Chapter 3 Distillation

3.1 Introduction Principle of distillation operation Main applications The boiling points of components in a miscible liquid mixture are different. Thus, by evaporating and condensing part of the mixture，the components can be separated with each other. Main applications Refinery of crude oil Purification of products Solvent recovery Treatment of waste liquids

Classification classified by operating method simple distillation equilibrium distillation fractional distillation special distillation extractive distillation azeotropic distillation dissolved-salt distillation classified by operating pressure classified by number of components classified by operating procedure

In simple distillation, all the hot vapors produced are immediately channeled into a condenser which cools and condenses the vapors. Therefore, the distillate will not be pure − its composition will be identical to the composition of the vapors at the given temperature and pressure.

Equilibrium distillation (Flash distillation) is a single stage separation technique. A liquid mixture feed is pumped through a heater to raise the temperature of the mixture. It then flows through a valve and the pressure is reduced, causing the liquid to partially vaporize. Once the mixture enters a big enough volume, the liquid and vapor separate. Because the vapor and liquid are in such close contact up until the "flash" occurs, the product liquid and vapor phases approach equilibrium.

Continuous fractional distillation tower separating one feed mixture stream into four distillate and one bottoms fractions For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Therefore, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column.

Batch Distillation In differential distillation a feed mixture (an initial charge) of a given composition is placed in a single stage separator and heated to boiling. The vapor is collected and condensed to a distillate. The composition of the remaining liquid and the distillate are functions of time. differential distillation 微分蒸馏 initial charge 初始进料 differential distillation 微分蒸馏 initial charge 初始进料

Reasons for running a batch process 1) Small capacity doesn’t warrant continuous operation 2) Separation is to be done only occasionally 3) Separation is preparative to produce a new product 4) Upstream operations are batch-wise or feed-stocks vary with time or from batch to batch 5) Feed materials are not appropriate for a continuous flow system

Continuous distillation A mixture is continuously fed into the process and separated fractions are removed continuously as output streams as time passes during the operation. In practice when there are multiple distillate fractions, each of the distillate exit points is located at different heights on a fractionating column. The bottoms fraction can be taken from the bottom of the distillation column or unit, but is often taken from a reboiler connected to the bottom of the column. Reasons for running a batch process

Continuous binary fractional distillation tower. Continuous fractional distillation tower separating one feed mixture stream into four distillate and one bottoms fractions Continuous binary fractional distillation tower. Chemical engineering schematic of Continuous Binary Fractional Distillation tower. A binary distillation separates a feed mixture stream into two fractions: one distillate and one bottoms fractions. Simplified chemical engineering schematic of Continuous Fractional Distillation tower separating one feed mixture stream into four distillate and one bottoms fractions

3.2 Multi-Component Distillation 1. Introduction Most of the distillation processes deal with multicomponent mixtures Principle the same as in two-component system. Calculation similar to that of two-component system, but more complicated.

While we can graphically solve a binary component distillation system using the McCabe-Thiele method, it is also possible to do a complete analytical solution using mass and energy balances with the equilibrium relationship. For multi-component systems, C>2, the number of equations obtained from mass and energy balances with the equilibrium relationship will always be one less than the number of unknowns. Consequently, one cannot do a complete analytical solution for multi-component distillation–it requires a trial-and-error solution with the additional unknown assumed to be known, as well as special considerations as to enhancing convergence of the solution. Multi-Component Distillation– The Problem

When dealing with multi-component systems, we introduce some new terminology in addition to the terms used in binary distillation: Fractional recoveries Key components Non-key components Splits Note that binary systems can be handled in the same terms. Some additional terms

Fractional Recoveries Fractional recoveries are often specified in MCD. A fractional recovery, FRi, is the amount or flow rate of component i in the distillate or bottoms stream with respect to the amount or flow rate of component i in the feed stream: These are also often specified simply as % recovery.

The Heavy Key component, HK Key Components In practice we usually choose two components separation of which serves as an good indication that a desired degree of separation is achieved. The components that have their distillate and bottoms composition specified are known as the key components. The Light Key component, LK The most volatile of the key components The Heavy Key component, HK The least volatile of the key components

Non-Key Components All other components not specified in the distillate or bottoms are termed non-key components (NK’s). The Light Non-Key component, LNK If a non-key component is more volatile than the light key, then it is termed a light non-key (LNK). The Heavy Non-Key component, HNK If a non-key component is less volatile than the heavy key, it is a heavy non-key (HNK).

Key Components There are different strategies to select these key components Choosing two components that are next to each other on the relative volatility scale often leads to all the components lighter than the light key components accumulating in the distillate and all the components heavier than the heavy key component accumulating in the bottoms product.

