Gas Absorption & Stripping in Chemical Engineering

Review gas absorption mass transfer fundamentals, including vapor pressure, Henry's law, convection, and key dimensionless numbers. Then model the gas-liquid interface and determine local and overall mass transfer coefficients.

Explain the equilibrium between vapor and liquid, where partial pressures balance and vapor pressure increases with temperature, and apply Henry's law and Dalton's law to relate liquid and gas concentrations.

Understand vapor pressure, the pressure a vapor exerts in thermodynamic equilibrium with its condensed solid or liquid in a closed system, and how temperature and volatility influence evaporation rates.

Raoult's law states that in an ideal liquid mixture, a component's partial pressure equals its vapor pressure times its mole fraction, predicting total vapor pressure in binary systems.

Examine how gas dissolves in nonvolatile liquids and how temperature and pressure affect solubility. Understand dynamic gas-liquid exchange, equilibrium distribution curves, and the role of solubility and the operation line.

compare hydrochloric acid, ammonia, and sulfur dioxide at 10 c to study solubility, noting hydrochloric acid highly soluble, sulfur dioxide less soluble, ammonia moderate, and rising temperature lowers solubility.

Explain Henry’s law, its applicability to real gases and dilute solutions, equilibrium between gas and liquid, partial pressures and Henry’s constant, and implications for gas absorption and film theory.

Explore diffusion and convection in mass transfer, explain how concentration gradients drive particle movement, and apply mass transfer coefficients and correlations to gas absorption.

Define flux as the rate of transport of species across a unit area, referenced to a fixed or moving frame, distinguishing mass transport from momentum transport.

Explore molecular diffusion driven by random thermal motion and concentration gradients. Compare molecular diffusion and eddies, and see how temperature speeds diffusion and pressure slows it in dye in water.

Explore diffusion applications in moving bulk systems, from pipe and reactor diffusion to spheres and variable-area geometries, focusing on steady-state and unsteady diffusion in liquids and solids.

Examine equimolar diffusion of ammonia and nitrogen between two tanks linked by a pipe under equal pressure and temperature, showing equal and opposite molar flux at steady state.

Explore unimolecular diffusion (UMD) in a binary gas system, where species A diffuses through stagnant B, driven by partial pressure differences, with steady-state flux and partial-pressure formulation.

Explore convection in mass transfer, contrasting diffusion at low temperatures with convection at higher temperatures, using correlations to estimate mass transfer coefficients and flux as h times delta C.

Explore the film concept in gas absorption, detailing how a liquid film forms at the gas–liquid interface and how the two-film theory simplifies mass transfer.

Apply the two-film theory with the equilibrium solubility curve to analyze gas absorption across a gas-liquid interface. The driving force equals the deviation from equilibrium and grows with concentration differences.

Explore the two-film theory and film mass transfer coefficients to predict gas–liquid absorption flux, using concentration-driven driving force and correlations or experiments to determine k values.

Learn to use overall mass transfer coefficients instead of local interface values, by relating gas and liquid phase concentrations to bulk properties through equilibrium conditions and driving forces.

Identify mass transfer resistance and its link to mass transfer coefficients, noting that increasing coefficients lowers resistance to favor transfer. Use a voltage-current analogy to locate and reduce controlling resistance.

Close the review on mass transfer and set the stage for gas absorption, refreshing fundamentals and guiding you toward transfer phenomena courses to support the gas absorption section.

Explore gas absorption basics, nomenclature, and real-life examples, and examine unit operations (tray, straight, marble, and packed columns) and design essentials (diameter, stages, and solvent quality).

Understand gas absorption as a gas–liquid contact that dissolves target components. Explore mass transfer driven by concentration gradients and unit operations like absorbers or scrubbers.

Define the nomenclature of absorption, detailing gas removal into a liquid phase, the roles of absorber and solvent, and common equipment like towers or bubble columns.

Explore gas absorption examples, including hydrogen sulfide removal from natural gas with MEA, ammonia scrubbing with water, and CO2 removal with amines; compare physical and chemical absorption and solvent regeneration.

Compare physical and chemical absorption: physical absorption causes no composition change with water or oils, while chemical absorption involves irreversible neutralization with sodium hydroxide, dissolving acid gas to form salts.

Examine the scope of nonreactive gas absorption focusing on a single gas component absorbed by nonvolatile liquids at low concentrations and steady state, with fixed pressure.

Desorption and stripping are reverse mass-transfer processes moving solute from liquid to gas. They show how absorption and stripping clean liquids or air, with CO2 exchanging between water and gas.

