Design of Heat Transfer for a Chemical Reactor - Research Paper Example

DESIGN OF HEAT TRANSFER FOR A CHEMICAL REACTOR Module Project Group no Group members' s: 299172 Design of Heat Transfer for a Chemical Reactor Abstract The present fluidized-bed reactors are not designed to perfection hence runs inefficiently giving poor results. This calls for the needs of coming up with accurate design parameters for the fluidized -bed reactor which can achieve maximum efficiency in operations. This research therefore entails the design of the process flow diagram and subsequent mass and energy calculations as well as designing appropriate shall-and tube heat exchanger. The piping and instrumentation (P&I) diagrams are also encompassed in the research report. The instrumentation part includes putting in place the correct system such as closed loop control system, sensors and pneumatic valves for control of flow rate and temperatures. Acknowledgment Much appreciation goes out to all the persons responsible in several ways for the success of this research, mostly to those who have made me gain much more than what the scholastic aspects of the course could have accorded. Much gratitude also goes to my lecturer for the basic knowledge he has provided in class. Table of Contents Page no Title .........1 Abstract.................... 2 Acknowledgement....2 Table of contents..... 3 Table of figures .......4 Chapter one Introduction..............3 Background Study.......3 Problem Statement...5 Justification..........6 Objectives......... 6 Chapter two Literature Review......6 Chapter three Research Methodology.......7 Reactor Section........ 7 Mass and energy calculations............8 Selection and design of appropriate shall and tube exchanger .........9 Instrumentation and control system for the heat exchanger.16 Chapter Four Discussions.......18 Conclusion.........18 Recommendations....18 Table of Figures Fig1. Piping and Instrumentation diagram8 Fig 2. Process flow diagram..17 Chapter One Introduction Background Study Production of Acrylic acid starts with propylene being partially oxidized in a fluidized -bed reactor. Propylene is broken down to acrolein in the process of getting catalytic gas at an oxidation stage. This takes place in presence of oxygen gas. The fluidized-bed reactor includes a packed-bed with a stirred tank that creates continuous flow reactors. It must posses some good characteristics of both heat and mass transfers. Substrates moves upwards through the bed which is immobilized with enzyme at high velocity which moves the particles up leading to though mixing. The reactor is normally used in highly exothermic reactions because it clears local hot-spots simply because of the mass and heat characteristics the fluidized -bed reactor has. The substrates are normally catalytic material where chemical reactants are given support. After achieving some optimum speed the reactor gets into a stage where the force of the fluid in the solids becomes enough to make a balance between the fluid and the solid materials. The contents of the bed-reactor begin to expand and swirl about in a manner equivalent to a agitated tank or a boiling pot of water after passing this level. This is where the reactor becomes a fluidized bed (Coulson, Richardson & Sinnott,1993). Pressure and temperature conditions in the process changes regarding the mode of reaction going on. Putting the fluidized-bed reactor in the higher section with higher temperatures and pressure than other areas provides good results since it promotes the needed reaction. The vapor velocity influences the rate of circulation for the catalyst. The velocity for the catalyst in inversely promotional to the speed of vapor. With the specific variables of the reaction process, a particular duration is needed to create stability in the entire reaction process and hence get a constant pressure drop through the fluidized-bed reactor. The reactor normally runs isothermally at some specified temperature. This leads to exothermic reaction which produced some heat at a certain rate. However the reactor has to be maintained at some isothermal conditions by extracting the reaction heat. This is achieved by moving around some heat-transfer fluid via a system of heat transfer in the reactor. It fluid gets into the reactor at some certain temperature and gets out at a higher temperature since it absorbs some heat from the reactor. A suitable shall and tube heat exchanger is used to cool the fluid back to the initial temperature before it is pumped back to the heat exchanger. Water is usually used here as the cooling medium (Peters& Timmerhaus, 1980). Problem Statement Current heat transfer systems for fluidized-bed reactors have not taken into account the necessary design and control parameters to ensure that the optimum temperatures, pressure, mass flow rate and energy balances operates efficiently. This has led to inefficiencies. Using the wrong flow rate and isothermal conditions significance fluidized-bed reactor means getting the wrong results for the end