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NAMP

Program for North American Mobility In Higher

Education

Module 9

Introduction to Steady State Simulation

PIECE

Introducing Process integration for Environmental

Control in Engineering Curricula

PIECE

Process integration for Environmental Control in

Engineering Curricula

NAMP

Program for North American Mobility in Higher

Education

Module 9

This module was created by

Amy Westgate

From

Host University

Richard Ezike

Project Summary

- Objectives
- Create web-based modules to assist universities

to address the introduction to Process

Integration into engineering curricula - Make these modules widely available in each of

the participating countries - Participating institutions
- Six universities in three countries (Canada,

Mexico and the USA) - Two research institutes in different industry

sectors petroleum (Mexico) and pulp and paper

(Canada) - Each of the six universities has sponsored 7

exchange students during the period of the grant

subsidised in part by each of the three

countries governments

Structure of Module 9

- What is the structure of this module?
- All modules are divided into 3 tiers, each with a

specific goal - Tier I Background Information
- Tier II Case Study Applications
- Tier III Open-Ended Design Problem
- These tiers are intended to be completed in that

particular order. In the first tier, students are

quizzed at various points to measure their degree

of understanding, before proceeding to the next

two tiers.

Purpose of Module 9

- What is the purpose of this module?
- It is the intent of this module to cover the

basic aspects of Steady State Simulation. It is

identified as a pre-requisite for other modules

related to the learning of Steady State

Simulation. - This module is intended for students familiar

with basic mass and energy balances and may have

had some training with thermodynamics and

transport processes.

Tier IBackground Information

- Statement of Intent
- Review basic chemical engineering concepts

employed in steady state simulation - Understand the purpose of steady-state simulation
- Learn how to develop models of processes in

steady-state - Discuss problem solving techniques

- Steady State Process
- We can use steady-state processes to determine

the optimum operation conditions for a process

that can be limited by safety, equipment

performance, and product quality constraints. - Concentration does not change with respect to
- time
- Accumulation term in mass balance set to zero
- Input Generation Output Consumption 0

- Three types of processes
- Batch

Continuous - Semi-batch

- A process where inputs and outputs flow

continuously through duration of process.

- A process where a set amount of input enters a

process, where it is removed from the process at

a later time.

- Neither batch or continuous, may be combination

of both.

In steady-state processes, we will be looking at

continuous processes.

- Batch Example
- Ammonia is produced from nitrogen and hydrogen.

At time t t0, nitrogen and hydrogen are added

to the reactor. No ammonia leaves the reactor

between t t0 and t tf. At tf, nf moles of

ammonia are released.

H2 N2

NH3

t to

t tf

- Semibatch Example
- Helium is pressurized in large tanks for storage.

When the tank valve is open, the gas diffuses out

due to the difference in pressure.

- Continuous Example
- Pump a methanol/water mixture into a distillation

column and withdraw the more volatile component

(methanol) from the top of the column and the

less volatile component (water) from the bottom

of the column.

Quiz 1

- Classify the following processes as batch,

continuous, or semibatch. - A balloon is filled with air at a steady rate of

2 m3/min. - Pump a mixture of liquids into a distillation

column at a constant rate and steadily withdraw

product streams from the top and bottom. - Slowly blend several liquids in a tank from which

nothing is being withdrawn.

- Block Diagrams
- When solving a problem, it is helpful to develop

a block diagram, such as the one below, that

defines what the process looks like as well as to

indicate all information about the process such

as flow rates and species compositions.

Process

Carbon (C) Air (79 N2, 21 O2)

Separator

Reactor

Degree Of Freedom Analysis

- Analysis done to determine if there is enough

information to solve a given problem. - Draw and completely label a flowchart
- Count the unknown variables, then the independent

equations relating them, - Subtract the number of equations from the number

of variables. This gives ndf, or the number of

degrees of freedom in the process.

Degree of Freedom Analysis

- If ndf 0 there are n independent equations in n

unknowns and the problem can be solved - If ndf gt0, there are more unknowns than

independent equations relating them, and at least

ndf additional variable values must be specified. - If ndf lt0, there are more independent equations

than unknowns. The flowchart is incompletely

labeled or inconsistent and redundant relations

exist.

Mass (Material) Balance A mass (material)

balance is an essential calculation that accounts

for the mass that enters and leaves a particular

process.

Accumulation of mass Mass flow rate in Mass

flow rate out

Mass (Material) Balance (continued) In the case

of a steady-state process we are able to set the

accumulation term to zero since it is a time

dependent term. Since steady-state does not

depend on time as it is constant, we are able to

eliminate this term Mass Flow Rate In Mass

Flow Rate Out Material Balance Procedure

- First Law of Thermodynamics (Energy Balance) for

a Steady State Open System

The net rate at which energy is transferred to

system as heat and/or shaft work equals the

difference between rates at which (enthalpy

kinetic energy potential energy) is transported

into and out of the system

Quiz 2

- Explain the Degree of Freedom analysis.
- What term goes to zero in a steady-state process?
- Is a continuous process closed or open? How about

a batch process?

