EXPERIMENT: Pressure drop over a bubble cap plate.
AIM : This experiment is conducted in an experimental test ring in which the effect of the variation in vapor and liquid flow rates on the pressure drop across bubble cap plates is simulated using air and water to represent the vapor and liquid respectively.
THEORY: In a pilot tube the relation between the gas velocity and pressure drop is
Where, is the differential pressure as expressed in the head of the fluid flowing.
Where,
’=level difference
= density of air= 1.21 kg/m3
’=density of liquid (water) =1000 kg/m3
.
Volumetric gas flow rate V (m3/s) is calculated by assuming a flat velocity profile inside the tube. This is justifiable as the flow is turbulent
V= v * s’
Where s’= pipe cross sectional area
Vapor velocity inside the distillation column (v) =
PROCEDURE:
• Allow water to pass through the equipment to ensure the plates are loaded with liquid and then stop the water flow. Wait till the excess water is drained from the plates.
• Set the inclined manometer to a suitable inclination to measure the differential pressure.
• Switch on the blower and measure across the pilot tube, keeping the air flow rate constant. Select the appropriate valves only (one “high” valve and one “low” valve) and measure the pressure drop across the three plates.
• Reuse the air flow rate by partially closing the inlet of the blower and repeat the above procedure to obtain pressure drop at various air i.e. vapor velocities.
• Repeat the above with two different water (i.e. liquid) flow rates which are kept constant.
• Measure the water flow rate by measuring the amount of water collected in a known time interval.
Discussion
A distillation column can use either trays or packing. Their mechanisms of mass transfer differ, but the key for both is a good approach to equilibrium through the generation of large amounts of interfacial area. In a trayed column, liquid flows down the column through down comers and then across the tray deck, while vapor flows upward through the liquid inventory on the tray.
The most common gas disperser for cross-flow plates has been the bubble-cap. This device has a built-in seal which prevents liquid drainage at low gas flow rates. Gas flows through a center riser, reverses the flow under the cap, passes downward through the annulus between riser and cap and finally passes into the liquid through a series of openings or “slots” in the lower side of the cap.
Trays and packing materials are widely used in distillation. Normally packed columns are used for gas-liquid and liquid-liquid contacting operations. That Means packed columns are used for distillation, gas absorption and liquid-liquid extraction. However, these Columns are used extensively for absorption and, to a limited extent, for distillation. But some time the packing materials are not suitable for the distillation process. Those times we can use trays to achieve the requirement.
The types of trays used in distillation
Bubble cap tray
Valve trays
Sieve trays
High Capacity Trays
Cartridge Trays
A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapour. Vapour rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.
Advantages of Bubble cap plate
• Bubble cap trays are used primarily where large turndown ratios are required.
• Their construction allows very low liquid rates to be handled with little or no leakage.
• Due to their ability to operate at low vapor and liquid rates, bubble cap trays are used in a significant portion of fractionation tray installations.
Disadvantages of bubble cap trays
• Capacity of perforated trays is often plotted as a function of percent hole area. Actually, the capacity of a perforated tray is not much affected by hole area unless the lack of hole area increases pressure drop and down comer backup to unacceptable values. For example, if a perforated tray has sufficient hole area to limit dry tray pressure drop to a reasonable value (about 2" to 3" liquid at 80% flood) the perforated tray will have the same capacity as a valve tray. A bubble cap tray cannot be designed to have as much hole area as a valve tray and will, therefore, have less capacity.
• Bubble cap trays cannot be used to achieve high flow rates.
• Not like the other trays, for bubble cap trays it has low efficiency.
Distillation process
Distillation is the dominant process for separating large multi component streams into high purity products. So, the chemical process industries’ ongoing quest to improve energy utilization, reduce capital costs, and boost operating flexibility is spurring increasing attention to distillation column optimization during design. Designers often approach column optimization in an iterative manner, heavily relying on vendor experience and information. A good understanding of mass-transfer and pressure-drop fundamentals, as they relate to optimization, will enable the column designer to independently judge vendor offerings and effectively determine the optimal equipment design. This article will address the following optimization goals:
(1) maximizing theoretical stages per height of section or column,
(2) minimizing pressure drop per theoretical stage,
(3) maximizing the operational range, turn-down, or turn-up.
