exhaust gas treament

Basic Engineering Workshop 1
Exhaust Gas Treatment

Index of contents

Introduction
Waste gas with harmful traces of gas must be burned in an afterburner. To destroy the harmful traces the temperature after the burner must achieve 900°C. The preheater can`t resist a temperature of 700°C. Therefore quench water in a mixer will be used to protect the heat exchanger. The waste gas has the following properties:
Waste gas composition: Mole fraction(= 0,9890, Mole fraction()=0,0110
Temperature = 20,00 °C
Pressure = 1,1 bar
Mass flow
The task is to calculate the optimal area of the heat exchanger as a function of t1. Spreadsheet calculations and Case Studies are used to show the results.
A set of assumptions is made concerning:
Overall heat transfer coefficient U
Costs of heat exchanger related to heat exchanger area
Operating costs for the after burner
Return of invest ROI after 20000 hours
Pressure drops are neglected
Background
The overall heat transfer coefficient U for gases around 1bar in literature1 is approximately a value between . On first approximation the value is used in this case.
The price for the heat exchanger related to the heat exchanger area is assumed . The operating cost for heat flow is assumed . The return of invest ROI is reached after 20000 hours. On first approach as an ideal process all pressure drops are neglected. The temperature and pressure of the quench water are both equal to the temperature and pressure of the waste gas.

Execution
The flow sheet is created and illustrated in the following figure 1:

Figure - Flow sheet Exhaust Gas Treatment

After creating the flow sheet and determining the given conditions the U*A-value is not provided but the temperature at t1 as t1=100°C is determined. In the next step the spreadsheet is created and the area A with the given data is calculated. With the given data the costs are calculated and a Case Study is created. The x-axis is determined as the temperature at t1 and the y-axis as the operation cost, heat exchanger cost and overall cost. The range for the x-axis is defined from 50°C to 699°C with a 10°C step size. Therefore the temperature is plotted in this rage. The optimal temperature is read from the overall cost curvature and set up in stream t1. After this the optimal area is calculated.

Results
The results are shown in the following diagram 1:

Figure – HE, operation and overall cost dependent on temperature

Figure - HE and operation cost dependent on temperature
Evaluation
The overall cost curvature in figure 2 shows on its minimum the optimal temperature for the system. The resulted data is shown in the following table1:
U / / °C / m²
C / €
20
500
99,66
269400
Table - U=20 data
With a low U-value the optimal temperature at t1 is low. Therefore the area has to be big and with that the overall cost is extremely high. With a bigger U-value the temperature is increasing and the area is decreasing. Therefore the overall is decreasing too.
For comparison and estimation of the influence of the U-value low and high U-values were evaluated and are shown in the following table 2 and in figure 4:
U / / °C / m²
C / €
5
200
63,93
364100
10
350
82,83
322800
15
450
97,89
289000
20
500
99,66
269400
25
525
93,98
253300
30
550
93,45
241100
35
560
86,34
231400
Table – Results for different U-values

Figure - area and overall cost dependent on temperature

List of table

List of figures

List of reference
Cerbe/Hoffmann. (1986). Einführung in die Wärmelehre.
VDI-Gesellschaft. (2006). VDI-Wärmeatlas.