Glossary

3.3 Multiphase Reactor Design

5 min read•july 22, 2024

Multiphase reactors are crucial in chemical engineering, involving interactions between different phases like gas-liquid or gas-solid systems. These reactors use various models to describe mass transfer, reaction kinetics, and heat transfer, considering factors like film theory and surface renewal.

Designing and optimizing multiphase reactors requires understanding phase equilibria, interfacial phenomena, and mixing characteristics. Engineers must balance conversion, energy consumption, and costs while considering reactor performance in bubble columns, packed columns, and stirred tanks. Advanced simulation tools and experimental techniques aid in this process.

Multiphase Reactor Modeling and Analysis

Models for multiphase reactors

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  • Gas-liquid systems
    • Two-film theory assumes stagnant films on both sides of the gas-liquid interface with mass transfer resistance
    • Penetration theory considers unsteady-state diffusion of solute into a stagnant liquid phase (falling film reactors)
    • Surface renewal theory assumes continuous replacement of liquid elements at the gas-liquid interface (bubble columns, stirred tanks)
  • Gas-solid systems
    • Shrinking core model describes reaction and diffusion in porous solid particles with a sharp interface between reacted and unreacted zones (gas-solid reactions, noncatalytic)
    • Grain model considers solid particles as a collection of small grains with reaction and diffusion occurring simultaneously (gas-solid reactions, catalytic)
    • Pore diffusion model accounts for diffusion and reaction in the pores of a catalyst particle (heterogeneous catalysis)
  • Liquid-liquid systems
    • Droplet dispersion model describes mass transfer and reaction in dispersed liquid droplets (liquid-liquid extraction, emulsion polymerization)
    • Emulsion polymerization model considers the polymerization reaction in monomer droplets dispersed in an aqueous phase (production of latex, rubber)
  • Conservation equations
    • Mass balance equations account for convection, diffusion, and reaction in each phase
    • Energy balance equations consider heat transfer between phases and heat of reaction
    • Momentum balance equations describe fluid flow and mixing in multiphase systems

Analysis of multiphase systems

  • Mass transfer correlations
    • Gas-liquid: Higbie penetration theory for short contact times, Dankwerts surface renewal theory for continuous phase renewal (bubble columns, stirred tanks)
    • Gas-solid: Sherwood number ShSh relates to particle size and diffusivity, Schmidt number ScSc represents the ratio of momentum to mass diffusivity, Reynolds number ReRe characterizes the flow regime
    • Liquid-liquid: Sauter mean diameter describes the average droplet size, Weber number WeWe relates inertial to surface tension forces (emulsification, dispersion)
  • Heat transfer correlations
    • Nusselt number NuNu relates heat transfer coefficient to thermal conductivity and characteristic length, Prandtl number PrPr represents the ratio of momentum to thermal diffusivity
    • Chilton-Colburn analogy relates heat and mass transfer coefficients based on the similarity between thermal and concentration boundary layers
  • Reaction kinetics
    • Intrinsic kinetics describe the true reaction rate without mass transfer limitations
    • Effectiveness factor η\eta represents the ratio of actual to intrinsic reaction rate in the presence of diffusion limitations
    • Thiele modulus ϕ\phi relates the reaction rate to the diffusion rate in a catalyst particle
  • Dimensionless numbers
    • Sherwood number ShSh: Sh=kcdpDABSh = \frac{k_c d_p}{D_{AB}} relates mass transfer coefficient kck_c to particle diameter dpd_p and diffusivity DABD_{AB}
    • Schmidt number ScSc: Sc=μρDABSc = \frac{\mu}{\rho D_{AB}} represents the ratio of momentum diffusivity μ/ρ\mu/\rho to mass diffusivity DABD_{AB}
    • Reynolds number ReRe: Re=ρudpμRe = \frac{\rho u d_p}{\mu} characterizes the flow regime based on fluid density ρ\rho, velocity uu, particle diameter dpd_p, and viscosity μ\mu
    • Weber number WeWe: We=ρu2dpσWe = \frac{\rho u^2 d_p}{\sigma} relates inertial forces ρu2\rho u^2 to surface tension forces σ/dp\sigma/d_p

