Introduction5+PRELAB+31.docx · Web viewThe rate of heat conducted depends on the thermal conductivity (k) of a substance. This constant is a measure of a substance’s resistance

Heat Transfer to a Fluid in a Stirred Tank University of Illinois

Heat Transfer to a Fluid in a Stirred Tank

The heat transfer coefficient of the wall of a tank of water and a jacket of steam is found in this laboratory. Heat

Transfer is measured in two sections. The first section is at steady state, when the temperature inside is time

independent. The second section is at unsteady state and the temperature is increasing.

1Unit Operations ChE 382 Group 5 Spring 2011 3/1/2011Damo, Duffy, Guerrero, Hsu, Kosak, Qamar, Tyska


Lab Prep Report

Unit Operations II Lab 2

March 1, 2011

Group 5

Andrew Duffy

Daniyal Qamar

Jeff Tyska

Bernard Hsu

Ryan Kosak

Tomi Damo

Alex Guerrero



Contents1. Introduction.........................................................................................................................................4

2. Literature Review/Theory....................................................................................................................5

3. Experimental.....................................................................................................................................13

3.1. Apparatus..................................................................................................................................13

3.2. Materials and Supplies...............................................................................................................18

3.3. Experimental Procedure............................................................................................................19

4. Anticipated Results............................................................................................................................20

5. References.........................................................................................................................................21

6. Appendix I: Job Safety Analysis..........................................................................................................22



1. Introduction

Stirred tanks are probably one of the most used pieces of chemical engineering

equipment that come is a wide variety for chemical processing applications. No two tanks are

alike and have to be designed specifically for the proper application. This involves looking into

the materials and operating parameters to ensure that the desired results occur in the safest and

most efficient way possible. The most common uses are for chemical reactions, leaching,

blending, and dispersing. Typically when looking into the system from a theoretical point of

view the tank is assumed to have perfect mixing or constant bulk properties. This is done to

make the calculations around the tank more manageable.

The main purpose of this lab is to determine the effects of heating and cooling on a

continuously stirred tank with multiple processes. Heating is performed by a steam jacket which

allows heat to be transferred to the internal liquid and as for the case of cooling the tank there is

an internal cooling coil system which uses cold water to remove the heat. There is also a

secondary system to cool the tank which directly pumps the liquid out of the bottom through a

heat exchanger that is also fed with cold water and then sends the cooled liquid back to the top of

the tank. Another aspect to this experiment is the use of baffles on the effect of mixing which are

implemented to enhance it.

The heat transfer coefficient between the fluid and the walls of the vessel will be the main

focus for calculations in this experiment based on its dependence of the impeller speed, fluid

properties, and the baffles. Heat transfer coefficient values are functions of the fluid flow field

and molecular transport properties of the fluid. Since a mixing tank has a very intricate flow 4

Unit Operations ChE 382 Group 5 Spring 2011 3/1/2011Damo, Duffy, Guerrero, Hsu, Kosak, Qamar, Tyska


pattern the heat transfer coefficient would be too complex to determine. Therefore the coefficient

must be determined experimentally by correlating it with dimensionless groups. In convective

flow the Reyonlds number (Re), Prandtl number (Pr), and geometric factors are needed to do so.

Four cases will be studied in this experiment. The first is a steady state case and the second is

also at steady state with the baffles placed in the tank. The third and fourth cases are both

unsteady state cases with increasing temperature where one is with the baffles.

2. Literature Review/Theory

Constant stirred tank reactors (CSTR) are widely used reactors in the industry. They are used

to carry out reactions that require intense agitation, such as the addition of gaseous reactants in a

liquid, a solid reactant in a liquid, or polymerization reactions. (Rawlings 5) Heat exchange in

CSTR reactors is a very important and well-studied division. A highly exothermic reaction or a

highly endothermic reaction both require that heat be taken out or put into the reactor

respectively.

