Image Descriptions

Reboiler

Figure 1.1. Schematic representation of kettle type reboiler as well as vertical and horizontal thermosiphon reboilers with bottom product control.

Kettle-type reboiler. Kettle-type reboiler showing liquid flowing (represented by blue arrow) from the column into the shell side of the boiling vessel on the right hand side in which there is a horizontal tube bundle submerged in the liquid. Boiling takes place on the outside of this bundle with vapour, represented by red arrow, flowing through a pipe section connecting the boiling vessel and the column to the bottom tray of the distillation column. The baffles in the boiling vessel promote liquid mixing by redirecting liquid flow which eventually overflows through weir and a pipe down to a pneumatic valve that is controlled by a level controller labeled as "LC". The level controller takes liquid level signal from two solid lines between the bottom and top of the boiling vessel. The heating fluid enters from top of the boiling vessel, flows through the tube bundle, and exit from the bottom.
Vertical thermosiphon reboiler. A vertical thermosiphon reboiler has liquid enter a secondary column from the distillation column. The source liquid is heated by a series of vertical tubes inside the secondary column. This tube bundle is heated by a heating fluid which passes through them. When the heating fluid has passed through all the tubes it exists through a pipe at the bottom. Once the source liquid is heated to the point where it is vapour, it passes back into the distillation column along with some liquid via a horizontal pipe, near the top, connecting the distillation and secondary columns. This vapour flows upwards while the liquid circulates back to the bottom of the column and the secondary column the distillation column via a tray with bubble caps, if the liquid from distillation tray above falls back into the distillation column. A level control in the secondary column allows for the release of liquid (labelled bottom products) via a valve in the case where too much liquid enters the secondary column.
Horizontal thermosiphon reboiler. A horizontal thermosiphon reboiler has liquid enter a secondary column from the distillation column. The source liquid is heated by a series of horizontal tubes inside the secondary column. This tube bundle is heated by a heating fluid which passes through these tubes. When the heating fluid has passed through all the tubes it exists through a pipe. Once the source liquid is heated to the point where it is vapour, it passes back into the distillation column via a pipe near the top, connecting the distillation and secondary columns along with some liquid. This vapour flows upwards into the distillation column through a bubble cap tray while the liquid circulates back to the bottom of the distillation column and the secondary boiling column. A level control controls the vapour leaving the secondary column at the top as well as the liquid leaving the secondary column (labelled bottom products) via pipe controlled by a valve at the bottom.

Figure 1.2. Schematic representation of a working vertical thermosiphon reboiler in relation to heat transfer from steam condensation.

Reboiler is represented by two containers that communicate with each other at the top and the bottom: the one on the left is a quarter full with liquid that moves towards the one on the right through a tube. The liquid boiling in the reboiler renders a two-phase mixture of liquid and vapour, which has a lower density than the liquid mixture at the bottom of the distillation column. The density difference creates a static head that drives the natural liquid circulation and a uniform boiling in the reboiler. The vapour bubble sizes and their distribution in the reboiler are related to heat transfer from the heating media through the temperature and heat flow rate. At the bottom of the boiling zone, small vapour bubbles coalesce to form large bubbles, and the large bubbles float up and break at top of the boiling zone, resulting in the unique profiles of vapour and temperature. The vapour and liquid mixture passes through the upper tube to the left container or the bottom of the distillation column. The vapour stream flows upwards into the bottom tray of the distillation column, while the liquid circulate back to the secondary container. The steam enters at the top of the right container, flows through the shell and exits at the bottom the vessel. The right container also has another tube at the bottom that moves through it the bottom product out. On the right side there is a graph where $ 0x $ axis is Vapour pressure and temperature and $ 0y $ axis is Heating tube height. The graph shows a red and a blue line both starting at the origin and looking like half a parabola. The blue line is labelled Vapour pressure and is inwards while the red line is labelled temperature. A vertical line is perpendicular on the right side of the $ 0x $ axis and it is labelled Steam temperature. The distance between the red line and the Steam temperature line is labelled $ \Delta T $ driving force.

