Distillation PDF - Unit Operations

Distillation Distillation is a separation process, separating components in a mixture by making use of the fact that...

Distillation Distillation is a separation process, separating components in a mixture by making use of the fact that some components vapourise more readily than others. When vapors are produced from a mixture, they Unit Operations contain the components of the original mixture, but in Distillation proportions which are determined by the relative volatilities of these components. The vapor is richer in those components that are more volatile, and so a separation occurs. In fractional distillation, the vapor is condensed and then re-- evaporated when a further separation occurs. Distillation Distillation ▪ The temperature at which any liquid ▪ It is difficult to prepare pure components in this way, boils is known as its boiling point. Different types of liquids have their but a degree of separation can easily be attained if the own unique boiling point. For example, volatilities are reasonably different. the boiling point of water is 100oC and ▪ Where great purity is required, successive distillations the boiling point of ethanol is 78oC. may be used. ▪ ▪ When a liquid begins to boil, the liquid ▪ Major uses of distillation in the food industry are for at the surface becomes a vapor. In the concentrating essential oils, flavours and alcoholic case of water this is known as steam and the process is called evaporation. beverages. If the vapor is then cooled, it returns to a liquid in a process called condensation. ▪ Evaporation and condensation are the basic principles on which distillation and refluxing work.  Distillation Distillation ▪ A normal distillation cannot separate ▪ Distillation is a commonly used method for removing a liquid from a two liquids perfectly. There will always reaction. First, the liquid in the reactor vessel is heated. When be a mixture of the two liquids in any the temperature of the liquid reaches its boiling point, vapor is fraction that is collected in the formed. This vapor leaves the reactor vessel and is transferred to a storage vessel. condenser, where it is cooled and converts to a liquid. The liquid is ▪ called the distillate and is collected separately in a holding vessel. ▪ A special type of distillation, called a fractional distillation, is the best ▪ Distillation can also be used to separate a mixture of two liquids method to use to achieve the purest that have different boiling points. The separation process fractions possible. A piece of depends on the difference in boiling points between the two liquids. equipment called a fractionating The greater the difference, the easier it is to separate them. column is added to the distillation apparatus. This device helps to purify the fractions. The bigger the fractionating column, the more effective the separation process. Simple Distillation ▪ If we wish to remove a single liquid, such as a solvent, from a vessel, then the Operation is called ‘Simple’ Distillation (or, where solvent is involved, ‘Solvent Recovery’). This is the process that is utilised in “batch distillation” ▪ A given volume of liquid (reaction mix) is heated to boiling and the vapour formed is condensed and routed to a Receiving Vessel. The composition of the liquid remaining in the still and the vapour collected change with time. A flow diagram for a typical “batch distillation” is shown on next slide. ▪ Used in some Whiskey Distilleries. 72  Distillation Equipment (Batch) – pot stills Distillation - Continuous ▪ In batch operation, the feed to the column is introduced batch-wise. Figure: Typical industrial distillation unit with a single feed and two product ▪ the column is charged with a 'batch' and then the distillation stream process is carried out. ▪ When the desired task is achieved, a next batch of feed is introduced. ▪ A pot still involves only one condensation process, whereas other types of distillation equipment have multiple stages which result in higher purification of the more volatile component namely the alcohol. Used in the manufacture of good quality whiskey Made from arsenic-free copper, have a swan neck fitted to the pot (shape: often called onion stills) Condenser is a heat exchanger also made of copper. Heat is applied to the pot by steam passing through coils or a jacket. Distillation - Continuous Distillation - Continuous Basic Operation and Terminology Basic Operation and Terminology ▪ The liquid mixture that is to be processed is known as the feed ▪ The vapour moves up the column, and as it exits the and this is introduced usually somewhere near the middle of the top of the unit, it is cooled by a condenser. column to a tray known as the feed tray. ▪ The condensed liquid is stored in a holding vessel ▪ The feed flows down the column where it is collected at the known as the reflux drum. bottom in the reboiler. ▪ Heat is supplied to the reboiler to generate vapour. ▪ Some of this liquid is recycled back to the top of the ❖ The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. column and this is called the reflux. ❖ The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column. ▪ The condensed liquid that is removed from the ❖ The liquid removed from the reboiler is known as the bottoms system is known as the distillate or top product. product or simply, bottoms. ▪ 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.  Distillation Equipment - Continuous Distillation-Continuous Basic Distillation Equipment and Operation Distillation columns are made up of several components, each ▪ The type of column internals: of which is used either to transfer heat energy or enhance material transfer. ❖Tray column – A typical distillation contains several major components: ❖trays of various designs are used to hold ▪ A vertical shell where the separation of liquid components is carried up the liquid to provide better contact out ▪ Column internals such as trays/plates and/or packings which are between vapour and liquid and hence used to enhance component separations better separation. ▪ A reboiler to provide the necessary vaporisation for the distillation process ▪ A condenser to cool and condense the vapour leaving the top of the column ❖Packed column – ▪ 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 ❖instead of trays, 'packings' are used to enhance contact between vapour and liquid. The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column. Link: http://www.gea- pen.nl/gpenl/cmsresources.nsf/filenames/P06e_Destillationstechnik_09.pdf/$file/P06e_Destillationstechnik_09.pdf Distillation Equipment Distillation Equipment ▪ Tray arrangement within column and type Packed column ▪ Instead of trays, packings are used to enhance contact between vapour and liquid.  Distillation Equipment Vacuum Distillation ▪ Distillation under reduced Pressure Column Reboilers ▪ In some cases, it is undesirable to bring the mixture to ▪ There are a number of designs of reboilers. the normal boiling temperature of the distillate. For ▪ They can be regarded as heat-exchangers that are example, the boiling point might be very high, or a required to transfer enough energy to bring the liquid heat-sensitive material might also be present in the at the bottom of the column to boiling point. reaction mixture. In such instances, it is possible to make the mixture boil at a lower temperature by connecting the system to a Vacuum Pump and putting it under a partial vacuum. ▪ One has to be careful in these cases because boiling can become violent and somewhat unpredictable at times, a situation that could result in ‘carryover’ (i.e. liquid rather than vapour coming across). Other Distillations Distillation - Azeotropes ▪ Vacuum Distillation ▪ reduce pressure to boil off solvent at lower temperature ▪ An azeotrope is a liquid mixture which when vaporised, produces the same composition as the liquid. ▪ Azeotropic Distillation ▪ Ethanol and water form an azeotrope of 95% at ▪ co-distillation of two compounds if their b.p. close 78.2oC. (95.6% ethanol in purity). It cannot be purified further by distillation. ▪ BP pure ethanol: 78.30C  Distillation - Applications ▪ Manufacture of Whisky ▪ Irish whiskey – 3 pot stills used ▪ Brandy distilled from fermented juice ▪ Rum distilled from fermented juice of sugar cane ▪ Manufacture of spirits ▪ Recovery of solvents from oil after extraction ▪ Extraction of essential oils from leaves and seeds (steam distillation)