One important property of a substance’s surface is adsorption. Like chemical kinetics, adsorption kinetics are also related to reaction rates. It’s just that the kinetics of adsorption is more specific, which only addresses the important properties of the surface of a substance.
Adsorption is used to state that other substances absorbed in that substance, for example, activated carbon can absorb acetic acid molecules in the solution. Each particle of adsorbant is surrounded by molecules that are absorbed due to interactions.
Dissolved substances can be adsorbed by solids, for example, CH3COOH by activated carbon, NH3 by activated carbon, phenolphthalein from acidic or basic solutions by activated carbon, Ag + or Cl- by AgCl. C better absorbs non-electrolytes and the greater the BM the better. Inorganic substances better absorb electrolytes. The selection of substances absorbed by blood charcoal, until the concentration rises.
Absorption is the process of separating the material from a gas mixture using fastening the material to the surface of the solvent c water followed by dissolution. The solubility of the gas to be absorbed can be caused only by physical forces (on physical absorption) or beside these forces also by chemical bonds (on chemical absorption). Gas components that can hold chemical bonds will be dissolved first and also at a higher speed.
What is Absorption
Absorption is the process of separating the material from the gas mixture unit by binding the material to the surface of the liquid absorbent followed by dissolution. Or the process of absorption of a substance by other substances. In this process, the absorbed substance enters the interior of the absorbent.
For example dissolution events (gas into liquid or solid), diffusion (liquid into solid), colors absorbed by an object (absorption color), absorption of refractive rays by a substance in twin refractive events (selective absorption) and absorption of energy by electrons in atomic units (absorption spectrum).
While the understanding of absorptiometry is an analytical method to determine the composition of a substance by measuring the light absorbed by the material. For example, by knowing the color frequency of the light absorbed, the type of absorbent can be determined.
Types Of Absorption
A. Physics Absorption
Physical absorption is absorption where the gas dissolved in the absorbent liquid is not accompanied by a chemical reaction. Absorption occurs because of physical interactions, diffusion of gases into water, or dissolution of gases into the liquid phase. Examples of this absorption is gas absorption of H2S with water, methanol, propylene, and carbonates.
Absorption occurs because of physical interactions, diffusion of gases into water, or dissolution of gases into the liquid phase. From this physical assumption, there are several theories for expressing the mechanism of mechanism, namely: film model theory, penetration theory, and updated surface theory.
B. Chemical Absorption
Chemical absorption is absorption in which gas is dissolved in an absorbent solution accompanied by a chemical reaction. An example of this chemical absorption is absorption in the presence of a solution of MEA, NaOH. K2CO3, and so on. Applications of chemical absorption can be found in the process of absorption of CO2 gas in ammonia plants.
The use of chemical absorption in the dry phase is often used to remove solutes more completely from the gas mixture. The advantage of chemical absorption is an increase in the mass transfer coefficient of gas, part of this change is due to the greater effective surface area. Chemical absorption can also take place in areas that are almost stagnant in addition to dynamic capture.
a. Definition of Absorbent
Absorbent is a liquid that can dissolve material to be absorbed on its surface, both physically and chemically. Absorbent is often referred to as washing liquid. Absorbent Requirements:
- Having the power to dissolve the material to be absorbed as much as possible (the need for less liquid, the volume of tools is smaller).
- Has a low steam pressure
- Not corrosive.
- Has a low viscosity
- Stable thermically.
The types of materials that can be used as absorbents are water (for soluble gases, or the separation of dust particles and liquid droplets), sodium hydroxide (for reacting gases such as acids) and sulfuric acid (for gases gas that can react like a base).
b. Absorbent properties
- A good absorbent must have a high solubility of the components to be transferred (solute). High solubility can be achieved by involving chemical reactions, but if a chemical reaction is used, the reaction must be reversible at high temperatures, so that the solute can be taken back from the absorbent.
- Absorbents should be non-volatile, to reduce the loss of absorbent with the gas.
