BKF3463: UNIT OPERATION 1

Dr. Hayder A. Abdul Bari

INTRODUCTION

What is chemical engineering? Chemical Engineering is a group of industrial processes in which row materials are changed or separated into useful products

Historical development: As the Industrial Revolution steamed along certain basic chemicals quickly became necessary to sustain growth

- Example: Sulfuric acid was first among these "industrial chemicals".

Chemistry:To create a new substanceTo study its propertiesTo investigate all possible pathways from one substance to another

Chemical Engineering:To design the most optimal technology

for production of a specified substance

from row materialsTo develop and discover new technological applications for materials

Feed

PERMEATE

RESIDUE

Caustic Scrubber

FURNACE

MEMBRANE

HYDROGEN

LIQUIDS

COOLING

SieveDryer

Light Hydrocarbons

Heavier Hydrocarbons

What is the meaning of a UNIT in chemical engineering ?

DETAILS SYLLABUS

1.0 Overview of Separation Processes

1.1 Purpose of Separation Process

1.2 Classification of Separation Processes

1.3 Mechanism of Separation Processes

1.4 Separation by Phase Addition or Creation

1.5 Separation by Barrier

1.6 Selection of Feasible Separation Processes

2.0 Evaporation

2.1Types of Evaporation Equipment and Operations Method

2.2 Overall Heat Transfer Coefficients in Evaporators

2.3 Calculation Methods for Single-Effect Evaporators

2.4 Calculation Methods for Multiple-Effect Evaporators

2.5 Condensers for Evaporators

2.6 Evaporations Using Vapor Recompression

3.0 Distillation

3.3 Simple Distillation Methods

3.4 Distillation with Reflux and McCabe-Thiele Method

3.5 Distillation and Absorption Tray Efficiencies

3.6 Fractional Distillation Using Enthalpy-Concentration Method

3.7 Distillation of Multi-components Mixtures

3.2 Single-Stage Equilibrium Contact Stages

3.1 Vapor-Liquid Equilibrium Relations

4.0 Absorption

4.3 Single and Multiple Equilibrium Contact Stages

4.4 Mass Transfer between Phases

4.5 Continuous Humidification Processes

4.6 Absorption in Plate and Packed Towers

4.7 Absorption of Concentrated Mixtures in Packed Towers

4.2 Equilibrium Relations between Phases

4.1 Types of Separation Process and Methods

4.8 Estimation of Mass Transfer Coefficients for Packed Towers

5.0 Extraction

5.3 Continuous Multistage Countercurrent Extraction

5.4 Introduction and Equipment for Liquid-Solid Leaching

5.5 Equilibrium Relations and Single Stage Leaching

5.6 Countercurrent Multistage Leaching

5.7 Properties of Pure Supercritical Fluids

5.2 Equipment Liquid-Liquid Extraction

5.1 Single-Stage Liquid-Liquid Extraction Processes

5.8 Process Concept in Supercritical Fluid Extraction

5.9Phase Equilibrium and Mass Transfer in Supercritical Fluid Extraction

Quizzes 10%

TEST 1 20%

TEST 2 20%

Assignments 10%

Project 10%

FINAL EXAM 30%

ASSEEEMENT PLAN

REFERENCES

Geankoplis., “Transport Processes and Unit Operations”, 3th Ed., Prentice Hall, USA, 1995.

McCabe, Smith, “Unit Operations of Chemical Engineering”, 5rd Ed., McGraw-Hill, Singapore, 1993.

Willian J. Thomson., “Introduction to Transport Phenomena”, Prentice Hall, USA, 2000

Unit Operations:

- Unit Operations is a method of analysis and design of chemical engineering processes in terms of individual tasks/operations

- It is a way of organizing chemical engineering knowledge into groups of individual tasks/operations

- A unit operation: basic step in a chemical engineering process

Unit Operations: Historical perspective

For all intents and purposes the chemical engineering profession began in 1888.

An effort in 1880, by George Davis to unite these varied professionals through a "Society of Chemical Engineers" proved unsuccessful.

However, this muddled state of affairs was changed in 1888, when Professor Lewis Norton of the Massachusetts Institute of Technology introduced "Course X" (ten), thereby uniting chemical engineers through a formal degree. Other schools, such as the University of Pennsylvania and Tulane University, quickly followed suit adding their own four year chemical engineering programs in 1892 and 1894 respectively.

AICHE: 1908IChemE: 1920

1.1 Purpose of separation Process

In chemistry and chemical engineering, a separation process is used to transform a mixture of substances into two or more compositionally-distinct products.

Barring a few exceptions, almost every element or compound is found naturally in an impure state such as a mixture of two or more substances. Many times the need to separate it into its individual components arises. Separation applications in the field of chemical engineering are very important.

A good example is that of crude oil. Crude oil is a mixture of various hydrocarbons and is valuable in this natural form. Demand is greater, however, for the purified various hydrocarbons such as natural gases, gasoline, diesel, jet fuel, lubricating oils, asphalt, etc.

