Chapter_4_distillation.pptx
- 1. Chapter 4 Distillation By Andom M. (M.Sc.) andom400@gmail.com
- 2. Introduction Distillation is defined as: • A process in which a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat. • Distillation is based on the fact that the vapour of a boiling mixture will be richer in the components that have lower boiling points. • Therefore, when this vapour is cooled and condensed, the condensate will contain more volatile components. At the same time, the original mixture will contain more of the less volatile material. • Distillation columns are designed to achieve this separation efficiently.
- 3. Cont.. Although many people have a fair idea what “distillation” means, the important aspects that seem to be missed from the manufacturing point of view are that: Distillation is the most common separation technique It consumes enormous amounts of energy, both in terms of cooling and heating requirements It can contribute to more than 50% of plant operating costs The best way to reduce operating costs of existing units, is to improve their efficiency and operation via process optimization and control. To achieve this improvement, a thorough understanding of distillation principles and how distillation systems are designed is essential.
- 4. TYPES OF DISTILLATION COLUMNS There are many types of distillation columns, each designed to perform specific types of separations, and each design differs in terms of complexity. Batch and Continuous Columns One way of classifying distillation column type is to look at how they are operated. Thus we have: batch and continuous columns. Batch Columns In batch operation, the feed to the column is introduced batch-wise. That is, the column is charged with a 'batch' and then the distillation process is carried out. When the desired task is achieved, a next batch of feed is introduced. Continuous Columns In contrast, continuous columns process a continuous feed stream. No interruptions occur unless there is a problem with the column or surrounding process units. They are capable of handling high throughputs and are the most common of the two types. We shall concentrate only on this class of columns.
- 5. Cont.. Types of Continuous Columns Continuous columns can be further classified according to: The nature of the feed that they are processing, binary column - feed contains only two components multi-component column - feed contains more than two components The number of product streams they have – multi-product column - column has more than two product streams where the extra feed exits when it is used to help with the separation, – extractive distillation - where the extra feed appears in the bottom product stream – Azeotropic distillation - where the extra feed appears at the top product stream The type of column internals – Tray column - where trays of various designs are used to hold up the liquid to provide better contact between vapour and liquid, hence better separation – Packed column - where instead of trays, 'packings' are used to enhance contact between vapour and liquid
- 6. Main Components of Distillation Columns Distillation columns are made up of several components, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation contains several major components: – A vertical shell where the separation of liquid components is carried out – column internals such as trays/plates and/or packing's which are used to enhance component separations – A Reboilers to provide the necessary vaporization for the distillation process – a condenser to cool and condense the vapour leaving the top of the 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
- 7. Cont.. • 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. 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 fluid, 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 product or simply,
- 8. Cont... • 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 condensed 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.
- 9. DISTILLATION PRINCIPLES • Separation of components from a liquid 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 pressure characteristics of liquid mixtures. Vapour Pressure and Boiling 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 • vapour pressure is related to boiling • a liquid is said to ‘boil’ when its vapour pressure equals the surrounding pressure • a liquid boils depends on its volatility • liquids with high vapour pressures (volatile liquids) will boil at lower temperatures • the vapour pressure and hence the boiling point of a liquid mixture depends on the relative amounts of the components in the mixture • distillation occurs because of the differences in the volatility of the components in the liquid
- 10. The Boiling Point Diagram (BPD) • The BPD 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 containing 2 components (A and B) - a binary mixture. This has the following boiling point diagram. • The boiling point of A is that at which the mole fraction of A is 1. The boiling point of B is that at which the mole fraction of A is 0. In this example, A is the more volatile component and therefore has a lower boiling point than B. The upper curve in the diagram is called the dew-point curve while the lower one is called the bubble- point curve. • The dew-point is the temperature at which the saturated vapour starts to condense. • The bubble-point is the temperature at which the liquid starts to boil.
