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Basic adsorption Techniques and mechanisms (by Asmare Tezera)
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Adsorption is a separation process in which certain components of a fluid phase are transferred to the surface of the solid adsorbent. The major applications had been in the separation of solutes from liquid streams and removal of impurities from gas streams. • In an adsorption process molecules or atoms or ions in a gas or liquid diffuse to the surface of a solid, where they bond with the solid surface or are held there by weak inter-molecular forces. The adsorbed solute is called the adsorbate, and the solid material is the adsorbent.
In order to achieve better efficiency a very large surface area is required so most of them are porous and the porous structure can account for up to 50% of the volume of the material. • During adsorption, the solid adsorbent becomes saturated or nearly saturated with the adsorbate. To recover the adsorbate and allow the adsorbent to be reused, it is regenerated by desorbing the adsorbed substances (i.e. the adsorbates). • Removal of adsorbates can also be achieved by changing the pressure or the temperature.
Accumulation of a substance over the surface of the other is known as adsorption. This surface phenomenon usually relies upon having two constituents, namely adsorbate and adsorbent. • The adsorbent is the solid substance that allows the other matter to get settled over its surface, whereas • Adsorbate is the entity that gets deposited over the surface and is usually gaseous or liquid in nature.
Adsorption processes Adsorption occurs whenever a solid surface is exposed to a gas or a liquid: it is defined as the enrichment of material or increase of density of the fluid in the vicinity of an interface Adsorbate Adsorbent (porous structure) Adsorbent (‘flat’ surface)
Surface Energy An explanation of why material accumulate interface is based on the excess energy associated with particles at interfaces. For example, in the pure water and air, the water molecules at the water interface have higher energy than water molecules in the interior of the water phase. The reason that these surface molecules have higher energy unlike the interior molecules, they have an unbalanced force component ( on the inside o the molecule). These surface molecules have additional energy to balance the forces. It takes energy to put molecules on the surface, since at least one of the intenral bonds must be broken to get the molecule to the surface. This excess energy is called surface tension.
Surface Energy Thermodynamics of surface adsorption In solutions certain particles tend to concentrate at the surface. These particles are those that have low affinity for the water (solvent). These are hydrophobic molecules. Because they have low affinity fore solvent they can get to the surface easily since they , have low bond energy in the bulk phase. The water system prefers to have these molecules at the surface because the placement at the surface requires less energy than a water molecule -- hydrophobic molecules decrease surface energy (surface tension) relative to a pure water system.
On the other hand if a particle has a high affinity or the solvent phase (hydrophilic) it will tend to remain in the bulk solution because of its strong bond with water. In fact, this bonding makes the water bonding stronger and, therefore, there is larger energy required to get water molecules to the surface-- therefore, hydrophilic molecules increase surface tension. As particles concentrate at surface there becomes a "surface excess".
Adsorption Is Useful When • The pollutant gas is noncombustible or difficult to burn • The pollutant is sufficiently valuable to warrant recovery • The pollutant is in very dilute concentration • It is also used for purification of gases containing only small amounts of pollutants that are difficult to clean by other means Depending upon the nature of forces which hold the molecules of the adsorbate (GAS) on the surface of the adsorbent(SOLID), adsorption is classified into two types: 1. Physical adsorption (Physisorption) 2. Chemical adsorption (Chemisorption)
Physisorption Chemisorption
Adsorption processes Mechanisms of selectivity * one component exhibits stronger interaction with adsorbent and therefore is selectively removed from the mixture * one component is not able to fit in the pores and therefore is excluded from adsorption * one component exhibits higher transport diffusion and is selectively removed from the mixture
AdsorptionPhenomenon The surface of a solid shows a strong affinity for molecules that come into contact with it.Certain solid materials concentrate specific substances from a solution onto their surfaces.
Regeneration • To remove unwanted particles from the adsorbent surface after the adsorption process • Using steam/hot inert gas • Steam condenses in the bed, raising the temp. of the solid, provide energy for desorption • The solvent is condensed, separated from water. • Then the bed is cooled and dried with inert gas
Factors affecting adsorption • Nature of adsorbate – A gas which has greater intermolecular attraction can be more readily adsorbed than a gas which has less intermolecular attraction . • Nature of adsorbent – Since adsorption is a surface phenomenon hence an adsorbent having larger surface area acts as a better adsorbent . • Temperature – Adsorption is exothermic . This is because ∆S for the process is –ve since gas getting adsorbed is becoming less random . To make ∆G=-ve the value of ∆H should be sufficiently negative . Hence adsorption generally decreases with increase in temperature . However chemisorptions first increases and then decreases with increase in temperature. This is because it requires energy of activation . But after it is achieved further increase in temperature decreases adsorption
Characteristics of adsorption(i) Adsorption refers to the existence of a higher concentration of any particular component at the surface of a liquid or a solid phase.(ii) Adsorption is accompanied by decrease in the (free energy change) of the system when adsorption equilibrium is said to be established. • (iii) Adsorption is invariably accompanied by evolution of heat, i.e. it is an exothermic process. In other words, ΔH of adsorption is always negative.(iv) When a gas is adsorbed, the freedom of movement of its molecules becomes restricted. On account of it decrease in the entropy of the gas after adsorption, i.e. ΔS is negative.