Heat is the transfer of energy from one substance to another. There are three types of heat

transfers; conduction, convection, and radiation. Heat conduction is the energy transfer at the

molecular level. As molecules collide and bounce off of each other they exchange energy, the

high energy particles loose energy to the low energy ones. Heat convection is the energy transfer

as the bulk fluid moves and radiation is the transfer of energy without a medium, it does not

required molecules or a bulk fluid to be transferred. In this lab we will be mainly studying

conduction and convection. (Bird 266)



The rate of heat conducted depends on the thermal conductivity (k) of a substance. This

constant is a measure of a substance’s resistance to heat conduction. The higher the k value the

easier it is to transfer heat through this substance. Temperature is the measure of energy a

substance holds and heat always transfers from a high temperature region to a low temperature

region. Heat is transferred according to the following law, known as Fourier’s Law (Bird 266-

332).

Q / A=k dTdx (1)

Q is the Heat transferred

A is the area this heat transfers through

K is the thermal conductivity

T is temperature

x is the distance this heat is transferred through

This law states that the heat flow per unit area is proportional to the temperature decrease dT

over a distance dx. The heat transfer, at a boundary, that takes place between a fluid and a solid

goes through a thin film. This heat transfer is not defined directly by the Fouriers law but is

defined by the Newton’s Law of cooling which is defined as follows: (Bird 266-332)



Q13=hA (T0−T b ) A (1a)

Q13 is the heat transferred

h is the heat transfer coefficient

T0 is the temperature of the surface

Tb is the temperature of the bulk fluid

A is the area of heat transfer

In this lab we will be studying the heat transfer through three different regions.

1) Heat transfer across the internal fluid to the wall of the stirred tank

2) Heat transfer across the tank wall

3) Heat transfer from the condensing steam to the tank wall

Since these three regions include the heat transfer through several different mediums a collective

heat transfer coefficients (U) can be derived. U is defined as the following for this lab:

U= 11hi

+ 1ho

(2)

hi is the heat transfers coefficient from the fluid in the CSTR to the tank wall. (W/m2−K ¿

ho is the heat transfer coefficient from the tank wall to the surrounding steam. (W/m2−K ¿

With the overall heat transfer coefficient, Newton’s law of cooling becomes:

Q13=U (T 1−T 3 ) A (1b)



U is the overall heat transfer coefficient (accounting for heat resistance of all three boundaries

listed above and described by equation 2) (W/m2−K ¿

T 1 is the temperature of the steam surrounding the CSTR (K)

T 3 is the temperature of the fluid in the CSTR (K)

A is the heat transfer area (m2)

Some assumptions that are needed to simplify the heat transfer are that the wall thickness is thin

compared to the tank so area is same for both values and that the tank wall has a very high k

value so it has no resistance to heat transfer.

The following picture can help to understand the derivation of the overall heat transfer

coefficient:

Figure 1: Temperature profile

As the heat transfer from the right to the left it first goes through a thin film with heat transfer

coefficient h0 then it flows through the solid with heat transfer coefficients kscale and kwall finally


QQQQQQQQQQQQQQQQQ


comes out on the left side where the heat transfer coefficient is hi. The heat transfer through in

the solid-fluid interface is described the equations:

Q = hiAi(t4 – t5) = h0A0(t1 – t2) (3)

h0 is the heat transfer coefficient across the solid-fluid boundary where temperature difference is

t1 – t2

hi is the heat transfer coefficient across the solid-fluid boundary where temperature difference is

t4 – t5

The heat transfer across the solids follows the Fourier law and is defined as follows:

Q = Ascalekscale(t2-t3)/xscale = Ascalekscale(t3-t4)/xwall (4)

Since the heat transfer is the same across all the walls at steady state, the x, h, and A values can

be combined to give an overall heat transfer coefficient:

1U i

= 1h i

+xscale

k scale∙ Ai

A scale+

xwall

k wall∙ Ai

Awall+ 1

ho

A i

A0 (5)

The amount of heat lost or gained by a substance depends on its heat capacity C.

Q=mC(T-T0) (6)

This equation determines how much heat is gained or lost by a substance as the temperature

drops or is raised (Packet).