Tray pressure drop
Typical tray pressure drops lie in the range of 250 - 1500 N/m*m (or 2.5 mbar - 15 mbar or 25 - 150 mm Water Column, in whatever units one prefers). Usually, the drop in pressure caused by gas flowing through a tray is small in comparison to the system pressure. Except for vacuum columns, where it can become quite substantial and the gas velocity in the perforations may become comparable to the velocity of sound. The tray pressure drop plays an important part in filling up the down comers. To compensate for the pressure drop, a liquid head builds up in the down comers, to enable the liquid to flow down against it. When the tray pressure drop becomes excessive with respect to the height of the down comers, flooding will be the result.
The tray pressure drop is composed of (at least) two (major) contributions:a pressure drop caused by the gas flowing through the perforations in the tray floor. This contribution depends on gas flow rate, fraction free area and the pressure drop coefficient of the particular perforations (or valves) being used. This pressure drop coefficient depends on relative hole thickness (i.e. the ratio of tray thickness over hole diameter), hole shape and nearness of other holes (ratio of hole pitch to hole diameter).a pressure drop caused by the liquid present on the tray. This liquid hold up effect primarily increases with an increase in outlet weir height, decreases with an increase in gas flowrate and increases with an increase in liquid flowrate. To a lesser extent, it depends on physical properties of the gas/liquid system.
Other kinds of trays that are used in industry
We use many kinds of trays to achieve some goals
The optimization goals are:
(1) Maximizing theoretical stages per section or column height,
(2) Minimizing pressure drop per theoretical stage, and
(3) Maximizing the operational range, turn-down, or turn-up.
Industries that used Bubble cap trays
• Glycol Dehydration
• Caustic Scrubber (Wash Section)
• Amine Columns (Wash Section)
- H2S or CO2 Removal
Errors involved in practical
• In the apparatus there wasn’t any meter to calculate the inclination of the manometer. So we faced great difficult to find the angle of manometer. By using an appropriate method to calculate the angle we can minimize the error.
• In the apparatus there wasn’t any method to change the water flow rate quickly. We have to collect water in to a bucket in a one minute of time period. That was not a good method, as we cannot change the water flow rate as we wish.
Tuesday, May 20, 2008
H.E.P.T. Distillation.
EXPERIMENT:
H.E.P.T. Distillation.
INTRODUCTION:
Distillation is the separation of a more volatile component from a less volatile component exploiting the differences in volatilities. In this experiment, we determine HEPT of pack column by changing the reflux ratio.
PROCEDURE:
First 10%, 30%, 50%, 70% and 90% IPA mixture were made using IPA and water. The Refractive Indexes of each and every sample were measured. The 50% IPA mixture was put in the Distillation apparatus. Electric heater was switched on. Allow this to achieve its steady state under total reflux. Reflux Ratio was adjusted to the 3 and measures the Refractive Index of both Top and Bottom product. This procedure was repeated for the reflux ratio of 2, 6.
Equipment Description
1) Heat Source
2) Rebolier: vaporizes the liquid that is sent to it
3) Feed Line: Transports the feed to the distillation unit
4) Packing (Glass Sand) : Provides maximum surface area for mass transfer
5) Reflux Tube: Place where overhead vapor is returned to column
6) Condenser: Cools and condenses the vapor leaving the top of the column
7) Water Line
8) Distillate Release Valve: allows sample to be taken from column. Also controls the
reflux ratio.
DISCUSSION
The purpose of the lab is to introduce batch distillation using a packed column.Batch distillation is a technique used for separating two or more miscible liquid or vapor mixtures that are separated into their component fractions of desired purity. This is accomplished by the application and removal of heat. The separation is based on the boiling points of the mixture components. You will operate this column at total reflux, periodically taking small distillate samples, and determine the number of theoretical trays
in the column.Distillation is the most common separation technique, however it does suffer from
some disadvantages (Refer to Table 1 for applications). It usually takes a large amount of heat, both in terms of heating and cooling, to run a distillation apparatus. This heat requirement can contribute to more than 50% of plant operating cost. The best way to cut down on operating costs is to improve the distillation unit’s efficiency and operation via process control and optimization.
Distillation is a process in which miscible liquids are separated based on their physical properties, specifically, relative volatilities. A liquid can be classified as volatile when it is readily vaporized at a relatively low temperature. The boiling of the more volatile components of the mixture drives the distillation process. When the vapor is cooled, the more volatile material condenses in a greater proportion than the less volatile material.
