University of Maiduguri Faculty of Engineering Seminar Series Volume 6, december 2015

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1 University of Maiduguri Faculty of Engineering Seminar Series Volume 6, december 2015 PINCH ANALYSIS OF BENZENE PRODUCTION PROCESS VIA THE HYDRODEALKYLATION OF TOLUENE I. M. Idriss*, H. I. Mohammed and B. K. Highina Department of Chemical Engineering, University of Maiduguri, Nigeria Abstract Chemical process industry has been facing increasing cost of energy, growing competition driven by rapid globalization, rising public concern for the environment, and increasing regulatory efforts of governments concerning the environment, health, and safety. In response to these conditions and with regard to the capital intensive nature of chemical process industries, constant optimization through redesign of existing production plants has emerged as a key strategy. In this study a typical process plant for production of benzene via hydrodealkylation of toluene is considered. Pinch analysis was carried out, energy flows and requirements were determined. Composite curve was generated. The results of the study show that the net temperature rise required by the heat exchangers collectively is 675 o C and the temperature of process streams collectively was in excess by o C which shows energy is wasted in the plant. Keywords: Composite curve, Heat Exchangers, hydrodealkylation of toluene, Pinch Analysis. 1.0 Introduction The increasing cost of energy and stringent environmental regulations force chemical process industries to search for possible ways of reducing energy consumption for its operations (Piagbo and Dagde, 201). Energy saving in plant design has essentially been a trial-and-error procedure between changes in structure and simulation until satisfactory reductions are achieved. A step by step system-oriented, thermodynamicsbased, integrated approach to the analysis, synthesis and retrofit of process plants is desired. Over the past two decades, the design tools of process integration has been developed to achieve process improvement, conservation in mass and energy resources, productivity enhancement, and reductions in the operating and capital costs of chemical processes using pinch analysis which is a very well recognized and proven technique for analysis of hydrogen, water and energy usage in industries such as chemicals, petrochemical, pulp and paper, oil refining and steel and metallurgy. The use of capable software is usually applied for analyzing large amount of data contained in pinch analysis. The application of software in the analysis offers many advantages like speedy design and modification of heat exchanger networks, eliminates avoidable errors from manual calculations, increase accuracy of results among others. Process revamp came into existence as the result of quest for energy savings and efficiency in production process. Retrofitting includes division of a process system into three subsystems: separation, heat exchange, and utility subsystems for proper heat integration within each subsystem and subsequent coordination between each subsystem. To carry out retrofitting of a process system, firstly there is need to perform Seminar Series Volume 6, 2015 Page 7

2 heat exchanger network (HEN) analysis starting by finding out the Pinch temperature of the HEN according to the relative position of the endothermic and exothermic temperatures with the Pinch temperature, different retrofit strategies for separators will be used (Feng and Liang, 201). 2.0 Process Integration To describe the systematic activities related to the design of chemical processes a holistic design of complete production system through the analysis and linkages of individual units to the overall system called process integration is used (Gundersen, 2000). Therefore, process integration applies the design, optimization and operational optimization of chemical and biochemical processes with strong emphasis on effective energy utilization and adherence to environmental laws. Hence, four major areas are identified as the pillars of process integration-efficiency: the use of raw materials, energy efficiency, minimization of waste emissions and good process operations (Hallale, 2001). Pinch analysis is defined as a systematic technique used in analyzing heat flow in an industrial process by maximizing the utilization of hot and cold utilities available within the process, this therefore reduces the use of external utilities based on fundamental principles and knowledge of thermodynamics, as the second law of thermodynamics requires that heat flows naturally from hot to cold body objects (Rossiter, 2010; Deepa and Ravishankar, 201). The principal objective of the pinch technology is to match cold and hot process streams with a network of exchangers, so that demands for externally supplied utilities are minimized. Therefore the starting point for the analysis is to identify in the process of interest, the process streams that requires heating and those that are for cooling. Identifying the streams flow rates and thermal properties, phase changes and the range of heating and cooling temperature is very essential in the analysis. The method is also used to counter-current heat exchange between the hot streams to be cooled down and cooled streams to be heated up in order to produce energy in the system. The judicious use of hot and cold streams within a given process with the aim of reducing or avoiding the use of external utilities is termed pinch analysis. It is method of minimizing energy consumption of a chemical process plant (Deepa and Ravishankar, 201). Pinch technology proffers systematic methods of energy analysis for energy savings in processes to total sites based on the principles of thermodynamics. The role of pinch technology in the overall process design cannot be over emphasized (Linnhoff,1998)..0 Materials and Method The material used in this study is the design of Production of Benzene via the Hydrodealkylation. Figure 1 shows the process flow diagram (PFD) of the plant (Turton et al. 2012). Aspen Energy Analyzer was used as the tool for the analysis..1 Process Description The production of Benzene via the hydrodealkylation is a process whereby Toluene and Hydrogen are charge into a furnace where they are been heated before Seminar Series, Volume 6, 2015 Page 74