Multiphase Reactor Design and Optimization

Design of multiphase reactors

  • Phase equilibria
    • Vapor-liquid equilibrium VLE determines the composition and properties of coexisting vapor and liquid phases (distillation, absorption)
    • Liquid-liquid equilibrium LLE describes the distribution of components between two immiscible liquid phases (extraction, separation)
    • Solid-liquid equilibrium SLE governs the solubility of solid components in a liquid phase (crystallization, precipitation)
  • Interfacial phenomena
    • Surface tension is the force per unit length acting at the interface between two phases (gas-liquid, liquid-liquid)
    • Wetting and contact angle describe the interaction between a liquid and a solid surface (packed columns, trickle bed reactors)
    • Marangoni effect is the mass transfer along an interface due to surface tension gradients (mass transfer enhancement)
  • Mixing characteristics
    • Mixing time is the time required to achieve a desired degree of homogeneity in a reactor (stirred tanks, bubble columns)
    • Circulation time represents the average time for a fluid element to complete one circulation loop in a reactor (loop reactors)
    • Power number NpN_p relates the power consumption to the impeller dimensions and fluid properties in stirred tanks
  • Optimization objectives
    • Maximize conversion or yield to achieve high reactor performance and product quality
    • Minimize energy consumption to reduce operating costs and environmental impact
    • Minimize capital and operating costs to improve the economic viability of the process

Performance of multiphase reactors

  • Bubble columns
    • Gas holdup is the volume fraction of gas in the reactor, influencing mass transfer and mixing
    • Bubble size distribution affects the and mass transfer rates (coalescence, breakup)
    • Axial dispersion coefficient quantifies the degree of mixing in the liquid phase (back-mixing, plug flow)
  • Packed columns
    • Pressure drop is the driving force for fluid flow and affects the operating cost (permeability, particle size)
    • Liquid holdup represents the volume fraction of liquid in the column, influencing residence time and mass transfer
    • Wetting efficiency describes the fraction of packing surface area wetted by the liquid phase (mass transfer, catalyst utilization)
  • Stirred tanks
    • Power consumption is the energy input required to maintain the desired mixing and mass transfer conditions (impeller type, size)
    • Mixing time characterizes the time required to achieve a desired degree of homogeneity (turbulence, circulation)
    • Gas-liquid mass transfer coefficient kLak_L a quantifies the rate of mass transfer between gas and liquid phases (bubble size, interfacial area)
  • Simulation tools
    • Computational Fluid Dynamics CFD simulates the detailed flow, mixing, and reaction behavior in multiphase reactors (velocity, concentration, temperature profiles)
    • Process simulators like Aspen Plus and HYSYS provide a framework for modeling and optimizing entire chemical processes, including multiphase reactors (mass and energy balances, unit operations)
  • Experimental techniques
    • Particle Image Velocimetry PIV measures the instantaneous velocity fields in a fluid by tracking the motion of seeded particles (flow visualization, turbulence)
    • Laser Doppler Anemometry LDA determines the local fluid velocity by measuring the Doppler shift of laser light scattered by particles (non-intrusive, point measurements)
    • Radioactive particle tracking RPT tracks the motion of radioactive tracer particles to obtain the velocity and mixing patterns in a reactor (Lagrangian approach, opaque systems)

Key Terms to Review (15)

