Thermodynamics MTX 311
Semester test 2 April 2016
Examiner
Dr AS Lexmond (University of Pretoria)
Instructions
1 Time: 90 min.
2 Answer all the questions
3 Full marks: 100
4 Neatness saves you points, especially when only part of the question is correct.
5 Give the equations before you start filling in numbers; for the questions marked with an
*, also give a short but clear solution strategy
6 You are allowed to take fundamentals of thermodynamics (7th or 8th edition), non-ideal
gas hand-out, one page, A4, with your own notes (both sides) and all the material from
the click-up page “lectures”
7 For all questions: use a reference temperature and pressure of T0=25°C, P0=1 bar
Q1 what is the exergy of:
1A (10) a process flow of 10g/s of saturated liquid ammonia at -15°C
1B (10) 10 kg of aluminium at 125°C, 1 bar
Organic waste has quite a high caloric value. The stored energy in the waste can be converted into a
useful product using a pyrolysis process. Part of the organic material is burned. The energy which is
released in the combustion process is used to heat the remainder of the material. This material
decomposes in the absence of air, forming a carbon monoxide rich combustion gas. Upon cooling,
the carbon monoxide reacts with the other substances in the gas, forming pyrolysis oil, which is the
main product of the process. Properties (and value) of high-quality pyrolysis oil are similar to
paraffin. The efficiency of the process is best when the organic waste contains little water. For that
reason, the feed material is usually first dried by evaporating as much water as possible. The wet air
that is formed is rarely used, and is a waste stream.
Since the process generates quite a lot of heat, integration with power production is a logical
extension to this process. In that way, the electricity which is needed for running the process control
computers, the fans, pumps, etc. can be generated by the process itself and the operator can
continue running the process during load shedding.
, A schematic of such a combined process is given in fig. 1. Properties of the process flows are given in
table 1.
Q2 (25) what are the irreversibility and second law efficiency of this process?
Q3 what are the irreversibility and second law efficiency of:
3A (10) heat exchanger D
3B (10) compressor C
3C (10) turbine I
As expected, by far the largest irreversibility of this process is caused by the pyrolysis process; both
the combustion process (in the shell, A) and the reformation of waste into oil are rather inefficient
processes. On top of this, there is quite some friction in both the pyrolysis both the furnace (B) and
the pyrolysis chamber (A), and the unit is not properly insulated. (although heat loss from the
pyrolysis chamber B is not an issue since it is inside the furnace).
4 (25) Make an irreversibility break-down of the various irreversibilities in the pyrolysis unit, A&B.
You do NOT have to make the split between process and equipment related irreversibilities , since
the unit is made for this process only.
Table 1: process flow properties of pyrolysis process
T P m H Ψ
(K) (Mpa) (kg/s) (kW) (kW)
1 air in for furnace 298.15 0.1 0.1 29.861 0.000
2 air compressed 431.9742 0.3 0.1 43.374 13.327
3 fuel for furnace 298.15 0.3 0.008 116.000 112.000
4 combustion off gas 900 0.28 0.108 100.780 40.863
5 wet feed in 298.15 0.1 0.008 96.233 91.600
6 pyrolisis feed 313.15 0.1 0.0065 94.250 91.000
7 heated pyr. Feed 690 0.25 0.0065 97.679 93.754
8 pyrolysis gas 720 0.22 0.0065 154.673 106.946
9 cooled pyr. Gas 400 0.22 0.0065 151.193 104.450
10 partially condensed product 350 0.0065 148.102 101.882
11 dry air in 298 0.04 12.100 0.400
12 wet air out 323 0.055 13.100 0.700
13 fully condensed product 300 0.6 1.17 146.932 100.809
14 turbine out 700 0.1 0.108 77.064 16.509
15 off gas 390 0.1 0.108 41.640 1.514
16 high P steam 650 1 0.0115 36.924 11.662
17 expanded steam 373 0.1 0.0115 30.775 5.610
18 low P water 313 0.1 0.0115 1.927 0.016
19 compressed water 313 1 0.0115 1.944 0.033
Semester test 2 April 2016
Examiner
Dr AS Lexmond (University of Pretoria)
