Project Topics
Get Complete Project Material File(s) Now! »
Non-ideal system
When the components have different properties the mixture is non-ideal and the interaction between the molecules differ greatly from the ones of a pure component. [6] [7] If the molecules in the mixture strongly repel each other the partial pressure is higher than ideal and then a minimum boiling azeotrope is formed. When the molecules strongly interact with each other a maximum boiling azeotrope is formed. With azeotropes it is impossible to separate the components with simple distillation. Other techniques i.e. azeotropic or extractive distillation can then be used. [2]
Relative volatility
The relative volatility (α) is a value that shows how much the volatility of different components in a mixture differs. The volatility of one component is analyzed in relation to another component, often the component with the highest boiling point is used as reference. Since the volatility of one component is defined as the partial pressure of that component divided by its mass fraction, the relative volatility can be described as in equation (6). 𝛼=𝑃𝑖/𝑥𝑖𝑃𝑟/𝑥𝑟 (6).
α: relative volatility.
Pi: partial pressure of lighter component.
Pr: partial pressure of reference component.
xi: mass fraction of lighter component.
xr: mass fraction of reference component.
Simple distillation
The simplest way of distilling a mixture is to just add heat to the mixture until it boils, condense the vapor and withdraw the condensed vapor as distillate. It is called simple distillation. The equilibrium between the liquid and gas phase is formed in a boiler and no fractionating column is used. Simple distillation can be used when the relative volatility is large or when the yield of the product is of less importance. This technique is called equilibrium distillation when it is done continuously and it is called differential distillation when it is done batch wise. When equilibrium distillation is performed the stream of the mixture i.e. the feed is added to the boiler, the distillate is taken out in the top and the bottom product is taken out in the bottom of the boiler continuously. When differential distillation is used the mixture is added to the boiler and the distillate is withdrawn in the top of the boiler. When the distillation has finished the bottom product which is left in the boiler is taken out.
Fractional distillation
When the yield of the product is of great importance or the boiling points between the components in the mixture is close it is better to use fractional distillation instead of simple distillation. In fractional distillation a column which contains multiple plates is used. On each plate equilibrium between liquid and gas phase is formed so that an enrichment of the more volatile component can be achieved. The more plates that are used, the more enriched the volatile component will be in the distillate.
Continuous distillation
When the distillation is performed continuously the mixture is added and the product is withdrawn continuously. The mixture is added to a determined position on the column in a steady flow, this is called the feed (Ḟ). The part of the column that is above the place for the feed is called the rectifying section and the lower part is called the stripping section. The composition of the flows is consistent.
The mass balance for the continuous process is described in equation (8). Ḟ=Ḋ+Ḃ (8).
Ḃ Flow rate of bottom product, continuous distillation [g/min].
Ḋ Flow rate of distillate [g/min].
Ḟ Flow rate of the Feed, continuous distillation [g/min].
When separating a mixture by continuous distillation a light component and a heavy component is chosen. The light component and the heavy component is chosen so the separation can be performed as easy as possible and at as low cost as possible. The light component is the component which has a lower boiling point, and thus is more volatile, than the heavy component. This means that the distillate, taken out in the top of the column, contains the lighter components and the bottom product, taken out in the bottom of the column, contains the heavier components. The distillate and bottom product components can then be further separated in a second distillation step respectively until all the wanted products have been achieved. For each step a light and heavy component is chosen until all components have been separated. For a mixture with N number of components, at least N-1 columns are needed to separate all the components. Alternatively a fractionating column with multiple outtakes can be used as is commonly done in the oil industry. The number of plates and reflux ratio is not varied within each separation step when distillation is performed continuously. [2] [9]
Batch distillation
In batch distillation a certain amount of the mixture to be distilled is added to the boiler and then the mixture is distilled until enough of the wanted components have been taken out as distillate at the top. This means that the composition of the distillate and the liquid in the boiler will change with time. After finished distillation the amount left in the boiler is taken out in the bottom. This procedure is then repeated. A simple distillation can be described by Rayeigh’s equation, see equation (9). ∫𝑑𝑥𝑖𝑦𝑖−𝑥𝑖𝑥(𝑡+1)𝑥(𝑡)=𝑙𝑛𝑊(𝑡)𝑊(𝑡+1) (9).