Heat transfer coefficients can be theoretically estimated using the following correlations:

Outer Wall Heat Transfer Coefficient Estimation

This estimates the h value on the outside of the vessel wall where steam condensation takes

place.

ho LT

k=0.925( L3 ρ2 g

μM )1/3

(6)

Where:

ho is the heat transfer coefficient through film of condensing steam. (W/m2−K )

LT is the vertical length of the tank. (m)

k is the thermal conductivity of the fluid. (W/K-m)

ρ is the density of the fluid. (kg/m3)

g is the gravitational constant. (m/s2)

μ is the viscosity of the fluid. (kg/s-m)

M is the mass rate of steam condensed per wetted perimeter described by:

M=mst

π D¿(6a)

mst is the mass rate of steam condensation. (kg/s)


Bernard Hsu, 03/01/11,

What?


Saturated Steam Heat Transfer Coefficient

ho=2960 ( D¿ /mst )1/3 (7)

Estimation Inner Wall Heat Transfer Coefficient Estimation

This system falls into the category of an unbaffled CSTR with Newtonian fluid. The heat transfer correlation can be estimated using the following correlations.

hi DT

k=0.36(C p µ

k )1 /3( Di

2 nρµ )

2/3

(3)

hi DT

k

(C p µk )

1 /3

( μμw )

0.14=0.36 ( Di

2nρµ )

2/3

(9)

Where:

hi is the heat transfer coefficient of the inner CSTR tank wall. (W/m2−K ¿

DT is the diameter of the tank (m)

μ is the viscosity of the fluid. (kg/s-m)

C p is the specific heat of the reactor fluid (J/kg-K)

k is the thermal conductivity of the fluid. (W/K-m)

ρ is the density of the reactor fluid. (kg/m3)

Di is the diameter of the impeller. (m)



n is the rate of revolution of the impeller. (RMP)

Correlation (3) can also be expressed in terms of dimensionless numbers:

Nut=0.36 (ℜimp )2 /3 ( Pr )1/3 (4)

Where:

Nut is the tank Nusselt number described by:

Nut=h DT

k(4a)

ℜimp is the impeller Reynolds number described by:

ℜimp=D I

2 nρμ

(4b)

Pr is the fluid Prandtl number described by:

Pr=C p µ

k(4c)

In this lab the convective heat transfer coefficient at the inner surface of the tank will be

experimentally determined. This value will then be compared with theoretically calculated values

using the equations listed above.



3. Experimental

3.1.Apparatus

Figure 2: Top Half of CSTR Apparatus


23

6

7

8

9

5

4

17

1


Figure 3: Bottom Half of CSTR Apparatus

The figures above depict the “Heat Transfer in a Stirred Tank” lab apparatus. It consists

of a constantly stirred tank that is equipped with a cooling coil and lined with a steam jacket.

After filling the tank with water, the pump located underneath continuously pumps water

out of the tank and into the shell side of a heat exchanger where cooling water removes heat.

The cooled tank water flows through the flowrator to display its flow rate and then re-enters


10

11

12

13

14

15

16


the tank. Dial thermometers are located at the inlet and outlet of the heat exchanger. The

cooling coils within the tank are fed with incoming cooling water. The flow rate of the

cooling water can be set and adjusted using a flowrator. The temperature of the cooling

water is displayed on dial thermometers located near the inlet and outlet of the cooling coils.

Steam is fed into the steam jacket of the tank to allow heat transfer into the system. Steam

condensate exits the steam jacket and enters a condenser, where cooling water flows through

it. The impeller, which stirs the water in the tank, is controlled using a variable power

supply. A strobotac is used to measure and display the speed of the impeller. The impeller

speed and the flowrate of water through the external heat exchanger will be studied in three

different experiments: 1) unbaffled stirred tank, 2) baffled stirred tank, and 3) unsteady-state

heat transfer in an unbaffled tank.

# Equipment Description Manufacturer Operating Range

1 Strobotac Displays rotational

speed (RPM) of

motor which powers

the impeller.