product. There is therefore the need to focus at the designing of a more accurate heat transfer system for the reactor as well as he shall and tube heat exchangers. Hypothesis Accurate design and instrumentation of heat transfer system for the fluidized-bed reactor which has all the necessary variables and control parameters would overcome the problem of inefficiency with the current reactors. Objectives Main objective is to design a Heat Transfer system for a fluidized-bed Reactor. Specific Objectives include designing of a suitable shall and tube heat exchanger, carrying out mass and energy equations and draw process flow and P& I diagrams. Literature Review Fluidized bed reactors have been min use of late in chemical engineering. The first reactor was put in use in 1920s by German Fritz Winkler. He came up with a fluidized bed gas generator. The Catalytic cracking nit was the first ever fluidized-bed reactor to be used in the US by Standard Oil Company now called Exxon Mobil. It was made for petrochemical and oil industries mainly to break the petroleum compounds into much more les complicated compounds via the process of cracking. This innovation resulted to higher levels of production of several fuels in the United States of America. Most recent reactors are used in gasoline and chemicals production of rubber, vinyl chloride and polyethylene. They are also applied in nuclear power plants and coal gasification in addition to waste water treatment plants. The fluidized-bed reactor has the merit of producing more cleaner and efficient process than most of he older processes. Most of these reactors are however not designed to perfection thus operate inefficiently and not much has been done to increase the efficiency of the reactor though creating a heat transfer system with control parameters to overcome its inefficiencies. The accurate design of the heat transfer system for the fluidized-bed reactor would therefore be of much significance in solving of these problems. Research Methodology The Reaction Process This is where the acrylic acid is manufactured. The reactor being is run isothermally at 310o C. The reaction produces an exothermic reaction that gives out heat energy at a rate of 3600000W. To maintain it at the isothermal conditions the heat of reaction has to be extracted by moving around some molten say usually HiTec TM. It is used as a fluid for heat transfer through the system and gets into the reactor at 200o c and leaves at 250o C. It enters the reactor shall and tube where it is cooled down to 200o c and back to the reactor in a closed loop control system. Water is used as the cooling medium at 20o C (Howard,1989). Fig.1 The process flow diagram Process reaction equations (1) Propylene acrylic acid water Other reactions: (2) Propylene Acrylic acid Mass and energy balance equations Determining the flow rate of HiTecTM ; (i) From this the rate of mass flow can be obtained by; Given that; Q=3600000W, T=250-200 Therefore in kg/h =30.8 kg/hr Thus from this equation the flow rate of HiTecTM is calculated as 30.8 kg/hr. This equation is also used in determining the flow rate of water in the shall and tube heat exchanger but the change of temperature of water and specific heat capacity (Cp) of water are employed instead. Making the assumptions that there is no heat lost in the fluidized bed reactor and the heat exchanger, the value of heat loss would be equivalent to calculating the heat loss from the HiTecTM (Tomas,1999). energy balance between the salt and the water would be equal to; = Q=3.6W= Therefore ==4.72kg/hr This means that the amount of water used is so small that it does have big temperature change. Design of shall and tube heat exchanger Finding the number of transfer units; Given Q=3600000KJ/h, U= coefficient of the overall heat transfer given to be =200w/m2oC A=Area of heat transfer T=Mean temperature With assumption of a counter current flow, =.. (iii) ==204oC From this, the Total Area=3600000/ (200x204) =88.24m2 With this equation the number of tubes can be calculated; =88.24/0.7665 =115.12=155 tubes The coefficient of heat transfer inside the tube, =1.57m/s This velocity can be used to calculate the Reynolds number Reynolds number (Re) = =813.5 x 1.57 x 0.0468/0.000222 =269246.51 Prandtl Number (pr) =.. (ix) ==5.46 Substituting the Re and Pr values in the dittus-boelter correlation: 0.023 x 269246.515x 5.460.4=856.0064 Hence, Convective heat transfer coefficient inside the shell, Volumetric flow rate= Mass flow rate of water, is calculated at Density of water=993.15 Volumetric flow rate therefore=57.39/993.15=0.0578m3/hr From this, velocity can be obtained by; V=Volumetric flow rate/cross flow area.(xi) Cross flow area Given Ds=Shell Diameter Pt=Tube Pith Baffle spacing Bundle diameter Given; Nt =tubes number Do =Outside diameter of tube K1 =0.156 (constant) n1 = 2.291 (constant) Therefore Bundle diameter for one tube (Db) = 0.05 =>Ds=Db+ Bundle Diametrical Clearance(xiv) =0.682+0.059=0.741m Baffle spacing =Ds x 0.5=0.370m Tube pith (Pt) =1.25 Do..