The two types of distillations utilized in industry are batch and continuous. Batch distillation is desirable when small quantities of high valued chemicals need to be separated. The biggest advantage to using a batch column is its flexibility. This allows one to deal with unknowns in the feed or product specifications. In a batch system, the column can handle different mixtures by simply changing its operating conditions. The main disadvantage to using a batch system is that the longer the components are exposed to high temperatures, the better the chances that the components are broken down via
thermal degradation. Along with this, the energy requirements are usually higher for a batch system. A column is built for separating a specific mixture in continuous distillation. Therefore, the distillation column apparatus needs to be modified for each new mixture that is to be separated.
A batch distillation apparatus consists of a distillation column, a condenser, and a reboiler. The distillation column provides an environment where the gas and liquid phases of each component can approach equilibrium. A column can contain either packing or trays. In both types of columns, an increase in surface area allows for better contact between the liquid and vapor phases. In a column containing trays there is a discrete distribution of surface area, whereas in a packed column the distribution of area is continuous. The continuous distribution found in a packed column maximizes the surface area available for mass transfer, therefore allowing for a more efficient separation. In order to provide the highest contact area, a column is filled with packing that has a large volumetric area and that has high porosity. The liquid trickles down the column and through the packing as small droplets. The gas is sent through the column in the upward direction. This countercurrent flow of liquid and vapor exists only in a packed column. Ideally, the porosity of the packing should not hinder the gas flow through the column. In this lab, the packing in the column is glass sand. The distillation
column also contains a condenser, which cools and condenses the vapor leaving the top of the column. A reboiler is connected to the bottom of the distillation apparatus and it provides the reboil heat that is necessary for distillation.
A useful way to determine a column’s effectiveness is to limit its operating
conditions. One way to accomplish this is to run the column at total reflux. In total
reflux, all of the overhead vapor (reflux) and all of the bottoms liquid (boilup) is returned
to the column. Total reflux conditions allow for assumptions that make calculations
easier and allow the student an easy way to graphically evaluate the column.
The Tx diagram shows how the equilibrium compositions of the components in a mixture vary with temperature at the column pressure. The dew point is the temperature at which the saturated vapor starts to condense. The bubble point is the temperature in which the liquid starts to boil. The Txy diagram for methanol in water can be produced using the UNIFAC method in Aspen. Data for this system is in the Appendix. Boiling point diagrams can help aid in the construction of a vapor-liquidequilibrium (VLE) curve. The VLE plot shows the bubble point and dew point at constant pressure. The equilibrium line describes the compositions of the liquid and vapor in equilibrium at some fixed pressure.
A packed distillation column allows for continuous contact of liquid and vapor, however it is convenient to analyze the column as if it were discontinuous (a staged tower). The packing in the column can be divided up into segments that are of equal height. Each of these segments can be looked at as a “stage”. It can be assumed that each stage allows the vapor and liquid to leave the stage in equilibrium with each other. This method of assuming that the packing can be broken down into stages is not physically accurate, but can be used for calculations. The following equation relates column height and the number of equilibrium stages to the height equivalent to a theoretical plate (HETP). HETP is defined as the height of packing needed to obtain the change in composition obtained with one theoretical contact. HETP is measured experimentally and usually can range from one to four feet. A small HETP indicates a small column and more efficient packing. To measure the HETP, the compositions of the top and bottoms streams must be found at total reflux and the number of equilibrium stages must be calculated. The following equation shows the relationship between the HETP, the height of packing and the number of theoretical stages.
There are two methods to find the number of theoretical stages in the column. Both techniques use the McCabe-Thiele analysis, which is a graph of vapor composition (mole fraction) verses liquid composition (mole fraction). The system operating line and the equilibrium line for the system are plotted. The operating line for a batch system with total reflux is y=x. This is true because the flow rate of liquid must be the same as the
flow rate of vapor. McCabe-Thiele nalysis can be performed in two ways. One method calculates theoretical stages numerically by expressing the equilibrium and operating lines mathematically and finding the number of vertical and horizontal intersections (stages) required to reach the desired separation. The other method allows the student to manually draw the stages on the VLE graph. The first method is more accurate because of the numerical calculations, however it is much more time consuming. Regardless of
the method used, the graph obtained will look similar to Figure 4, which can be produced using Aspen.