- Heat Transfer
- Also classified as energy transfer
- Three types of heat transfer modes
- Conduction
- Convection
- Radiation

- Conduction
- Accomplished in two ways
- Molecular interaction
- Free electrons
- Conduction equation called Fouriers Law
- qy heat transfer area in y direction (W)
- A area normal to direction flow (m2)
- dT/dy temperature gradient (oC/m)
- k thermal conductivity (W/moC)

- Convection
- Accomplished in two ways
- Natural convection
- Forced convection
- Convection equation called Newtons Law
- qy rate of convective heat transfer (W)
- A area normal to direction flow (m2)
- ?T temperature gradient (oC)
- h convective heat transfer coefficient (W/m2 oC)

Radiation (Thermal)

- Exhibits same optical properties as optical light
- May be absorbed, reflected, or transmitted

Total radiation for unit area of opaque body of

area A1, emissivity e1, and absolute temperature

T1, and a universal constant s

Radiation Between Surfaces

- Simplest type occurs where each surface can see

only the other and where both surfaces are black - Energy emitted by first plane is sT14 the

second plane emits sT24 - if T1 gt T2, then net loss energy per unit area

by first plane and net gain by second are sT14-

sT24, or s(T14-T24)

Cold surface

Note this is only in ideal cases no surface is

exactly black, and emissivities must be considered

Hot surface

- Mass Transfer
- The transport of one constituent from a region of

higher concentration to a region of lower

concentration - Molecular mass transfer
- Random molecular motion in quiescent fluid
- Convective mass transfer
- From a surface into a moving fluid or vice-versa

- Flux
- - (overall density)(diffusion

coefficient)(concentration gradient) - Fick rate equation (restricted to

isothermal/isobaric systems) - de Groot equation is more general

Units JA mol A/m2s CA- mol A/m3 DAB- m2/s

- Molar flux of species A in binary system (A B)

c concentration DAB diffusivity of species A

in B change of molar species in y with

respect to a specified direction NA, NB molar

fluxes of components

Quiz 3

- What are two ways in which conduction occurs?
- Define natural and forced convection.
- What is the restriction to the use of Ficks Law?

- Modeling
- What is Modeling?
- Steady-State vs. Dynamic Modeling
- Empirical vs. Mechanistic Modeling
- Derivation of a Steady State Model
- Modeling and Process Design Implications

What is a Model?

- A model is an depiction of a process operation
- used to design, change, improve or control a

process. - Uses of Model
- Equipment Design, Size and Selection
- Comparison of Different Process Configurations
- Evaluation of Process Performance Against

Limitations - Optimization

- Models vary by
- Phenomena represented
- Energy, phase changes
- Level of details
- Assumptions (perfect mixing, heat loss)
- Inputs required
- Functions performed (satisfaction of constraints,

optimization) - Outputs generated

- Requirements of a good model
- Accuracy the model should be close to the target

description. - Validity model must have a solid foundation and

ability to be easily justified. - Complexity the level of the model should be

considered and easy to understand. - Computational efficiency models should be

calculable using reasonable amounts of time and

computing resources.

Time-based Modeling

Steady State

Dynamic

Model

Empirical

Mechanistic

Hybrid

Level of Knowledge-based Modeling

Steady State

Dynamic

Balance at equilibrium condition Time dependent results

Equilibrium results for all unit operations Equilibrium conditions not assumed for all units

Equipment sizes not needed Equipment sizes needed

Amount of information required small to medium Amount of information required medium to large

Steady State Example

Continuous Stirred Tank Reactor (CSTR)

Concentration profile at one point in reactor

does not change with time

Ca

t

Dynamic Example

Batch Reactor

Concentration profile at one point in reactor

does change with time

ca

t

- Empirical Modeling
- Definition
- a model that is based on data whether it has been

collected from a process or some other source. - Key Notes
- Derived from observation
- Often simple
- May or may not have theoretical foundation
- Valid only within range of observation

- Procedure Empirical Modeling
- Obtain data from process you wish to model.
- Temperature, pressure, flow, etc
- Perform appropriate statistical analysis and

develop accurate correlations from data. - Develop mathematical equations to accurately

represent the data and the correlations found in

step 2, and determine which equations are useful

in the development of the model. - Check for correctness in your analysis and

equations, and determine if the model is

satisfactory. - Statistical Analysis with Excel

- Example
- The figure below depicts a heat exchanger. Heat

exchangers function as a medium to transfer

energy (in the form of heat) from a hotter stream

to a cooler stream. Lets say we have a hot

stream of fluid coming into the exchanger at Th1,

leaves at Th2 and a cool stream coming in at Tc1

and leaving at Tc2. If the physical properties of

the fluids are the same, then the temperature

difference describes the amount of energy

transferred.

- We do not know Tc2, but we can take various

measurements of Th1, Th2 and Tc1 to find Tc2 .

