Packed bed

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Super-Raschig rings Anelli Raschig.jpg
Super-Raschig rings
Structured packing Riempimento strutturato.jpg
Structured packing

In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packed bed can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing. Packed beds may also contain catalyst particles or adsorbents such as zeolite pellets, granular activated carbon, etc.

Contents

The purpose of a packed bed is typically to improve contact between two phases in a chemical or similar process. Packed beds can be used in a chemical reactor, a distillation process, or a scrubber, but packed beds have also been used to store heat in chemical plants. In this case, hot gases are allowed to escape through a vessel that is packed with a refractory 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

A packed bed used to perform separation processes, such as absorption, stripping, and distillation is known as a packed column. [1] Columns used in certain types of chromatography consisting of a tube filled with packing material can also be called packed columns and their structure has similarities to packed beds.

The column bed can be filled with randomly dumped packing material (creating a random packed bed) or with structured packing sections, which are arranged in a way that force fluids to take complicated paths through the bed (creating a structured packed bed). In the column, liquids tend to wet the surface of the packing material and the vapors pass across this wetted surface, where mass transfer takes place. Packing materials 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 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. [1] 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, 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.

Packed Bed Reactors (PBRs)

Packed bed reactors are reactor vessels containing a fixed bed of catalytic material, they are widely used in the chemical process industry and find primary use in heterogeneous, gas-phase, catalytic reactions. The advantages of using a packed bed reactor include the high conversion of reactants per unit mass of catalyst, relatively low operating costs, and continuous operation. Disadvantages include the presence of thermal gradients throughout the bed, poor temperature control, and difficult servicing of the reactor. [2]

Theory

The Ergun equation can be used to predict the pressure drop along the length of a packed bed given the fluid velocity, the packing size, and the viscosity and density of the fluid.

The Ergun equation, while reliable for systems on the surface of the earth, is unreliable for predicting the behavior of systems in microgravity. Experiments are currently underway aboard the International Space Station to collect data and develop reliable models for in-orbit packed-bed reactors. [3]

Monitoring

The performance of a packed bed is highly dependent on the flow of material through it, which in turn is dependent on the packing and how the flow is managed. Tomographic techniques such as near-infrared, x-ray, gamma ray, electrical capacitance, electrical resistance tomography are used to quantify liquid distribution patterns in packed columns; choice of tomographic technique depends on the primary measurement of interest, randomness of packing, safety requirements, desired data acquisition rate, and budget. [4] [5] [6] [7] [8] [9]

See also

Bibliography

Related Research Articles

<span class="mw-page-title-main">Distillation</span> Method of separating mixtures

Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heating of solid materials to produce gaseous products ; this may involve chemical changes such as destructive distillation or cracking. Distillation may result in essentially complete separation, or it may be a partial separation that increases the concentration of selected components; in either case, the process exploits differences in the relative volatility of the mixture's components. In industrial applications, distillation is a unit operation of practically universal importance, but is a physical separation process, not a chemical reaction. An installation used for distillation, especially of distilled beverages, is a distillery. Distillation includes the following applications:

Mass transfer is the net movement of mass from one location to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, and distillation. Mass transfer is used by different scientific disciplines for different processes and mechanisms. The phrase is commonly used in engineering for physical processes that involve diffusive and convective transport of chemical species within physical systems.

Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize. It uses distillation to fractionate. Generally the component parts have boiling points that differ by less than 25 °C (45 °F) from each other under a pressure of one atmosphere. If the difference in boiling points is greater than 25 °C, a simple distillation is typically used. It is used to refine crude oil.

<span class="mw-page-title-main">Raschig ring</span>

A Raschig ring is a piece of tube, approximately equal in length and diameter, used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. They are usually ceramic, metal or glass and provide a large surface area within the volume of the column for interaction between liquid and gas vapours.

