"Mastering Distillation: Basic Principles, Operations, and Troubleshooting"

 

Plants like this one spend millions of dollars every year to produce marketable, usable products. And just about all of these products will at some stage go through a process called distillation.

 It's a process based on the simple principle that different substances can be separated by their respective boiling points. In practice, however, this separation process can become quite complex as many different variables enter the picture.

 In this blog, we'll go through the basic principles of distillation and then focus on what you, the operator, can do to make the operation work more safely and efficiently.

 

But let's start with the basics. The properties of various substances are determined to a large extent by the size and structure of their molecules. In distillation, the property we're most concerned with is boiling temperature. Substances composed of large heavy molecules tend to boil at high temperatures, while smaller, lighter compounds have lower boiling points. Distillation uses heat to separate a mixture of different compounds by their respective boiling points or boiling point ranges.

 Let's see how this works. When we add heat to this 50-50 mixture of butane and pentane, more of the light components, butane, boils off and goes into a vaporized state.

 If we cool and condense these vapors back to liquid, we get a fraction that is enriched in butane. The liquid that doesn't vaporize on the other hand forms a heavy fraction enriched in pentane.

 We can get an even cleaner separation by passing this mixture through a series of stills. The top stills operate at progressively lower temperatures, so less of the heavy components, pentane, is vaporized as we progress from one still to the next.

 This results in a relatively pure butane product being formed in the top still. The bottom stills operate at increasingly higher temperatures, so more of the light components, butane, is boiled out of liquid as it is passed down to each lower still. This gives us a relatively pure pentane product out of the bottom still.

 This is essentially what happens inside a distillation column, except that the process continually repeats itself on each tray or in each section of the tower.

 Hot vapors rising up the tower heat the liquid on each tray. As a result of this exchange, light components in the liquid vaporize and heavy components in the vapor condense. This cycle gradually isolates a different fraction on each tray. The composition of each fraction is determined by the tray temperature.

 The lowest temperature occurs at the top of the column where a cool liquid called reflux is returned, so a light fraction is formed here. The highest temperature is at the bottom of the column where hot vapors are generated, so a heavy fraction develops in this part of the tower.

 Although there are many different types of distillation columns in any refinery or chemical plant, the goal of each is the same, to produce quality products that meet specifications at the lowest possible cost. Setting specifications for products allows us to control the type or range of molecules that are included in any one product.

 To determine product purity, we use boiling point tests or actually measure the percent of heavy and light key components in the product. Other specifications are set for flash point, API gravity or specific gravity, and color. Products are periodically tested to make sure that they are meeting specifications. When products are off specification, a change must be made in tower operations to bring the products back on spec. This is usually done by adjusting the heat and material balance inside the column. If we change the amount of heat that is put into or taken out of a distillation column, products composition and quantity will also change. In most distillation columns, instruments are used to automatically control tower operations.

Temperature, pressure, liquid level and flow are the four primary variables that affect the heat and material balance inside a distillation column. Instruments function to keep these variables at a desired level or set point.

 

 

Let's review how the distillation process works and how a tower is controlled to produce products that meet specifications.

 

 During normal operations, control room instrumentation can automatically adjust to most changes in process conditions to keep the tower producing on specification products. An operator is responsible for adjusting these instruments, collecting data and making sure the tower operates at maximum efficiency.

 Distillation is a complicated process. In order to maximize production, a good operator must understand what happens inside a distillation column and why.

 When we say an operator should use logical thinking, we mean he should go through the following steps when working with a distillation column.

 The first thing he does is collect all relevant data on tower operating variables. Then he analyzes this data to determine if there is an operating problem. If so, he considers all the different ways it can be corrected, chooses the best solution and makes the necessary adjustment. Finally, he checks the results of his action to make sure the problem has been resolved.

Suppose lab tests show an overhead product has an endpoint 15 degrees higher than called for by specifications. An operator quickly increases the reflux rate to remove heavy material from the overhead product. In this situation, has the operator used logical thinking and made the correct adjustment?

 No.

 The operator in this example did not check all the data on the tower operating variables before increasing the reflux rate. This chart shows that the current tower operating conditions have not changed from the previous readings.

 Have the operator analyzed this data? He might have suspected that the endpoint test was inaccurate because none of the tower operating variables had changed. The most logical thing to do in this situation would be to retest the overhead product before increasing the reflux rate.

 During the normal operation of a distillation column, there is a certain set of procedures that an operator should follow.

 Let's go over these control procedures and then apply them to some normal operating situations that an operator is likely to encounter.

 An operator is usually responsible for sampling or testing distillation products at regular time intervals. He also checks instrument readings at the same time these samples are taken. On some distillation columns, process analyzers automatically monitor product specifications. These test results and instrument readings are recorded in the unit logbook and are then compared with previously logged instrument readings and test results. If the test results indicate a product is not meeting specifications or an operating variable has changed, it may be necessary to adjust the tower operation. But before making any adjustments, consider the possibility that the tests may be inaccurate or that an instrument or automatic controller is malfunctioning or that there is some other mechanical problem.