Figure 1.3. T-xy diagram of vapour-liquid equilibrium for binary ethanol-water system at atmospheric pressure (the red square line represents bubble point curve or liquid composition, and blue circle line represents dew point curve or vapour composition).

A graph with horizontal axis as “Mole Fraction of ethanol in liquid and vapour”, and the vertical axis as “Boiling temperature (°C)”. The horizontal axis ranges from 0 to 1, and the vertical axis ranges from 75 to 105 degrees. One curve represents the “Liquid or bubble point” and has points plotted on the curve denoted by square shapes which are spaced out equally horizontally by an interval of 0.02. The other curve represents the “Vapour or dew point” and has points plotted on the curve denoted by circles. Both curves start at $(0,100)$. The curves are described as follows:

The “Liquid or bubble point” curve has a rapidly decreasing slope that looks like a decaying exponential curve, it levels out around 79 degrees when it reaches 0.8 on the horizontal axis.
The “Vapour or dew point” curve is a decreasing slope resembling an exponential decay curve that starts off concave down, but has an inflection point at $ (0.5,85)$, which continues a decrease slope that eventually levels out around 79 degrees as well when it reached 0.8 on horizontal axis.

Figure 1.4. Specific heat transfer rate or reboiler duty as a function of driving force in a vertical thermosiphone reboiler.

A graph with temperature difference $ \Delta T = T_w-T_s $ ranging from 0°C to 100°C on the horizontal axis. The vertical axis, $ Q_R/A \text{ kW/m}^2 $, ranges from 0 to 60 ${ kW/m}^2$. Nucleate boiling happens between approximately 4°C and 10°C where the graph increases rapidly from $ (4, 2)$ to a maximum of about $(10, 42)$. Transition boiling happens between roughly 10°C 1and 59°C where the graph decreases in a shallow curve from about $(10, 42)$ to $(50, 22)$. Film boiling occurs between 50°C and 95°C where the graph increases almost linearly from $(50,22)$ to $(95, 48)$.

Figure 1.3. Solution: T-xy diagram of vapour-liquid equilibrium for binary ethanol-water system at atmospheric pressure (the red square line represents bubble point curve or liquid composition, and blue circle line represents dew point curve or vapour composition).

The “Liquid or bubble point” curve has a rapidly decreasing slope that looks like a decaying exponential curve, it levels out around 79 degrees when it reaches 0.8 on the horizontal axis.
The “Vapour or dew point” curve is a decreasing slope resembling an exponential decay curve that starts off concave down, but has an inflection point at $ (0.5,85)$, which continues a decrease slope that eventually levels out around 79 degrees as well when it reached 0.8 on horizontal axis.
The vertical green line from the liquid mole fraction (0.14) intercepts with the bubble point line where the boiling point reads 85° C, the horizontal purple line from the intercepts intercepts with the dew point line at which the vapour mole fraction reads 0.5 from the intercepts back to the horizontal axis.

Distillation Column

Figure 2.1. Schematic representation of a packed distillation column with random packings.

On the right side a reboiler is shown: 3 quarters full of liquid and also steam shown coming at the top and leaving as steam condensate at the bottom right, while from the bottom of it bottom product is leaving the reboiler. The reboiler is connected through two conduits at the top and the bottom to the distillation column which contains just a bit of liquid at the bottom of it. Right above the top conduit that comes from the reboiler there is a packing support plate that holds Random packing all the way to the top. At the top and in the middle there is liquid distributors. On the left side of the distillation column, midway, there is a conduit coming it and labelled Feed. Above the top liquid distributor, still inside the column, there is a conduit that is labelled Reflux and connects to the Reflux drum (which is a vertical container half-full with liquid) and continues with the Top product conduit. The top of the reflux drum is connecting at the top to the condenser which then connects through another conduit to the top of the distillation column.

Figure 2.2. Schematic representation of a tray distillation column with multiple trays in cascade and cross-flow pattern.

Three components make up the system: the reboiler, the distillation column, and the condenser.