N/AN/A

2 Electric Motor Powers the impeller. Eastern Industries 1/4 HP

3 Variable Transformer Controls the speed of

the electric motor.Powerstat

0-140 V

4 Cooling Coil Inlet

Flowrator Tube

Displays the flow

rate of the cooling

coil water.

F & P CO. 0-100% Max of

0.810 GPM



5 Cooling Water Inlet

Valve

Controls the flow rate

of the cooling coil

water.

Crane CO. N/A

6 Recycle Water

Flowrator

Displays the flow

rate of the recycle

water.

F & P CO.

0-100% Max of

0.810 GPM

7 Recycle Water

Thermometer

Displays the

temperature of the

recycle water.

MFG CO. 15-105 oC

8 Cooling Water Inlet

Thermometer

Displays the

temperature of the

cooling coil water at

the inlet.

Moeller Instrument CO.0-250 oF

9 Tank Thermometer Displays the

temperature of the

fluid in the tank.

Trend Instruments INC.

50-300 oF

10 Tank Contains the fluid, as

well as the impeller

and steam jacket.

N/A N/A

11 Tank Outlet

Thermometer

Displays the

temperature of the

condensate exiting

Trend Instruments INC.



the steam jacket

before it enters the

condenser.

0-200 oF

12 Condenser Cools the steam

condensate.N/A

N/A

13 Pump Continually re-

circulates water

through the tank.

Dayton Electric MFG

CO.1/3 Hp

14 Condenser

Thermometer

Displays the

temperature of the

cooling water

entering the

condenser

Trend Instruments INC. 0-200 oF

15 Recycle thermometer Displays the

temperature of the

water that exits the

tank before it enters

the heat exchanger.

MFG CO.

15-105 oF

16 Heat Exchanger Cools the recycled

water before it re-

enters the tank.

N/A

N/A

17 Pump Power Switch Turns on the pump. Appleton Electric



Products N/A

3.2.Materials and Supplies

Material/Supply Description

Water Used throughout the system where

measurements will be taken from.

Graduated Cylinder Used to measure amount of water

collected from the condenser.

Baffles Directs the flow of fluids for maximum

efficiency in stir tank.

Mop Used to clean up any liquid spills.

Stop Watch Time the amount of fluid exiting the

condenser.

3.3.Experimental Procedure



Begin the steady state experiment -

1. Open the main water valve

2. Open the valve to the tank, and fill it up to around 2 inches from the top of the tank.

3. Measure the height of the water in the tank.

4. Turn on the impellor, and try to adjust it’s speed to 100 rpm, +- 50 rpm

5. Turn on the cooling water to the heat exchanger and the cooling coils inside the tank.

Note the flowrate of cooling water.

6. Make sure the pump bypass valve and pump suction valve is on, and turn on the pump.

7. Open the valve so that the cooling water goes to the steam condenser.

8. Open the steam inlet valve, and wait for the process to reach steady state.

9. Measure the temperatures, flow rates, and impellor speed.

10. Increase the impellor speed to a different rpm

11. Measure the same variables as in part 9

12. Increase the temperature inside the tank

13. Measure the same variables as in part 9

14. Turn off the impellor, and put baffles inside the tank, being careful not to get burnt.

15. Repeat steps 9 through 13.

16. Turn off the cooling water and steam

17. Drain the tank

Note – This ends the steady state portion of the experiment, the unsteady state procedure

is now started.

18. Add fresh water, whose temperature should be less than 40 degrees C.



19. Run cooling water through the steam condenser.

20. Open the condenser water valve

21. Partially open the steam valve.

22. Turn the impellor on, and adjust it’s speed to 100 rpm.

23. Open the steam valve completely

24. Measure the temperature around every 20 seconds (time intervals must be constant, but

intervals need not be 20 seconds)

25. Record this data until the water reaches 85 degrees Celsius

26. Turn off the steam, and record the temperature of the water as it cools down with the

same time intervals.

