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Journal of Science (JOS) Vol. 2, No. 4, 2012, ISSN 2324-9854 Copyright © World Science Publisher, United States www.worldsciencepublisher.org

179

HYSYS Simulation of a Sulfuric Acid Plant and Optimization Approach of Annual Profit 1

Niaz Bahar Chowdhury, 1 Zahid Hasan and 1 A. H. M. Biplob

1

Chemical Engineering Department, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh Email: [email protected]

Abstract – Sulfuric acid is a basic chemical used intensively all over the world. One can determine the prosperity of a nation by measuring the annual use of sulfuric acid of that country. Also sulfuric acid is a very important basic chemical, widely used in different industrial sector. The main purpose of the study is to simulate and optimize the annual profit 98.4% sulfuric acid plant by Aspen HYSYS 3.2. In this project, a simplified sulfuric acid production process is simulated and optimized. In order to simulate this process some process operational data of the Sulfuric Acid plant of WATA CHEMICALS LIMITED are used. The optimization criterion of the process is to maximize the annual profit. This study will be helpful for the entrepreneurs who are interested to build a sulfuric acid plant. Also this study will be very helpful for the plant operators to run the factory efficiently by minimizing the process system requirement. Keywords – Sulfuric Acid, Converters, Aspen HYSYS, Annual Profit, Optimization

1. Introduction Aspen HYSYS is a market-leading process modeling tool for conceptual design, optimization, business planning, asset management, and performance monitoring for oil & gas production, gas processing, petroleum refining, and air separation industries. Aspen HYSYS is a core element of Aspen Tech's aspen ONE® Engineering applications. It has vast importance for chemical engineers to simulate a process. This project contains a simplified simulation of a sulfuric acid production plant which also simulates an optimized annual profit. Sulfuric acid is one of the most widely used and important technical products. It is employed in the manufacture of fertilizers, leather and tin plate, in the refining of petroleum, and in the dyeing of fabrics [1]

The reactions involved in this simulation are S+ O2 = SO2, 2 SO2+ O2 = 2 SO3, SO3+ H2O = H2SO4

2.3. Fluid Package In order to simulate the process as accurately as possible COM thermo is selected as advanced thermodynamics databank. In model phase selection NRTL was selected for liquid phase and Peng-Robinson was selected for vapor phase. 2.4. Process Condition

2. Methodology and Simulation

The process conditions of this simulation is described below

Simulation is done by Aspen HYSYS 3.2. Procedure is described below [2].

2.4.1. Streams

2.1. Components

The conditions of the prime streams are Table1. Stream Conditions

The components used in this simulation are Liquid Sulfur, Oxygen, Nitrogen, Sulfur Dioxide, Sulfur Trioxide, Sulfuric Acid, and Water

2.2. Reaction involved

Condition

Pure Liquid Sulfur

Temperature

105.0

Moist Air DM (65% Water RH) 30.0

30.0

Product

75.0

Niaz Bahar Chowdhury, et al., JOS, Vol. 2, No. 4, pp. 179-182, 2012

(°C) Pressure (atm) Flow rate (kmole/hr)

180

Figure1. Drying Tower and Sulfur Burner 2.0

2.5

1.0

1.0

25.0

257.2

30

29.8

2.4.2. Converters

The sulfur dioxide is converted to sulfur trioxide by passing through 4 converter beds. The sulfur dioxide from the sulfur burner is passed through a waste heat boiler to lower the temperature before entering the 1st converter bed.

The conditions of various converters used in this simulation are Out let Inlet Pass Conversion temperature temperature Number (%) (ͦC) (ͦC) 1st 2nd 3rd 4th

410.0 438.0 432.0 427.0

616.8 489.7 444.1 435.8

74.0 70.8 56.6 93.9

2.5. Unit Operations The unit operations used in this simulation is a. b. c. d. e. f.

One Waste Heat Boiler Three Interbed Coolers Three Coolers One Circulation Tank Two Splitters One Drying Tower

Figure2. Four Single Pass Converter Beds Between each of two consecutive converter beds, there is also an interbed cooler for the same purpose. Finally, the outlet gas from the 4th converter bed is passed through a cooler.

2.6. Unit Process The unit process used in this simulation is a. One Waste Heat Boiler b. Three Interbed Coolers c. Three Coolers d. One Circulation Tank e. Two Splitters f. One Drying Tower

3. Process Description Type Moist air is dried in the drying tower using 98% sulfuric acid. The resulting dry air, along with liquid sulfur, is fed to a sulfur burner to produce sulfur di oxide.