Explore stripping of volatile organic solvents from water by increasing airflow to drive concentration gradients, illustrating absorption and stripping, sulfur removal, and solvent recovery for reuse.

Explain absorption and stripping for sweetening sour gas, removing hydrogen sulfide and CO2 with amine solutions. Demonstrate lean and rich amines in countercurrent contact and regeneration to produce sweet gas.

Explore gas-liquid dispersion techniques, comparing gas dispersed in liquid and liquid dispersed in gas, to maximize contact time and surface area for efficient mass transfer in absorption.

Explore bubble column gas absorption, where gas disperses as bubbles in a continuous liquid to maximize mass transfer area, driven by pressure and gravity, despite high pressure drops.

Explore tray column basics for gas absorption and stripping, covering gas inlet, liquid inlet, bubble caps and sieve trays, vapor–liquid interaction, and equilibrium-stage concepts.

Assess solvent selection by maximizing gas solubility in the liquid to boost absorption, while noting the choice between physical and chemical absorption and solvent recyclability.

Choose solvents that avoid corrosiveness in gas absorption systems to protect piping and vessels. Corrosion refers to reactions that gradually destroy piping, containers, and vessels over time.

Explore the applications of gas absorption, its operation types, and solvent selection, clarifying nomenclature and single versus multiple stage concepts to build a practical understanding of absorption processes.

Identify the equipment used in tray columns, including the upper and bottom equipment, and explore real-life foam and bubble formation, gas-liquid contact, and steady-state operation.

Understand tray spacing in a chemical column, with guidelines of 18–24 inches (about 40–60 cm) balancing maintenance access and safety. Increasing spacing raises column height and requires bigger, costlier structures.

Determine tray diameter and hole configuration for gas absorption and stripping trays, using standard producer specifications and the column diameter, height, and number of stages as sizing drivers.

Understand tray layouts for gas absorption and stripping, focusing on contraflow columns with liquid down and vapor up, and recognize why single pass designs are simple, cost-efficient, and preferred.

Explore tray manways as maintenance access points that allow a human to enter, clean, and add materials to trays, with one worker per tray pass coordinating access from opposite sides.

Compare costs across tray types in gas absorption and stripping, noting that trays are the cheapest and bubble cap trays are the most expensive due to installation and maintenance considerations.

Explain tray maintenance in gas absorption and stripping, emphasizing bubble-cap trays as the most complex to clean due to plugging and downtime, with hole or jet trays for fallen streams.

Sieve trays are the cheapest and recommended for most applications; avoid bubble cap trays due to cost, and consider turndown ratios for gas–liquid flow.

Examine the internal features of gas-liquid contact zones, including holes that allow gas flow and liquid interaction. Learn how liquid falls and gas rises within these structures.

Cross-flow guides liquid downward and vapour upward, promoting bubbling and foaming with high interfacial area for strong gas–liquid mass transfer, as described by the two-film theory.

Identify the contact area between liquid and vapor, emphasize a disengagement space, and optimize tray spacing to allow droplets to fall and avoid carrying liquid to the next tray.

Explore weir trays in gas absorption and stripping, focusing on maintaining a desired liquid level and tray height, and how hold-up affects residence time and pressure drop in low-pressure columns.

The downcomer provides a controlled path for liquid to descend between trays, preventing flooding and promoting smooth flow. It must be properly sized to avoid choking and support vapor disengagement.

Explore countercurrent operation for dilute gas–liquid systems, where gas flows upward and liquid flows downward, enabling absorption via vapor–liquid contact and mass transfer from gas to liquid.

The operation line for a countercurrent absorber comes from a mass balance with constant L and G; slope L/G sets the line, intercept Yb links X1, Y1 to X2, Y2.

Draw the operation line for a carbon monoxide gas absorption process using gas and solvent; identify bottom and top points, ensure a positive slope, and choose countercurrent versus concurrent operation.

Compute the analytical minimum solvent flow rate for a contraflow gas absorber removing 97% of a 3% pentane feed, using equilibrium lines to yield Lmin ≈ 8.68 kmol/h.

Explore the cases in gas absorption and stripping, focusing on countercurrent flows, absorption versus stripping configurations, and how flow orientations affect operations.

Explore coal counter-current gas stripping, the reverse of absorption, where a gas rich in solid contacts a liquid that becomes lean in solute, driving mass transfer from liquid to gas.

Determine the number of equilibrium stages in gas–liquid separations using both graphical and algebraic methods, and apply equilibrium efficiency with real-life stage analogies.

Explore the diluted absorption factor A and its link to the L/G ratio, show that A<1 implies infinite stages while A>1 gives finite stages, and introduce the Krenzler equation.