Separation processes can essentially be termed as mass transfer processes. The classification can be based on the means of separation, mechanical or

chemical. The choice of separation depends on the pros and cons of each. Mechanical separations are usually favored if possible due to the lower cost

of the operations as compared to chemical separations. Systems that can not be separated by purely mechanical means (e.g. crude oil), chemical separation is the remaining solution.

The mixture at hand could exist as a combination of any two or more states: solid-solid, solid-liquid, solid-gas, liquid-liquid, liquid-gas, gas-gas, solid-liquid-gas mixture, etc.

1.2 Classification of Separation Processes

In the chemical and other physical processing industries and the food and biological processing industries, many similarities exist in the manner in which the entering feed materials are modified or processed into final materials of chemical and biological products.

We can take these seemingly different chemical, physical, or biological processes and break them down into a series of separate and distinct steps that were originally called unit operations.

However, the term “unit operations” has largely been superseded by the more modern and descriptive term “separation processes.” These separation processes are common to all types of diverse process industries.

1.2.1 Fundamental Transport Processes

1. Momentum transfer. This is concerned with the transfer of momentum which occurs inmoving media, such as in the separation processes of fluid flow, sedimentation, mixing, and filtration.

2. Heat transfer. In this fundamental process, we are concerned with the transfer of heatfrom one place to another; it occurs in the separation processes of drying, evaporation, distillation, and others.

3. Mass transfer. Here mass is being transferred from one phase to another distinct phase; the basic mechanism is the same whether the phases are gas, solid, or liquid. This includes distillation, absorption, liquid–liquid extraction, membrane separation, adsorption, crystallization, and leaching.

1.2.2 Classification of Separation Processes

1. Evaporation. This refers to the evaporation of a volatile solvent such as water from anonvolatile solute such as salt or any other material in solution.

2. Drying. In this operation volatile liquids, usually water, are removed from solid materials.

3. Distillation. This is an operation whereby components of a liquid mixture are separated by boiling because of their differences in vapor pressure.

4. Absorption. In this process a component is removed from a gas stream by treatment with a liquid.

5. Membrane separation. This process involves the separation of a solute from a fluid by diffusion of this solute from a liquid or gas through a semi permeable membrane barrier to another fluid.

6. Liquid–liquid extraction. In this case a solute in a liquid solution is removed by contacting with another liquid solvent that is relatively immiscible with the solution.

7. Adsorption. In this process a component of a gas or liquid stream is removed and adsorbed by a solid adsorbent.

8. Ion exchange. Certain ions in solution are removed from a liquid by an ion-exchange solid.

9. Liquid–solid leaching. This involves treating a finely divided solid with a liquid that dissolves out and removes a solute contained in the solid.

10. Crystallization. This concerns the removal of a solute such as a salt from a solution by precipitating the solute from the solution.

11. Mechanical–physical separations. These involve separation of solids, liquids, or gases by mechanical means, such as filtration, settling, centrifugation, and size reduction.

1.3 Mechanism of Separation Processes

A mixture to be separated usually originates as a single, of that mixture into its constituent chemical species, is not a spontaneous process; it homogenous phase (solid, liquid, or gas).

If it exists as two or more immiscible phases, it is often best to first use some mechanical means based on gravity, centrifugal force, pressure reduction, or an electric and/or magnetic field to separate the phases. Then, appropriate separation techniques are applied to each phase.

A schematic diagram of a general separation process is shown in the Figure .The feed mixture can be vapor, liquid, or solid, while the two or more products

may differ in composition from each other and the feed and may differ in phase state from each other and/or from the feed.

The separation is accomplished by forcing the different chemical species (components) in the feed into different spatial locations by any of five general separation techniques.

Separation ProcessFeed mixture to be

separated

Product 1

Product 2

Product N-1

Product N

as shown in Figure 1.7 The most common industrial technique, Figure a, involves the creation of a second phase (vapor, liquid, or solid ) that is immiscible with the feed phase . The creation is accomplished by energy (heat ore shaft work ) transfer to or from the or by pressure reduction

Phase Creation Feed

Phase 1

Phase 2

(a)

A second technique, Figure b. is to introduce the second phase into the system in the form of a solvent that selectively dissolves some of the species in the feed mixture.

Feed

Phase 1

Phase 2MSA

Mass Separating Agent (b)

Less common, but of growing importance, is the use of a barrier, Figure 1.7c, which restricts and/or enhances the movement of certain chemical species with respect to other species.

Feed

Phase 1

Phase 2

Barrier

(c)

Also of growing importance are techniques that involve the addition of solid particles, Figure 1.7d, which act directly or as inert carriers for other substances so as to cause separation

Feed

Phase 1

Phase 2

(d)

Finally, external fields, Figure 1.7e, of various types are sometimes applied for specialized separations.

Force field or gradientFeed

Phase 1

Phase 2

(e)

For all of the general techniques of Figure 1.7 the separations are achieved by enhancing the rate of mass transfer by diffusion of certain species relative to mass transfer of all species by bulk movement within a particular phase. The driving force and direction of mass transfer by diffusion is governed by thermodynamics, with the usual limitations of equilibrium. Thus, both transport and thermodynamic considerations are crucial in separation

1.4 Separation by Phase Addition or Creation

If the feed mixture is a homogeneous, single-phase solution (gas, liquid, or solid), a second immiscible phase must often be developed or added before separation of chemical species can be achieved.