Figure3. Absorption Tower and Circulation Tank This cool gas, containing sulfur trioxide, is fed to an absorption tower where it reacts with 98% sulfuric acid to form 98.5% sulfuric acid. The stack gas from the absorption tower consists predominantly of nitrogen. The 98.5% sulfuric acid is fed to circulation tank along with demineralized water and, 97.5% sulfuric acid which comes from the drying tower. The resulting concentration of the sulfuric acid exiting from the circulation tank is 98.1%, which is split into two portions. One portion is cooled and recycled back to the absorption tower. The other portion is also cooled and further split into two portions; one of which is the final product (98% sulfuric acid) and the other portion is recycled back to the drying tower [3]. Therefore, the above is a brief description of the sulfuric acid production process simulated by us.

4. Results and Discussion

Niaz Bahar Chowdhury, et al., JOS, Vol. 2, No. 4, pp. 179-182, 2012

181

The result of this simulation is discussed in this section

4.1. Optimized Function The criterion of the optimization is to maximize the profit. The function used for optimization is:







{(Duty1+ Duty2 + Duty3 + Duty4 + Duty5 + Duty6 + 0.000293 3 }] 24 ( ) Duty7) 300 (



)

Figure5. Optimization Variables

4.3. Profit Maximization The window which shows the profit maximization is given below

Figure4: Optimization Spreadsheet

4.2. Variables The a. b. c. d. e. f. g.

optimization variables are temperatures of – Cool 1st Converter Bed Inlet Cool 2 nd Converter Bed Inlet Cool 3 rd Converter Bed Inlet Cool 4 th Converter Bed Inlet Absorber Inlet Cool Recycle 1 Cool Final Product

Figure6. Maximized Profit

4.4. Product Composition

Niaz Bahar Chowdhury, et al., JOS, Vol. 2, No. 4, pp. 179-182, 2012

182

f.

acid. In absence of that, the unit operation named ‘mixer’ is used as a circulation tank. To avoid complexity, all cooling actions are done by simple coolers instead of shell and tube heat exchanges or air coolers.

7. Conclusion By doing this simulation project, the main features of industrial production of sulfuric acid were represented in a Process Flow Diagram. Satisfactory results are obtained in optimizing the process, keeping in mind the fact that the profit maximization is done in a rather simple way. On the whole, using this simulation approach will be helpful for the process plant to optimize the annual profit.

Acknowledgements Figure8. Product Composition after Optimization

5. Limitations a.

b.

c.

d.

e.

In our simulation, all reactions are considered as conversion reactions, though they are actually equilibrium reactions. This is done because sufficient data for equilibrium type of reaction in Hysys were not available. In practice, solid sulfur is the raw material input to the process, which is consequently melted in a sulfur melter. But, in Hysys, there is no such unit operation as sulfur melter. As a result, liquid sulfur is directly fed as the raw material for the process. In practice, one 4-pass converter or two 2pass converters in series are used to convert sulfur dioxide into sulfur trioxide. But in Hysys, there is no provision for 4-pass or 2pass converter. So four single pass converters in series are used in the simulation [4] In Hysys, there is no absorber where reaction can take place. This type of unit process is needed to convert sulfur trioxide to sulfuric acid. In absence of that, conversion reactor is used as an absorption tower. In Hysys, there is no circulation tank where mixing action take place without flashing. This type of unit operation is needed to convert 98.5% sulfuric acid to 98.1% sulfuric

We express our gratitude to Wata Chemicals Limited for providing some basic information on Sulfuric acid plant

References [1] W. G. Davenport and M. J. King, Sulfuric Acid Manufacture: Analysis, Control and Optimization, 3rd edition, Elsevier, New York, 2006, pp 33-38 [2] www.aspentech.com [3] George T. Austin, Shreve’s Chemical Process Industries, 5th Edition, McGraw-Hill, New York, 2008, pp 320-345 [4] W. W. Duecker, and J. R. West, The Manufacture of Sulfuric Acid, Reinhold Publishing Corporation, New York, 1966, pp 167-178

Vitae Mr. Niaz Bahar Chowdhury was born in Chittagong, Bangladesh. He obtained a B. Sc degree in 2012 in Chemical Engineering department from Bangladesh University of Engineering in Technology. He worked as a Research Assistant in the above department. His research interest includes LPG, Process Engineering, Coal Gasification, and Thermal Engineering.