Using certain statistical procedures, it can be

determined that Tc2 is related to the other three

temperatures by this equation - Tc2 Tc1 a(Th2-Th1)
- We have empirically determined a value for a, but

only for the specific fluids and conditions

tested.

- If we knew, say, the mass flow rates and heat

capacities of the two fluids, we can use them to

determine the mechanistic model that relates the

four temperatures for any combination of two

fluids.

- Mechanistic Modeling
- Definition
- a model that is derived from fundamental physical

laws or basic principles - Key Notes
- Model construction time-consuming and costly
- Most reliable, but often not enough data available

- Procedure Mechanistic Modeling
- Know physical and chemical properties of the

process. - Determine the appropriate process model using

mass and/or heat balance. - Determine appropriate model run conditions and

parameters - Complete runs and use output data to compare

against the predicted model results - Develop an acceptable conclusion for the model.

Should the conclusion not be acceptable,

re-examine the assumptions, process and the

physical and chemical properties made in Step 1.

Make appropriate modifications and repeat Steps

2-4.

- Let us go back to the heat exchanger. Now we know

that the empiricism a that we determined earlier

is related to the mass flow and heat capacity of

the two fluids. This knowledge allows us to model

a heat exchanger for any two fluids. The model is

determined to be

Steady state model derivation

- 1. Define Goals.
- a) Specific design decisions.
- b) Numerical values.
- c) Functional relationships.
- d) Required accuracy.

2. Prepare information. a) Sketch

process. b) Identify variables of

interest. c) State assumptions and data.

Steady state model derivation

3. Formulate model a) Conservation

balances. b) Constitutive equations. c) Rationaliz

e (combine equations and collect terms). d) Check

degrees of freedom.

- 4. Determine solution
- a) Analytical
- b) Numerical

Steady state model derivation

- 5. Analyze results
- a) Check results for correctness
- Accuracy of numerical/analytical methods
- Plot solution
- Relate results to data and assumptions
- Answer what if questions
- Compare with experimental results

Process insights resulting from modeling

- Identification If we know the input (I) and

output (O) parameters, we can determine the

structure (R) of the model.

I

O

R?

Process insights resulting from modeling

2. Simulation If we know the structure of the

model, we can simulate what the output of the

process will be for a given input.

I

O?

R

Process insights resulting from modeling

3. Control/Optimization If we know the desired

output (O) and the structure (R) of the model, we

can determine what the input (I) should be to

optimize the process.

I?

O

R

Quiz 4

- What are some uses of modeling?
- Name and explain three requirements of a good

model. - What distinguishes a steady-state model and a

dynamic model? - Review the procedures for developing a

mechanistic and empirical model. What are some

differences between the two procedures? - Discuss the control/optimization insight of

modeling.

- Solving Problems
- Analytical Methods
- Process Design
- Methods
- Spreadsheets
- Simulation Software
- Solution Determination

- Curve fitting
- Try to find the best fit of a curve through the

data such that the distribution of the data

points on either side of the line is equal - Possible errors
- Measurement error
- Precision error
- Systematic error
- Calculation error
- Error propagation
- Curve Fitting Example

- Least Squares
- The best curve through the data is the one that

minimizes the sum of the squares of the residuals

(differences between predicted and experimental

values) - Least Squares Method

Process Design

Process design

- The design of chemical products begins with the

identification and creation of potential

opportunities to satisfy societal needs and to

generate profit. The scope of chemical product is

extremely broad. They can be roughly classified

as - Basic chemical products.
- Industrial products.
- Consumer products.

Process design

Motivation for Process Design

- Desires of customers for chemicals with improved

properties for many applications. - Discovery of a new inexpensive source of a raw

material with comparable physical and chemical

properties to the old source. - New markets are discovered.

Steps in a Process Design

- Process Design Questions to Answer
- Is the chemical structure known?
- Is a process required to produce the chemicals?
- Is the gross profit favorable?
- Is the process still promising after further

elaboration? - Is the process and/or product feasible?

Steps in a Process Design

- Process Design Steps

- Develop objective(s).
- Find inputs that have the desired properties and

performance. - Create process.
- Develop a base case for which to conduct initial

testing on process. - (does it stay stable at steady state?)
- Improve/maintain process

Stability of the process

- When a process is disturbed from an initial

steady state, it will generally respond in one of

3 ways. - Proceed to a steady state and remain there.

Stability of the process

- Fail to attain to a steady state condition

because its output grows indefinitely. The system

is unstable.

Stability of the process

- Fail to attain a steady state condition because

the output of the process oscillates indefinitely

with a constant amplitude. The system is at the

limit of stability.

Quiz 5

- What are some errors that may arise when

attempting to fit a curve? - What are the three products developed from

process design? Provide an example of each

product. - What happens to an unstable system over time?

- Spreadsheet
- A computer program (Microsoft Excel) used to

store and calculate information in a structured

array of data cells. By defining relationships

between information in cells, a user can see the

effects of certain data changes on other data in

other parts of the spreadsheet. - Provides an easy, efficient method for solving

sets of equations and other forms of data that

are not too numerous but complex enough that it

would be difficult to solve by hand.

