Lec14 Updated

8/8/2019 Lec14 Updated

1/6

CHE 3172CHEMICAL ENGINEERING THERMODYNAMICS

First Law of Thermo, open systems. Unsteady-state,uniform systems. Second Law of Thermo

Reading: Ch. 2, 3 Koretsky

Lecture 14

1

Copyright 2010 F. R. Hung, LSU, CHE 3172. Permission is granted to use the material related to this course and posted on http://moodle.lsu.edu forpersonal or your own instructional use. All other rights to the material presented here are reserved. You may not use this material for any commercialpurposes, nor may you redistribute it in any fashion.

2

Announcements Problem sessions next week (usual days, times and place)


2/6

3

In-class exercise

Last class we saw the video How thermodynamicists make ice cream:

http://www3.imperial.ac.uk/people/e.muller/teaching/icecream

Lets analyze the process that happens to the CO2 from the thermo point ofview. A commercial CO2 fire extinguisher contains about 5 kg of CO2,

typically a liquid in equilibrium with its vapor (lets assume that the quality isabout 1%) at room temperature (say, 20 C).

When one operates a conventional fire extinguisher (see figure), a small

tube extracts liquid from the bottom of the extinguisher. The vacuum left bythe liquid is filled by vapor coming from the evaporation of part of the liquidstill present. For that to happen, some heat must be transferred from thesurroundings. Therefore, the pressure of CO2 within the extinguisher willremain constant as long as there is liquid inside it. Lets assume that thetemperature is always constant.

(a) Explain how dry ice (solid CO2) forms inside that pillowcase in the video.Assume that the extinguisher discharges CO2 at atmospheric pressure(lets say 0.98 bar)

(b) How much CO2 can be extracted from the extinguisher before its internalpressure starts dropping?

(c) How much heat must be transferred from the surroundings?

http://www.firemarkext.co.uk/ds-co2.htm

4

http://www.union.dk/media/CO2-GB.pdf


3/6

http://www.union.dk/media/CO2-GB.pdf

Question: can we use thermo data from different sources?

51 2 3 4 5

20% 20% 20%20%20%

1. OK for all properties we have seen so

far (T, P, v, u, h)2. OK only for T, P, v

3. OK only for u, h

4. Not OK for any of T, P, v, u, h

5. OK for some other thermo property

6

In-class exercise

Last class we saw the video How thermodynamicists make ice cream:

http://www3.imperial.ac.uk/people/e.muller/teaching/icecream

Lets analyze the process that happens to the CO2 from the thermo point ofview. A commercial CO2 fire extinguisher contains about 5 kg of CO2,typically a liquid in equilibrium with its vapor (lets assume that the quality isabout 1%) at room temperature (say, 20 C).

When one operates a conventional fire extinguisher (see figure), a smalltube extracts liquid from the bottom of the extinguisher. The vacuum left bythe liquid is filled by vapor coming from the evaporation of part of the liquid

still present. For that to happen, some heat must be transferred from thesurroundings. Therefore, the pressure of CO2 within the extinguisher willremain constant as long as there is liquid inside it. Lets assume that thetemperature is always constant.

(a) Explain how dry ice (solid CO2) forms inside that pillowcase in the video.Assume that the extinguisher discharges CO2 at atmospheric pressure(lets say 0.98 bar)

(b) How much CO2 can be extracted from the extinguisher before its internalpressure starts dropping?

(c) How much heat must be transferred from the surroundings?

http://www.firemarkext.co.uk/ds-co2.htm


4/6

7

Second Law of Thermodynamics

First law = conservation of energy processes where energy is created ordestroyed are not possible

However, not all processes where energy are conserved are possible.

The Second Lawtells which processes are naturally possible/impossible. Thislaw can be stated in many ways:

Taken from Levenspiels book

8





5/6

9




10





6/6

11


The Second Lawwas stated back in the 1800s, by scientists dealing with theproduction of (useful) work from heat.

The partial conversion of heat into work is the basis for nearly allcommercial production of power nowadays(with the exception of

electricity produced from water dams, and wind power)

To develop a quantitative expression for the Second Law, lets first take a lookat heat engines:

Adapted from Levenspiels book

These are devices or machines that operate in a cyclicprocessand produce work from heat

H2O

Heat supplied byburning fuel (QH)is partiallyconverted intouseful work (W,i.e., mechanical

energy that canbe converted inelectricity).

Figure taken fromSVNA 7th ed.

Documents

Lec14 Updated