27. Repeat steps 22-26 with different impellor speeds (same ones as used in step 10).

4. Anticipated Results

The objective of this lab is to measure the heat transfer coefficient of the wall of a tank of

water and a jacket of steam. The heat transfers will be measured in two sections: first as steady

state, when the temperature inside has leveled off, and second during unsteady state, when the

temperature is still increasing.

The first section will measure the heat transfer coefficient with baffles and without

baffles. These baffles are anticipated to increase the transfer of heat between the walls because

the area of tank water exposed to the hot steam is increase therefore more of the water will pick

up the heat as it is exposed to the heat. The baffles do this by increasing the agitation in the tank



without the need for increasing the impeller speed. It is anticipated that the heat transfer

coefficient for the baffle runs will be larger than the one without. All the temperatures and flow

rates will be collected in order to do an energy balance and use equation 8 and 1a to figure out

the overall heat transfer coefficient. As high as possible transfer coefficient is desired in order to

transfer heat more rapidly but one thing that will slow this down is fouling. The wall inside the

tank is lined will water build up than will inhibit heat transfer this will lower the transfer

coefficient and make the tank take longer to heat.

During the unsteady state the tanks temperatures are measure when the heating begins not

when it has reached the desired value. In this section the goal is to determine the relationship

between temperature and the heat transfer coefficient. It is anticipated that initially this will be

high due to the larger temperature gradient (sink) between the two streams.

5. References

Bird, R. B., Warren E. Stewart, and Edwin N. Lightfoot. Transport Phenomena. 2nd ed. New York, NY: Jonh Wiley & Sons, Inc., 2002

Packet, Heat transfer to a Fluid in a Stirred Tank

6. Appendix I: Job Safety Analysis



What is the purpose of this experiment?

The purpose of this experiment is to determine the effects of mixing with and without baffles on

a stirred tank system. Four cases will be studied: steady state without baffles, steady state with

baffles, unsteady state without baffles, and unsteady state with baffles.

What are the hazards associated with the experiment?

Since high temperature steam is used to heat the tank it is necessary to be aware of which pieces

of the equipment will be at high temperatures. Another hazard is the impeller which should be

avoided when in use. Water is continuously passed through the system therefore there is a

spilling and slipping hazard.

How will the experiment be conducted in a safe manner?

The experiment will be constantly monitored to ensure that everything is running properly and

safety. When filling the tank it will be necessary to be sure that it doesn’t overflow and cause

slipping hazard. It will also be necessary to ensure that all the cooling water is draining properly

and not spilling on the floor. Since electricity is used around a system that uses a lot of water it

will be necessary to be aware this to avoid any electric shocks. As always, close-toed shoes and

safety goggles will be worn at all times in the experiment area to decrease the chance of potential

hazards.

What safety controls are in place?



The stirred tank is warped with insulation to ensure no one gets burned by accidently touching

the side when moving around the apparatus. There is a recycle system for the pump. Most

importantly, the laboratory area around the experiment will be kept dry using paper towels and a

mop to clean any spilled liquid promptly to avoid wet working conditions. The water levels will

be monitored to ensure that the water does not overflow and cause a spill in the laboratory.

Safety goggles will be worn at all times to ensure eye protection.

Describe safe and unsafe ranges of operations.

Since steam is used it would be unsafe to run the system at very high temperatures so the vessel

should be kept below 85°C at all times. The impeller should not exceed 200 rpm during the

duration of the experiment. Never run the pump dry.

I have read relevant background material for the Unit Operations Laboratory entitled:

“Heat Transfer to a Fluid in a Stirred Tank” and understand the hazards associated with

conducting this experiment. I have planned out my experimental work in accordance to

standards and acceptable safety practices and will conduct all of my experimental work in a

careful and safe manner. I will also be aware of my surroundings, my group members, and other

lab students, and will look out for their safety as well.

Electronic Signatures:



Bernard Hsu

Daniya l Qamar

Jeff Tyska

Alex Guerrero

Tomi Damo

Ryan Kosak

Andrew Duffy


Documents

Introduction5+PRELAB+31.docx · Web viewThe rate of heat conducted depends on the thermal conductivity (k) of a substance. This constant is a measure of a substance’s resistance