The HETP is dependant upon the packing type and size, the gas flow rate and the chemicals being separated. The higher the HETP the lower the efficiency of the packing. If the gas flow rate is low, the HETP will generally be higher because the packing is not completely wet. The HETP can be calculated as explained in the above method, however literature values are usually much more accurate. One approximation that can be used is to set the HETP equal to the column diameter.
There are many types of distillation columns, each designed to perform specific types of separations, and each design differs in terms of complexity.
One way of classifying distillation column type is to look at how they are operated.
* Batch Columns
* Continuous Columns
Batch Columns
In batch operation, the feed to the column is introduced batch-wise. When the desired task is achieved 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.
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 where the extra feed exits when it is used to help with the separation.
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.
• Column internals such as trays/plates and/or packings which are used to enhance component separations
• A reboiler to provide the necessary vaporization for the distillation process
• A condenser to cool and condense the vapor leaving the top of the column
• A reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycled back to the column
The selection of distillation equipment is largely a matter of economics. Continuous operation stills must be properly engineered and are costly to construct. However, the advantages of automated operation and low labor requirements make them very attractive. For operations producing a large amount of fuel, a continuous still clearly makes sense. Batch stills, although labor intensive, can be built by the layman with relative ease and for a small amount of money
The type of column internals
* Packed column - where instead of trays, packing are used to enhance contact between vapor and liquid
* Tray column - where trays of various designs are used to hold up the liquid to provide better contact between vapor and liquid, hence better separation
Packed columns are called continuous-contact columns while tray columns are called staged-contact columns because of the manner in which vapour and liquid are contacted
Packing
Packing are passive devices that are designed to increase the interfacial area for vapor-liquid contact. The following pictures show 3 different types of packing.
These strangely shaped pieces are supposed to impart good vapor-liquid contact when a particular type is placed together in numbers, without causing excessive pressure-drop across a packed section. This is important because a high pressure drop would mean that more energy is required to drive the vapor up the distillation column.
Bubble cap trays
A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapor. Vapor rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.
Valve trays
In valve trays, perforations are covered by liftable caps. Vapor flows lifts the caps, thus self creating a flow area for the passage of vapor. The lifting cap directs the vapor to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays.
Sieve trays
Sieve trays are simply metal plates with holes in them. Vapor passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters
Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many applications.
Packing Vs Trays
A tray column that is facing throughput problems may be de-bottlenecked by replacing a section of trays with packing. This is because:
* Packing provide extra inter-facial area for liquid-vapour contact
* Efficiency of separation is increased for the same column height
* Packed columns are shorter than tray columns
The performance of a distillation column is determined by many factors, for example:
• Feed conditions
• state of feed
• composition of feed
• internal liquid and fluid flow conditions
• state of trays (packings)
• weather conditions
• Reflux
Feed Conditions
The state of the feed mixture and feed composition affects the operating lines and hence the number of stages required for separation. It also affects the location of feed tray. During operation, if the deviations from design specifications are excessive, then the column may no longer be able handle the separation task. To overcome the problems associated with the feed, some column are designed to have multiple feed points when the feed is expected to containing varying amounts of components.
Reflux
Reflux ratio is increasing means more and more liquid that is rich in the more volatile components are being recycled back into the column. Separation then becomes better and thus fewer trays are needed to achieve the same degree of separation. Minimum trays are required under total reflux conditions.
On the other hand, as reflux is decreased, the operating line for the rectification section moves towards the equilibrium line. The ‘pinch’ between operating and equilibrium lines becomes more pronounced and more and more trays are required.
Vapor Flow Conditions
Adverse vapor flow conditions can cause
• Foaming
• Entrainment
• Weeping/dumping
• Flooding
State of Trays and Packing
The actual number of trays required for a particular separation duty is determined by the efficiency of the plate, and the packing if packing are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the column. Tray efficiencies are affected by fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus appropriate materials should be specified for tray construction.
As the feed stage is moved lower down the column, the top composition becomes less rich in the more volatile component while the bottoms contains more of the more volatile component. However, the changes in top composition are not as marked as the bottoms composition.
Condensers
The vapors going into the condenser are impeded in any great degree; pressure could build up inside the column and boiler. Therefore, the diameter of tubing in a condenser for a 3-inch column should be no smaller than 3/8 inch diameter. Note that the effective diameter of a condenser can be increased by connecting two or more condensing coils in parallel.