- Absorbents must also be cheap because the loss of absorbent amounts is unavoidable.
- Absorbents must be non-corrosive, inert, except for solutes.
- Has a low viscosity under operating conditions,
- Has a low freezing point
Absorption in the industrial world is used to increase the use-value of a substance by changing its phase.
1. The Process of Making Formalin
Liquid formalin derived from formaldehyde which has a gas phase can be produced through an absorption process. The process technology for making formaldehyde as input gas is fed into the reactor. The output of the reactor in the form of gas having a temperature of 182º C is cooled in the condenser to a temperature of 55º C, put into the absorber.
Output from absorber at level I contains formaldehyde solution with formaldehyde content around 37 – 40%. The largest part of methanol, water, and formaldehyde is condensed under the cooling water part of the tower, and almost all removal of residual methanol and formaldehyde from gas occurs at the top of the absorber with counter-current contact with process water.
2. Nitric Acid Manufacturing Process
Manufacture of nitric acid (NO and NO 2 absorption ). The process of making nitric acid the final stage of the process of making nitric acid takes place in the absorption column. At every level of the oxidation reaction occurs column NO to NO2 and the absorption reaction of NO2 by the water to nitric acid. The absorption column has four in and two out fluxes.
Four incoming fluxes are absorber feed water, bleach air, process gas, and weak acids. The two fluxes are product nitric acid and flue gas. The absorption column is designed to produce nitric acid with a concentration of 60% by weight and NO x exhaust gas content of not more than 200 ppm.
Other absorption applications such as urea manufacturing process, ethanol production, carbonated drinks, fire extinguisher, dry ice, supercritical carbon dioxide and many more absorption applications in the industry. Besides this absorption is also used to purify the gas produced from the fermentation of cow dung. CO 2 gas reacts immediately with NaOH solution while CH 4 does not.
With the reduction of CO 2 concentration as a result of the reaction with NaOH, the ratio of CH 4 concentrations with CO 2 becomes greater for CH 4 concentrations. The absorption of CO 2 from the biogas mixture into the NaOH solution can be described as follows:
- NaHCO 3 (aq) ==> CO2 (g) combine with NaOH (aq)
- NaHCO 3 + NaOH (aq) ==> Na2CO 3 (s) + CO2 (g) + 2NaOH (aq) + HO (l) ==> Na2CO3 (s) + H2O (l)
Under alkaline or basic conditions, the formation of bicarbonate can be ignored because bicarbonate reacts with OH – forming CO3 2-.
Factors That Influenced Process Absorption
- The flow rate of water: the more large absorption, the better.
- Composition in water flow: If there are compounds that are capable of acting with CO 2 (for example NaOH) then better absorption.
- Operating temperature: the lower the operating temperature, the better the absorption.
- Operating pressure: the higher the operating pressure, the better the absorption to a certain extent. Above the maximum pressure (for hydrocarbons usually 4000-5000 kPa), absorption is worse.
- The flow rate of gas: the greater the gas flow rate, the absorption is getting worse.
What is Adsorption
Adsorption is the separation of material from a mixture of gas or liquid where the material to be separated is pulled by the surface of the solid.
Thus it can be concluded:
- Adsorbate: dissolved compounds that can be absorbed ( in the form of a mixture of gas or liquid).
- Adsorbent: solids where the accumulation of compounds is absorbed (in the form of solids).
Types of Adsorts
Based on the process of occurrence there are two types of adsorption, namely chemical adsorption and physical adsorption. The following are each explanation.
Physical Adsorption (Physisorption)
The interaction that occurs between adsorbent and adsorbate is Van der Walls force where when the molecular tensile strength between the solution and the surface of the media is greater than the tensile force of the dissolved substance and solution, the dissolved substance will be absorbed by the surface of the media.
This physics adsorption has a relatively small force of attraction Van der Walls. The bound molecule is very weak and the energy released at physical adsorption is relatively low at around 20 kJ/mol.
Example: Adsorption by activated carbon.
