An Integrating Case Studyammonia Synthesis

This book briefly reviews ammonia synthesis, its importance in the chemical process industry, and safety precautions. This case study is integrated into several chapters in the text. See the Introduction for further details. Additionally, solutions to problems are presented in the text and the accompanying CD contains computer programs (Microsoft Excel spreadsheet and software) for solving modeling problems using numerical methods. The CD also contains colored snapshots on computational fluid...

Safety In Chemical Reaction

Equipment failures or operator errors often cause increases in process pressures beyond safe levels. A high increase in pressure may exceed the set pressure in pipelines and process vessels, resulting in equipment rupture and causing major releases of toxic or flammable chemicals. A proper control system or installation of relief systems can prevent excessive pressures from developing. The relief system consists of the relief device and the associated downstream process equipment (e.g.,...

Jvs

(1 - XA )(0B - 3XA ) 1 - XA 0B - 3X 1 - A(0b - 3XA )+B(1 - XA ) Equating the coefficients of the constant and XA gives Const 1 A0b + B (3-268) Adding simultaneous Equations 3-268 and 3-269 gives 1 A(0B - 3). Therefore, A 1 (0B - 3) and B -3A -3 (0B - 3). Equation 3267 is now expressed as Cao I 0 ( B -3)(1 -Xa) 0 (0B -3)(0b -3Xa)

The Dispersion Model

Plug Flow Reactor With Axial Dispersion

The design of chemical flow reactors depends on the overall rate at which the reaction proceeds, and the extent of backmixing occurring in the reactors. The first factor relates to the determination of the rate equations as reviewed in Chapter 2. The second factor, backmixing or dispersion, is used to represent the combined action of all phenomena, namely molecular diffusion, turbulent mixing, and non-uniform velocities, which give rise to a distribution of residence times in the reactor. The...

Fluidized Bed System

Fluid Catalytic Cracking Unit Images

The fluidized bed in Figure 4-10 is another common type of catalytic reactor. The fluidized bed is analogous to the CFSTR in that its contents though heterogeneous are well mixed, resulting in an even temperature distribution throughout the bed. In a fluidized bed reactor, the solid material in the form of fine particles is contained in a vertical cylindrical vessel. The fluid stream Figure 4-10. Fluid catalytic cracking unit. Figure 4-10. Fluid catalytic cracking unit. is passed up through the...

Residence Time Distribution In A Static Mixer

Static mixers, as reviewed in Chapter 7, contain mixing elements enclosed in a tubular housing through which radial mixing is achieved. They redistribute fluid across the flow channel and consequently rearrange temperature and composition distributions. They are often used to promote mixing in laminar flow systems thus having a pronounced effect on the RTD. A major unknown variable in reactor design is the scale of imperfection in the flow pattern. This is caused by various factors such as the...

Vessel Design And Aspect Ratio

Stirring Design Reactors

A vertical cylindrical, and mechanical agitated pressure vessel, equipped with baffles to prevent vortex formation is the most widely used fermenter configuration. The baffles are typically one-tenth of the fermenter diameter in width, and are welded to supports that extend from the sidewall. A small space between the sidewall and the baffle enables cleaning. Internal heat transfer tube bundles can also be used as baffles. The vessels must withstand a 45 psig internal pressure and full vacuum...

Dynamic Behavior Of Ideal Systems

The review of the performance equations for the ideal system has been for the steady state situation. This occurs when the process has begun and all transient conditions have died out (that is, no parameters vary with time). In all flow reactors, parameters such as the flowrate, temperature, and feed composition can vary with time at the beginning of the process. It is important for designers to review this situation with respect to fluctuating conditions and the overall control and Figure...

Effect Of Micromixing On Conversion

Mixing System Reactors

As stated earlier, the actual state of micromixing lies between two extremes. However, further details are extremely difficult to use either theoretically or experimentally. Kramers and Westerterp 2 gave results for the extreme case of a perfectly macromixed reactor with various reactions occurring. Figures 9-8 and 9-9 illustrate a comparison between the segregated flow and complete mixing for n 0 and n > 1, respectively. It can be seen in Figure 9-9 that the differences between Figure 9-8....

Twodimensional Tubular Plug Flow Reactor

Momentum Balance Plug Flow Reactor

Consider a reaction A products in a tubular reactor with heat exchanged between the reactor and the surroundings. If the reaction is exothermic and heat is removed at the walls, a radial temperature gradient occurs because the temperature at this point is greater than at any other radial position. Reactants are readily consumed at the center resulting in a steep transverse concentration gradient. The reactant diffuses toward the tube axis with a corresponding outward flow of the products. The...