This second phase is created by an energy-separating agent (ESA) and! or added as a mass-separating agent (MSA).

When Two immiscible fluid phases are contacted, intimate mixing of the two phases is important in enhancing mass transfer rates so that the thermodynamic maximum degree of partitioning of species can be approached more rapidly.

After phase contact, the separation operation is completed by employing gravity and/or an enhanced technique, such as centrifugal force, to disengage the two phases.

1. When the feed mixture includes species that differ widely in their tendency to vaporize and condense, partial condensation or partial vaporization, Separation Operation may be adequate to achieve the desired separation or recovery of a particular component.

2. The degree of species separation achieved by a single partial vaporization or partial condensation step is inadequate because the volatility differences among species in the feed mixture are not sufficiently large. In that case, it may still be possible to achieve a desired separation of the species in the feed mixture, without introducing an MSA, by employing Distillation

3. When volatility differences between species to be separated are so small as to necessitate more than about 100 trays in a distillation operation, extractive distillation, is often considered. Here, an MSA, acting as a solvent, is used to increase volatility differences between selected species of the feed, thereby reducing the number of required trays to a reasonable value.

4. If condensation of vapor leaving the top of a distillation column is not easily accomplished by heat transfer to cooling water or a refrigerant, a liquid MSA called an absorbent may be introduced to the top tray in place of reflux. The resulting separation operation is called reboiled absorption

For more information on the rest of operations , please see P11-13 ( J.D. Seader ,Separation Process principles)

1.5 Separation by Barrier

The use of microporous and nonporous membranes as semipermeable barriers for application to difficult and highly selective separations is rapidly gaining adherents in industrial separation processes.

Membranes are usually fabricated from natural fibers, synthetic polymers, ceramics, or metals, but they may also consist of liquid films. Solid membranes are fabricated into flat sheets, tubes, hollow fibers, or spiral-wound sheets.

For the microporous membranes, separation is effected by differing rates of diffusion through the pores; while for nonporous membranes, separation occurs because of differences in both solubility in’ the membrane and the rate of diffusion through the membrane.

The following Table lists the more widely used membrane separation operations.

1. Osmosis, Operation in the Table, involves the transfer, by a concentration gradient, of a solvent through a membrane into a mixture of solute and solvent. The membrane is almost nonpermeable to the solute.

2. In Reverse Osmosis, transport of solvent in the opposite direction is effected by imposing a pressure, higher than the osmotic pressure, on the feed side. Using a nonporous membrane, reverse osmosis successfully desalts water.

3. Thalysis, is the transport, by a Concentration gradient, of small solute molecules, sometimes called crystalloids, through a porous membrane. The molecules unable to pass through the membrane are small, insoluble, nondiffusible particles, sometimes referred to as colloids.

4. Microfiltration, refers to the retention of molecules typically in the size range from 0.02 to 10 m.

5. Ultrafiltration, refers to the range from 1 to 20 nm. To retain even smaller molecules, reverse osmosis, sometimes called hyperfiltration, can be used down to 0.1 nm.

6. Although reverse osmosis can be used to separate organic and aqueous-organic liquid mixtures, high pressures are required. Alternatively, pervaporation, in which the species being absorbed by and transported through the nonporous membrane are evaporated, can be used. This method, which uses much lower pressures than reverse osmosis, but where the heat of vaporization must be supplied, is used to separate azeotropic mixtures.

7. Liquid membranes, of only a few molecules in thickness can be formed from surfactant-containing mixtures that locate at the interface between two fluid phases. With such a membrane, aromatic hydrocarbons can be separated from paraffinic hydrocarbons. Alternatively, the membrane can be formed by imbibing the micropores with liquids that are doped with additives to facilitate transport of certain solutes, such as CO2 and H2S.

1.6 Selection Of Feasible Separation Process

The selection of a best separation process must frequently be made from among a number of feasible candidates.

When the feed mixture is to be separated into more than two products, a combination of two or more operations may be best.

Even when only two products are to be produced, a hybrid process of two or more operations may be most economical.

A. Feed conditions1. Composition, particularly concentration of species to be recovered or separated2. Flow rate3. Temperature4. Pressure5. Phase state (solid, liquid, and/or gas)

B. Product conditions1. Required purities of products2. Temperatures3. Pressures4. Phase states

C. Property differences that may be exploited1. Molecular2. Thermodynamic3. Transport

D. Characteristics of separation operation1. Ease of scale-up2. Ease of staging3. Temperature, pressure, and phase-state requirements4. Physical size limitations5. Energy requirements

Factors That Influence the Selection of Feasible Separation Operations

Some separation operations are well understood and can be readily designed from a mathematical model and/or scaled up to a commercial size from laboratory data.

The results of a survey shown in the following figure , show that the degree to which a separation operation is technologically mature correlates well with its commercial use.