CHAPTER 5AGITATION LIQUIDS

CHE 503 FLUID FLOW

OVERVIEW

Introduction and Definition Purpose of Agitation & Mixing Agitated Equipment Types of Impeller Flow Pattern in Agitated Vessel Standard Turbine Design

AGITATION – forcing of “homogenous” material by mechanical method to flow in a circulatory or other pattern inside a vessel.

MIXING – involve the taking of two or more separate phases, such as a fluid and a powdered solid, or 2 fluids, and causing them to be randomly distributed through one another.

DEFINITIONS

PURPOSES OFAGITATION / MIXING

Blending miscible liquids (e.g ethyl alcohol & water) Dispersing a gas through the liquid (fine bubbles) Dispersing a second liquid to form an emulsion or

suspension Dissolving solids in liquids (e.g. salt in water) Agitation of the fluid to increase heat transfer

between fluid and coil or jacket in vessel wall.

AGITATION AND MIXING

AGITATION AND MIXING

AGITATION AND MIXING

INTRODUCTION TO MIXING

Most common operations carried out in the chemical, processing and allied industries.

Applied to the processes used to reduce the degree of non-uniformity, or gradient of a property in a system such as concentration, viscosity, temperature and so on.

Achieved by moving material from one region to another and also when achieving a desired degree of homogeneity

Used to promote heat and mass transfer, often where a system is undergoing a chemical reaction.

TYPE OF MIXING

Single-phase liquid mixing Mixing of immiscible liquids Gas-liquid mixing Liquid-solids mixing Gas-liquid-solids mixing Solids-solids mixing

TASK (15 MINUTES)

Discuss process that get involve in all this mixing types:

Single-phase liquid mixingMixing of immiscible liquids Gas-liquid mixingLiquid-solids mixingGas-liquid-solids mixingSolids-solids mixing

INTRODUCTION TO MIXING

1. Single-phase liquid mixing:

Two or more miscible liquids must be mixed to give a product of a desired specification.

This is the simplest type of mixing as it involves neither heat nor mass transfer, nor indeed a chemical reaction.

Example:

1. The use of mechanical agitation to enhance the rates of heat and mass transfer between the wall of a vessel, or a coil, and the liquid (brine solution= HCl+H2O).

2. In the blending of petroleum products of different viscosities.

MIXING

2. Mixing of immiscible liquids: When two immiscible liquids are stirred together, one

phase becomes dispersed as tiny droplets in the second liquid which forms a continuous phase.

Example: Liquid-liquid extraction, a process using successive mixing and settling stages.

The liquids are brought into contact with a solvent that will selectively dissolve one of the components present in the mixture.

Vigorous agitation causes one phase to disperse in the other and, if the droplet size is small, a high interfacial area is created for interphase mass transfer.

When the agitation is stopped, phase separation takes place, but care must be taken to ensure that the droplets are not so small that a diffuse layer appears in the region of the interface; this can remain in a semi-stable state over a long period of time and prevent effective separation from occurring.

MIXING

3. Gas-liquid mixing: Numerous processing operations involving chemical

reactions, such as aerobic fermentation, wastewater treatment, oxidation of hydrocarbons, and so on, require good contacting between a gas and a liquid.

The purpose of mixing here is to produce a high interfacial area by dispersing the gas phase in the form of bubbles into the liquid.

Generally, gas-liquid mixtures or dispersions are unstable and separate rapidly if agitation is stopped.

MIXING

4. Liquid-solids mixing: Mechanical agitation may be used to suspend particles in a

liquid in order to promote mass transfer or a chemical reaction.

The liquids involved in such applications are usually of low viscosity, and the particles will settle out when agitation ceases.

5. Gas-liquid-solids mixing: In some applications such as catalytic hydrogenation of

vegetable oils, slurry reactors, froth flotation, evaporative crystallization, and so on, the success and efficiency of the process is directly influenced by the extent of mixing between the three phases.

MIXING

6. Solids-solids mixing: Mixing together of particulate solids, sometimes referred to

as blending, is a very complex process in that it is very dependent, not only on the character of the particles — density, size, size distribution, shape and surface properties.

Mixing of sand, cement and aggregate to form concrete and of the ingredients in gunpowder preparation are examples of the mixing of solids.

Other industrial sectors employing solids mixing include food, drugs, and the glass industries.

MIXING

Miscellaneous mixing applications:

For example, the rotational speed of an impeller in a mixing vessel is selected to achieve a required rate of heat transfer, and the agitation may then be more than sufficient for the mixing duty.

Excessive or overmixing should be avoided as it is not only wasteful of energy but may be detrimental to product quality.

In mixing, two problems need to be considered — how to design and select mixing equipment for a given duty, and how to assess whether a mixer is suitable for a particular application.

Aspects of the mixing process that should be understood:

(i) Mechanisms of mixing,

(ii) Scale-up or similarity criteria,

(iii) Power consumption,

(iv) Flow patterns,

(v) Rate of mixing and mixing time,

(vi) The range of mixing equipment available and its selection.

INTRODUCTION TO MIXING (CONT’)

MIXING MECHANISMS In order to produce a uniform mixture, it is necessary to

understand how liquids move and approach this condition.

In liquid mixing devices, two requirements must be satisfied.