- Columns are designated by letters, rows by numbers

http//www.instrunet.com/images/Direct20To20Exce

l20Spreadsheet.png

Goalseek

- Under Tools Menu
- want to know input value formula needs to

determine result - Excel varies value in cell specified until

dependent formula returns value you want

- Spreadsheet Drawbacks
- Entering the equations yourself could lead to

false answers as you can make a mistake. Mistakes

can become unmanageable very quickly causing

debugging to be difficult. - Excel can handle large amounts of data but there

is a point where Excel may have difficulty in

solving a system of equations.

- Simulation
- Predicts behavior of a process by solving

mathematical relationships that describe the

behavior of the process components. - Involves performance of experiments with a

process model

- Simulation Software Why use it?
- economical way for engineers to construct or

modify a process before doing a test in reality. - Can determine optimum operating conditions
- Quantify equipment, raw materials required with

accuracy - Can discover process problems
- Make accurate changes in process without

sacrificing money or safety - Determine composition of streams and simplify

complex unit operations

- Simulation Software What does it allow?
- Manipulation and comparison of previous data as

well as for research - Manipulation of a process until a desired target

is reached - Allows complex processes to be easily calculated
- Can easily change conditions and see how the

output is changed and the equipment behaves

- Simulation Issues and Considerations
- Built-in assumptions in programs must be taken

into account and validated - Can make mistakes in calculations do mass

balances over process as a check over - Number of variables involved
- Physical properties of streams
- Size of process being simulated

Process Flowsheet (Block Diagram)

- A process flowsheet is a collection of icons to

represent process units and arrows to represent

the flow of materials to and from the units.

Fresh feed

steam

Calculation Order

- In most process simulators, the units are

computed one at a time. The calculation order is

automatically computed to be consistent with the

flow of information in the simulation flowsheet,

where the information flow depends on the

specifications for the chemical process.

1

2

3

4

Recycle Flows

- A simulation flowsheet usually contains

information recycle loops. That is, there are

variables that are not known which prevent the

equations in the process model from being solved

completely. These variables are recycled back to

the initial calculation point.

1

2

3

4

For these processes, a solution technique is

needed to solve the equations for all the units

in the recycle loop.

- Iteration
- Initial guess is taken at the input and a

solution is determined for the system - Second, a more educated guess is made and the

system is solved based on initial solution - Iterations continue until solution converges to

one value

Convergence

- Is the process to compare the guessed value with

the computed value until a value is found within

the tolerance range.

Guessed value

Yes

No

Guessed value calculated value lt Tolerance

Convergence

When the criterion is achieved, the solution is

found and no more iteration needs to be done.

Process synthesis methodologies

- Total account of an explicit process is the most

obvious. Here we generate and evaluate every

alternative design. We locate the better

alternative by directly comparing the

evaluations. - Evolution of design follow from the generation

of a good base case design. Designers can then

make many small changes, a few at a time, to

improve the design incrementally. - Structured Decision Making following a plan that

contains all the alternatives. - Design to target we design and specify unit

operations to operate according to the desired

target operation of the process.

- Solution Determination
- Sequential Solution
- Work backwards from one point in a sequential

order solving one equation at a time - Iterative Method
- Simultaneous Solution
- Have to solve multiple equations with multiple

variables all at same time - Generally requires simulation software

Some advice when running a simulation

- 1. Talk with trained professionals (chemists,

vendors, other engineers in the field). - 2. Beware of using estimated parameters and

interaction parameters when screening process

alternatives. - 3. Go see the plant. Plant personnel are usually

helpful. Their insight and your knowledge of

modeling can help solve problems efficiently.

With a simulator, one day of process operation

can be simulated in just seconds, and make as

many changes as you want.

Change in Reactor Properties

Change composition in feed

Change in Column Properties

Commercial Simulation Software Packages

- There are many of them, some of them are
- Excel (spreadsheet) Excel Tutorial
- Matlab MATLAB homepage
- Fortran and C (programming languages)
- Aspen AspenTech
- HYSYS HYSYS
- WinGEMS WinGEMS
- SuperPro Designer SuperPro Designer
- IDEAS (Simons)

Final Quiz

- What is a drawback of using spreadsheets?
- What are two functions that simulation allows

for? - How are units calculated within a simulation

process? - Explain how iteration works and why you should

use it. - You are an engineer who has been tabbed to design

a new chemical process for a company. What are

some steps you can take to help you in your

design?

