Transport phenomena in chemical processes

part II

Michał Araszkiewicz PhD

What the transport means?

In casual situation, when we say transport, we think about movement of one thing from one place to the different one.

In chemical engineering nomenclature we understand transport as a process of transfering:• Liquid• Gas• SolidsFor example:

Tank Reactor

The transport processes can be divided into two main groups:

A Transport A

Transport process without chemical reaction

Substance A remains intact during the transport process

Transport process with chemical reaction

A Transport + reaction B

Substance A undergo the chemical reaction during the transport process and become substance B at the end

And that kind of transport process is no longer in our field of interest.

• The transport definition can not be limited to the mentioned before mechanical or even chemical transfer processes.

• In this course I would like to focus on a more complicated situation, which is not only limited to the simply material transfer during chemical processes.

• We will discuss the transport phenomena that can be connected or take place along the processes mentioned before.

• So, we have to clearly separate the three kinds of processes:– Mechanical transport– Chemical reaction– Mass, heat and momentum transport

Processes

Mechanical Chemical reaction

Mass, Heat, Momentum

• In order to complicate the situation all mentioned before processes can take place together. In such case the analysis of the process course, balance and efficiency becomes more difficult and requires more effort.

We will:• State the definition of each of these processes• Present the obvious and more mysterious

driving forces for each case. • Present the most important rudiments of each

of them• Look at the boundary and initial conditions• Present the main governing equations of

these three processes.

So, its time for some very general definitions:

Momentum transfer

The momentum transfer is connected with analysis of various forces that work on the single element or flow of fluids. In fact – the momentum concept means product of mass and velocity

Heat transfer

Heat transfer is about transferring the energy that can be manifested by increasing or decreasing material temperature. The most common driving force of heat transfer is temperature gradient within the material.

Mass transfer

The mass transfer is concerned with the transport of chemical particles from one place (where their concentration is high) to another one (with lower concentration). That process is also very important, when the two different compounds react together (when the chemical reaction between them takes place). The reaction rate depends on the quality of contact between two (or more ) reagents in that case.

Processes Chemical engineering is kind of science about physical and chemical phenomena that take place in chemical apparatus. These phenomena (which sometimes run at the same time) are producing processes. Finally all is about of conversion of substrates into products.

Phenomena• Physical• Chemical

Process

Processes

• The number of processes are linked in one exemplary course of production.

Phenomena Process Production

Substrate 1

Substrate 2

Chemical reaction

Preparing /division

ofproducts

Main product

By - products

preparation

preparation

Example of production scheme

And now for something completely different we will take a closer look at the processes in chemical engineering:

Processes in chemical

engineering

Basic processes

Unit operations

Processes with chemical

reaction

Basic processes

Momentum transfer

Steady Unsteady

Heat transfer

Steady Unsteady

Mass transfer

Steady Unsteady

Momentum transfer

movement of fluids in:

one phase system

multi phase system

movement of the particles in force field

Heat transfer

Conduction Convection Radiation

Unit operations

Transfer of materials

Temperature and phase changes of materials

Shape changes of

solidSeparation

Solids

Liquids

Gases

Mixing

Solids

Liquids

Gases

• The all three basic processes (momentum, heat and mass tranfer) can be present in the unit operations.

• Acording to the dominant basic process the unit operations can be divided into various types:

Unit operations

Mechanical operations

Thermal operations

Diffusivity operations

Diffusive – thermal

operations

Thermodynamic

operations

Mechanical operations

Transport

Fragmentation

Classification

Flotation

Dedusting

Separation Mixing

Thermal operations

Cooling Heating Condensation Boiling

Solution concentratio

n

Diffusivity operations

Extraction Absorption Desorption Adsorption

Diffusive – thermal operations

Distillation Crystallization Drying Humidificati

on Permeation

Heat transferHeat transfer

Conduction Convection Radiation

Conduction

Energy transfer by conduction can take place in two ways. First one is connected with molecule interaction, where molecules with higher energy levels are transferring energy to the molecules with lower energy level. That kind of mechanism is common for situations where the temperature gradient exists and take place in solids, liquids and gases.

Conduction

Second mechanism of conduction heat transfer is connected with free electrons movement. That kind of phenomena take place mostly in pure –metallic solids. The concentration of free electrons varies considerably for alloys and becomes very low for non metallic solids. That is the reason that metallic solids are mostly so good heat conductors.

Convection

Convection is a kind of heat transfer that involves the energy exchange between a surface and adjacent fluid. The convection can be forced – where a fluid is made to flow past a solid surface by an external force.

Convection

Free/natural convection take place when fluid with significant temperature placed next to the solid boundary causes circulation in order to the differences in fluid density due to the temperature gradients within the fluid volume.

Radiation

The radiation mechanism is different to the two previously mentioned. In that case no physical medium is required for its propagation. The most efficient radiation heat transfer takes place in case of existing vacuum between two solids.

Mass transfer

Mass transfer

Diffusion

Molecular diffusion Convective mass transfer

Mass transfer

Mass transfer can take place by random molecular movement in fluids or can be connected with moleculer transport from solid surface into the moving fluid, when its dynamics depends on the fluid flow characteristic. That two kinds of mass transfer are very similar to the heat transfer mechanisms: conduction and convection respectively.

Molecular diffusion

There is a phenomemon that in closed system with one or more compounds where the concentration vary there is a natural tendency to level the compound concentration within the system.

Convective mass transferConvective mass transfer takes place in situation when:• Fluid moves near the solid surface and mass

transfer takes place between them• Two immiscible fluids separated by a mobile

interface (kind of contractor)The mass transfer always goes with concentration gradient from higher to lower concentration. Convective mass transfer depends on both transport properties and the fluid flow characteristic.

To be continued…Thank you for today.