Reboiler
Heat is supplied to the reboiler to generate vapor. 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 distillation equipment described so far uses the principle of adding heat to boil the beer and provide vapor for the distillation process. Alternately, vapor can also be produced by reducing pressure. Alcohol/water mixtures can be boiled at "room" temperature and below simply by reducing pressure. The equipment consists of a vacuum pump, condenser, and a still pot built to withstand the external pressure created by the vacuum. Although the energy required to run the vacuum pump is probably equal to the amount of energy required to operate a conventional still, this type of equipment merits consideration
Designing a column to match a simulation depends on many factors
o Pure Component Properties
o Deciding Between Trays and Packing
o Selection of Tray / Packing Type
o Column Sizing:
o Pressure Drop
o Tray or Packing Efficiency
o Condenser and Reboiler Design
H.E.P.T. Distillation.
INTRODUCTION:
Distillation is the separation of a more volatile component from a less volatile component exploiting the differences in volatilities. In this experiment, we determine HEPT of pack column by changing the reflux ratio.
PROCEDURE:
First 10%, 30%, 50%, 70% and 90% IPA mixture were made using IPA and water. The Refractive Indexes of each and every sample were measured. The 50% IPA mixture was put in the Distillation apparatus. Electric heater was switched on. Allow this to achieve its steady state under total reflux. Reflux Ratio was adjusted to the 3 and measures the Refractive Index of both Top and Bottom product. This procedure was repeated for the reflux ratio of 2, 6.
Equipment Description
1) Heat Source
2) Rebolier: vaporizes the liquid that is sent to it
3) Feed Line: Transports the feed to the distillation unit
4) Packing (Glass Sand) : Provides maximum surface area for mass transfer
5) Reflux Tube: Place where overhead vapor is returned to column
6) Condenser: Cools and condenses the vapor leaving the top of the column
7) Water Line
8) Distillate Release Valve: allows sample to be taken from column. Also controls the
reflux ratio.
DISCUSSION
The purpose of the lab is to introduce batch distillation using a packed column.Batch distillation is a technique used for separating two or more miscible liquid or vapor mixtures that are separated into their component fractions of desired purity. This is accomplished by the application and removal of heat. The separation is based on the boiling points of the mixture components. You will operate this column at total reflux, periodically taking small distillate samples, and determine the number of theoretical trays
in the column.Distillation is the most common separation technique, however it does suffer from
some disadvantages (Refer to Table 1 for applications). It usually takes a large amount of heat, both in terms of heating and cooling, to run a distillation apparatus. This heat requirement can contribute to more than 50% of plant operating cost. The best way to cut down on operating costs is to improve the distillation unit’s efficiency and operation via process control and optimization.
Distillation is a process in which miscible liquids are separated based on their physical properties, specifically, relative volatilities. A liquid can be classified as volatile when it is readily vaporized at a relatively low temperature. The boiling of the more volatile components of the mixture drives the distillation process. When the vapor is cooled, the more volatile material condenses in a greater proportion than the less volatile material.
The two types of distillations utilized in industry are batch and continuous. Batch distillation is desirable when small quantities of high valued chemicals need to be separated. The biggest advantage to using a batch column is its flexibility. This allows one to deal with unknowns in the feed or product specifications. In a batch system, the column can handle different mixtures by simply changing its operating conditions. The main disadvantage to using a batch system is that the longer the components are exposed to high temperatures, the better the chances that the components are broken down via
thermal degradation. Along with this, the energy requirements are usually higher for a batch system. A column is built for separating a specific mixture in continuous distillation. Therefore, the distillation column apparatus needs to be modified for each new mixture that is to be separated.
A batch distillation apparatus consists of a distillation column, a condenser, and a reboiler. The distillation column provides an environment where the gas and liquid phases of each component can approach equilibrium. A column can contain either packing or trays. In both types of columns, an increase in surface area allows for better contact between the liquid and vapor phases. In a column containing trays there is a discrete distribution of surface area, whereas in a packed column the distribution of area is continuous. The continuous distribution found in a packed column maximizes the surface area available for mass transfer, therefore allowing for a more efficient separation. In order to provide the highest contact area, a column is filled with packing that has a large volumetric area and that has high porosity. The liquid trickles down the column and through the packing as small droplets. The gas is sent through the column in the upward direction. This countercurrent flow of liquid and vapor exists only in a packed column. Ideally, the porosity of the packing should not hinder the gas flow through the column. In this lab, the packing in the column is glass sand. The distillation
column also contains a condenser, which cools and condenses the vapor leaving the top of the column. A reboiler is connected to the bottom of the distillation apparatus and it provides the reboil heat that is necessary for distillation.