The Reboiler: The reboiler has liquid filling approximately three-quarters of the volume. A steam-filled pipe enters near the top of the liquid level and coils down the reboiler before leaving as an output (steam condensate). At the bottom of the reboiler, the bottom product is directed away. The reboiler has two connections with the distillation column. One connection is to the bottom of the distillation column and is full of liquid. The other connection is near the top of the reboiler and is void of liquid.
The Distillation Column: Shaped like a vertical cylinder, the column is made up of a series of five trays arranged in an offset manner where every other tray is parallel to one another. Each tray has an area holding a boiling liquid represented by liquid droplets with small circles within continuous liquid phase. Red arrows pointing up towards the base of each tray represent upward flowing vapour. To the side of each tray, downward facing blue arrows represent flowing liquid to the tray below it. The bottom of the distillation column is filled with liquid. In the middle of the distillation column a feed arrow points to the middle tray. Near the top of the distillation column, reflux is directed at the topmost tray. At the top of the distillation column, red arrows direct the vapour flow towards a pipe connected to the condenser.
The Condenser: Represented as a rectangle with multiple horizontal lines and a liquid filled tank directly underneath, the condenser takes input from the distillation column and has arrows representing cooling water to and from the rectangular area. At the bottom of the liquid filled tank, arrows direct the top product away from the apparatus while reflux is redirected to the distillation column.

Figure 2.3. Bubble cap tray: design, vapour and liquid flow path, and performance.

An illustration of a cross-section of a bubble cap tray, which consists of

a perforated horizontal plate with an upward protruding conduit (riser) affixed to the perforation;
a cap, larger and longer than the riser, sits loosely on the riser so that there is a uniform gap between the riser and the cap on all sides; and
the space between the cap and riser is partially blocked but vented via smaller slots at the bottom of the cap.

The following dimensions are noted:

$ h_s $, the slot height, is the distance from the plate to about half the riser height.
$ h_w $, the weir height, is the distance between the plate and the upper edge of the riser.
$ h_l $, the clear liquid height, is approximately the distance between the bottom plate and the upper edge of the cap.
$ h_f $, the froth height, measured from the plate, is approximately twice the distance between the upper edge of the cap and the plate.

Note that $ h_f \lt h_l \lt h_w \lt h_s $. Flow rises from below the plate, through the riser, flow down the gap in between the riser and the cap, and bubble into the area surrounding the cap, through the vapour bubbles.

Figure 2.4. Schematic representation of McCabe-Thiele shortcut method.

A rectangle representing Tray n has inflows and outflows represented as arrows pointing into and away from the rectangle.

Inflow $ V,y_{n+1}$
Outflow $ V,y_n $
Inflow $ L,x_{n-1}$
Outflow $ L,x_n $

Figure 2.5. Schematic representation of McCabe-Thiele shortcut method for ethanol-water separation.

A graph with “Ethanol mole fraction in liquid, x” on the horizontal axis ranges from 0 to 1. The vertical axis, “Ethanol mole fraction in vapour, y” ranges from 0 to 1 as well. The following lines/functions are on the graph.

A straight red line denoted by the letter B represents “45 degree line for L = V” which is an identity function. A blue curve function denoted by the letter A represents the “Equilibrium line”, which almost appears to have a logarithmic shape that quickly diverges above the line B from 0 to 0.3 on the horizontal axis, but an inflection point occurs approximately around $(0.3,0.56)$, then the line A gradually starts to line up with line B by the time it reaches 1 on the horizontal axis.
Line C, known as the “Enriching operation line” is a straight line function that starts at $ (0.14, 0.44)$ and ends at $(0.64)$. Line C is nearly parallel to line A and is in between the curve A and line B.
Line D is known as the “Stripping operating line”, and it is a straight line that starts at $ (0.04,0.04) $ and ends at $ 0.14,0.44 $, it also lies between line A and B, and the end of the line D connects with the beginning of line C.
Line E, known as the “Feed or q line”, is a vertical line that starts at $ (0.14, 0.14) $, and ends at $ (0.14,0.44) $, which is the beginning of line C.
Line F, known as the “Equilibrium stages”, is a staircase function which lies between line A and lines D and C. The staircase function corners touch both line when rising, and lines D or C when going towards the right. It starts at $ 0.04,0.04 $, then goes vertically up to $ 0.04, 0.3 $, then to the right to $ 0.1,0.3 $ touching line D, then vertically again to approximately $ 0.1, 0.45 $, then to the right to approximately $ 0.16, 0.45 $, then vertically to $ 0.16, 0.5 $, then to the right to $ 0.26, 0.5 $, then up to approximately $0.26, 0.55$, then right to approximately $ 0.41, 0.55 $, then up to approximately $ 0.41,0.62 $, then to the right and ends at $ 0.64,0.64 $.