Tier IIWorked Examples

- Statement of Intent
- Review basic chemical engineering concepts

employed in steady state simulation through

examples - Understand how to develop a steady-state

simulation problem in Excel

First Example A Single Effect Evaporator (to be

done in Excel)

Evaporation

Function is to concentrate solution

- What affects evaporation?
- Rate at which heat is transferred to the liquid
- Quantity of heat required to evaporate mass of

water - Maximum allowable temperature of liquid
- Pressure which evaporation takes place

Single Effect Vertical Evaporator

- Three functional sections
- Heat exchanger
- Evaporation section
- liquid boils and evaporates
- Separator
- vapor leaves liquid and passes off to other

equipment

Three sections contained in a vertical cylinder

- In the heat exchanger section (calandria), steam

condenses in the outer jacket - Liquid being evaporated boils on inside of the

tubes and in the space above the upper tube stack - As evaporation proceeds, the remaining liquors

become more concentrated

Diagram of Single Effect Evaporator

Vapor V

Tv, yv, Hv, ?V

Tf, xf, hf, ?f

U J/m2 s oC

Feed F

P kPa

Ts, Hs, ?s

A ? m2

Condensate S

Ts, hs, ?s

Steam S

Concentrated liquid L

TL, xL, hL, ?L

Material and Heat Balances

q UA?T ?T Ts TL Heat given off by

vapor ? Hs hs ?FhF ?sHs ?LhL ?VHV

?shs ?FhF ?s? ?LhL ?VHV q ?s(Hs-hs)

?s? ?s? ideal heat transferred in evaporator

?F ?L ?V ?FxF ?LxL ?VyV

Finding the Latent Heat of Evaporation of

Solution and the Enthalpies

- Using the temperature of the boiling solution TL,

the latent heat of evaporation can be found - The heat capacities of the liquid feed (CpF) and

product (CpL) are used to calculate the

enthalpies of the solution.

Property Effects on the Evaporator

- Feed Temperature
- Large effect
- Preheating can reduce heat transfer area

requirements - Pressure
- Reduction
- Reduction in boiling point of solution
- Increased temperature gradient
- Lower heating surface area requirements
- Effect of Steam Pressure
- Increased temperature gradient when higher

pressure steam is used.

Boiling-Point Rise of Solutions

- Increase in boiling point over that of water is

known as the boiling point elevation (BPE) of

solution - BPE is found using Duhrings Rule
- Boiling point of a given solution is a linear

function of the boiling point of pure water at

the same pressure

Duhring lines (sodium chloride)

http//www.nzifst.org.nz/unitoperations/evaporatio

n4.htm

Problem Statement (McCabe 16.1 modified)

A single-effect evaporator is used to concentrate

9070 kg/h of a 5 solution of sodium chloride to

20 solids. The gauge pressure of the steam is

1.37 atm the absolute pressure in the vapor

space is 100 mm Hg. The overall heat transfer

coefficient is estimated to be 1400 W/m2 oC. The

feed temperature is 0oC. Calculate the amount of

steam consumed, the economy, and required heating

surface.

First Example Excel Spreadsheet

1. Draw Diagram and Label Streams

Vapor V

9070 kg/h feed, 0oC, 5 solids, hF

Tv, 0 solids, Hv, ?v

U 1400 W/m2 oC

Feed F

P 100 mm Hg

Ts, Hs, 1.37 atm gauge, ?s

q?

Condensate S

Ts, hs, ?s

A?

Steam S

TL, 20 solids, hL, ?L

Liquor L

2. Perform Mass Balances

- ?F ?L ?V
- 9070 kg/h ?L kg/h ?V kg/h
- ?FxF ?LxL ?VyV (note that yv is zero

because only vapor is present, no solids) - 0.05 9070 kg/h 0.2 ?L kg/h 0
- Can solve for ?v and ?L
- ?V 6802.5 kg/h, ?L 2267.5 kg/h

3. Perform Heat Balances to find the Economy

The economy is defined as the mass of water

evaporated per mass of steam supplied.

?FhF ?SHS ?LhL ?VHV ?ShS ?FhF ?S?

?LhL ?VHV q ?S(HS- hS) ?S?

Needed Data

- Boiling point of water at 100 mm Hg 51oC (from

steam tables) - www.nzifst.org.nz/unitoperations/appendix8.htm
- Boiling point of solution 88oC (from Duhring

lines) - http//www.nzifst.org.nz/unitoperations/evaporatio

n4.htm - Boiling point elevation 88 51 37oC
- Enthalpy of vapor leaving evaporator (enthalpy

of superheated vapor at 88oC and 100 mm Hg .133

bar) 2664 kJ/kg (FR, p.650) also called the

latent heat of evaporation - Heat of vaporization of steam (Hs-hs ? ) at

1.37 atm gauge 20 lbf/in2 939 Btu/lb 2182

kJ/kg (McCabe, App.7, p.1073)

Finding the enthalpy of the feed

yNaCl0.05 ywater0.95 Cp,water4.18 kJ/kgoC

Cp,NaCl0.85 kJ/kgoC

- Find the heat capacity of the liquid feed
- feed is 5 sodium chloride, 95 water