A useful way to determine a column’s effectiveness is to limit its operating
conditions. One way to accomplish this is to run the column at total reflux. In total
reflux, all of the overhead vapor (reflux) and all of the bottoms liquid (boilup) is returned
to the column. Total reflux conditions allow for assumptions that make calculations
easier and allow the student an easy way to graphically evaluate the column.
The Tx diagram shows how the equilibrium compositions of the components in a mixture vary with temperature at the column pressure. The dew point is the temperature at which the saturated vapor starts to condense. The bubble point is the temperature in which the liquid starts to boil. The Txy diagram for methanol in water can be produced using the UNIFAC method in Aspen. Data for this system is in the Appendix. Boiling point diagrams can help aid in the construction of a vapor-liquidequilibrium (VLE) curve. The VLE plot shows the bubble point and dew point at constant pressure. The equilibrium line describes the compositions of the liquid and vapor in equilibrium at some fixed pressure.
A packed distillation column allows for continuous contact of liquid and vapor, however it is convenient to analyze the column as if it were discontinuous (a staged tower). The packing in the column can be divided up into segments that are of equal height. Each of these segments can be looked at as a “stage”. It can be assumed that each stage allows the vapor and liquid to leave the stage in equilibrium with each other. This method of assuming that the packing can be broken down into stages is not physically accurate, but can be used for calculations. The following equation relates column height and the number of equilibrium stages to the height equivalent to a theoretical plate (HETP). HETP is defined as the height of packing needed to obtain the change in composition obtained with one theoretical contact. HETP is measured experimentally and usually can range from one to four feet. A small HETP indicates a small column and more efficient packing. To measure the HETP, the compositions of the top and bottoms streams must be found at total reflux and the number of equilibrium stages must be calculated. The following equation shows the relationship between the HETP, the height of packing and the number of theoretical stages.
There are two methods to find the number of theoretical stages in the column. Both techniques use the McCabe-Thiele analysis, which is a graph of vapor composition (mole fraction) verses liquid composition (mole fraction). The system operating line and the equilibrium line for the system are plotted. The operating line for a batch system with total reflux is y=x. This is true because the flow rate of liquid must be the same as the
flow rate of vapor. McCabe-Thiele nalysis can be performed in two ways. One method calculates theoretical stages numerically by expressing the equilibrium and operating lines mathematically and finding the number of vertical and horizontal intersections (stages) required to reach the desired separation. The other method allows the student to manually draw the stages on the VLE graph. The first method is more accurate because of the numerical calculations, however it is much more time consuming. Regardless of
the method used, the graph obtained will look similar to Figure 4, which can be produced using Aspen.
The HETP is dependant upon the packing type and size, the gas flow rate and the chemicals being separated. The higher the HETP the lower the efficiency of the packing. If the gas flow rate is low, the HETP will generally be higher because the packing is not completely wet. The HETP can be calculated as explained in the above method, however literature values are usually much more accurate. One approximation that can be used is to set the HETP equal to the column diameter.
There are many types of distillation columns, each designed to perform specific types of separations, and each design differs in terms of complexity.
One way of classifying distillation column type is to look at how they are operated.
* Batch Columns
* Continuous Columns
Batch Columns
In batch operation, the feed to the column is introduced batch-wise. When the desired task is achieved 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.
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 where the extra feed exits when it is used to help with the separation.
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.
• Column internals such as trays/plates and/or packings which are used to enhance component separations
• A reboiler to provide the necessary vaporization for the distillation process
• A condenser to cool and condense the vapor leaving the top of the column
• A reflux drum to hold the condensed vapor from the top of the column so that liquid (reflux) can be recycled back to the column
The selection of distillation equipment is largely a matter of economics. Continuous operation stills must be properly engineered and are costly to construct. However, the advantages of automated operation and low labor requirements make them very attractive. For operations producing a large amount of fuel, a continuous still clearly makes sense. Batch stills, although labor intensive, can be built by the layman with relative ease and for a small amount of money
The type of column internals
* Packed column - where instead of trays, packing are used to enhance contact between vapor and liquid
* Tray column - where trays of various designs are used to hold up the liquid to provide better contact between vapor and liquid, hence better separation
Packed columns are called continuous-contact columns while tray columns are called staged-contact columns because of the manner in which vapour and liquid are contacted
Packing
Packing are passive devices that are designed to increase the interfacial area for vapor-liquid contact. The following pictures show 3 different types of packing.