Figure 2.6. Variations of distillation column performance and hydraulic characteristics with operation conditions.

The chart illustrates the effects of liquid flow rate on the horizontal axis and vapour flow rate on the vertical axis on distillation column hydraulics and efficiency. A diagonal dashed line passing through the origin indicates constant L/V, i.e., equal vapour and liquid flow rate. A trapezoidal-shaped box shows the effect of different vapour and liquid flow rates on the following column hydraulic characteristics:

Increasing weeping along the bottom edge of the box with increasing liquid flow rate at lower vapour flow rate.
Increasing entrainment along the left edge of the box with increasing vapour flow rate at lower liquid flow rate.
Increasing jet flooding along the top edge of the box with decreasing liquid flow rate at higher vapour flow rate.
Increasing downcomer flooding along the right edge of the box with increasing vapour flow rate at higher liquid flow rate. As all these hydraulic characteristics along the edges reduce column efficiency, the optimal operation zone for vapour and liquid flow rates is in the center of the box with the blank circle indicating the optimal operation zone. The circle is bisected by the diagonal constant L/V line.

Figure 2.7. Fair’s flooding correlation for distillation column with cross-flow trays: column capacity factor as a function of flow parameter for different tray spacing values.

A line graph comparing the flow parameter ($ F_{LV} $) versus plotting the flooding capacity factor versus ($ C_{SB} $) for different tray spacing in millimeters. axes start at a value of 0.01. The six lines on the graph represent six different tray spacings in millimetres labelled ‘A’ to ‘F’. The tray spacing for A is 152.4 millimetres, B is 228.6, C is 304.8, D is 457.2, E is 609.6, and F is 914.4. The higher the tray spacing the higher the starting value of the flooding capacity factor with A beginning at just above 0.045 and F starting just below 0.14. All values of the flooding capacity factor decrease as the flow parameter increases following a negative exponential pattern (equation 23) with the lines never crossing.

Condenser

Figure 3.1. Schematic representation of distillation condensers: shell-and-tube heat exchanger (a) and shell-and-coil heat exchanger (b).

Shell-and-tube heat exchanger: The shell-and-tube heat exchanger or condenser intakes cooling water, circulates it through the unit in tubes where it outflows on the same side as where it enters. Vapour enters at the top of the unit and passes the shell side through a system of baffles. Overhead liquid exits the system on the bottom of the unit.
Shell-and-coil heat exchanger: The shell-and-coil heat exchanger has cooling water flow through coiled tubes from one side of the unit to the other. Vapour enters on the top of the unit and overhead liquid exits on the bottom.

Feed Line

Figure 4.1. Schematic diagram of a typical feed line process unit for distillation column with (a) pump, (b) pneumatic valve, (c) flow meter, (d) heat exchanger, and (e) manifold and valves.

A schematic diagram illustrating the feed flow rate control and reconditioning for distillation operation. A pump (a) below the feed storage reservoir pumps liquid feed at ambient temperature to a pneumatic valve (b) that is controlled by a flow controller labeled as FC. The flow controller takes input flow signal from a flow meter (c) labeled as FM, and adjusts the pneumatic to control the feed flow to a desired flow rate. The feed flow then flows through a heat exchanger (d) with heating fluid entering and exiting through the red arrows being adjusted to achieve desired feed temperature or condition. The conditioned feed is then directed to an individual feed tray through the valves on a manifold (e).