Chemisorption occurs when chemical bonds are formed (not van Dar Wallis bonds) between compounds dissolved in solution and molecules in the media.
Chemisorption occurs beginning with physical adsorption, ie adsorbate particles are attracted to the surface of the adsorbent through the Van der Walls force or can be via hydrogen bonds. In Chemisorbption the particles attach to the surface by forming chemical bonds (usually covalent bonds) and tend to find a place that maximizes the coordination number with the substrate.
Example: Ion exchange.
What is Understanding
Adsorbents are solids that can absorb fluid particles in an adsorption process. Adsorbents are specific and are made from porous materials. The choice of adsorbent in the adsorption process must be adjusted to the nature and state of the substance to be adsorbed and its commercial value. The following are the types of adsorbents :
- Polar Adsorbents
Polar adsorbents are also called hydrophilic. Type adsorbents including in this group are silica gel, activated alumina, and zeloit.
- Non-Polar Adsorbents
Nonpolar adsorbents are also called hydrophobic. Types of adsorbents included in this group are polymer adsorbents and activated carbon.
The most widely used adsorbent to absorb substances in solution is charcoal. Activated carbon which is an example of adsorption, which is usually made by burning coconut shell or wood with a limited supply of air (oxygen).
Each adsorbent particle is surrounded by molecules that are absorbed due to attractive interactions. This substance is widely used in factories to remove dyes in solution. Absorption is selective, which is absorbed only by solutes or solvents very similar to the absorption of gases by solids. Some types of adsorbents commonly used are:
a) Carbon active/active charcoal/norit
Carbon active is a porous solid containing 85-95% carbon, produced from materials containing carbon by heating at high temperatures. When heating takes place, efforts are made to avoid air leaks in the heating chamber so that the material containing carbon is only carbonized and not oxidized.
Besides being used as fuel, it can also be used as an adsorbent. Absorption is determined by the surface area of the particles and this ability can be higher if the charcoal is activated by active factor chemicals or by heating at high temperatures. Thus, charcoal will experience changes in physical and chemical properties. Such charcoal is referred to as activated charcoal.
Activated charcoal can adsorb certain gases and chemical compounds or selective adsorption properties, depending on the size or volume of pores and surface area. Absorption of activated charcoal is very large, ie 25-10 00% to the weight of activated charcoal. Activated charcoal is divided into 2 types, namely activated charcoal as a pale and as an absorbent of steam.
Activated charcoal as a pale, usually in the form of a powder which is very common, pore diameter reaches 1000 Aº, used in the liquid phase, serves to remove substances that cause unexpected color and odor, freeing solvents from the agents of interference and uses another is in the chemical industry and new industries. Obtained from sawdust pollen, pulp making or from raw materials that have a small density and have a weak structure.
Activated charcoal as a steam absorber, usually in the form of granular or pellet which is very hard, the pore diameter ranges from 10-200 Aº, a finer pore type, used in the gas phase, functions to recover solvent, catalyst, separation, and purification of gas. Obtained from coconut shells, bones, bricks or raw materials that have raw materials that have a hard structure.
Bentonite is a term for clay containing montmorillonite in the world of commerce and belongs to the dioctahedral group. The naming of clay types depends on the inventor or researcher, for example, geologists, mineralogists, industrial minerals and others. Bentonite can be divided into 2 groups based on the content of hydrous aluminum silicate pestle, namely activated clay and fuller’s earth.
Activated clay is clay that has less pale power, but its pale power can be increased through certain processing. Meanwhile, fuller’s earth is used in fulling or cleaning wool material from fat. The nature of the Benton as an adsorbent has a large surface area, solid acid, ion exchange, and catalyst.
Zeolite minerals are not a single mineral, but rather a group of minerals consisting of several types of elements. In general, zeolite minerals are aluminosilicate hydrate compounds with alkaline earth metals. and has the following chemical formula:
M2x/nSi 1-x AlxO2 . yH2O With M = eg Na, K, Li, Ag, NH, H, Ca, Ba Al-Si-O ion bonding is the structure of the crystal structure, whereas the alkali metal is a cation that is easily swapped. The number of water molecules indicates the number of pores or volume of vacuum that will be formed when the zeolite crystal cell unit is heated.
