Heterogeneous Reactors & Electrochemical Processes
Reactors
1. Length of Residence, Space-Time, and Space Velocity: Definition, Significance, and Relationship
Residence time: The interval that an element of fluid spends inside the reactor. For an ideal plug flow reactor (PFR), the residence time is the same for all elements of the effluent fluid. It is calculated as:
Tr = Vr / Vf
- Where:
- Tr = Residence time
- Vr = Reactor volume
- Vf = Volumetric flow rate of fluid
Space-time: Reactor volume divided by the volumetric flow rate of fluid, taking the proper dimensions.
Space velocity: The reciprocal of space-time.
2. Types of Heterogeneous Reactors: General Characteristics
Fixed bed reactor: The catalyst is packed in duplicate to allow for regeneration. The operation and size of the particles are important; they should not be too small. Key features include:
- Significant gradients
- Small diameter
- Moderate pressure variation
- Catalysts with long life
- Easy to operate
- Easy to scale up
- Relatively low cost
Fluidized bed reactor: Allows for catalyst regeneration outside the reactor. Key features include:
- Practically isothermal
- Large diameter
- Small particle size
- Large pressure variation
- Short-lived catalysts
- Significant abrasion
- Difficult to operate
- Difficult to scale up
- Relatively low cost
Other reactors, depending on the conditions of each reaction, include moving bed reactors, slurry reactors, and fixed bed reactors.
Electrochemistry
Electrochemistry is based on the use or production of electrical energy through chemical reactions that occur in redox processes. The most important electrochemical processes are electrosynthesis and electrodialysis.
The selectivity of a compound in an electrosynthesis process is achieved by controlling the electrode potential.
3. Electrosynthesis: Characteristics and Components of a Cell
Electrosynthesis: A technology that allows a chemical reaction to occur in a solution containing inorganic or organic substances, which can be reduced and oxidized through the application of a continuous power source. This creates a potential difference, producing an electric current that passes between two electrodes in the solution.
Characteristics:
- Heterogeneous processes where one phase is a solid electrode and the other is a liquid electrolyte.
- The reaction occurs by the direct action of a continuous electrical current on the components of the solution.
- Controlling the electrode potential allows for the selectivity of a compound without causing byproducts.
- Environmental problems are minimized as it is easier to remove electrons and selectively control the reaction.
- Products can be obtained that would be almost impossible to achieve via conventional chemistry.
Components:
- Electrochemical reactor or electrochemical cell (containing the following three components)
- Cathode
- Anode
- Diaphragm
- Turbulence promoters
- Anolyte
- Rectifier
4. Electrodialysis
Electromembrane technology that enables, under the influence of a continuous electric field, the extraction of ionized organic and inorganic substances dissolved in an aqueous solution through selective ion-exchange membranes.
5. Electric Current Density: Definition
Current density: Influences the processing speed, selectivity, current efficiency, specific energy consumption, and corrosion rate of an electrode. From an industrial standpoint, it is important that the process of interest is carried out at a high current density and low overpotential to maximize productivity and minimize energy consumption.