Packed bed

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Packed bed

In chemical process

Chemical process

In a "Process " sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by somebody....
ing, a packed bed is a hollow tube

Tubing

Tubing may refer to:*Tubing , flexible hose or pipe *Tubing , the act of riding an Tire#Inner_tube*hollow structural section*plumbing tube used in domestic water systems...
, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig ring

Raschig ring

Raschig rings are pieces of tube used in large numbers as a packed bed within columns for distillation columns and other chemical engineering processes....
s or else it can be a specifically designed structured packing

Structured packing

The term structured packing refers to a range of specially designed materials for use in absorption and distillation columns and chemical reactors....
.

The purpose of a packed bed is typically to improve contact between two phases

Phase (matter)

In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, refractive index, and chemical composition....
in a chemical or similar process.

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Encyclopedia

In chemical process

Chemical process

Tubing

Raschig ring

Structured packing

Phase (matter)

Chemical reactor

In chemical engineering, chemical reactors are vessels designed to contain chemical reactions. The design of a chemical reactor deals with multiple aspects of chemical engineering....
, a distillation

Distillation

Distillation is a method of separation process mixtures based on differences in their Volatility in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
process, or a scrubber

Scrubber

Scrubber systems are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams....
, but packed beds have also been used to store heat

Heat

In physics and thermodynamics, heat is any transfer of energy from one body or thermodynamic system to another due to a difference in temperature....
in chemical plants. In this case, hot gases are allowed to escape through a vessel that is packed with a refractory

Refractory

A refractory material is one that retains its strength at high temperatures. ASTM International C71 defines refractories as "non-metallic materials having those chemical and physical properties that made them applicable for structures, or as components of systems, that are exposed to environments above 1000 ?F "....
material until the packing is hot. Air or other cool gas is then fed back to the plant through the hot bed, thereby pre-heating the air or gas feed.

Applications

In industry, a packed column is a type of packed bed used to perform separation process

Separation process

In chemistry and chemical engineering, a separation process is used to transform a mixture of substances into two or more distinct products. The separated products could differ in chemical properties or some physical property, such as size, or crystal modification or other separation into different components....
es, such as absorption

Absorption (chemistry)

File:Absorber.svgAbsorption, in chemistry, is a physical or chemical phenomenon or a Process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material....
, stripping

Stripping (chemistry)

Stripping is a physical separation process where one or more components are removed from a liquid stream by a vapor stream. In industrial applications the liquid and vapor streams can have concurrent or countercurrent flows....
, and distillation

Distillation

Pressure vessel

A pressure vessel is a closed container designed to hold gases or liquids at a pressure different from the ambient pressure.The pressure differential is potentially dangerous and many fatal accidents have occurred in the history of their development and operation....
that has a packed section. The column can be filled with random dumped packing or structured packing

Structured packing

The term structured packing refers to a range of specially designed materials for use in absorption and distillation columns and chemical reactors....
sections, which are arranged or stacked. In the column, liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer

Mass transfer

Mass transfer is the transfer of mass from high concentration to low concentration. The phrase is commonly used in engineering for physical processes that involve molecule and convection transport of atoms and molecules within systems....
takes place. Packing material can be used instead of trays to improve separation in distillation columns. Packing offers the advantage of a lower pressure drop across the column (when compared to plates or trays), which is beneficial while operating under vacuum. Differently shaped packing materials have different surface areas and void space between the packing. Both of these factors affect packing performance.

Another factor in performance, in addition to the packing shape and surface area, is the liquid and vapor distribution that enters the packed bed. The number of theoretical stages

Theoretical plate

A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an Vapor-liquid equilibrium with each other....
required to make a given separation is calculated using a specific vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapor ratio will not be correct and the required separation will not be achieved. The packing will appear to not be working properly. The height equivalent to a theoretical plate (HETP) will be greater than expected. The problem is not the packing itself but the mal-distribution of the fluids entering the packed bed. These columns can contain liquid distributors and redistributors which help to distribute the liquid evenly over a section of packing, increasing the efficiency of the mass transfer. The design of the liquid distributors used to introduce the feed and reflux to a packed bed is critical to making the packing perform at maximum efficiency.

Packed columns have a continuous vapor-equilibrium curve, unlike conventional tray distillation in which every tray represents a separate point of vapor-liquid equilibrium. However, when modeling packed columns it is useful to compute a number of theoretical plates to denote the separation efficiency of the packed column with respect to more traditional trays. In design, the number of necessary theoretical equilibrium stages is first determined and then the packing height equivalent to a theoretical equilibrium stage

Theoretical plate

A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an Vapor-liquid equilibrium with each other....
, known as the height equivalent to a theoretical plate (HETP), is also determined. The total packing height required is the number theoretical stages multiplied by the HETP.

Columns used in certain types of chromatography

Chromatography

Chromatography is the collective term for a family of laboratory techniques for the separation of mixtures. It involves passing a mixture dissolved in a "mobile phase" through a stationary phase, which separates the analyte to be measured from other molecules in the mixture and allows it to be isolated....
consisting of a tube filled with packing material can also be called packed columns and their structure has similarities to packed beds.

Packed bed reactors

Packed bed reactors can be used in chemical reaction. These reactors are tubular and are filled with solid catalyst particles, most often used to catalyze gas reactions. The chemical reaction takes place on the surface of the catalyst. The advantage of using a packed bed reactor is the higher conversion per weight of catalyst than other catalytic reactors. The reaction rate

Reaction rate

The reaction rate or rate of reaction for a reactant or product in a particular chemical reaction is intuitively defined as how fast a reaction takes place....
is based on the amount of the solid catalyst rather than the volume of the reactor.

Theory

The Ergun equation

Sabri Ergun

Sabri Ergun was a chemical engineer.He is widely recognized for his enthusiasm and many contributions to the fields of chemical engineering and solid state physics....
can be used to predict the pressure drop along the length of a packed bed given the fluid velocity

Velocity

In physics, velocity is defined as the Derivative of Position vector. It is a vector physical quantity; both speed and direction are required to define it....
, the packing size, and the viscosity

Viscosity

Viscosity is a measure of the Drag of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness"....
and density

Density

The density of a material is defined as its mass per unit volume. The symbol of density is ....
of the fluid.

Packed bed

Applications

Packed bed reactors

Theory

See also

Bibliography

External links