<span class="mw-page-title-main">Fractionating column</span> Equipment to separate liquids by distillation

A fractionating column or fractional column is equipment used in the distillation of liquid mixtures to separate the mixture into its component parts, or fractions, based on their differences in volatility. Fractionating columns are used in small-scale laboratory distillations as well as large-scale industrial distillations.

<span class="mw-page-title-main">Vacuum distillation</span> Low-pressure and low-temperature distillation method

Vacuum distillation or Distillation under reduced pressure is a type of distillation performed under reduced pressure, which allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This technique separates compounds based on differences in their boiling points. This technique is used when the boiling point of the desired compound is difficult to achieve or will cause the compound to decompose. Reduced pressures decrease the boiling point of compounds. The reduction in boiling point can be calculated using a temperature-pressure nomograph using the Clausius–Clapeyron relation.

<span class="mw-page-title-main">Unit operation</span>

In chemical engineering and related fields, a unit operation is a basic step in a process. Unit operations involve a physical change or chemical transformation such as separation, crystallization, evaporation, filtration, polymerization, isomerization, and other reactions. For example, in milk processing, the following unit operations are involved: homogenization, pasteurization, and packaging. These unit operations are connected to create the overall process. A process may require many unit operations to obtain the desired product from the starting materials, or feedstocks.

<span class="mw-page-title-main">Chemical reactor</span> Enclosed volume where interconversion of compounds takes place

A chemical reactor is an enclosed volume in which a chemical reaction takes place. In chemical engineering, it is generally understood to be a process vessel used to carry out a chemical reaction, which is one of the classic unit operations in chemical process analysis. The design of a chemical reactor deals with multiple aspects of chemical engineering. Chemical engineers design reactors to maximize net present value for the given reaction. Designers ensure that the reaction proceeds with the highest efficiency towards the desired output product, producing the highest yield of product while requiring the least amount of money to purchase and operate. Normal operating expenses include energy input, energy removal, raw material costs, labor, etc. Energy changes can come in the form of heating or cooling, pumping to increase pressure, frictional pressure loss or agitation.

Reactive distillation is a process where the chemical reactor is also the still. Separation of the product from the reaction mixture does not need a separate distillation step which saves energy and materials. This technique can be useful for equilibrium-limited reactions such as esterification and ester hydrolysis reactions. Conversion can be increased beyond what is expected by the equilibrium due to the continuous removal of reaction products from the reactive zone. This approach can also reduce capital and investment costs.

<span class="mw-page-title-main">Continuous distillation</span> Form of distillation

Continuous distillation, a form of distillation, is an ongoing separation in which a mixture is continuously fed into the process and separated fractions are removed continuously as output streams. Distillation is the separation or partial separation of a liquid feed mixture into components or fractions by selective boiling and condensation. The process produces at least two output fractions. These fractions include at least one volatile distillate fraction, which has boiled and been separately captured as a vapor condensed to a liquid, and practically always a bottoms fraction, which is the least volatile residue that has not been separately captured as a condensed vapor.

<span class="mw-page-title-main">Van Deemter equation</span>

The van Deemter equation in chromatography, named for Jan van Deemter, relates the variance per unit length of a separation column to the linear mobile phase velocity by considering physical, kinetic, and thermodynamic properties of a separation. These properties include pathways within the column, diffusion, and mass transfer kinetics between stationary and mobile phases. In liquid chromatography, the mobile phase velocity is taken as the exit velocity, that is, the ratio of the flow rate in ml/second to the cross-sectional area of the ‘column-exit flow path.’ For a packed column, the cross-sectional area of the column exit flow path is usually taken as 0.6 times the cross-sectional area of the column. Alternatively, the linear velocity can be taken as the ratio of the column length to the dead time. If the mobile phase is a gas, then the pressure correction must be applied. The variance per unit length of the column is taken as the ratio of the column length to the column efficiency in theoretical plates. The van Deemter equation is a hyperbolic function that predicts that there is an optimum velocity at which there will be the minimum variance per unit column length and, thence, a maximum efficiency. The van Deemter equation was the result of the first application of rate theory to the chromatography elution process.