Explain the analytical Kremser equation for diluted gas–liquid systems, linking operating line and equilibrium line via Henry's law and the absorption factor A to determine stages in absorption and stripping.

Apply the Kremser equation to determine the total number of ideal stages in a gas absorption scrubber using water, with equilibrium data, yielding about seven stages.

Verify that the graphical method matches the analytical result for gas absorption, giving about seven as the outlet, aligning with exercise 07's 6.46.

Engage with a dynamic simulation of a gas absorber, illustrating how countercurrent flow, temperature, and solvent rate shift equilibrium and operating lines to reduce contaminants.

Dynamic stripping column analysis shows how the equilibrium line, governed by temperature and pressure, and the operation line, defined by targets, determine the required number of stages.

Explore stage efficiency in gas absorption and stripping, contrasting real-stage performance with theoretical equilibrium, defining efficiency as actual change over theoretical change and discussing methods to predict overall column efficiency.

Apply Murphree's efficiency to each tray, comparing actual concentration changes to equilibrium predictions under well-mixed gas and liquid phases, and use the logarithm of the efficiency to determine total efficiency.

Industrial bubble cap columns show stage efficiencies rarely above 50 percent, varying with tray count, pressure, and temperature. Lower pressure reduces mass transfer, while stripping can yield higher efficiencies.

Calculate Murphree stage efficiencies from ideal theoretical stages and an average absorption factor to estimate real stage performance in a gas absorption column.

Examine column operation in gas absorption and stripping, detailing operating cases with varying gas and liquid flows, including normal operation, flooding, priming, froth, koening, whipping, and dumping.

Priming increases gas flow to suspend liquid droplets, extending gas-liquid contact and mass transfer in column operation, though higher gas velocities raise liquid hold up and may carry droplets upward.

Analyze tray diameter sizing by linking gas superficial velocity to pressure drop, flooding, and weeping, and determine cross-sectional area needed to handle gas and liquid rates with empirical correlations.

Defines how to calculate the diameter of a tray column for countercurrent absorption of ethanol in water, using flooding, superficial velocity, flow parameter, and iterative spacing.

Determine the equivalent height of clear liquid holdup on a tray and its impact on gas–liquid contact and pressure drop, using density, superficial gas velocity, and tray height.

Explore how surface tension drives headloss and gas bubble formation in perforated liquids, causing a pressure drop, and apply the correlation with density, perforation diameter, and surface tension.

Compute the pressure drop in a tray column by summing gas-side head loss, liquid head loss, and surface tension effects, using hole area and superficial velocity.

Explore gas absorption and stripping basics, including why we need them, how to select a good solvent, and the types of operation, with note on nomenclature to be covered later.

Explore packed towers for gas absorption, where a liquid distributor wets packing to maximize gas–liquid contact and interaction, while noting pressure loss and importance of column height and packing diameter.

Explore the equipment, packaging, and packing support required for practical packed columns, including installation considerations for the packaging and packing components.

Explore packing types for gas liquid contact—random, structured, and grid packings—and the criteria for inert, corrosion resistant, high surface area, low weight materials.

Compare metal, plastics, and ceramics for packing in gas absorption and stripping, focusing on wetting behavior, strength, temperature resistance, and reliability.

Explore random packing in chemical columns, where randomly arranged packing provides voids that enable gas and liquid flow and maximize contact between solid, liquid, and gas.

Liquid/vapour distributors redistribute intermediate liquid feeds in distillation columns and between packed sections. They help mix and equalize liquid and vapor compositions across the column, reducing distribution problems.

Explore how a vortex breaker prevents vortex formation in a gas absorption and stripping column by redistributing liquid flow to the bottom, reducing chaotic flow for a controlled stream.

Install packing supports to hold the packing and prevent downward migration into the gas phase. Ensure open area supports the unrestricted flow of liquid and paper, preferring fixed corrugated supports.

Relate pressure drop to flow parameter, packing, velocity, and liquid properties via an exponential equation, and outline steps to compute gas and liquid densities and mass flow rates.

Determine gas velocity and absorber diameter with a fractional approach, using densities, viscosity, and packing parameters to compute velocity, then derive diameter from flow rate at 50–70% of reference velocity.

Determine column diameter for an ethanol absorber packed with 50 mm hi-flow rings under a 300 Pa/m pressure drop, iterating to meet 97% ethanol recovery with flooding and X–Y calculations.

Learn to estimate column height using the height equivalent to a theoretical plate (HETP), relate packing height to stages, and assess how flow rates and packing type affect HETP.