(xv) =1.25 x 0.05=0.0625m => Cross flow area (As)= =0.055m Velocity= 57.39/(993.15 x 0.055)=1.051m/s The equivalent diameter; =0.0495m From this value we can determine the Reynolds number 993.15 x 1.015 x 0.0495/0.0007745=66711.59739 Prandtl number= Using this figures in dittus-boelter correlation (the viscosity correction is neglected) => Specifications for Tubes - Material used= Cupro-Nickel Inside Diameter = 0.05 m Outside Diameter = 0.0468 m Thickness of the tube = 0.0016 m Length of tube= 4.88 m Total no. of Tubes = 115 Bundle Type = Pitch Square Arrangement of the Bundle = Floating Head for Split-Ring Diameter of the Bundle = 0.735 m Shell - Material = Stainless Steel Diameter of the Shell= 0.865 m Clearance = 0.054 m Baffle Spacing % = 48.9 % Baffle Spacing = 0.389 m Baffle Cut % = 27 % Dimensions Total Area = 47.94 m2 Instrumentation and Control system for the heat exchanger To control the whole system flow a suitable motor is required. It must have sufficient force to move the salt via the heat exchanger and the reactor. Sensors would be needed to check on the speed of the motor which is started manually. There is a control panel room where the button switch is placed. The sensors would help in sensing the temperatures of the HiTec salt and the results indicated in the control panel which has a seven segment indication reading. To control the relative motion of the fluid in the pipe to the desired velocity, there has to be a flow rate sensor that detects the volumetric flow rate of the salt. In case this changes it has to alert the control panel. The sensor has some features that helps it to trigger a valve which is being ran pneumatically to automatically adjust the rate of flow of the HiTec salt. To ensure precise if not accurate control of the salt passing via the exchanger the closed loop control system is used. This also check whether the salt is cooled o the required temperatures before being pumped back to the reactor. It must be control to run at a temperature of 200o C at the shall and tube heat exchanger (Callahan & Miller, 2009). Fig. 2. Piping and Instrumentation diagram The flow rate of the water I also regulated. It temperature, pressure and mass flow rate has to be checked through out. This is achieved through the use of pressure and temperature sensors which must not be in closed loop controlled. Incase more water is to be allowed in this would be done by the flow rate control valve of the water as well as its sensors. In case of faults such as water temperature rising, the temperature alarm is triggered and sends the message to the control panel where action is taken immediately. This can be overcome by increasing the flow rate of the water by making slight adjustments to the pneumatic valve. If much failure occurs, the heat exchanger should have installed watch glass for checking whether the cooling water contains impurities and as well assist the technician to find the leakages. Discussion According to research by American Chemical Society (2007), the accurate design of the heat transfer system for the reactor would have the following merits that would increase the efficiency of the system and its use in the chemical engineering technology. Uniform temperature gradients and appropriate mass flow rates; Many reactors require hat addition and removal to avoid creation of cold spot or local hot spot in the reactor that results to degradation of the end product. The accurately designed and controlled heat transfer system would see to it that these challenges are overcome since it would be automatically operated. This would also mean doing away with axial and radial gradients concentration for good interaction between the solids and the fluid. Conclusion Accurate design and instrumentation of a transfer system for the reactor would help overcome most the problems of inefficiencies experienced by current systems hence increase its productivity in the industrial application. Recommendations More research is required in this technology to study more on the characteristics of interaction stages in the reaction bed and also come up with standard design parameters concerning temperature, pressure and mass flow rates of the heat exchanger. References American Chemical Society. (2007). Industrial Advances. (Online). Available at http://acswebcontent.acs.org/landmarks/industrial_tl.html (Accessed 18 May 2009). Callahan, J. L. & Miller, A. F. (2009). Combination fixed-fluid bed reactor. (Online). Available at http://www.che.cemr.wvu.edu/publications/projects/acrylic/acrylic-c.PDF (Accessed 18 May 2009). Tomas B. C. (1999). Pipe and Instrumentation Diagram (Online). Available at http://ocw.kfupm.edu.sa/user/SE43901/P&I_diagram.pdf(Accessed 18 May 2009). Coulson, J. M., Richardson, J. F. & Sinnott, R. K. (1993). Chemical Engineering: Design. Pergamon Press Ltd, Oxford. Howard, J. R. (1989).Fluidized Bed Technology: Principles and Applications. New York, NY: Adam Higler. Peters, M. S., & Timmerhaus, K. D. (1980). Plant Design and Economics for Chemical Engineers. McGraw-Hill: New York. Read More