<span class="mw-page-title-main">Structured packing</span>

The term structured packing refers to a range of specially designed materials for use in absorption and distillation columns and chemical reactors. Structured packings typically consist of thin corrugated metal plates or gauzes arranged in a way that force fluids to take complicated paths through the column, thereby creating a large surface area for contact between different phases.

<i>Distillation Design</i> Handbook for design of industrial distillation columns

Distillation Design is a book which provides complete coverage of the design of industrial distillation columns for the petroleum refining, chemical and petrochemical plants, natural gas processing, pharmaceutical, food and alcohol distilling industries. It has been a classical chemical engineering textbook since it was first published in February 1992.

The McCabe–Thiele method is a technique that is commonly employed in the field of chemical engineering to model the separation of two substances by a distillation column. It uses the fact that the composition at each theoretical tray is completely determined by the mole fraction of one of the two components. This method is based on the assumptions that the distillation column is isobaric - i.e the pressure remains constant - and that the flow rates of liquid and vapor do not change throughout the column. The assumption of constant molar overflow requires that:

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 equilibrium with each other. Such equilibrium stages may also be referred to as an equilibrium stage, ideal stage, or a theoretical tray. The performance of many separation processes depends on having series of equilibrium stages and is enhanced by providing more such stages. In other words, having more theoretical plates increases the efficiency of the separation process be it either a distillation, absorption, chromatographic, adsorption or similar process.

<span class="mw-page-title-main">Air stripping</span>

Air stripping is the transferring of volatile components of a liquid into an air stream. It is an environmental engineering technology used for the purification of groundwaters and wastewaters containing volatile compounds.

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 co-current or countercurrent flows. Stripping is usually carried out in either a packed or trayed column.

<span class="mw-page-title-main">Reflux</span> Condensation of vapors and their return to where they originated

Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply energy to reactions over a long period of time.

Dixon rings are a form of random packing used in chemical processing. They consist of a stainless steel mesh formed into a ring with a central divider, and are intended to be packed randomly into a packed column. Dixon rings provide a large surface area and low pressure drop while maintaining a high mass transfer rate, making them useful for distillations and many other applications.

Random column packing is the practice of packing a distillation column with randomly fitting filtration material in order to optimize surface area over which reactants can interact while minimizing the complexity of construction of such columns. Random column packing is an alternative to structured column packing.

References

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  3. St. Onge, Tom. "PBRE". Space Flight Systems. Glenn Research Center. Archived from the original on 5 September 2015. Retrieved 13 December 2015.
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  6. Johansen, G.A. (2015), "Gamma-ray tomography", Industrial Tomography, Elsevier, pp. 197–222, doi:10.1016/b978-1-78242-118-4.00007-1, ISBN   978-1-78242-118-4 , retrieved 2023-10-22
  7. Schubert, Markus; Hessel, Günther; Zippe, Cornelius; Lange, Rüdiger; Hampel, Uwe (2008-07-01). "Liquid flow texture analysis in trickle bed reactors using high-resolution gamma ray tomography". Chemical Engineering Journal. 140 (1): 332–340. doi:10.1016/j.cej.2007.10.006. ISSN   1385-8947.
  8. Wu, Hao; Buschle, Bill; Yang, Yunjie; Tan, Chao; Dong, Feng; Jia, Jiabin; Lucquiaud, Mathieu (2018-12-01). "Liquid distribution and hold-up measurement in counter current flow packed column by electrical capacitance tomography". Chemical Engineering Journal. 353: 519–532. doi: 10.1016/j.cej.2018.07.016 . ISSN   1385-8947.
  9. Eda, Takeshi; Sapkota, Achyut; Haruta, Jun; Nishio, Masayuki; Takei, Masahiro (2013). "Experimental Study on Liquid Spread and Maldistribution in the Trickle Bed Reactor Using Electrical Resistance Tomography". Journal of Power and Energy Systems. 7 (2): 94–105. doi: 10.1299/jpes.7.94 .