Annular Flow: Annular flow is a type of fluid flow pattern characterized by a core of one fluid surrounded by another fluid in a cylindrical configuration, often observed in multiphase systems. This configuration is significant in the context of reactor design, as it allows for effective mass transfer and reaction rates while maintaining stability in the system. Understanding annular flow is crucial for optimizing multiphase reactions, especially in processes involving gas-liquid or liquid-solid interactions.
Bubble Column: A bubble column is a type of reactor designed to facilitate gas-liquid interactions by introducing gas into a liquid through a distributor, creating a column filled with rising bubbles. This design allows for efficient mass transfer between the phases, making it ideal for various multiphase reactions, particularly those involving gas and liquid systems. In these reactors, the bubble dynamics and flow patterns significantly influence reaction rates and overall process efficiency.
Continuum modeling: Continuum modeling is a mathematical approach used to represent physical systems where properties are distributed continuously throughout the material, rather than being concentrated at discrete points. This method simplifies the analysis of complex processes, particularly in multiphase systems, by treating materials as continuous media rather than as a collection of particles or discrete phases, allowing for more manageable equations and simulations.
Emission Control: Emission control refers to the measures and technologies used to reduce or eliminate the release of harmful pollutants into the environment, particularly from industrial processes and vehicles. In the context of multiphase reactor design, effective emission control is essential for minimizing environmental impact while maximizing the efficiency and safety of chemical reactions. Understanding how to implement these controls can significantly influence reactor design choices, operational parameters, and overall sustainability of chemical production processes.
Feed Distribution: Feed distribution refers to the method by which reactants are introduced into a reactor, particularly in multiphase systems, ensuring an even and efficient mixing of phases. Proper feed distribution is essential for optimizing reaction conditions, enhancing mass transfer rates, and minimizing concentration gradients that can lead to inefficiencies or unwanted side reactions.
Heterogeneous Reaction: A heterogeneous reaction is a chemical reaction that occurs between reactants in different phases, typically involving solid, liquid, and gas components. This type of reaction is characterized by the distinct physical states of the reactants and products, which can significantly influence the reaction rate, mechanism, and overall efficiency. Understanding heterogeneous reactions is crucial in multiphase reactor design, as it helps engineers optimize conditions for effective mass transfer and reaction kinetics.
Homogeneous reaction: A homogeneous reaction is a chemical reaction that occurs within a single phase, typically involving reactants and products that are all in the same physical state, such as gas, liquid, or solid. This uniformity allows for more consistent reaction rates and easier analysis of reaction mechanisms, making it an essential concept in understanding multiphase reactor design, where the phase behavior significantly impacts efficiency and performance.
Hydraulic Residence Time: Hydraulic residence time is the average time that a fluid element spends inside a reactor or a system, calculated as the volume of the reactor divided by the flow rate of the fluid. This concept is crucial for understanding how long reactants are in contact within multiphase reactors, influencing the conversion rates and efficiency of chemical reactions. By analyzing hydraulic residence time, engineers can optimize reactor design and operation to improve yield and selectivity in multiphase systems.
Interfacial Area: Interfacial area refers to the surface area between two immiscible phases, such as liquid-liquid, gas-liquid, or solid-gas interfaces, where mass and energy transfer can occur. This area is critical in processes where reactions or separations take place, as it directly impacts the efficiency and rate of these processes by influencing contact between the phases.
Mass Transfer Coefficient: The mass transfer coefficient is a key parameter that quantifies the rate at which mass is transferred between phases, often expressed in terms of concentration change over time and area. It reflects the efficiency of mass transfer in processes such as absorption, distillation, and chemical reactions within multiphase systems. A higher mass transfer coefficient indicates a more effective transfer, which is critical for optimizing reactor designs, particularly in gas-liquid systems and advanced mass transfer applications.
Process Safety: Process safety refers to the discipline focused on preventing and mitigating the consequences of catastrophic failures in chemical processes. It encompasses a wide range of practices, principles, and technologies aimed at ensuring that hazardous materials are managed safely throughout their lifecycle, from design to operation. This concept is crucial for minimizing risks in systems involving multiple phases, where reactions can be complex and conditions may change rapidly.
Scalability: Scalability refers to the ability of a process or technology to adapt and perform effectively when the scale of operation changes, particularly when increasing production or capacity. It is crucial for ensuring that systems can meet growing demand without sacrificing performance, efficiency, or safety. Scalability is influenced by various factors, such as design, materials, and operational strategies, all of which play a role in how well a process can be expanded or adapted.
Slug Flow: Slug flow is a type of two-phase flow pattern in which large, distinct plugs or slugs of liquid travel through a gas-filled pipeline. This flow regime is characterized by alternating sections of liquid and gas, leading to intermittent contact between the phases. Understanding slug flow is crucial in multiphase reactor design, as it affects mass transfer, reaction rates, and the overall efficiency of chemical processes.
Temperature Control: Temperature control refers to the management of thermal conditions within a reactor to maintain optimal reaction rates and product quality. It is crucial for ensuring that reactions occur at desired temperatures, preventing unwanted side reactions, and enhancing energy efficiency. Effective temperature control directly influences the overall performance and safety of multiphase reactors, where temperature variations can significantly affect mass transfer and reaction kinetics.
Trickle Bed Reactor: A trickle bed reactor is a type of multiphase reactor that allows liquid and gas to flow through a packed bed of solid catalyst, typically in a downward direction. This configuration promotes efficient mass transfer between the phases and facilitates various chemical reactions, making it especially useful in applications like hydrogenation, oxidation, and catalytic cracking.
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