Fractionating column
The rectification column is a procedural apparatus for thermal separation of mixtures. Although in common parlance most of distillation columns is mentioned, the principle of rectification is used for the majority of industrial equipment, which has consequences for the design and operation. For the difference and the thermodynamic principles of distillation and rectification Reference is made to the relevant articles.
In the chromatography comparable device column is called.
- 2.1 Discontinuous operation
- 2.2 Continuous Operation
- 3.1 dividing wall column
- 3.2 reactive distillation
- 3.3 columns with side draw
Basic structure
Although it is mostly spoken by a "column", belong to a reasonably usable rectification column a number of other components, first of all does not matter whether it is a rectification or distillation column ( see also the diagrams below):
Column body
The actual column is to avoid heat losses insulated cylindrical tube, which usually exists in the industry of steel or higher alloy stainless steels, while in the laboratory mostly glass is employed. Plastic is used due to its limited thermal stability only in rare exceptions. The height of the column body is mainly dictated by the required quality of the separation; the diameter of the flow rate of the mixture to be separated. Due to the usually large amount in columns as " shots " designated segments are divided.
Vaporizer
To evaporate the mixture is at the lower end of the column ( in the bottom ), an evaporator, also called Swamp cooker. This heat exchanger can be designed differently depending on the properties of the mixture and other conditions: As a directly integrated into the bottom tube bundles, double jacketed, external natural circulation evaporator ( as shown in the schemes below) or forced circulation evaporator. As the operating medium, almost all possible heat sources may be used. In most cases, it is steam, but it can just as well hot water, flue gas, electricity or microwaves to be.
Capacitor
To liquefy the steam mixture at the top end of the column ( at the top), a capacitor is required. As the evaporator can be integrated directly in the head or run as an external apparatus next to the column and this heat exchanger. It also can be operated with a variety of possible Betriebsmediem such as water, brine, refrigerant or air.
Fixtures
To intensify the heat and mass transfer between gas and liquid phase of the column body is provided with fixtures that due to the properties of the mixture (eg, high foaming ) and operation of the column ( eg vacuum ) can be selected. Because they have decisive influence on the hydraulic and thus directly the separation behavior of the column, they are simultaneously used to characterize the type of column,
There are three types:
A uniform distribution of the gas and liquid phase over the entire cross section of the column are necessary for the reliable and efficient operation of all types of column. In soil columns, this makes it suitable already guaranteed, but with structured packings and specially packed columns, additional measures are required. In this case, after passing through a given height of packing the whole liquid phase is collected and re-distributed over the cross section. The gas flow is automatically also evened out again. Otherwise, the liquid tends to flow to the inner walls of the column, downwardly so that the gas phase in the middle only a related contact is still possible, which significantly reduces the separation efficiency.
Modes of operation
Discontinuous operation
The discontinuous mode ( engl. batch) is mainly used to separate a single column with a multicomponent mixture into its various components - to fractionate. Here, the mixture is placed in the bottom of the column and vaporized in the evaporator. First, the component is preferably at the lowest boiling point in the gas phase and rises in the column body. The baffles continuous condensation and re- evaporation takes place, which leads to an enrichment of the lowest boiling component of the gas phase and of the components with higher boiling points in the liquid phase.
In the case of distillation, the entire liquefied in the condenser at the top mixture is drawn off and collected as fraction until an increase in head temperature indicating the next component. Then usually a so-called intermediate fraction is formed, which consists of a mixture of two components, and is treated separately before the next component is finally collected. Theoretically, it is possible to obtain successively all the individual compounds in pure form, to the whole of the original mixture evaporates. In practice, however, beyond a certain minimum level, either fresh mixture is refilled or removed for further treatment of the residue from the bottom.
In a rectification is basically do the same, but with the crucial difference that only a small portion of the overhead condensate is withdrawn and collected, while the majority is fed back as a so-called reflux into the column head. In this way, the separation efficiency is significantly improved, and can achieve significantly higher purities of the individual components can be achieved.
Continuous operation
While mostly worked in the laboratory in batch, a large part of the industrial columns are continuously operated. Here, the mixture to be separated is fed into a constant current to a fixed point of the column (feed), from where it trickles over the internals down in the swamp. In the intense heat and mass exchange with the rising of vapors below the lower-boiling components are " stripped " from the liquid and rise in the gas phase back up. The higher-boiling components flow further down until they are eventually highly enriched continuously withdrawn from the bottom. The area between the swamp and the mixture feed is also referred to as " output " of the column ( engl. stripping section).
Above the feed causes the liquid trickling down from above a " washing out " the remaining higher-boiling components from the gas in the liquid phase with which they are transported into the swamp. At the head of the highly enriched low-boiling components are liquefied and partly as reflux fed back again into his head taken off partly in a constant stream. This reintroduction is in continuous operation is essential to ensure the effectiveness of the " reinforcing member " (English rectification section) between feed and head.
In contrast to a batch can only be separated into at best a pure substance ( heavy or Leichhardt test boilers) and a mixture of the other components in a continuous column, a multi-component mixture, which must be treated in further columns. However, it is much easier to handle and automate.
Special columns
In addition to the variants described above, there are now a number of developments, on the one hand, for example, greater energy efficiency, or lower investment costs cause, but are sure to drive less flexible and more difficult due to the increased specialization.
Partition column
In this column extends over part of the height of the column, a vertical partition which divides the cross section into two sections. Above the partition wall, the liquid phase is collected and distributed in a selectable ratio to the two column sections. With proper design and operation, it is theoretically possible for a ternary mixture in a single column into its three pure components to dissect what usually two conventional columns would be needed. By saving an entire column with their peripheral significantly higher investment cost of a dividing wall column will be more than offset. In addition, the energy consumption due to the thermodynamic effects, is less than would be required for the system consisting of two columns. However, the interpretation, driving and control of these columns represent a challenge to the process engineering dar.
Reactive distillation
This column is a combination of a chemical reactor and a rectification column dar. in it a reaction at usually coated with a catalyst special packages, while at the same time, a separation of the reaction products. In this way it is possible, for example for a normally only up to a certain equilibrium reaction which proceeds in a single apparatus to obtain a complete conversion of the raw materials. However, also applies here that the design and operation of such a column is associated with a significantly higher cost than for a simple column.
Columns with side draw
In some mixtures it may be necessary at one or more locations along the column height to remove, to prevent excessive accumulation of a partial stream of the column. However, it is not possible to get to this side draws a pure component, but only a mixture with a specific content of low - and high boilers