(Cp)F .050.85 .954.18 4.01 kJ/kgoC

2. Calculate Enthalpy (neglecting heats of

dilution)

hF 4.01 kJ/kgoC (0 - 0 oC) 0 kJ/kg

Finding the enthalpy of the liquor

yNaCl0.20 ywater0.80 Cp,water4.18

kJ/kgoC Cp,NaCl0.85 kJ/kgoC

- Find the heat capacity of the liquor
- feed is 20 sodium chloride, 80 water

Cp,L .200.85 .804.18 3.51 kJ/kgoC

2. Calculate Enthalpy (neglecting heats of

dilution)

hL 3.51 kJ/kgoC (88-0 oC) 309 kJ/kg

Heat Balances

?LhL ?VHV - ?FhF ?SHS - ?ShS ?S(HS- hS)

?S? ? (HS-hS) 2182 kJ/kg (2267.5 kg/h

309.23 kJ/kg) (6802.5 kg/h 2664 kJ/kg) (0)

?S (HS-hS) q ?S (2182 kJ/kg)

?s8626.5 kg/h

q 8626.5 kg/h2182 kJ/kg 1.88x107 kJ/h

5228621 W 5.23 MW

Find the Economy

?V/?S

4. Calculate Required Heating Surface

Condensing temperature of steam (1.37 atm gauge

126.1oC

q UA?T A q/U?T

Click on the Hyperlink and click on the Final

Solution tab to see the final answer for the

system.

First Example Final Solution

Second Example Simulation of Cyclic Process

(Felder and Rousseau, Example 10.2-3, pp.

516-519) (to be done in Excel)

Problem Statement

The gas-phase dehydrogenation of isobutane (A)

to isobutene (B) is carried out in a continuous

reactor. A stream of pure isobutane (the fresh

feed to the process) is mixed adiabatically with

a recycle stream containing 90 mole isobutane

and the balance isobutene, and the combined

stream goes to a catalytic reactor. The effluent

from this process goes through a multistage

separation process one product stream containing

all the hydrogen (C) and 10 of the isobutane

leaving the reactor as well as some isobutene is

sent to another part of the plant for additional

processing, and the other product stream is the

recycle to the reactor. The conversion of

isobutane in the reactor is 35. Assume a fresh

feed of 100 mol isobutane. Simulate the process

using a spreadsheet to find the desired process

variables.

Diagram of Process

Second Example Cyclic Process

Notes

- A will denote isobutane, B denotes isobutene, C

denotes hydrogen - All streams are gases, is the required rate

of heat transfer to the reactor and is the

net rate of heat transfer to the separation

process - Specific enthalpies are for the gaseous species

at the stream temperatures relative to 25oC - - Heats of formation are taken from Table B.1,

and heat capacity formulas are taken from Table

B.2 in Felder and Rousseau

1. Perform Degree of Freedom Analysis

Review Degrees of Freedom

- Draw and completely label a flowchart
- Count the unknown variables, then the independent

equations relating them, - Subtract the number of equations from the number

of variables. This gives ndf, or the number of

degrees of freedom in the process.

Degree of Freedom Analysis

- If ndf 0 there are n independent equations in n

unknowns and the problem can be solved - If ndf gt0, there are more unknowns than

independent equations relating them, and at least

ndf additional variable values must be specified. - If ndf lt0, there are more independent equations

than unknowns. The flowchart is incompletely

labeled or inconsistent and redundant relations

exist.

Degree of Freedom Analysis Mixing Point

4 unknowns (?A1, ?B1, ?4,T1) - 3 balances (2

material balances, 1 energy balance) 1 local

degree of freedom

Degree of Freedom Analysis Reactor

- 7 unknowns (?A1, ?B1, ?A2, ?B2, ?C2, T1, )
- 4 balances (3 molecular species balances, 1

energy balance) - 1 additional relation (35 conversion of

isobutane) - 1 independent chemical reaction
- 3 local degrees of freedom

Degree of Freedom Analysis Separator

- 8 unknowns (?A2, ?B2, ?C2, ?A3, ?B3, ?C3, ?4,

) - 4 balances (3 material balances, 1 energy

balance) - - 1 additional relation (isobutane split)
- 3 local degrees of freedom

Net Degree of Freedom Analysis Overall Process

7 local degrees of freedoms (133) - 7 ties

(?A1, ?B1, ?A2, ?B2, ?C2, ?4, and T1 were counted

twice) 0 net degrees of freedom

The problem can be solved for all labeled

variables.

2. Equation Based Solution Process

Tearing the Cycle

- Cant solve system in a unit-to-unit manner

without trial and error - tear between two units
- Purpose is to have the least number of variables

that have to be determined by trial and error - We tear between separation process and mixing

unit - - Only have to determine ?4 by trial and error.