There must be bulk or convective flow → no dead (stagnant) zones.

There must be a zone of intensive or high-shear mixing → inhomogeneities are broken down.

Both processes are energy-consuming and ultimately the mechanical energy is dissipated as heat; the proportion of energy attributable to each varies from one application to another.

Depending on the fluid properties (e.g. viscosity), the flow in mixing vessels may be laminar or turbulent, with a substantial is transition zone and frequently both flow types will occur simultaneously in different parts of the vessel.

MIXING / AGITATED EQUIPMENT

The wide range of mixing equipment available commercially reflects the enormous variety of mixing duties encountered in the processing industries.

It is reasonable to expect therefore that no single item of mixing equipment will be able to carry out such a range of duties effectively.

This has led to the development of a number of distinct types of mixer over the years.

The choice of a mixer type and its design is therefore primarily governed by experience. In the following sections, the main mechanical features of commonly used types of equipment together with their range of applications are described qualitatively.

STIRRED TANK DESIGN

Energy required to achieve amount of agitation or quality of mixing are based on:

• Fluid properties• Size of vessel• Dimensions and arrangement of impellers, baffles • Other internals factors.

The internal arrangements depend on the objectives of the operation: whether it is to maintain homogeneity of a reacting mixture or to keep a solid suspended or a gas dispersed or to enhance heat or mass transfer.

A BASIC STIRRED TANK DESIGN

THE VESSEL: A dished bottom requires less power than a flat one. When a

single impeller is used, DT=H, with the impeller located at the center for an all-liquid system.

BAFFLES: Baffles are needed to prevent vortexing and rotation of

the liquid mass as a whole. A baffle width, WB = DT/10 Often fitted to the walls of the vessel. Generally not required for high viscosity liquids

because the viscous shear is then sufficiently great to damp out the rotary motion.

A BASIC STIRRED TANK DESIGN (CONT’)

IMPELLERS:

A rotating impeller in a fluid cause flow and shear to it, that shear resulting from the flow of one portion of the fluid past another.

The liquids will flow in the axial or radial directions and the impellers are classified conveniently according to which flows is dominant.

Those generate currents parallel with the axis of the impeller shaft are called axial-flow impeller and those that generate currents in a radial or tangential direction are called radial flow impeller.

A BASIC STIRRED TANK DESIGN (CONT’)

AGITATION AND MIXING

Axial flow impeller Radial flow impeller

PROPELLER An axial-flow, high speed (400 – 1750 rpm) impeller for

liquids of low viscosity (< 3000 cp). The direction of rotation is usually chosen to force the liquid

downward, and the flow currents leaving the impeller continue until deflected the floor of the vessel.

Because of the persistence of the flow currents → effective in very large vessels.

For deep tank- two or more maybe mounted on the same shaft.

Type: Standard 3-blade marine propeller with square pitch (common in used), four blade, toothed/other designed.

TURBINES Type 1: The turbine with flat vertical blades extending to

the shaft is suited to the vast majority of mixing duties up to 100,000 CP or so at high pumping capacity. The currents it generates travel outward to the vessel wall and then flow either upward or downward. Such impellers are sometimes called paddles.

Type 2 & 3: Create zones of high shear rate. Good in dispersing gas in a liquid (gas is forced at high shear rate to flow radially to the blade tips)

HIGH- EFFICIENCY IMPELLER

The pitched-blade turbine have been developed to provide more uniform axial flow in addition to radial flow for better mixing, as well as to reduce the power required for a given flow rate.

These impeller are widely used to mix low or moderate viscosity liquids, but they are not recommended for very viscous liquids or for dispersing gases.

Fluid foil impeller

Use for liquid with viscosities more than 20 Pa.s or 20,000 cP

Diameter helix approximately to inner diameter of tank Provide good agitation near the floor of the tank;

No vertical motion Promotes good heat transfer to/from the vessel.

a) Double-flight helical-ribbon impeller b) Anchor impeller

HIGHLY VISCOUS LIQUID IMPELLER

FLOW PATTERNThe way of liquid moves in an agitated vessel depends on;

a) the type of impeller;

b) the characteristics of the liquid, especially its viscosity;

c) the size and proportions of the tank, baffles and impeller.

The liquid velocity at any point in the tank has three components:

The first velocity component - radial and acts in a direction perpendicular to the shaft of the impeller.

The second component- longitudinal and acts in a direction parallel with the shaft.

The third component- tangential, or rotational, and acts in a direction tangent to a circular path around the shaft.

FLOW PATTERN

FLOW PATTERN: 3-BLADE PROPELLER AGITATOR

Axial flow – fluids flow axially down the center axis or propeller shaft and up on the sides of the tank

FLOW PATTERN: TURBINE AGITATOR

ASSIGNMENT 2 – (4 MEMBERS/GROUP)

MIXING OPERATIONS AVAILABLE IN INDUSTRY

Choose 1 type of mixing operation available in industry and your discussion shall includes;

1. AGITATION AND MIXING PROCESSES BACKGROUND

2. DESIGN OF MIXING TANK

- Production Rate

- Tank Dimension (D and H), Agitated System, No of baffles,

3. THE FLOW PATTERNS

4. CALCULATION OF POWER CONSUMPTION

Date of Report Submission: 19/11/2012

Date of Presentation: 26/11/2012