Characteristics of adsorptionGenerally adsorption has the chara/tics • It is a surface phenomenon. • It takes place due to the presence of residual surface forces. • It is dependent upon temperature and pressure. • It is affected by the surface area of adsorbent. • It is in an exothermic process. • It is a reversible process and a state of dynamic equilibrium is attained.Example: Accumulation of acetic acid molecules on the surfaceby charcoal.
Application of adsorption(1) Production of high vacuum The remaining traces of air can be adsorbed by charcoal from a vessel evacuated by a vacuum pump to give a very high vacuum. (2) In Gas masks : This apparatus is used to adsorb poisonous gases (e.g. oxide of sulphur etc.) and thus purify the air for breathing.
Application of adsorption(3) For desiccation or dehumidification : These substances can be used to reduce/remove water vapors or moisture present in the air. Silica gel and alumina are used for dehumidification in electronic equipment.(4) Removal of coloring matter from solution : • Animal charcoal removes colors of solutions by adsorbing colored impurities. • Animal charcoal is used as decolorizer in the manufacture of cane sugar. (5) Heterogeneous catalysis : Mostly heterogeneous catalytic reactions proceed through the adsorption of gaseous reactants on solid catalyst.
Application of adsorption(6) Separation of inert gases : Due to the difference in degree of adsorption of gases by charcoal, a mixture of inert gases can be separated by adsorption on coconut charcoal at different low temperatures.(7) Softening of hard water(i) The hard water is made to pass through a column packed with zeolite (sodium aluminium silicate) (ii) Ca---++, Mg++ ions which are responsible for hardness, get adsorbed on zeolite, exchanging sodium ions. (iii) The exhausted zeolite is regenerated with 10% of sodium chloride solution.
Application of adsorption (8) In curing diseases : A number of drugs are adsorbed on the germs and kill them or these are adsorbed on the tissues and heat them (9) Froth floatation process. A low grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method (10) In dyeing : Many dyes get adsorbed on the cloth eitherdirectly or by the use of mordants
Nature of Adsorbent • Porous material - Large surface area per unit mass - internal surface area greater than the external surface area - often 500 to 1000 m2/g. • Separation occurs because differences in molecular weight, shape or polarity of components • Rate of mass transfer is dependent on the void fraction within the pores • Granular (50μm - 12 mm diameter) • Suitable for packed bed use • Activated carbon, silica gel, alumina, zeolites
Factors effecting Carbon Adsorption • Physical and chemical characteristics of carbon (surface area, pore size) • Physical and chemical characteristics of adsorbate ? (molecular size, molecular polarity, chemical composition) Higher molecular weight more easily adsorbed Molecular weight Size • Concentration of adsorbate in the liquid phase (solution)Characteristics of the liquid phase ?(pH, temperature)Contact timeIncreasing solubility of the solute in the liquid carrier decreases adsorbability. Branched chains are usually more adsorbable than straight chains
Causes of Adsorption • Dislike of Water Phase – ‘Hydrophobicity’ • Attraction to the Sorbent Surface • Van der Waals forces: physical attraction • Electrostatic forces (surface charge interaction) • Chemical forces (e.g., - and hydrogen bonding)
Adsorption • Selectivity • High capacity • Chemical and thermal stability • Low solubility in the carrier solvent • Hardness and mechanical strenght • Low cost
Mechanism of Adsorption • Adsorption is due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. • Inside the adsorbent, all the forces acting between the particles are mutually balanced but on the surface the particles are not surrounded by atoms or molecules on all sides. • They possess unbalanced force or residual attractive force. • These forces of the adsorbent are responsible for attracting the adsorbate particles on its surface. • The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure.
Mechanism of Adsorption • Adsorption is surface phenomenon. • It arises because of unbalanced forces on the surface of solids and liquids. • The surface is under tension due to unbalanced forces. • The surface of the solid or liquid tends to satisfy their residual forces by attracting and retaining the molecules of other species when brought in contact with them.
Mechanism of Adsorption Forces of adsorption of relevant interest here is the van der Waals forces, which exists between all atoms and molecules and can be classified into three groups.a) Dipole-dipole forcesb) Dipole-induced dipole forces.c) Dispersion forces.
Mechanism of Adsorption Step3: Monolayer build of adsorbent Step2: Migration into pores of adsorbent Step1: Diffusion to adsorbent surface Contaminants