Figure 4.2. Effects of various feed conditions on the binary distillation of ethanol-water system.

The graph depicts the effects of feed conditions on distillation, with $ x $ on the horizontal axis ranging from 0 to 1 and $ y $ ranging from 0 to 1. The following lines are on the graph.

A straight red line represents 45 degree line with $ x=y $ along the line.
A blue curved line function represents the “Equilibrium line” which spans from $ x=0 $ to $ x=1 $.
A dashed line labeled as $ x_F $ starts from feed mole fraction (0.3) and intercepts with starting 45 degree line.
Line A represents feed line starting from the intercept to the equilibrium line for the feed as superheated vapour with q value less than 0.
Line B represents feed line starting from the intercept to the equilibrium line for the feed as saturated vapour with $ q $ equal to 0.
Line C represents feed line starting from the intercept to the equilibrium line for the feed as partial vapourized liquid with $ q $ value between 0 and 1.
Line D represents feed line starting from the intercept to the equilibrium line for the feed as saturated liquid with $ q $ value equal to 1.
Line E represents feed line starting from the intercept to the equilibrium line for the feed as subcooled liquid with $ q $ value greater than 1.

Figure 4.3. Schematic representation of the effects of feed conditions on the liquid and vapour flow rates in rectifying and stripping sections of distillation column.

Flow rates on feed tray are divided into two vertically arranged sections: rectifying and stripping sections, with the rectifying section situated above the stripping section. Liquid flows down from the rectifying section into the stripping section, whereas vapour flows from the stripping section to the rectifying section. Feed stream is introduced to the column at the interface between the rectifying and the stripping sections. Molar flow in the feed stream, $ F, q $, is split into vapour flow, $ 1-qF $, and liquid flow, $ qF $. Rate of liquid coming into the interface from the rectifying section is $ L $ and, after the addition of the molar flow in the feed stream, the new flow rate $ \bar{L} $, going out of the interface into the stripping section is $ \bar{L}=L+qF $. Rate of vapour coming into the interface from the stripping section is $ \bar{V} $ and, after the addition of the molar flow from the feed stream, the new flow rate $ V $, going out of the interface into the rectifying section is $ V=\bar{V}+(1-q)F $.

Overhead

Figure 5.1. Schematic diagram of overhead reservoir and flow control for distillation process with (a) pump, (b) pneumatic valves, level sensor (LS), level controller (LC), flow meter (FM), and flow controller (FC).

A schematic diagram of an overhead reservoir and flow control system for the distillation process. A pump below the overhead reservoir creates directional flow through the distillation process. Level sensors (LS) at the overhead reservoir measure the liquid level and signal to a level controller (LC) which controls a pneumatic valve permitting a fixed amount of liquid through the process. The liquid that gets let through this pneumatic valve separated into either distillate or reflux. Both the distillate and reflux are measured with individual flow meters (FM) but the flow meter associated with the reflux sends a signal to a flow controller (FC) controlling a pneumatic valve located once the reflux is separated. These pneumatic valves can ensure the liquid flow rate out of the reservoir is the same as the overhead flow rate coming out of the condenser.

Bottom

Figure 6.1 shows a typical diagram for the bottom flow control.

Reboiler is shown in the form of a cup with a conduit connected to its bottom. The conduit connects to a pump inlet. The pump outlet connects to a pneumatic valve inlet, which is controlled by the level controller (LC) using signal from a level sensor (LS) located at the bottom of the reboiler. The pneumatic valve outlet connects to a flow meter (FM) inlet, whose outlet carries away the bottom product (B). Bottom product flows from the reboiler, through the pump, valve, flow sensor, on to its destination.

Performance

Distillation Performance: A Tray in Action showing the cross-flow pattern of vapour and liquid on the tray, the hydraulic characteristics such as clear liquid height and froth height. It is these hydraulic characteristics that determine the liquid-vapour contact time and tray efficiency performance.