3 being fed into a reactor. The reactor product is cooled and the unreacted toluene and hydrogen recycled back to the reactor while the end product is channeled to the separator, the unit that separates the benzene from a flue gas. Equation 1 shows the reaction of the process. Figure 1: Process Flow Diagram for the production of benzene via Hydrodealkylation (Turton et al. 2012). By observing the PFD of the process we can highlight a significant enthalpic content of outlet stream from the reactor. Such stream is quenched to block any further side conversion of toluene to biphenyl and its enthalpic content is exploited to preheat the inlet stream to the reaction section (although it is not sufficient and some further heating is provided in the furnace to comply with the process specification). Table 1: Cold and Streams data for production of benzene via Hydrodealkylation (Turton et al. 2012). Stream Temp Hydrogen(kmol/h) Methane(kmol/h) Benzene(kmol/h) Toluene(kmol/h) Seminar Series, Volume 6, 2015 Page 75

4 Stream Temp Hydrogen(kmol/h) Methane(kmol/h) Benzene(kmol/h) Toluene(kmol/h) Results and Discussion The summary of the results extracted from the work book of aspen energy analyzer is as shown in table 2 and also the composite curve generated is depicted in figure 2. Looking at the composite curve, the results shows that there exists energy difference that gives way for energy integration. Table 2 shows that in one of the heat exchangers the stream is heated from o C to 600 o C, so also in many of the process streams the heating needed is not that much. Figure 2 shows that the maximum temperature required in the process is in excess of 600 o C but not up to 700 o C. However table 2 shows that a higher temperature as 181 o C is been realized by certain hot stream which is wasted without utilization. The difference between the hot and cold composite curves indicates the possibility of heat recovery in the process. Seminar Series, Volume 6, 2015 Page 76

5 Table 2: Data summary of the pinch analysis of the process Heat Exchanger Cold Stream Cool T in Tie d Cold out Tied Stream in Tie out Tie d Load (kj/h) dt Min dt Min Cold E-107 H/M/B/ T T T Fired Heater e E-109 Cooling H/M/B/T T e E-111 Cooling H/M/B/T T e E-108 H/M/B/ T T H/M/B/T T e T E-11 H/M/B/ T T Fired e T2 Heater E-105 H/M/B/ T T T H/M/B/T T e E-114 Cooling B/T T 8.00 T 1.204e E-112 Cooling H/M/B/T 79.0 T 8.00 T 6.141e E-106 H/M/B/ T T H/M/B/T T e T5 E-110 B/T T T H/M/B/T T e Figure : Composite curve of the process 5.0 Conclusion The results show that there is excess heat emitted from the system, this can be seen from table table 2 and the plot in figure 2 as well. This shows that the analysis has established that that some energy streams are wasted. The cumulative temperature requirement of the process streams was 675 o C and the total outlet temperature attained by the entire Seminar Series, Volume 6, 2015 Page 77

6 process was o C, which gives excess temperature of 1912 o C. It can be concluded that the entire process revamp is feasible in order to make some energy savings. References 1. Deepa and Ravishankar (201) Reducing and Cold Utility Requirements for Finishing Column Section Using Pinch Analysis Techniques, International Journal of Engineering Research and Applications. (4), Feng X. and Liang C. (201) Strategy for Total Energy System Retrofit of a Chemical Plant, Chemical Engineering Transactions, 5, Gundersen T. (2000) A Process Integration PRIMER, SINTEF Energy Research Dept. of Thermal Energy and Hydro Power Trondheim, Norway- IEA Tutorial on Process Integration. 4. Hallale N. (2001) Burning Bright Trends in Process Integration, ASPENTECH LTD. 5. Linnhoff M. (1998) Introduction to Pinch Technology, Targeting House Gadbrook Park Northwich, Cheshire CW9 7UZ, England. 6. Piagbo B. K.and Dagde K. K. (201) Heat Exchanger Network Retrofit Design by Eliminating Cross Pinch Heat Exchangers), American Journal of Engineering Research (AJER). 2(), Rossiter A. P. (2010) Improve Energy Efficiency via Heat Integration, American Institute of Chemical Engineers (AlChE). 8. Turton R., Bailie R. C., Whiting W. B., Shaeiwitz J. A. and Bhattacharyya D. (2012) Analysis, Synthesis and Design of Chemical Processes, Fourth Edition, Prentice Hall International Series in the Physical and Chemical Engineering Series. Seminar Series, Volume 6, 2015 Page 78