neutralization

  • Reactions of acids and bases

Nitration

  • Generally, refers to treatment of organic compounds with nitric acid to produce either nitrates or nitro compounds

ROH  +  HNO3 →  RONO2 +  H2O

RH     +  HNO3 →  RNO2 +  H2O

  • Plays important role as an intermediate reaction
  • Hastened by the presence of sulfuric acid
  • Used in the manufacture of nitrocellulose (protective coating), explosives, etc

Oxidation (Controlled)

  • Oxygen is most often used as oxidizing agents
  • In liquid-phase reactions, permanganates, dichromates, chloric anyhydrides, hypochlorites, chlorates, lead peroxide and hydrogen peroxides are used
  • Reaction is basically exothermic
  • Heat transfer system are carefully designed to prevent controlled oxidation becoming to combustion
  • Example, production of phthalic anhydride by air oxidation of naphthalene at 698 K in the presence of V2O5 catalyst

Polymerization

  • Simple molecules (monomer) react to form larger molecules
  • Classified as polymerization through
    • Condensation reactions occur between two groups to form a new group not present in the reactant with a small compound split out like water usually be elimination of water or alcohol from bifunctional molecules
    • Addition by opening of a multiple bond without elimination of any part of molecule

H2C=CHX  +  H2C=CHX  →  H2C-CHXCH2CHX

  • Can be carried out in bulk, solution, emulsion or suspension

Pyrolysis

  • Unit process where heat is employed to decompose compounds
  • Some reactions take place at low temperature in the presence of solvents
  • Used in industry to break down large molecules into smaller molecules
  • Examples are
    • Wood and coal are heated in a chamber in the absence of air
    • Thermal cracking of heavy hydrocarbon to lighter fractions like gasoline and diesel

Source:

Jose, W.  Introduction to Chemical Engineering. UP Press, 1985.

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