Scaleup Of Bioreactors

Reactors Instrument Diagram

Fermenters are generally designed to provide more oxygen (i.e., for a higher oxygen transfer rate OTR) than is required at the peak oxygen load of the organism. This usually occurs during the log or exponential growth phase. The oxygen uptake rate (OUR) value of a specific fermentation system is required to design the system, and these requirements are different for each microorganism. Employing the OTR criterion, a high respiration rate of the organism enables successful scale-up of aerobic...

Differential Reactor

The differential reactor is used to evaluate the reaction rate as a function of concentration for a heterogeneous system. It consists of a tube that contains a small amount of catalyst as shown schematically in Figure 4-17. The conversion of the reactants in the bed is extremely small due to the small amount of catalyst used, as is the change in reactant concentration through the bed. The result is that the reactant concentration through the reactor is constant and nearly equal to the Criteria...

Vent Sizing Package

The vent sizing package (VSP) was developed by Fauskes & Associates, Inc. The VSP and its latest version VSP2 employ the low thermal mass test cell stainless steel 304 and Hastelloy test cell with a volume of 120 ml contained in a 4-l, high-pressure vessel as shown in Figure 12-13. The typical -factor is 1.05-1.08 for a test cell wall thickness of 0.127-0.178 mm. Measurements consist of sample temperature T1 and pressure P1, and external guard temperature T2 and Figure 12-13. Vent sizing...

Case Study

Refrigeration Trucks Temperatuge Table

As an introduction to the modeling of chemical kinetics and reactor design, consider the manufacture of ammonia. The synthesis of ammonia is performed on a large scale with over 100 million tons produced each year. Computer simulation of the plant is increasingly employed as the first stage in identifying which parameters control the conversion rate, the product purity, the energy expended, and the production rate. The economic considerations that affect the reduction of costs with increased...

Simular Reaction Calorimeter

Many chemical reactions in the CPI are performed in a semi-batch process under isothermal conditions. The DIERS methodology in sizing the relief system for runaway reactions is based on adiabatic conditions using the batch process. There are instances where a semi-batch process under isothermal condition is advantageous in significantly reducing the vent size or even possibly eliminating runaway reaction and the need for pressure relief. Singh 20 performed studies involving the esterification...

Equal Size Cfstr In Series

Cfstr Tank

Consider a system of N continuous flow stirred tank reactors in series as shown in Figure 5-24. Although the concentration is uniform from one tank to another, there is a change in concentration as the fluid traverses between the CFSTRs. This is illustrated in Figure 5-25. The drop in concentration implies that the larger the number of CFSTRs in series, the closer the system would behave as plug flow. Figure 5-24. System of N equal size CFSTRs in series. Figure 5-24. System of N equal size...

Conversion In Ammonia Synthesis

Autothermal Reactor

In reviewing thermodynamics, the nature of the heat of reaction and whether the reaction is reversible can be investigated. In the synthesis of ammonia, reactors operate at about 200 atm and 350oC and approach the equilibrium conversion of about 70 in each pass. The ammonia is separated from unreacted H2 and N2, and these are recycled back to the reactor. The overall process of a tubular reactor plus separation and recycle produces essentially 100 NH3 conversion. Water-gas shift CO H2O CO2 H2...

Nonlinear Regression Using The Solver

In this chapter, the Microsoft Solver Excel's powerful optimization package is used to perform non-linear least squares curve fitting. Procedures for using the Solver are illustrated in Appendix B. The Solver provides results that are comparable with those from the commercial software packages. The Solver determines the sets of least-squares regression coefficients very quickly and efficiently. The slight differences between the Solver and commercial software packages arise from the fact that...

A

Plug Flow Reactor Design Equation

K 1 - Xa Cbo - CaoXa 5-189 With 0B CBO CAO Equation 5-189 becomes X4 Equation 5-190 enables the residence time to be determined for a given conversion of A and the initial concentrations of A and B in a CFSTR. Figure 5-22 illustrates the graphical performance equations for CFSTRs. A continuous flow stirred tank reactor is used to decompose a dilute solution of species A. The decomposition is irreversible and first order with velocity constant at 2.5 hr-1. The reactor volume is 15 m3. What...