- 11. Relative volatility • Relative volatility is a measure of the differences in volatility between 2 components, and hence their boiling points. It indicates how easy or difficult a particular separation will be. The relative volatility of component ‘i’ with respect to component ‘j’ is defined as αij = 𝑌𝑖 𝑋𝑖 𝑌𝑗 𝑋𝑗 yi = mole fraction of component ‘i’ in the vapour xi = mole fraction of component ‘i’ in the liquid • Thus if the relative volatility between 2 components is very close to one, it is an indication that they have very similar vapour pressure characteristics. This means that they have very similar boiling points and therefore, it will be difficult to separate the two components via distillation.
- 12. Some optimal relative volatility that are used for distillation process design
- 13. Vapour-Liquid Equilibrium (VLE) A two phase multicomponent mixture is said to be in equilibrium if; The temperature Tv of the vapor phase is equal to Tl of the liquid phase. The total pressure Pv throughout the vapor phase is equal to the total pressure Pl throughout the liquid phase. The tendency of each component to escape from the liquid phase to the vapor phase is exactly equal to its tendency to escape from the vapor phase to the liquid phase. In the following analysis it is assumed that Tv = Tl = T, Pv = Pl = P and the escaping tendencies are equal. A special case of the third condition for equilibrium is represented by Raoult’s law. P yi = Pi xi Where xi and yi are liquid and vapor mole fractions of component i at temperature T of the system. • For a binary system having components A and B; the equilibrium relationships are given as follows : P yA = PA xA , P yB = PB xB, yA + yB = 1, xA + xB = 1.
- 14. Cont.. • For a binary mixture we usually omit the component index for the light component, • i.e. we write x=xA (light component) and xB=1-x (heavy component). Then the VLE relationship becomes:
- 15. Multi-stage Continuous distillation column design • The vapour-liquid equilibrium characteristics (indicated by the shape of the equilibrium curve) of the mixture will determine the number of stages, and hence the number of trays, required for the separation. This is illustrated clearly by applying the McCabe-Thiele method to design a binary column. McCABE-THIELE DESIGN METHOD • The McCabe-Thiele approach is a graphical one, and uses the VLE plot to determine the theoretical number of stages required to effect the separation of a binary mixture. It assumes constant molar overflow and this implies that: Molar heats of vaporization of the components are roughly the same Heat effects (heats of solution, heat losses to and from column, etc.) are negligible For every mole of vapour condensed, 1 mole of liquid is vaporized
- 16. Cont... The design procedure is simple. Given the VLE diagram of the binary mixture, operating lines are drawn first. – Operating lines define the mass balance relationships between the liquid and vapour phases in the column. – There is one operating line for the bottom (stripping) section of the column, and one for the top (rectification or enriching) section of the column. – Use of the constant molar overflow assumption also ensures the operating lines are straight lines.
- 17. Cont.. The steps to be followed to determine the number of theoretical stages by McCabe- Thiele Method: 1. Determination of the Rectifying section operating line (ROL). 2. Determination the feed condition (q). 3. Determination of the feed section operating line (q-line). 4. Determination of required reflux ratio (R). 5. Determination of the stripping section operating line (SOL). 6. Determination of number of theoretical stage.
- 18. Determination of the Rectifying section operating line (ROL) Material balance on the rectifying section • Overall or total balance Vn+1 = Ln + D (5.13) • Component balance for more volatile component Vn+1yn+1 = Lnxn+ DxD (5.14) • From equation (5.13) and (5.14), it can be written as ( Ln + D) yn+1 = Lnxn+ DxD (5.15) • Consider the constant molar flow in the column, and then one can write: L1 =L2 =….. Ln-1 = Ln =Ln+1 =L=constant V1 =V2 =….Vn-1 =Vn =Vn+1 =V=constant ( L + D) yn+1 = Lxn+ DxD …(5.16)
- 19. Cont..
- 20. Cont.. • Rectifying operating line is expressed without subscript of n or n+1. Without subscript the ROL is expresses as:
- 21. Determination of the feed condition (q) To be continued