Instructions
1 Time: 90 min.
2 Answer all the questions
3 Full marks: 100
4 Neatness saves you points, especially when only part of the question is correct.
5 Give the equations before you start filling in numbers; for the questions marked with an
*, also give a short but clear solution strategy
6 You are allowed to take fundamentals of thermodynamics (7th or 8th edition), non-ideal
gas hand-out, one page, A4, with your own notes (both sides) and all the material from
the click-up page “lectures”
7 For all questions: use a reference temperature and pressure of T0=25°C, P0=1 bar
Q1 what is the exergy of:
1A (10) a process flow of 10g/s of saturated liquid ammonia at -15°C
1B (10) 10 kg of aluminium at 125°C, 1 bar
Organic waste has quite a high caloric value. The stored energy in the waste can be converted into a
useful product using a pyrolysis process. Part of the organic material is burned. The energy which is
released in the combustion process is used to heat the remainder of the material. This material
decomposes in the absence of air, forming a carbon monoxide rich combustion gas. Upon cooling,
the carbon monoxide reacts with the other substances in the gas, forming pyrolysis oil, which is the
main product of the process. Properties (and value) of high-quality pyrolysis oil are similar to
paraffin. The efficiency of the process is best when the organic waste contains little water. For that
reason, the feed material is usually first dried by evaporating as much water as possible. The wet air
that is formed is rarely used, and is a waste stream.
Since the process generates quite a lot of heat, integration with power production is a logical
extension to this process. In that way, the electricity which is needed for running the process control
computers, the fans, pumps, etc. can be generated by the process itself and the operator can
continue running the process during load shedding.
, A schematic of such a combined process is given in fig. 1. Properties of the process flows are given in
table 1.
Q2 (25) what are the irreversibility and second law efficiency of this process?
Q3 what are the irreversibility and second law efficiency of:
3A (10) heat exchanger D
3B (10) compressor C
3C (10) turbine I
As expected, by far the largest irreversibility of this process is caused by the pyrolysis process; both
the combustion process (in the shell, A) and the reformation of waste into oil are rather inefficient
processes. On top of this, there is quite some friction in both the pyrolysis both the furnace (B) and
the pyrolysis chamber (A), and the unit is not properly insulated. (although heat loss from the
pyrolysis chamber B is not an issue since it is inside the furnace).
4 (25) Make an irreversibility break-down of the various irreversibilities in the pyrolysis unit, A&B.
You do NOT have to make the split between process and equipment related irreversibilities , since
the unit is made for this process only.
Table 1: process flow properties of pyrolysis process
T P m H Ψ
(K) (Mpa) (kg/s) (kW) (kW)
1 air in for furnace 298.15 0.1 0.1 29.861 0.000
2 air compressed 431.9742 0.3 0.1 43.374 13.327
3 fuel for furnace 298.15 0.3 0.008 116.000 112.000
4 combustion off gas 900 0.28 0.108 100.780 40.863
5 wet feed in 298.15 0.1 0.008 96.233 91.600
6 pyrolisis feed 313.15 0.1 0.0065 94.250 91.000
7 heated pyr. Feed 690 0.25 0.0065 97.679 93.754
8 pyrolysis gas 720 0.22 0.0065 154.673 106.946
9 cooled pyr. Gas 400 0.22 0.0065 151.193 104.450
10 partially condensed product 350 0.0065 148.102 101.882
11 dry air in 298 0.04 12.100 0.400
12 wet air out 323 0.055 13.100 0.700
13 fully condensed product 300 0.6 1.17 146.932 100.809
14 turbine out 700 0.1 0.108 77.064 16.509
15 off gas 390 0.1 0.108 41.640 1.514
16 high P steam 650 1 0.0115 36.924 11.662
17 expanded steam 373 0.1 0.0115 30.775 5.610
18 low P water 313 0.1 0.0115 1.927 0.016
19 compressed water 313 1 0.0115 1.944 0.033
