Transcript
Increased heat recovery process at the sulphuric acid plant at Kemira Kemi AB Rasmus Hansson Department of Chemical Engineering Lund Institute of Technology February 2010 Abstract A simulation model of a humidifier and an absorption tower was built to investigate how much the heat production could be increased at the sulphuric acid plant at Kemira Kemi AB in Helsingborg. 3 3 The results show that if the present water flow of 48 m /h was increased to 248 m /h the heat production in the drying towers increase with 1.53 MW. It has also been investigated how the heat production increase if the air flow through the humidifier was increased and if an extra humidifier was built. If the air flow was increased the heat production increase with 1.78 MW and if also the humidifier area was increased with a factor of two the heat production increase with 2.84 MW. The simulations have also shown that the inlet air and water temperature have a major influence on the heat production. cooled by the district heat network of Helsingborg.
Introduction Sulphuric acid is one of the most used chemical substances today. It is largely used in the phosphate fertilizer industry, but also in the petroleum industry, pigment production, plastics production, paper industry etc. The production today is performed according to the contact process. The process can be divided into four different steps which are gas drying, catalytic oxidation of SO2 to SO3, absorption of the SO3 in sulphuric acid and acid cooling. When liquid sulphur is burn to form SO2 large amounts of heat is released which is used to produce high pressure steam. In the two drying towers heat is produced as heat of condensation and heat of dilution when water in the air condenses. In the absorption tower heat is produced when SO3 reacts with water to form H2SO4. With the increased demand of energy today it could be in great interest to increase the heat production in the process. [1]
For about 10 years ago a project was done at the sulphuric acid plant. The aim of the project was to increase the heat production by increasing the moisture content in the air before one of the drying towers. This resulted in more heat could be produced in the drying tower. To increase the moisture content a humidifier was built there the air passes through the humidifier countercurrent to a water flow at low heat grade. The temperature of the water decrease down through the humidifier and the air temperature and humidity increase up through the humidifier. [2] At the nearby ECOX plant large amounts of water at low heat grade is cooled with sea water today. This water could instead be used in the humidifier which would lead to higher moisture content and temperature of the air that exits the humidifier. Aim of project The aim of this project is to build a simulation model of the humidifier and drying towers. The model could be used to simulate the process and see what can be done to increase the heat production by increasing the water flow, changing water temperature and change the equipment dimensions.
Background The air used in the process has to be completely dry when it enters the combustion chamber, to manage that the air passes through two parallel absorption towers countercurrent to sulphuric acid. The water in the air condense and heat of dilution and heat of condensation is released and absorbed by the acid. The acid is later on
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Model
The equation was input to MATLAB and the height of the towers was discretised to manage calculating the countercurrent flows.
The simulation model of the humidifier was built on mass and energy balance between the air and water. The following equation describes the processes taking place in the humidifier.
Results and discussion Three different cases have been investigated and these are the following Increased water flow through the humidifier Increased water- and air flow through the humidifier Increased water and air flow through the humidifier at a larger humidifier area In Figure 1 a description of the present process and the three different cases is shown. In Case 1 only the water flow is connected to the existing water loop.
Eq- 1
Eq- 2
Eq- 3 [3] Equation 1 describes the temperature change of the air, equation 2 describes the temperature change of the water and equation 3 describes the moisture content change in the air. [1] For the air dryers mass and energy balance between the air and sulphuric acid was used to build the model. The following equations describes the processes taking place. Equation 4 describes the change in air temperature in the air dryer Eq- 4
Figure 1 Process description, - .-.- Case 2, - - case 3
Equation 5 describes the change in acid temperature in the dryer.
Before the different cases were investigated the simulation model was calibrated and validated to process data, that can be seen in Figure 2
Eq- 5
3,5
Twater Tair Large dryingtower Small dryingtower
Temperature error °C
2,5
Equation 6 describes the change in moisture content in the air through the dryer.
1,5
0,5
Eq- 6
-0,5
Equation 7 describes the change in acid flow due to the condensing water.
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-1,5 -2,5
Eq- 7
-3,5
Equation 8 describes the change in acid concentration due to the condensing water.
Validation run
Figure 2 Temperature error of simulated values compared to the process values.
