08-02-2013, 03:32 PM
Packed bed
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INTRODUCTION
In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material.
The packing 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.
PURPOSE
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.
PRINCIPLE
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.[2] 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 is based on the amount of the solid catalyst rather than the volume of the reactor.
TYPES OF PACKED BED REACTORS
Packed column
In industry, a packed column is a type of packed bed used to perform separation processes, such as absorption, stripping, and distillation. A packed column is a pressure vessel that has a packed section.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.
Column structure: random and stacked packed columns
The column can be filled with random dumped packing (creating a random packed column) or with structured packing sections, which are arranged or stacked (creating a stacked packed column). In the column, liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer 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.
Liquid and vapor distribution (vapor to liquid ratio)
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.
Continuous Packed Bed Reactor
Continuous packed bed reactors are the most widely used reactors for immobilized enzymes and immobilized microbial cells. In these systems, it is necessary to consider the pressure drop across the packed bed or column, and the effect of the column dimensions on the reaction rate. There are three substrate flow possibilities in a packed bed and they are illustrated below:
1. Downward flow method
2. Upward flow method
3. Recycling method