Solution Process

- Assume value of recycle flow rate (?4A 100

mol/s) - Assume mixing point outlet temperature (T1

50oC) - Vary ?4A until calculated recycle flow rate

(?4C) equals assumed value in ?4A - - Will be done by driving (?4A - ?4C) using

Goalseek - Mixing point temperature (T1) will be varied to

determine the value that drives ??mix to zero

(remember, the mixer is adiabatic)

Known Values

XA 0.35 (fractional conversion of A) 100 mol/s

(basis of calculation) Feed temperature

20oC Reactor Effluent Temperature 90oC Product

Stream Temperature 30oC Guess for recycle

stream flow rate (?A4) 100 mol/s Mole fraction

of A in recycle stream 0.9 Mole fraction of B

in recycle stream 0.1 Temperature of recycle

stream 85oC Initial guess for combined stream

temperature 50oC

Mass Balances (based on initial guesses)

?A1 100 mol/s feed (100 mol/s recycle 0.9

mol fraction 190 mol/s) ?B1 100 mol/s recycle

0.1 mol fraction 10 mol/s ?A2 ?A1 (1-XA)

123.5 mol/s ?B2 ?B1 (?A1XA) 76.5 mol/s ?C2

?A1 XA 66.5 mol/s ?A3 0.01 ?A2 1.24

mol/s ?C4 (?A2- ?A3)/0.9 mol fraction 135.85

mol/s ?B3 ?B2 (0.1 mol fraction ?C4 ) 62.9

mol/s ?C3 ?C2 66.5 mol/s

Calculation of Specific Enthalpies (Tables B.1

and B.2, Felder and Rousseau)

- (heats of formation) are located in Table B.1

of FR

A (isobutane g) -134.5 kJ/mol B (isobutene

g) 1.17 kJ/mol C (hydrogen g) 0 kJ/mol

Calculation of Specific Enthalpies (Tables B.1

and B.2, Felder and Rousseau)

- heat capacity of component i (kJ/moloC)

a bT cT-2 dT-3 , where T is temperature in

oC

Chemicals A 103 B 105 C 108 D 1012

isobutane 89.46 30.13 -18.91 49.87

isobutene 82.88 25.64 -17.27 50.50

hydrogen 28.84 0.00765 0.3288 -0.8698

Heat Balances (based on initial guesses)

??mix ?A1HA1 ?B1HB1 100 mol/sHA0 -

(?A40.9 mol A/mol HA4) - (?A40.1 mol

fractionHB4) -78.64 kJ/mol

?A2HA2 ?B2HB2 ?C2HC2 - ?A1HA1

?B1HB1 9980.4 kJ/s

?A3HA3 ?B3HB3 ?C3HC3(?A40.900 mol

fractionHA4)(?B40.100 mol fractionHB4)

?A2HA2 - ?B2HB2 - ?C2HC2 -568.4 kJ/s

Click on the Hyperlink and click on the Final

Solution tab to see the final answer for the

system.

Second Example Final Solution

Tier IIIOpen-ended problem

- Approach to open-ended problem
- Case Study.

- Statement of Intent
- Learn how to approach open-ended design problems
- Solve a problem on your own

How to approach openended problems

- State the problem clearly, including goals,

constraints, and data requirements. - Define the trade-offs necessary.
- Define the criteria for a valid solution.
- Develop a set of cases to simulate possible

solutions. - Perform the simulation and evaluate results

against solution criteria. - Evaluate solutions against environmental, safety

and financial considerations.

The Use of Limestone Slurry Scrubbing to Remove

Sulfur Dioxide from Power Plant Flue Gases

Prepared by Ronald W. Rousseau and Jack Winnick,

Georgia Tech Department of Chemical Engineering,

and Norman Kaplan, National Risk Management

Research Laboratory, United States EPA

About Coal

- Protection of environment through process

development is an important responsibility for

chemical engineers - Coal is an abundant source of energy and source

of raw materials in production - Predominately carbon, but contains other

elements and hydrocarbon volatile matter

- burned in many of worlds power plants to

produce electricity - can produce a lot of pollution if gases not

treated, like soot and ash - sulfur dioxide emissions regulated in the U.S.

by the Environmental Protection Agency - current regulations are no more than 520 ng SO2

per joule of heating value of the fuel fed to the

furnace - plants must remove 90 of SO2 released when

coal-burning

About Commercial Processing

- SO2 removal is classified as regenerative or

throwaway - throwaway processing can be modified to produce

gypsum - throwaway processing uses separating agent to

remove SO2 from stack gases followed by disposal

of SO2 innocuously (CaSO3 ½ H2O) and a slurried

separating agent of calcium carbonate

Process Description

- want to produce 500 MWe (megawatts of

electricity) - properties of coal given in table on next slide
- coal fed at 25oC to furnace, burned with 15

excess air - sulfur reacts to form SO2 and negligible SO3
- carbon, hydrogen oxidized completely to CO2 and

water - nitrogen in coal leaves furnace as N2
- ash in coal leaves furnace in two streams
- 80 leaves as fly ash in furnace flue gas
- remainder as bottom ash at 900oC

Component Dry Weight

Carbon 75.2

Hydrogen 5.0

Nitrogen 1.6

Sulfur 3.5

Oxygen 7.5

Ash 7.2

Moisture 4.8 kg/100 kg dry coal

HHV 30780 KJ/kg dry coal

Cp dry coal 1.046 kJ/(kgoC)

Cp ash 0.921 KJ/(kgoC)