An illustration of a cross-section of a bubble cap tray, which consists of:

a perforated horizontal plate with an upward protruding conduit (riser) affixed to the perforation;
a cap, larger and longer than the riser, sits loosely on the riser so that there is a uniform gap between the riser and the cap on all sides; and
the space between the cap and riser is partially blocked but vented via smaller slots at the bottom of the cap.

The following dimensions are noted:

$ h_s $, the slot height, is the distance from the plate to about half the riser height.
$ h_w $, the weir height, is the distance between the plate and the upper edge of the riser.
$ h_l $, the clear liquid height, is approximately the distance between the bottom plate and the upper edge of the cap.
$ h_f $, the froth height, measured from the plate, is approximately twice the distance between the upper edge of the cap and the plate.

Design and Optimization

Figure 1. General distillation column design and design methods.

Flow chart starts on the left with Design Specifications and Objectives box. One arrow leads to a box called Column Height box and the second to the Column Diameter box.

The Column height box lists the following information:

Number of trays
Reflux ratio
Optimum feed location
Tray spacing

From the Column Height box there are two more arrows which lead to two boxes. The first box is Shortcut Method which lists the following:

Vapour-liquid equilibrium
McCabe-Thiele method
FUG Method
Distillation efficiency

The second box is Rigorous Method which lists the following:

Vapour-Liquid equilibrium
Tray hydraulics
Distillation efficiency
Steady state simulation
Unsteady state simulation

The Column Diameter box lists the following:

Tray spacing
Column diameter as per design vapour flow

The Column Diameter box leads to the Fair’s Design Method box which lists the following:

Column flooding
Tray geometry

Figure 2. Iterative distillation design scheme based on experimental design and validation.

Flow chart starts with a Design Specifications and Objectives box on top. One arrow leads to a box called Design Theories and Correlations.

There are two boxes below the Design Theories and Correlations box: Experimental Objectives & Design and Implementation of the Design.

An arrow points from Design Theories and Correlations to Experimental Objectives & Design which lists the following

Measure design parameters
Generate operation data for design validations
Gather experimental observation

A double sided arrow points from Design Theories and Correlations to Implementation of the Design which lists the following:

Determine design parameters
Validate design correlations
Execute the design using validated correlations
Justify the final design as per experimental observation

A double sided arrow also connects the Experimental Objectives & Design and Implementation of the Design boxes.

Figure 3. Materials and enthalpy flow on Tray j. V=vapour molar flow, L=Liquid molar flow, x=liquid mole fraction, y=vapour mole fraction, h is the enthalpy of the liquid, H is the enthalpy of vapour, F=feed, and M= liquid mole holdup on the tray.

A rectangle representing Stage $ j, M_j $ has the following material and enthalpy flows represented as arrows pointing into or away from the stage:

Inflow $V_{j+1}, y_{i,y+1}, H_{j+1}$
Outflow $V_{j}, y_{i,j}, H_j$
Inflow $ L_{j-1}, x_{i,j-1}, h_{j-1} $
Outflow $ L_j, x_{i,j}, h_j $
Inflow $ F,x_F,h_F $

Figure 4. Distillation design optimization based on annualized cost analysis.

A graph of the Reflux ratio R on the horizontal axis and the Annualized cost on the vertical axis showing only the first quadrant. Three curves labelled A, B, and C appear on the graph. All three curves have a starting location on a vertical line labelled $\text{R}_{\text{min}}$.

Curve A is the annualized capital cost and starts above Curve B and below Curve C with a negative slope and a concave up shape. It eventually flattens out, intersecting curve B, before continuing with a slightly positive slope.
Curve B is the annualized operation cost and starts below Curves A and C. It has a linear shape. It does not intersect Curve C.
Curve C is the total annualized cost. It starts above both curves A and B. It starts with a negative slope, is concave up, and where it reaches a minimum has a vertical line drawn to the horizontal axis labelled $ \text{R}_{\text{optimum}} $. The curve then continues with a positive slope.