Activated Carbon Adsorption Isotherm & Kinetics PDF

ACTIVATED CARBON ADSORPTION ISOTHERM & KINETICS (OPEN-ENDED LAB)  SEPARATION PROCESS II ACTIVATED CARBON ADSORPTIO N, ISOTHERMS, KINETICS & CONTINUOUS-FLOW OPERATION 1.0 Introduction Adsorption is a unit operation in which surface-active materials in true solution are removed fr...

ACTIVATED CARBON ADSORPTION ISOTHERM & KINETICS (OPEN-ENDED LAB)  SEPARATION PROCESS II ACTIVATED CARBON ADSORPTIO N, ISOTHERMS, KINETICS & CONTINUOUS-FLOW OPERATION 1.0 Introduction Adsorption is a unit operation in which surface-active materials in true solution are removed from the solvent by inter-phase transfer to the surfaces of an adsorbent particle. This process is used in environmental engineering practice for removal of various pollutants such as soluble organics, dyes, pesticides, lignin, etc., from wastewaters and for removal of colour and taste and odour-producing substances from natural waters that are to be used as potable water supplies. Activated carbon in very fine powder or granular form is useful to purify both water and air. It is an extremely porous material with high ratios of surface area to unit weight up to 100 acres per pound. Activated carbon has particular affinity to organic materials such as solvents used in printing inks and common coatings. When the carbon particle becomes saturated with the contaminant, the exit stream will evidence a "breakthrough" of that contaminant, at which time the canister will be replaced and/or reactivated (usually by heat). The objective of this experiment is yo investigate the adsorption properties of activated carbon by studying the equilibrium isotherms, adsorption kinetics and operational characteristics of a lab-scale packed-bed reactor. 2.0 Theory Activated carbon is a highly porous carbonaceous substance with a wide range of applications in gas, vapor, and liquid treatment. The use of activated carbon dates back to 1500 BC where its use was discovered in an Egyptians papyrus for medicinal purposes. Activated carbon is used successfully today, especially in water treatment to remove organic compounds that impart color, taste and odor to the water. Contaminant removal is achieved through a process called adsorption by which contaminants adhere to the surface of the carbon and are thus removed from the water. Adsorption is the process by which Activated Carbon removes substances from water. Defined, adsorption is "the collection of a substance onto the surface of adsorbent solids." It is a removal process where certain particles are bound to an adsorbent particle surface by either chemical or physical attraction. Adsorption is often confused with Absorption, where the substance being collected or removed actually penetrates into the other solid. The reason that activated carbon is such an effective adsorbent material is due to its large number of cavernous pores. These provide a large surface area relative to the size of the actual carbon particle and its visible exterior surface. An approximate ratio is 1 gram = 100 m2 of surface area. Activated carbon adsorption proceeds through 3 basic steps: 1. Substances adsorb to the exterior of the carbon granules 2. Substances move into the carbon pores 3. Substances adsorb to the interior walls of the carbon Adsorption efficiency decreases over time and eventually activated carbon will need to be replaced or reactivated. Isotherms are empirical relations, which are used to predict how much solute can be adsorbed by activated carbon. The three most well-known isotherms are the Freundlich, Langmuir and Linear. In environmental engineering and specifically drinking water treatment application the most commonly used isotherm is the Freundlich. Shown to the right is the Freundlich isotherm equation in general form. The empirical equation for describing Freudlich isotherm is shown in eq (1), where qe is the equilibrium concentration of adsorbate on adsorbent (mass of solute adsorbed/mass of adsorbent), Ce is the equilibrium concentration of solute, and KF and n are the experimental constant. 1 qe  K F * Ce n (1) The two graphs below illustrate a general Freundlich isotherm equation and a sample breakthrough curve. Each individual type of GAC has its own isotherm curve and breakpoint characteristics. These help to predict the adsorptive capacity of particular activated carbons and give a design estimate for adsorptive life. Reactivation becomes necessary once the breakpoint has been reached.  The Langmuir isotherm has a theoretical basis and is given by the eq (2), where qe is the equilibrium concentration of adsorbate on adsorbent (mass of solute adsorbed/mass of adsorbent), qmax is the maximum concentration adsorbate on adsorbent (mass of solute adsorbed/mass of adsorbent), Ce is the equilibrium concentration of solute, and KL is the experimental constant. q max K L Ce qe  (2) 1  K L Ce The adsorption kinetics of any adsorption process is governed by the diffusion of the adsorbate and the surface interaction between adsorbate and adsorbent surface which could be purely physical, chemical or mixed of both processes. An ideal adsorbent required a large adsorption capacity and fast adsorption rate. Therefore, in adsorption kinetics studies explained how fast chemical reaction occurs and factors affecting the adsorption reaction rate. Generally, the adsorption kinetics in most cases follows either the pseudo-first (Lagergren model) or pseudo-second order (Ho and McKay model) rate equation. Table 1 shows the widely used models available for the kinetic of adsorption process. Table 1: Adsorption kinetic models Kinetic models Equation model Linearized form Pseudo-first order dq t qt = K1 (q e − q t ) ln (1 − ) = ln q e − K1 t dt qe Pseudo-second order dq t t 1 1 = K 2 (q e − q t )2 = 2 + ( )t dt qt K2qe qe qt = concentration of adsorbate on adsorbent at time t (mass of solute adsorbed/mass of adsorbent) qe = equilibrium concentration of adsorbate on adsorbent at time t (mass of solute adsorbed/mass of adsorbent) K1 = kinetic constant for pseudo-first-order K2 = kinetic constant for pseudo-second-order 3.0 Experiment A locally-based textile company has recently fined by the Ministry of Natural Resources and Environment Malaysia due to over discharge of methylene blue to the water bodies and resulted to serious water pollution. The discharge range of methylene blue by the company was 10-40 mg/L. As a junior engineer in the company, you and your team members are assigned overcome this problem by understanding the fundamental behavior of adsorption using activated carbon. Task B: a) Design a proper experimental procedures to achieve objective in part (b) and attain at least 70 % removal efficiency of methylene blue by using activated carbon. b) Determine the adsorption kinetics (Pseudo-first-order or Pseudo-second-order) and its respective kinetic constant(s). c) Based on the result in part (a) and (b), discuss and justify your answer. Note: 1. Each group of students are requested to submit a proposal of experimental procedures to the respective Graduate Assistant on the day of the experiment (before conducting the experiment). Your proposal will be evaluated and graded accordingly.