Compute the height equivalent theoretical plates (HETP) for an absorption column using eight trays and a three-meter height, illustrating a packed-system reduction and yielding HETP around 0.0375 m per tray.

Choose the best packing between X and X for a flow number of 2.5, calculate HETP and real trays, yielding about 20 stages and a height near 8 m.

Explains the method of transfer units for modeling gas absorption and stripping, using differential packing height, gas phase transfer units, HTU and GTU to relate mass balances and transfer coefficients.

Analyze number of transfer units (NTU) as a measure of separation difficulty in towers, where higher NTU indicates a taller column, and relate NTU to vertical trades and equilibrium stages.

Explain converting interface concentrations to overall concentrations to calculate overall mass transfer coefficients in MTU, using Henry's law and equilibrium for gas and liquid phases.

Explain gas phase mass transfer using MTU and the gas phase mass transfer coefficient, linking packing area, Henry's law, and operating lines to driving forces Delta Y and Delta X.

Use the mass transfer unit method on a packed absorber to estimate packing height for 90% ethanol removal from a CO2-rich gas, applying Henry's law.

Explains the difference between HETP and MTU in packed columns, showing HETP uses the overall mass transfer coefficient while MTU reflects the total height of transfer units.

Raising purity to 99.5 percent dramatically increases packed column height and stages, with non-linear growth near equilibrium; 90% purity ~3 m, 95% ~9 m, 99% ~15 m.

Explore non-diluted cases in tray columns, highlighting how constant gas and solvent flows challenge dilute assumptions and how uppercase X and Y modify equilibrium and mass balances.

Analyze a concentrated tray column absorber removing 65% of CO2 from a gas stream using mass balance, equilibrium data, L/G ratio optimization, and Murphree efficiency to estimate 1.8–2 stages and 3 trays.

Explore multicomponent absorption in stack gas treatment, including removal of CO2, CO, NOx, and SOx, and compare Henry's law and Meche methodology for dilute and higher concentrations.

Explore multicomponent absorption using Horton-Franklin mass-balance equations to model stage-by-stage gas–liquid equilibrium, noting temperature changes and heat exchange that drive iterations to determine stage compositions.

case study analyzes multicomponent gas absorption of methane, ethane, propane, and butane. it examines a three-stage adiabatic absorber with a butane impurity liquid and propane removal goals.

Explore reactive absorption, where gas reacts with the liquid to boost mass transfer. See CO2 absorption with sodium hydroxide forming sodium carbonate, aiding exhaust gas cleaning through enhanced interfacial performance.

Conclude section six with an overview of advanced reactive, component, and concentrated absorption cases, noting that engineers rely on simulation software to model and analyze problems.

Explores advanced gas absorption and stripping cases using a process simulation, evaluating column size, pressure drops, tray versus packed columns, and implementation costs to keep the engineer in control.

Explore how Aspen Plus models chemical processes by building physical property environments and unit operation diagrams to solve material and energy balances and optimize absorption and stripping processes.

Learn how Aspen Plus models simulate gas absorption in columns using mass-transfer based accum stages with Murphree or vaporization efficiencies and options for trays, packing, and multistage absorption and stripping.

Analyze tray spacing effects on pressure drop and column performance in a gas absorbed with water and acetone, illustrating how spacing adjustments influence diameter and 80 percent efficiency.

Explore how loss and simulation enable shifting from dilute to concentrated gas absorption, optimizing LNG ratio, convergence, and temperature tweaks to achieve impurity drops below 0.5 percent.

Explore models of absorption columns in Aspen HYSYS, from a simple absorber to complex templates with condensers, feeds, outlets, and converged solution behavior.

Explore the case study two problem statement on sulfur dioxide absorption into water, detailing 20 stages at one atmosphere and comparing tray and packing column designs in Aspen Plus.

Explore case study 2 on simulations and results for gas absorption using Aspen Hazes, comparing absorber configurations, optimizing pressure and temperature to maximize SO2 removal and achieve high gas purity.

Explore multicomponent absorption by adding acetone and ethanol and compare diluted and concentrated cases, while observing how composition and pressure changes affect water interactions and acid formation, recalculation not required.

Review section seven covers modeling gas absorption cases, including simple, concentrated, and multiple component absorbers, and explores what-if scenarios using a process simulator like Aspen.

Concludes the course on gas absorption and stripping, reviewing mass transfer, dimensionless numbers, operation conditions, and design of packed and tray columns with Aspen Plus and Aspen Ices.

Explore the top unit operation questions across pumps, tanks, heat exchangers, and absorption and distillation, with practical insights on pump curves, system curves, and mass and heat transfer.