The use of zeolite is quite a lot, for example for the paper industry, rubber, plastics, light aggregate, puzzolana cement, fertilizer, pollution prevention, acid gas manufacturing, dental pavement, mineral point of exploration, coal production, natural gas purification, oxygen industry, petrochemical industry.
Under normal circumstances, the vacuum in zeolite crystals is filled with free water molecules that form a sphere in the cation. If the crystal is heated for several hours, usually at a temperature of 250-900º C, then the zeolite crystals which have the function of absorbing gas or liquid. The absorbance of zeolites depends on the amount of vacuum and surface area.
Usually, zeolite minerals have a surface area of several hundred square meters for every gram of weight. Some types of zeolite minerals can absorb gas as much as 30% of its weight in a dry state. Drying zeolites are usually carried out in a vacuum using gas or dry air nitrogen or methane to reduce the vapor pressure of the zeolite itself.
- Must have a large internal surface area.
- The area must be accessible through pores large enough to recognize molecules to be adsorbed. This is a bonus if the pores are also small enough to exclude unwanted molecules to absorb.
- Adsorbents must be able to be easily regenerated.
- Adsorbents should not experience rapid aging, which loses its absorption capacity through continuous recycling.
- Must be a mechanical adsorbent strong enough to withstand the mass and vibration handling that is a feature of every industry unit.
- Bleaching cane sugar. Colored sugar is dissolved in water and then flowed through the diatoms soil and bone char. Dyes in sugar will be adsorbed so that a clean white sugar is obtained.
- Norit tablet which is made from norit activated carbon. In the intestine, it forms a colloid system that can adsorb gas / toxic substances.
- Purifying water by adding alum/aluminum sulfate (will be hydrolyzed to form Al(OH)3 in the form of colloid). This colloid can adsorb dyes/pollutants in water.
Factors Affecting Adsorption
1. Contact Time
Contact time is a very determining factor in the adsorption process. The contact time allows the diffusion process and the attachment of adsorbate molecules to take place better.
2. Characteristics of Adsorbents
Particle size is an important requirement of activated charcoal to be used as an adsorbent. Charcoal particle size affects the speed at which adsorption occurs. The adsorption speed increases with decreasing particle size.
3. Surface Area
The more surface area of the adsorbent, the more adsorbate is absorbed, so that the adsorption process can be more effective. The smaller the diameter of the adsorbent, the more surface area. The total adsorption capacity of an adsorbate depends on the total surface area of the adsorbent.
4. Solubility of adsorbate
For adsorption to occur, a molecule must be separate from the solution. Soluble compounds have a strong affinity for their solutions and are therefore more difficult to adsorb than insoluble compounds. However, there are exclusions because many compounds with low solubility are difficult to adsorb, while some very soluble compounds are easily absorbed. Attempts to find a quantitative relationship between the ability of adsorption and solubility are few that succeed.
5. Size of adsorbate
The size of the adsorbate molecule is really important in the adsorption process when the molecule enters the micropore of a charcoal particle to be absorbed. Adsorption is strongest when the size of the adsorbent pores is large enough to allow the adsorbate molecule to enter.
The pH at which the adsorption process takes place shows a great influence on the adsorption itself. This is because hydrogen ions themselves are adsorbed strongly, partly because pH affects ionization and therefore also affects the absorption of some compounds. Organic acids are more easily adsorbed at low pH, whereas organic base adsorption occurs easily at high pH. The optimum pH for most adsorption processes must be determined by laboratory tests.
The temperature at which the adsorption process occurs will affect the speed and amount of adsorption that occurs. The adsorption velocity increases with increasing temperature and decreases with decreasing temperature. However, when adsorption is an exothermic process, the degree of adsorption increases at lower temperatures and decreases at higher temperatures.