Eq- 8 [4]
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As can be seen in the figure the simulation model fits the process data good enough. In case 1 the water flow is increased from 48 m3/h to 248 m3/h. The temperature of the inlet 3 water is 50 °C. The air flow is 68221 m /h with a temperature of 8 °C which is the annual mean temperature in south Sweden. The humidity is set to 70 %. The result from the simulation can be seen in Figure 3 and Figure 4.
warmed in the acid. There is also less heat production due to the moisture content is very low, which result in very low heat of condensation and heat of dilution. In case number two the water flow is increased so also the air, because the air that enters the small drying tower is also humidified. This leads 3 to an air flow of 90000 m /h and the heat production increase as can be seen in Table 1. Case three is the same as case two with the difference that the humidifier area is increase with a factor 2. The air is then dried in the two drying towers. The heat production in this case is the largest, but this case has also the largest investment costs. In Table 1 the results from all the three cases is shown.
Figure 3 Profile of the temperature and moisture content of the air and water in the humidifier
Table 1 Heat production in the three different cases there ΔQ is the increased heat production compared to the present heat production. Case 1 2 3
Q[MW] 2.58 2.85
ΔQ[MW] 1.53 1.78
3.91
2.84
It also became evident during the simulations that the inlet water temperature and air temperature has e major affect on the heat production. The air temperature has not been investigated very much because it changes with weather and seasons. The water temperature on the other hand has been investigated for the different cases. The result can be seen in Figure 5.
Figure 4 Profile of the temperature and moisture content in air and sulphuric acid. In Figure 3 it can be seen that the temperature of the water decrease down through the humidifier and the air temperature and moisture content increase up through the humidifier. In Figure 4 the upper two figures represents the profile of the large drying tower. It can be seen that the temperature of the acid increase down through the tower due to heat of condensation and heat of dilution. It can also be seen that the drying capacity is good enough when the moisture content has been increased in the entering air. The moisture content after the drying tower is kg water/ kg dry air. In the lower part of the figure the profile in the small drying tower is shown, there the inlet air is not humidified. It can be seen that the acid temperature decrease in the tower, this due to the entering air is not humidified and being
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Heat prodduction [MW]
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Water flow today Increased Water flow Increased Water- and Air flow Increased Area, Water- and Air flow
3.5 3 2.5 2 1.5 1 0.5 45
46
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48
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Inlet water temperature [oC]
Figure 5 Heat production at different inlet water temperature for the cases. The black line represents the present process, the blues line represents case 1, green line = case 2 and red line = case 3.
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References
It can be seen in the figure that the inlet water temperature affects the heat production. Therefore could it be very important to maintain a high water temperature when the water is transported from the ECOX plant to the sulphuric acid plant. It can also be seen in the figure that case three is the one most dependent of the inlet water temperature.
[1] H. Müller, sulfuric acid and sulfur trioxide, Ullmann´s Enchlopedia of Industrial Chemistry, Wiley –VCH verlag Gmbh & Co, 2005. [2] L. Pålssson, Luftförvärmning, Kemira Kemi AB, 1998, Helsingborg. [3] V.D Papaefthimiou et.al, Thermodynamic study of wet cooling tower performance, Int. J. Energy Res., 2006, 30, 411-426. [4] N. Fumo, Study of an aqueous lithium chloride desiccant system: air dehumidification and desiccant regeneration, Elsevier Science Ltd., 2002, Florida, USA.
Conclusions At this study, it has been shown that there is a great potential to increase the heat production at the sulphuric acid plant. A first step to increase the heat production is to build a pipeline that connects the ECOX plant with the sulphuric acid plant. By increasing the water flow through the humidifier the heat production increase with 1.53 MW and more heat could be delivered to the district heating network. It can be seen in Table 1 that case 3 gives the largest heat production but this case has also the largest investment costs which should be considered. It has also been shown that it is important to maintain a high temperature of the entering water or else the heat production will decrease. The air temperature affects also the process. In future work could it therefore be interesting to see how the heat production change if the air is preheated before the humidifier.
Nomenclature
A – Area [m2] Cp – Specific enthalpy [J/kg °C] h – Enthalpy [J/kg] K – Mass transfer coefficient [kg/m3s] m – Mass flow [kg/s] Q – Heat production [MW] T – Temperature [°C] w – Moisture content [kg water / kg dry air] Z – Height [m] X – Concentration acid [wt %] 3 αLA – Heat transfer coefficient [W/m K] Index A – Air Dil - dilution W – Water S – Sulphuric acid Sat – Saturation V – Vapour m – Mean value
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