Transcript
Table of Contents
Introduction 3
Theoretical Background 4
Experimental/Simulation Procedure 6
Results and Discussion 8
Figure 1: HYSIS Simulation of dehydrogenation of n-heptane without recycle operator 8
Figure 2: HYSIS Simulation of dehydrogenation of n-heptane with recycle operator 8
Table 1: Comparison of Energy Streams 9
Conclusion and Recommendations 10
References 11
Appendices 11
Raw Data 11
Sample Calculations 11
Introduction
Aspen HYSYS is an easy-to-use process modeling environment that enables optimization of conceptual design and operations Aspen HYSYS is used for simulations of processes to act upon situations in a real manner. Designing processes or setting parameters is costly due to downtime or capital costs. HYSYS is used to simulate a run-through without a negative impact. It is more efficient to predict certain situations therefore allowing for engineers to tweak and alter parameters. The purposes of this experiment is to learn to install and converge: reaction and reactors and install a separator with a recycle line using logical operations.
The objective of this experiment is to produce toluene by modelling an acyclic process for dehydrogenation of n-heptane using a Cr2O3 catalyst. Two approaches were done in the modelling of an Acyclic Process. First approach was to superheat the product stream, feed it to a catalytic reactor, then allowed to be cooled for later separation. The second approach was to very similar to the first but with the installation of a recycle operator. This utilizes the energy from the product stream by running it through the shell of the heat exchanger for high energy efficiency.
According to the recorded data, the modelling of the acyclic process of dehydrogenating of n-heptane fared well, with the recycle line reducing the energy from the heater from 5.567x106 Btu/hr to 1.429x106 Btu/hr and from the cooler from 5.917x106 Btu/hr to 1.479x106 Btu/hr. In summary, a more energy efficient system can be modelled for the acyclic process using a recycle line for the product stream.
Theoretical Background
Chemical engineering is a branch of engineering that applies sciences and mathematics, together with economics to produce, transform, transport, and properly use chemicals, materials and energy. It essentially deals with the engineering of chemicals, energy and the processes that create and/or convert them. Chemical reactors are vessels where chemical reactions take place. Engineers design these reactors to ensure that the reaction proceeds to the highest efficiency towards the desired product, producing the highest yield while requiring the least amount of total operating cost.
In this experiment, Toluene is being produced by the hydrogenation over a Cr2O3 catalyst absorbed on Al2O3 by the following reaction:
C7H16 ?C6H5CH3+4H2
Toluene is an aromatic hydrocarbon consisting of a methyl group attached to a benzene ring. It is a volatile liquid that is highly flammable explosive. In order to calculate the stream compositions for different temperatures and pressures, Pen-Robinson equation of state is used shown by the following:
P=RTVm-b-aVm2+2Vmb-b2
a=0.457236?R2Tc2Pc
b=0.0777961RTcPc
?=(1+0.37464+1.54226?-0.26992?21-TTC2
(Nasir,2009)
In the HYSIS program, it is used to calculate the changes in composition as a process stream flows from one unit operation to the next and gets heated, cooled then separated.
Experimental/Simulation Procedure
Part A: Without the Recycle
Define components in the simulation; n-heptane, toluene, hydrogen
Add Super heater with inlet feed from initial n-heptane feed, R-feed and H-duty as energy stream.
Pressure was set for Super Heater and Temperature of the R-feed
Catalytic reactor was added with the inlet feed from the R-feed, outlet feeds Reactor Vapor and Reactor Liquid and with an R-duty energy stream
Temperature and pressures were set for the reactor and streams
Reaction type was defined with stoichiometric coefficients and conversations
Cooler was added with inlet feed from Reactor vapor and outlet feed for the S-feed and C-duty energy stream. Temperatures and pressures were set for reactor streams
Separator was added, separating S-feed to outlet feeds for liquid outlet, LIQ, and vapor outlet, VAP, feeds
Reaction was set and all calculated properties of each fluid were printed out in Workbook.
Part B: With a Recycle Stream
HYSIS process in part A was rearranged for better visual configuration
Heat Exchanger was added and placed before Superheater with initial n-heptane feed passing through one tube shell.
Outlet stream is then passed through the Superheater before being fed to the catalytic reactor.
Temperature and pressure was set for heat exchanger
Recycle Operator was added and placed over Catalytic Reactor to recycle reactor vapor back into heat exchanger
Recyc stream is then added to the other inlet of the Heat Exchanger shell to be feed to the cooler, then to the Separator.
Results and Discussion
Figure 1: HYSIS Simulation of dehydrogenation of n-heptane without recycle operator
Figure 2: HYSIS Simulation of dehydrogenation of n-heptane with recycle operator
Table 1: Comparison of Energy Streams
Heat Flow (Btu/hr)
Simulation without Recycle Operator
Simulation with Recycle Operator
H-Duty
5.867x106 Btu/hr
1.429x106 Btu/hr
C-duty
5.917x106 Btu/hr
1.47x106 Btu/hr
R-duty
1.627x106 Btu/hr
1.627x106 Btu/hr
Without the recycle stream, the feed (65°F) does through a super heater before it enters the catalytic reactor (800°F). Once the feed has reacted within the reactor, the flow has to be cooled back down to (65°F) to allow for proper separation. Knowing that the feeding streams needed to be first heated and then cooled again for separation, it is ideal to place a Recycle Operator to utilize the heat from the feed going into the reactor to reduce the overall heat needed to heat the other inlet streams. The increase of efficiency can be attributed to the recycle stream where the hot gas is used to heat up the product simultaneously cooling it before it enters the separator. In this case, we can use the heat exchanger to cool the gas then pass it through a cooler which will require less consumption of energy.
When Comparing the two system, from the results shown in Table 1, it shows that the original process that went without the recycle operator was very inefficient when compared to the process that had the recycle operator. In the simulation diagram, the energy streams coming from the superheater is 5.867x106 Btu/hr, the energy stream coming from the reactor is 1.627x106 Btu/hr and finally the energy going into the cooler is 5.917x106 Btu/hr.
By adding a recycle stream, as well as the heat exchanger, energy streams are greatly lowered, with H-duty being 1.429x106 Btu/hr, C-duty being 1.47x106 Btu/hr and R-duty being 1.627x106 Btu/hr. This shows that the presence of the recycle operator and the heat exchanger increases the efficiency of the overall system, by about 4 times, as the process without the recycle stream. It is clear to see that from this simulation, it can be very beneficial to re-use heats from fluid streams to substantially lower energy consumption of the system and therefore decreasing the overall operating cost.
Conclusion
In conclusion, the overall experiment was quite successful. The objective, which was to produce toluene from N-heptane using HYSIS modelling of an Acyclic Process, was achieve in two approaches. The first approach was a simulation without a recycle component, and second was a simulation with one. As both simulations resulted in the production of toluene with the same amount of molar flow rate, the data also shows that the presence of a recycle feed, and heat exchanger and greatly affect energy consumption. As the results show, the simulation with the recycle component yield lower energy consumption, which results to higher energy efficient system.
References
Dhib, R. (Fall 2008). Process Control Laboratory Manual. Toronto: Ryerson University.
Perry, H.H., and D. Green, Perry’s Chemical Engineers’ Handbook, 6th Ed., McGraw-Hill Inc., New York, 1984
Nasri, Z., & Binous, H. (2009). Applications of the Peng-Robinson Equation of State using MATLAB.43(2). Retrieved from http://cache.org/site/news_stand/summer09/summer09 Binous applic of equation.pdf
Seader, J.D. and E.J. Henley, Separation Process Principles, John Wiley & Sons, 1998
