Answer to Question #236439 in Chemical Engineering for VijaY

Question

Design of a reactor with given reaction :- hydrodesulfurization reaction to convert benzothiophene into ethylbenzene with a cobalt-molybdenum catalyst supported

on alumina

Answer 1

Enthalpy of Reaction

The heat released in a chemical process is determined by the enthalpies of the constituent chemical reactions, which are given for standard temperature and pressure (1 atm, 25 C). Standard heats of reaction values can be found listed in the literature or derived from temperatures of production or combustion. It is critical to specify the foundation for the heat of reaction and the states of the reactants and products.

Reaction Mechanisms, Rate Equations, and Rate Constants

The major process reaction rate equations and rate constants cannot be anticipated from first principles in most situations and must be approximated (Towler and Sinnott, 2013). This is because of the following:

Using heterogeneous catalysis or enzymes that result in Langmuir-Hinshelwood-Hougen-Watson or Michaelis-Menten kinetics.

Mass transfer between two liquid phases or vapor and liquid

Multistep processes with rate expressions that deviate from overall reaction stoichiometry

Conflicting side effects

As a result, to estimate the residence time necessary for a target conversion, the major process reaction is generally modeled as first- or second-order across a restricted range of process parameters (temperature, pressure, species concentrations). Rate equations are always a good match for experimental data and should thus be utilized for intra-data interpolation. When extrapolating, it is critical to acquire more data, especially for exothermic processes that have the potential for runaway.

The choice of operating circumstances is a significant deciding element in reactor type selection. Optimal process operation generally entails maximizing process yield rather than reactor yield. A target range of yields and selectivities might be selected based on the preliminary economic analysis. To guarantee that goal yields and selectivities are attained, the final reaction conditions must be empirically validated.

Reactions that are chemical or biochemical in nature

If a biochemical process generates the intended product, the circumstances used must ensure the biological agent's survival (e.g., microorganisms or enzymes). Proteins denature when exposed to temperatures and pH levels outside of their particular ranges. Still, live organisms require precise oxygen and other solutes to thrive and cannot endure high shear rates. More information on the design of bioreactors may be found at bioreactors.

Catalyst

A catalyst is used to improve the pace of the reaction by decreasing the activation energy without being wasted in the process. Because the catalyst must sustain activity for a length of time between catalyst regenerations, the usage imposes operating condition limitations. High temperatures, as well as impurities in the feed or recycling streams, might hasten catalyst deactivation.

Temperature Increasing the temperature of the reaction increases the reaction rate, diffusivities, and mass transfer rates. The equilibrium constant is also affected by temperature: higher temperatures raise the equilibrium constant for endothermic reactions while decreasing it for exothermic processes.

Pressure

When selecting the reactor pressure, the key factor in keeping the reaction in the correct phase for the given temperature. The pressure can also be adjusted to allow for vaporization of a component, simpler product separation, changing the reaction equilibrium, or eliminating heat from the reactor. Increasing the pressure for gas-phase processes improves reactant activity and hence the reaction rate. Le Chatelier's principle governs reactor yields: for reactions that increase moles, lower pressure increases equilibrium conversion; for reactions that reduce the number of moles, lower pressure decreases equilibrium conversion. 

Phase of Reaction

Because fluids are simpler to handle, heat and cool, and transport than solids, reactions are often carried out in liquid or gas phases. A suspension in liquid or gas is typically utilized for reagents or products in the solid phase. The temperature and pressure of the reactor generally influence the phase of the reaction. Because of the highest concentrations and maximum compactness, the liquid-phase operation is typically favored. However, there can be no liquid phase at temperatures above the critical point. Sometimes the pressure may be regulated to maintain all chemicals in the liquid phase, but a multiphase reactor is required when this is not feasible.

Because the reactor is generally a minor portion of the total capital cost, only a modest amount of work should be dedicated to optimizing the reactor cost. However, if the conversion, yield, and selectivity targets are not attained, the process economics may suffer considerably. As a result, steps 2 through 6 should be performed once more until the minimum criteria are fulfilled.