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Chemical Reactor Concepts

Chemical Reactor Concepts. Problem: Characterize a chemical reactor for the purposes of: 1) Predicting reactor performance 2) Designing the reactor. PURPOSE OF A CHEMICAL REACTOR. The purpose of a chemical reactor is to convert less desirable feed components into

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Chemical Reactor Concepts

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  1. Chemical Reactor Concepts Problem: Characterize a chemical reactor for the purposes of: 1) Predicting reactor performance 2) Designing the reactor Chemical Reactor Concepts

  2. PURPOSE OF A CHEMICAL REACTOR The purpose of a chemical reactor is to convert less desirable feed components into more desirable output components. • Convert a cheap raw material into a more valuable products, e.g., natural gasinto ammonia, ethylene into polyethylene, etc. • Convert hazardous compounds into ones that are environmentally benign, e.g., hydrogen sulfide into sulfur, carbon monoxide into carbon dioxide, PCB’s into CO2, H2O, and HCl. Chemical Reactor Concepts

  3. THE REALITY OF CHEMICAL REACTORS • Chemical reaction engineering is at least as much of an art as it is an exact science, the textbooks to the contrary notwithstanding. • A reliable kinetic model is the last thing to become available in the course of developing a new reactor system • Do not use a kinetic model outside the operating range and conditions for it was developed (Murphy’s Law of Kinetic Models: Almost all kinetic models reported in the literature have been developed for conditions other than those at which you want to operate your reactor.) • The continuous stirred tank (CSTR) and plug flow (PFR) reactor mixing models are idealizations can only be realized in practice through careful design and some expense, if at all. Chemical Reactor Concepts

  4. WHAT WE CAN EXPECT TO KNOW ABOUT A NEW REACTION SYSTEM What we can expect to know about a reaction system is learned in the laboratory and the pilot plant. It generally includes: • The reaction chemistry, at least for the major reactions • The type of reactor used to produce the data and its performance for at least one set of operating conditions: • Feed composition • Operating pressure and temperature • Phase (gas, liquid) • Type of catalyst • Reactor size and geometry • Residence time or space velocity • Conversion and selectivity Chemical Reactor Concepts

  5. WHAT WE PROBABLY WON’T KNOW ABOUT A NEW REACTION SYSTEM • The performance of the reactor over a wide range of feed compositions and operating conditions Result: considerable uncertainty as to how the reactor will respond to perturbations in feed composition and operating conditions • An exact characterization of the mixing regime and the residence time distribution Result: scale up to commercial size reactors may be extremely risky • Unless all of the components are well-known, the heats and free energy changes of reactions must be estimated. Result: potentially significant errors in the reactor energy balance and the approach to chemical equilibrium Get the Picture? Reaction Engineering is not an exact science Chemical Reactor Concepts

  6. MAJOR CONCERNS IN REACTOR CHARACTERIZATION • The Reactor Material Balance, i.e., what selectivity do we get for given operating conditions and conversion? Is this selectivity acceptable? • The Reactor Energy Balance • The preferable mode of operation is adiabatic. Question: What is the adiabatic temperature change across the reactor? Is is small enough to ignore? Rule of thumb: Can ignore it Tadibatic <  25 C. • If not, some means of heating or cooling the reactor must be provided • Boiling reactor (cooling) • Internal heat transfer surface (heating or cooling) • Pump-around heat transfer (heating or cooling) • Diluent (heating or cooling) Chemical Reactor Concepts

  7. Chemical Reactor Concepts

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