Introduction to Separation Processes PDF

Introduction to Separation Processes Separation Processes Lecture 1 Chemical process plant operations Mostchemical process plant operations involve at least one chemical reactor and one or more separation operations. Separation processes A separation process refers to any method which converts...

Introduction to Separation Processes Separation Processes Lecture 1 Chemical process plant operations Mostchemical process plant operations involve at least one chemical reactor and one or more separation operations. Separation processes A separation process refers to any method which converts an initial mixture into two or more distinct product mixtures. When are separation processes needed? Pre-purification of raw materials Separation of unreacted feed from products for recycling Purification of product Case: Hydration of Ethylene Reaction: C2H4 + H2O  C2H5OH Case 1: Single-pass reactor  5% overall conversion Case 2: Unreacted C2H4 recovery via partial condensation and recycling  ~100% overall conversion Case: Hydration of Ethylene Ideal block flow diagram This is only realized when the feedstock is ethylene and no side reactions occur. More separation processes are often required.  Case: Hydration of Ethylene Industrial product of ethanol via hydration of ethylene Classification of Separation Processes (1) Phase creation (2) Phase addition (3) Separation by barrier (4) Separation by an external force field Phase creation - Most common type of separation processes employed in the industry - Involves inducing the chemical species in a feed to partition among distinct product phases via transfer of energy or energy separating agents (ESA) - ESA includes (1) heat or shaft work by compression and (2) pressure reduction through a nozzle. Phase Creation Processes Phase Creation Techniques Partial condensation or vaporization – for vapor or liquid feed with different volatilities; heat is transferred to or from feed in heat exchanger followed by gravity separation Flashvaporization – partially vaporizes liquid by reducing feed pressure with valve followed by phase separation Phase Creation Techniques Distillation – employed when differences in volatility are sufficiently large (eg. benzene and toluene) - Most common separation technique in industry - Distillation involves vigorous mixing during contact between countercurrently flowing liquid and vapor phases  Phase addition A single-phase feed is introduced to a second phase called a mass separating agent (MSA) to allow separation of a component from the feed phase to the MSA phase. Phase addition techniques are often used when physical limitations prevent the use of phase creation techniques. Phase Addition There are various disadvantages however in the use of phase addition techniques. This include: (1) Need for additional separator for MSA recovery (2) Need for MSA makeup (3) Possible MSA contamination in the product (4) More complex design procedures Ultimately, these reasons make phase addition techniques generally more expensive compared to phase creation techniques. Phase Addition Techniques Phase Addition Techniques Absorption – a component of a vapor feed is transferred to a liquid MSA contacted with the feed Stripping – a component of a liquid feed is transferred to a gaseous MSA contacted with the feed Phase Addition Techniques Liquid-liquidextraction – solute from a feed mixture is transferred to another solvent (MSA) which is immiscible or partially miscible with the feed solvent Adsorption – a component from a gaseous or liquid feed binds itself physically or chemically to a solid material (MSA) Barrier Separation Involvesthe use of microporous and nonporous membranes as semipermeable barriers for separating vapor and liquid mixtures Porous membrane: separates based on differences in particle size Nonporous membrane: separates based on differences in solubility and diffusivity Barrier Separation Retentate Feed Permeate Barrier Separation Techniques Barrier Separation Techniques – separation of suspended colloidal particles Dialysis from a mixture by means of unequal diffusion rates through the pores of a semipermeable microporous membranes Reverse osmosis – selective transport of solvent through a microporous membrane by increasing feed pressure to a value higher than the osmotic pressure of the solution – used in water purification Barrier Separation Techniques Gas permeation – separation of gases by introducing a pressure gradient to a nonporous membrane with selective permeability; used in recovering hydrocarbons from gas streams and producing oxygen-enriched air Pervaporation – certain species in liquid feed diffuse through nonporous membrane, where they are evaporated before exiting as permeate ; uses low pressure for vaporization Separation by an external force field Involves the separation of a certain chemical species in a feed by preferential attraction with an external force field such as gravity, electric field, etc. Techniques involving separation by an external force field Centrifugation: separation of particles from a solution according to size, shape, density, viscosity of the medium, and rotor speed via application of a centrifugal force Electrophoresis: separation based on the differences in migration velocities of charged colloidal or suspended particles in a electric field Other techniques include thermal diffusion, electrolysis, and electrodialysis. What method of separation to use? The following factors are vital in choosing the method of separation to be used for a particular process: (1) Fixed and operating cost (2) Technological maturity (3) Ease of providing multiple stages (4) Ease of scale-up from laboratory experiments (5) Need of parallel units to handle large capacities Equilibrium and Stagewise Operations Equilibrium Consider liquid and vapor phases in a closed container with each phase containing two components A and B. Liquid molecules are continually vaporizing and gas molecules are continually condensing. Initially, rates of condensation and vaporization are different. When is equilibrium achieved?  At equilibrium, three conditions occur in the closed system: Equilibrium (1) Thermal equilibrium Heat transfer between the two phases stops and the temperature of the two phases are equal. (2) Mechanical equilibrium Forces between the vapor and liquid phases are balanced and the pressure in the liquid and vapor phase are equal. (3) Phase equilibrium Rates of evaporation is equal to the rates of condensation. There is no macroscopic change in the composition of both phases.  Equilibrium Stage An equilibrium stage involves the Lin Vout contact of two streams or phases for a sufficient amount of time to establish equilibrium between the phases leaving the stage. Equilibrium stage The equilibrium stage is also called ideal or theoretical stage. Stage efficiencies of actual stages are based on comparison with the degree of Lout Vin separation achieved from an equilibrium stage. Equilibrium Stage The following processes can be treated as a single equilibrium stage since mass transfer is so rapid such that phase equilibrium is approached. Stagewise operations In many cases, a cascade of multiple-contact equilibrium stages are necessary to achieve a desired separation Mode of contact can be countercurrent, co-current or cross-current. …. Stagewise Operations The following processes often require stage cascades to achieve a desired level of separation. Stagewise Operations Co-current stages cascade Countercurrent stages cascade Cross-current stages cascade