《基本蒸馏设备与操作》英语原文.docx

1、基本蒸馏设备与操作英语原文DistilltionBasic Distillation Equipment And OperationMain Components of Distillation ColumnsDistillation columns are made up of several components, each of which is used either to tranfer heat energy or enhance materail transfer. A typical distillation contains several major components:

2、1. a vertical shell where the separation of liquid components is carried out 2. column internals such as trays/plates and/or packings which are used to enhance component separations 3. a reboiler to provide the necessary vaporisation for the distillation process 4. a condenser to cool and condense t

3、he vapour leaving the top of the column 5. a reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column. A

4、 schematic of a typical distillation unit with a single feed and two product streams is shown below: Basic Operation and Terminology The liquid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray known as the feed tray

5、. The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler. Heat is supplied to the reboiler to generate vapour. The source of heat input can be any suitable f

6、luid, although in most chemical plants this is normally steam. In refineries, the heating source may be the output streams of other columns. The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottoms prod

7、uct or simply, bottoms.The vapour moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The conde

8、nsed liquid that is removed from the system is known as the distillate or top product. Thus, there are internal flows of vapour and liquid within the column as well as external flows of feeds and product streams, into and out of the column.Column InternalsTrays and PlatesThe terms trays and plates a

9、re used interchangeably. There are many types of tray designs, but the most common ones are : Bubble cap trays A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapo

10、ur. Vapour rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.Valve trays In valve trays, perforations are covered by liftable caps. Vapour flows lifts the caps, thus self creating a flow ar

11、ea for the passage of vapour. The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays. Valve trays (photos courtesy of Paul Phillips)Sieve trays Sieve trays are simply metal plates with holes in them. Vapour passes straigh

12、t upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters. Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many applications

13、.Liquid and Vapour Flows in a Tray ColumnThe next few figures show the direction of vapour and liquid flow across a tray, and across a column.Each tray has 2 conduits, one on each side, called downcomers. Liquid falls through the downcomers by gravity from one tray to the one below it. The flow acro

14、ss each plate is shown in the above diagram on the right.A weir on the tray ensures that there is always some liquid (holdup) on the tray and is designed such that the the holdup is at a suitable height, e.g. such that the bubble caps are covered by liquid.Being lighter, vapour flows up the column a

15、nd is forced to pass through the liquid, via the openings on each tray. The area allowed for the passage of vapour on each tray is called the active tray area. The pictureon the left is a photograph of a section of a pilot scale column equiped with bubble capped trays. The tops of the 4 bubble caps

16、on the tray can just be seen. The down- comer in this case is a pipe, and is shown on the right. The frothing of the liquid on the active tray area is due to both passage of vapour from the tray below as well as boiling.As the hotter vapour passes through the liquid on the tray above, it transfers h

17、eat to the liquid. In doing so, some of the vapour condenses adding to the liquid on the tray. The condensate, however, is richer in the less volatile components than is in the vapour. Additionally, because of the heat input from the vapour, the liquid on the tray boils, generating more vapour. This

18、 vapour, which moves up to the next tray in the column, is richer in the more volatile components. This continuous contacting between vapour and liquid occurs on each tray in the column and brings about the separation between low boiling point components and those with higher boiling points.Tray Des

19、ignsA tray essentially acts as a mini-column, each accomplishing a fraction of the separation task. From this we can deduce that the more trays there are, the better the degree of separation and that overall separation efficiency will depend significantly on the design of the tray. Trays are designe

20、d to maximise vapour-liquid contact by considering the liquid distribution and vapour distribution on the tray. This is because better vapour-liquid contact means better separation at each tray, translating to better column performance. Less trays will be required to achieve the same degree of separ

21、ation. Attendant benefits include less energy usage and lower construction costs.Liquid distributors - Gravity (left), Spray (right)(photos courtesy of Paul Phillips)PackingsThere is a clear trend to improve separations by supplementing the use of trays by additions of packings. Packings are passive

22、 devices that are designed to increase the interfacial area for vapour-liquid contact. The following pictures show 3 different types of packings。These strangely shaped pieces are supposed to impart good vapour-liquid contact when a particular type is placed together in numbers, without causing exces

23、sive pressure-drop across a packed section. This is important because a high pressure drop would mean that more energy is required to drive the vapour up the distillation column. Structured packing (photo courtesy of Paul Phillips)Packings versus TraysA tray column that is facing throughput problems

24、 may be de-bottlenecked by replacing a section of trays with packings. This is because: 1. packings provide extra inter-facial area for liquid-vapour contact 2. efficiency of separation is increased for the same column height 3. packed columns are shorter than trayed columnsPacked columns are called

25、 continuous-contact columns while trayed columns are called staged-contact columns because of the manner in which vapour and liquid are contacted.Column ReboilersThere are a number of designs of reboilers. It is beyond the scope of this set of introductory notes to delve into their design principles

26、. However, they can be regarded as heat-exchangers that are required to transfer enough energy to bring the liquid at the bottom of the column to boiling boint. The following are examples of typical reboiler types. Photo courtesy of Brian Kennedy Photo courtesy of Brian Kennedy Tube bundle(photo cou

27、rtesy of Paul Phillips)Tube bundle(photo courtesy of Paul Phillips)Tubesheet(photo courtesy of Paul Phillips)Fitting a Shell & Tube Heat Exchanger(photo courtesy of Paul Phillips)A novel development in reboiler design is the self-cleaning shell-and-tube heat exchangers by Klarex Technology for appli

28、cations where heat exchange surfaces are prone to fouling by the process fluid. Particles are introduced into the process stream and these produce a scouring action on the heat exchange surfaces. An example is shown in the diagram on the left.Distillation PrinciplesSeparation of components from a li

29、quid mixture via distillation depends on the differences in boiling points of the individual components. Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. Therefore, distillation processes depends on the vapour pres

30、sure characteristics of liquid mixtures.Vapour Pressure and Boiling1. The vapour pressure of a liquid at a particular temperature is the equilibrium pressure exerted by molecules leaving and entering the liquid surface. Here are some important points regarding vapour pressure:2. energy input raises

31、vapour pressure3. vapour pressure is related to boiling 4. a liquid is said to boil when its vapour pressure equals the surrounding pressure5. the ease with which a liquid boils depends on its volatility6. liquids with high vapour pressures (volatile liquids) will boil at lower temperatures7. the va

32、pour pressure and hence the boiling point of a liquid mixture depends on the relative amounts of the components in the mixture8. distillation occurs because of the differences in the volatility of the components in the liquid mixtureThe Boiling Point DiagramThe boiling point diagram shows how the equilibrium compositions of the components in a liquid mixture vary with temperature at a fixed pressure. Consider an example of a liquid mixture containin