These strangely shaped pieces are supposed to impart good vapor-liquid contact when a particular type is placed together in numbers, without causing excessive pressure-drop across a packed section. This is important because a high pressure drop would mean that more energy is required to drive the vapor up the distillation column.
Bubble cap trays
A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapor. Vapor rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray.
Valve trays
In valve trays, perforations are covered by liftable caps. Vapor flows lifts the caps, thus self creating a flow area for the passage of vapor. The lifting cap directs the vapor to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays.
Sieve trays
Sieve trays are simply metal plates with holes in them. Vapor passes straight upward through the liquid on the plate. The arrangement, number and size of the holes are design parameters
Because of their efficiency, wide operating range, ease of maintenance and cost factors, sieve and valve trays have replaced the once highly thought of bubble cap trays in many applications.
Packing Vs Trays
A tray column that is facing throughput problems may be de-bottlenecked by replacing a section of trays with packing. This is because:
* Packing provide extra inter-facial area for liquid-vapour contact
* Efficiency of separation is increased for the same column height
* Packed columns are shorter than tray columns
The performance of a distillation column is determined by many factors, for example:
• Feed conditions
• state of feed
• composition of feed
• internal liquid and fluid flow conditions
• state of trays (packings)
• weather conditions
• Reflux
Feed Conditions
The state of the feed mixture and feed composition affects the operating lines and hence the number of stages required for separation. It also affects the location of feed tray. During operation, if the deviations from design specifications are excessive, then the column may no longer be able handle the separation task. To overcome the problems associated with the feed, some column are designed to have multiple feed points when the feed is expected to containing varying amounts of components.
Reflux
Reflux ratio is increasing means more and more liquid that is rich in the more volatile components are being recycled back into the column. Separation then becomes better and thus fewer trays are needed to achieve the same degree of separation. Minimum trays are required under total reflux conditions.
On the other hand, as reflux is decreased, the operating line for the rectification section moves towards the equilibrium line. The ‘pinch’ between operating and equilibrium lines becomes more pronounced and more and more trays are required.
Vapor Flow Conditions
Adverse vapor flow conditions can cause
• Foaming
• Entrainment
• Weeping/dumping
• Flooding
State of Trays and Packing
The actual number of trays required for a particular separation duty is determined by the efficiency of the plate, and the packing if packing are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the column. Tray efficiencies are affected by fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus appropriate materials should be specified for tray construction.
As the feed stage is moved lower down the column, the top composition becomes less rich in the more volatile component while the bottoms contains more of the more volatile component. However, the changes in top composition are not as marked as the bottoms composition.
Condensers
The vapors going into the condenser are impeded in any great degree; pressure could build up inside the column and boiler. Therefore, the diameter of tubing in a condenser for a 3-inch column should be no smaller than 3/8 inch diameter. Note that the effective diameter of a condenser can be increased by connecting two or more condensing coils in parallel.
Reboiler
Heat is supplied to the reboiler to generate vapor. 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 distillation equipment described so far uses the principle of adding heat to boil the beer and provide vapor for the distillation process. Alternately, vapor can also be produced by reducing pressure. Alcohol/water mixtures can be boiled at "room" temperature and below simply by reducing pressure. The equipment consists of a vacuum pump, condenser, and a still pot built to withstand the external pressure created by the vacuum. Although the energy required to run the vacuum pump is probably equal to the amount of energy required to operate a conventional still, this type of equipment merits consideration
Designing a column to match a simulation depends on many factors
o Pure Component Properties
o Deciding Between Trays and Packing
o Selection of Tray / Packing Type
o Column Sizing:
o Pressure Drop
o Tray or Packing Efficiency
o Condenser and Reboiler Design
Monday, May 19, 2008
Helps Mora
Hi Friends,
This site will give you all an additional help in making your academic career a SUCCESS.
cheers!!!
Helps Mora
This site will give you all an additional help in making your academic career a SUCCESS.
cheers!!!
Helps Mora
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