- combustion air brought into process at 25oC, 50

RH - air sent to heat exchanger, temperature

increased to 315oC - air then fed to boiler, reacts with coal
- flue gas leaves furnace at 330oC, goes to

electrostatic precipitator - 99.9 of particulate material removed
- goes to air preheater, exchanges heat with

combustion air - leaves air preheater and split into two equal

streams - each stream is feed to one of two identical

scrubber trains - trains sized to process 60 of flue gas

- divided gas stream fed to scrubber, contacts

aqueous slurry of limestone, undergoes adiabatic

cooling to 53oC. - sulfur dioxide absorbed in the slurry and reacts

with the limestone - CaCO3 SO2 ½ H2O CaSO3 ½ H2O

CO2 - solid/liquid slurry enters scrubber at 50oC
- liquid slurry flows at 15.2 kg liquid/kg inlet

gas - solid to liquid ratio in the slurry is 19 by

weight - liquid saturated with CaCO3 and CaSO3
- cleaned flue gas
- meets EPA SO2 requirements
- leaves scrubber with saturated water at 53oC

- cleaned flue gas contains CO2 generated in

scrubbing but no fly ash - cleaned flue gas reheated to 80oC, blended with

clean flue gas stream from other train, and

sent to be released to atmosphere - solids in spent aqueous slurry
- unreacted CaCO3, flyash from flue gas, inert

materials, CaSO3 - liquid portion of slurry saturated with CaCO3,

CaSO3 - specific gravity of 0.988
- spent slurry split in two
- one stream sent to a blending tank, mixed with

freshly ground limestone, makeup water, and

recycle stream - fresh slurry stream from blending tank fed to

top of scrubber

- second stream sent to filter where wet solids

containing fly ash, inert materials, CaSO3 and

CaCO3 are separated from filtrate - filtrate saturated with CaSO3, CaCO3, and is the

recycle stream fed to the blending tank - wet solids contain 50.2 liquid that has similar

composition to filtrate - fresh ground limestone fed to blending tank at

rate of 5.2 excess of that is required to react

with SO2 absorbed from flue gas - limestone 92.1 CaCO3 and rest is insoluble

inert material

- Boiler generates steam at supercritical

conditions - 540oC and 24.1 MPa absolute
- mechanical work derived by expanding steam

through a power- generating system of turbines - low pressure steam extracted from power system

contains 27.5 liquid water at 6.55 kPa absolute - heat removed from wet low pressure steam in a

condenser by cooling water - cooling water enters condenser at 25oC and

leaves at 28oC - saturated condensate at 38oC is produced by

condenser and pumped back to boiler

- Assume a basis of 100 kg dry coal/min fed to the

furnace. - Construct a flowchart of the process and

completely label the streams. Show the details of

only one train in the scrubber operation. Do this

in Excel. - Estimate the molar flow rate (kmol/min) of each

element in the coal (other than those in the

ash). - Determine the feed rate (kmol/min) of O2 required

for complete combustion of the coal.

- If 15 excess oxygen is fed to combustion

furnace, estimate the following - The oxygen and nitrogen feed rates (kmol/min)
- The mole fraction of water in the wet air, the

average molecular weight, and the molar flow rate

of water in the air stream (kmol/min) - The air feed rate (kmol/min, m3/min)
- Estimate flow rate (kmol/min, kg/min) of each

component and composition (mole frac) of furnace

flue gas (ignore fly ash). At what rate (kg/min)

is fly ash removed from flue gas by the

electrostatic precipitator?

6. If system is assumed to meet standard 90 SO2

removal released upon combustion a. Determine

flow rate (kg/min and kmol/min) of each

component in the flue gas leaving scrubber b.

Determine flow rate (kg/min) of slurry entering

scrubber c. Estimate solid-to-liquid mass

ratio in slurry leaving scrubber. d.

Estimate feed rate (kg/min) of fresh ground

limestone to the blending tank.

6. (continued) e. What are flow rates (kg/min)

of inerts, CaSO3, CaCO3, fly ash, and water, in

the wet solids removed from the filter? f.

Estimate rate (kg/min, L/min) at which filtrate

is recycled to blending tank. At what rate

(kg/min, L/min) is makeup water added to

blending tank? 7. At what rate is heat removed

from the furnace? Estimate the rate of steam

generation in the power cycle, assuming all the

heat removed from the furnace is used to make

steam.

References

- Felder, R.F. and Rousseau, R.W. Elementary

Principles of Chemical Processes, Third Edition.

New York, John Wiley and Sons, 2000. - Smith, J.C. and Harriott, Peter. Unit Operations

of Chemical Engineering, Sixth Edition. Boston,

McGraw Hill, 2001. - Earle, R.L. Unit Operations in Food Processing,

Second Edition. http//www.nzifst.org.nz/unitopera

tions/index.htm - Thibault, Jules. Notes, CHE 4311 Unit

Operations. University of Ottawa, August 2002. - Genzer, Jan. Notes, CHE 225 Chemical Process

Systems. North Carolina State University, August

2002.

- Source on pictures for slides 13, 41,