Continuous Multiphase Reactors

Continuous Reactor

Figure 4-8 shows a continuous reactor used for bubbling gaseous reactants through a liquid catalyst. This reactor allows for close temperature control. The fixed-bed packed-bed reactor is a tubular reactor that is packed with solid catalyst particles. The catalyst of the reactor may be placed in one or more fixed beds i.e., layers across the reactor or may be distributed in a series of parallel long tubes. The latter type of fixed-bed reactor is widely used in industry e.g., ammonia synthesis...

Info

Parallel Reaction Chemistry

Douglas, Conceptual Design of Chemical Processes, McGraw-Hill, Inc., 1988. Source J. M. Douglas, Conceptual Design of Chemical Processes, McGraw-Hill, Inc., 1988. in the generated heat rate. When the rate of heat generation is greater than the rate of heat absorbed, a thermal runaway will occur with disastrous consequences as the incidents at Seveso and Bhopal have shown . A review of these incidents in batch reactors reveals the main causes are Inadequate control systems and...

FC

Methanation Reaction Rate Constant

Schematics of the differential method for data analysis. Figure 3-17. Schematics of the differential method for data analysis. 3. Plot the calculated values as shown in Figure 3-18 to give a straight line of slope k. 4. From experimentally determined values of the integral of Equation 3-226, plot these at corresponding times as shown in Figure 3-18. 5. If the data yield a satisfactory straight line passing through the origin, then the reaction rate equation assumed in step 1 is...

Jc

Micromixing Mixing among molecules of different ages i.e., mixing between macrofluid clumps . Mixing on a scale smaller than the minimum eddy size or minimum striation thickness by molecular diffusion. Mixing, ideal or complete A state of complete uniformity of composition and temperature in a vessel. In a flow system, the residence time distribution is exponential, ranging from zero to infinity. Open end vessel One in which there are no discontinuities abrupt changes in concentration at the...

Principles Of Operation

Static Mixer Reactor

The Kenics static mixer unit is a series of fixed, helical elements enclosed within a tubular housing. The fixed geometric design of the unit produces the following unique patterns of flow division and radial mixing simultaneously. In laminar flow, a processed material divides at the leading edge of each element and follows the channels created by the element shape. At each succeeding element, the two channels are further divided, resulting in an exponential increase in stratification. Figure...

A1E1 1 Xa

Equation 6-173 suggests that for every value of XA, there is a corresponding temperature Topt on the optimum temperature progression OTP. k1 Consider a first order reversible reaction A R, where the rate expression is -rA k1CA - k2CR. Therefore, a1 P1 uR 1. For a feed concentration, Cao 1.0 mol l with no product 0R 0 , using XA 0.485 with the following kinetic parameters gives E1 11,600 cal mol E2 29,600 cal mol R 1.987 cal mol K A1 exp 17.2 A2 exp 41.9 The optimum temperature progression Topt...

Jacket Zoning Of A Batch Reactor

The heat transfer performance of a batch reactor system equipped with a partial pipe jacket can be improved using multiple zoning. This involves dividing the jacket into two or more zones. In this instance, each independent zone receives the heat transfer medium at the cooling or heating supply temperature at the inlets. The outlets from these zones are combined, and the flows of the medium blend to create a stream of some mixture temperature before the combined flow returns to the cooling or...

Stage 2 Shift And Methanation Conversion

Ammonia Synthesis Converter

The shift conversion involves two stages. The first stage employs a high-temperature catalyst, and the second uses a low-temperature catalyst. The shift converters remove the carbon monoxide produced in the reforming stage by converting it to carbon dioxide by the reaction Figure 2. Catalytic reforming flowsheet. Used with permission of Simulation Sciences Inc. Figure 2. Catalytic reforming flowsheet. Used with permission of Simulation Sciences Inc. The reaction produces additional hydrogen for...

Chemical Equilibrium

Consider a single homogeneous phase of one component of unchanging composition. If it undergoes an isothermal reversible change and does work, then from the first law of thermodynamics q dU pdV or dU q - pdV 2-1 From the second law of thermodynamics Combining the first and second laws Equations 2-1 and 2-2 gives Now consider a homogeneous phase containing different substances or components. Its phase contains For a constant composition, it is known that dU TdS -a variable composition, U U S, V,...

Ammonia Process Flow Diagram

Flow Diagram Ammonia Plant

Compressors with interstage cooling. Used with permission of Simulation Sciences Inc. excess of 21 bar are required to achieve sufficient conversion. Conversions of 20 -25 ammonia per pass are achieved. However, the conversion of hydrogen per pass is still less than 30 , therefore, the process requires a large recycle of unreacted gases. The converted vapor product is cooled by ammonia refrigeration in the primary separator to condense the ammonia product. A purge is removed from the...