W(t), W(t+1): mass of liquid in the boiler at a certain time [g].
The calculation for a fractional batch distillation is more complicated. Fenske, Underwood and Gilliland has defined equations (modified by Eduljee) that can describe a fractional batch distillation, see Appendix 3.
Batch distillation is often used when production volumes are not very large or when a multicomponent system is to be separated since it in general is more flexible and easier to control than a continuous process. One component at a time may be separated from the mixture and this is useful if the composition of the feed changes over time. Another important aspect of using batch distillation is traceability where each batch has a unique identity. [8]
Azeotropic and Extractive distillation
In case of an azeotrope in the system two techniques are used to separate the components; azeotropic and extractive distillation. In both techniques an entrainer is used to alter the relative volatility between the components in the azeotrope. The difference is how the entrainer is applied to the system. In azeotropic distillation the entrainer is added to the mixture and a new azeotrope is formed that lets one of the components of the former azeotrope to be distilled off. Both batch and continuous distillation can be used. With extractive distillation the entrainer is added to the system in the top of the column and then only continuous distillation can be used.
Reflux divider and reflux ratio
A switch, called reflux divider, for controlling the reflux ratio is often used and it is placed between the column and the condenser, see Figure 6. There are different types of reflux dividers. One type has an arm onto which the condensed liquid is led down from the condenser. The arm can be turned into two positions, one where the liquid drips back into the column and another where the liquid is taken out as distillate. The switch is regulated by time. A magnet is turned on at determined time intervals to position the switch over the outlet and back over the column. Another type is a switch that divides the vapor to two different condensers, one for the reflux and one for the withdrawal. The third type is most common in large scale columns: the condensed distillate is taken in a separate pipe and that flow is divided by a three way control valve. The ratio between the amount of distillate returned and taken out is called the reflux ratio.
Table of contents :
1 INTRODUCTION
1.1 BACKGROUND
1.2 AIM OF THE PROJECT
1.3 PROJECT OBJECTIVES
2 THEORY
2.1 PHASE EQUILIBRIUM
2.1.1 Ideal system
2.1.2 Non-ideal system
2.1.3 Relative volatility
2.2 DISTILLATION
2.2.1 Simple distillation
2.2.2 Fractional distillation
2.2.3 Continuous distillation
2.2.4 Batch distillation
2.2.5 Azeotropic and Extractive distillation
2.3 EQUIPMENT
2.3.1 Columns and packing materials
2.3.2 Reflux divider and reflux ratio
2.4 MCCABE THIELES GRAPHICAL METHOD
2.5 SHORT CUT METHOD
3 EXPERIMENTAL METHODOLOGY
3.1 INITIAL TESTS WITH BINARY SYSTEMS
3.1.1 Bubble cap column
3.1.2 Initial evaluation method
3.1.3 Random packed column
3.1.4 Sampling from the boiler using a tube and a syringe
3.1.5 Sampling in the boiler using an IR-probe
3.2 TEST WITH MULTICOMPONENT SYSTEMS
4 RESULT AND DISCUSSION
4.1 INSULATION OF THE EQUIPMENT
4.2 EQUILIBRATION TIME
4.3 DISTILLATION VELOCITY
4.4 SAMPLING
4.5 MEASUREMENT WITH IR
4.6 MCCABE THIELE GRAPHICAL METHOD
4.7 SHORTCUT METHOD
4.8 RELATIVE VOLATILITY COMPARISON OBSERVED AND LITERATURE
4.9 AZEOTROPES AND MULTICOMPONENT SYSTEMS
5 CONCLUSION
6 REFERENCES .