Non-Key Component Splits The split of the non–key components is generally defined as to where the non–key components are obtained with respect to the distillate or bottoms stream. One can have two types of situations concerning the split of the non–key components: Sharp split – Non-distribution of non-keys Split – Distribution of non-keys

Distributed and undistributed components Components that are present in both the distillate and the bottoms product are called distributed components. - The key components are always distributed components Components with negligible concentration (<10-6) in one of the products are called undistributed components. A B C D E G light non-distributed components (will end up in the overhead product) key components heavy non-distributed components (will end up in bottoms product)

Non-distribution of NK’s Non–distribution of non–keys means that essentially all of the non–keys are obtained in either the distillate stream or the bottoms stream. Non–distribution of non–keys can be assumed when: All of the non-keys are either HNK’s or LNK’s The fractional recoveries of the LK in the distillate and HK in the bottoms are relatively large.

Distribution of NK’s Distribution of non–keys means that the non-keys are not sharply split between the distillate stream or the bottoms stream. Distribution of non–keys occurs when: Not all of the non-keys are either HNK’s or LNK’s – we have NK’s. The fractional recoveries of the LK in the distillate and HK in the bottoms are not relatively large.

How to determine the keys (LK and HK) and the non–keys (LNK’s, HNK’s and NK’s) in MCD? The classification of components in MCD can be determined from their relative volatilities. Relative volatility is defined as the ratio of the K values for two components, which is trivial for a binary system. In order to use relative volatilities in MCD, we choose a reference component and define all other component volatilities with respect to the reference component.

How to determine the keys (LK and HK) and the non–keys (LNK’s, HNK’s and NK’s) in MCD? The relative volatility for the reference component, of course, will be 1. We can then define relative volatilities using equilibrium coefficient K values for each component, e.g., from the DePriester charts for hydrocarbon systems. The choice of the reference component depends upon the problem, but in general it will be the HK component since it is less volatile than the LK component.

Key and Non-Key Example Consider a distillation column with the following feed components: propane n–butane n–pentane n–hexane The recoveries for n–butane and n–pentane are specified for the distillation. What are the key and non–key designations for this separation?

Component volatilities can be determined from the K values. From the DePriester charts, the order of volatility is: propane > n–butane > n–pentane > n–hexane Since the recoveries of n–butane and n–pentane are specified…

We have: Volatilities propane > n–butane > n–pentane > n–hexane Component Designation Propane Light Non–Key n–butane Light Key n–pentane Heavy Key n–hexane Heavy Non–Key

If the recoveries of n-butane and n-hexane are specified: Volatilities propane > n-butane > n-pentane > n-hexane Component Designation Propane Light Non-Key n-butane Light Key n-pentane Non-Key n-hexane Heavy Key

If only the recovery of n–butane is specified: Volatilities propane > n–butane > n–pentane > n–hexane Component Designation Propane Light Non–Key n–butane Key n–pentane Non–Key n–hexane Non–Key

2. Calculation of the amounts of distillate and bottom products in sharp splits Clear distribution: Selected components are neighbors with large . All components heavier than the heavy key component are distributed in bottom product and others in top product. The concentration of other components can be calculated with mass balance. Strictly, clear distribution is only a simplification for convenient and not a real situation.

The total equations The total variables Material balance Sum equations We have designing variables

For LNK For HNK flow rate of i component in feed flow rate of i component in distillate flow rate of i component in bottoms For HNK

The flow rate of distillate The flow rate of bottom

Distillate composition (xD)

Bottom composition (xW)

⑴ The distillate composition of LK (xLK,D) and bottom composition of HK (xHK,W) are specified. ⑵ The distillate composition of HK (xHK,D) and bottom composition of LK (xLK,W ) are specified.

⑶ The distillate and bottom compositions of LK (xLK,D , xLK,W ) are specified. ⑷ The distillate composition and recovery of LK (xLK,D, φLK ) are specified.

3. Calculation of multi-component distillation Short-cut method Method simplified to two-component distillation of two crucial components (pseudo-binary separation). Basic Equations: Fenske Eq., Underwood Eq. and Gilliland correlation

(1) Minimum Number of stages by Fenske’s Equation: Material balance

Phase equilibrium Operating line For second equilibrium stage For Nth stage For j component

For condenser Average relative volatility

Geometric mean relative volatility i- LK，j- HK Geometric mean relative volatility 芬斯克公式还可用摩尔、体积或重量之比表示

Check of Non-key distribution 式中，i为非关键组分；r为关键组分或参考组分；αir为i相对于r的相对挥发度

(2) Minimum Reflux Ratio by Underwood’s Equations:

(3) Erbar-Maddox correlation

(4) Fedd stage For rectifying stages For stripping stages NR和NS