 The data should fit together. If the total picture of the operating condition seems unreasonable, you should question whether the data shows the actual state of tower conditions. Suppose instrument readings tell you that the flow of steam to the reboiler has increased and that the temperature at the bottom of the column has decreased. Other tower variables remain the same.

 Does this data fit together?

 If you answered no, you are correct. An increase in the flow of steam to the reboiler should result in an increase in temperature at the bottom of the tower. The reading of the steam flow rates or the tower bottom temperature may be wrong.

 Another possibility is that the reboiler itself is not operating properly. When it is necessary to change tower operations, always make the change that will bring the product back on specification with the least effect on other tower conditions and with energy efficiency in mind. These adjustments should always be made gradually. Sudden changes will cause a tower to behave erratically, reducing fractionating efficiency. A distillation column does not respond immediately to operational changes. When we adjust the heat balance inside a column, there is a gradual change in product composition.

 This chart summarizes what happens when we increase the reflux rate to remove butane from an overhead product.

% BUTANE                         TIME

11                                             3.00 PM

9.8                                          3.10 PM

8.5                                           3.25 PM

7.1                                            3.40 PM

6.0                                          3.50 PM

 You can see that it takes 50 minutes to reduce the butane content in the top product from 11% to 6%. The amount of time needed for a tower to adjust to operational changes will vary from column to column. When making cut point changes, always give the tower time to adjust to these changes. Then take new tests and instrument readings to see if the product is meeting specifications.

 Let's look at another situation. In this column, the end point for the overhead product has increased from 100 degrees Fahrenheit to 105 degrees Fahrenheit. You know that heavy molecules boil at high temperatures, so the new end point indicates the overhead product contains material that is too heavy. Looking at the instrument readings for this same column, you can see that tower top temperature is the only variable that has changed. An increase in the amount of heavy material on the top trays is responsible for the temperature increase. Before correcting the operation, logically think through the problem.

One way to remove heavy material from the top product is to increase the reflux rate.

 Another way is to decrease the reboiler temperature. In this example, you would probably decrease the reboiler temperature because of the energy savings. However, increasing the reflux rate may be the quickest way to remove heavy material from the overhead product.

 

 

Let's review the control procedures that are used during the normal operation of a distillation column and work through some more problems.

 During the normal operation of a distillation column, an operator makes minor adjustments to the heat and material balance to keep the column producing on specification products. These adjustments are fairly routine, so when a tower is operating normally or at steady state, it is relatively easy to control. An operator is faced with a greater challenge when a tower is operating abnormally.

 There may be several different reasons for this, so an operator must develop troubleshooting skills to identify these types of problems.

One abnormal operating situation is flooded trays. When a group of trays in a distillation column become completely filled with liquid, the tower is said to be flooded. Too much liquid flowing down the tower can result in flooding. When the capacity of the downcomers and trays is exceeded, liquid backs up in the tower and the column floods. A vapor rate that is too high can also cause flooding. The pressure from excessive vapor rates resists the flow of liquid down the tower. If this pressure is high enough, liquid will back up the tower and flooding occurs.

 For vapor to move up a tower, there must be a difference in pressure between the bottom and top of the column. In a tower that is operating normally, the highest pressure occurs at the bottom and then gradually decreases as you move towards the top.

 This differential pressure is the driving force that pushes vapors up through risers and bubble caps and through the liquid that is on each tray. The driving force that moves liquid down a column is called hydrostatic head. This is simply the depth of liquid that exists in the downcomer. The higher the head, the greater the downward force. To travel down a tower, a liquid must overcome the resistance caused by the upward pressure of the vapors as well as friction in the downcomers.

 In a tower that is operating normally, the differential pressure of the vapors and the hydrostatic head of the liquid are in balance and there is a steady two-way flow of fluids through each of the trays in the column. Excessive vapor or liquid rates through any one of the trays can upset this balance for the entire column.

 Let us see what happens when either vapor or liquid rates are increased, holding other operating conditions constant.

 You can see that this causes the hydrostatic head of the liquid to increase, which can lead to flooding. There are several ways to identify a tower with flooded trays.

 First, you will find that there is very little temperature difference or gradient across the flooded section. This occurs because the composition of the liquid is essentially the same on all the flooded trays.

 Second, a tower with flooded trays produces poor quality products. This is due to poor vapor-liquid contact in the flooded area, which reduces fractionating efficiency.

 Third, in a flooded section of a tower, rising vapors have to push against a very high hydrostatic head of liquid. As a result, differential pressure in this area of the column is larger than normal.

 And fourth, the pressure tends to fluctuate inside a tower that is flooded. This is because the vapors underneath the flooded section must build up a very high pressure before they can overcome the resistance of the liquid.

 When the vapors reach this pressure, they suddenly blow through the liquid and pressure below the flooded section quickly falls. This build up and blow through cycle is what causes pressure to fluctuate in a flooded tower. High liquid levels sometimes occur in the bottom of a distillation column because too much liquid is entering the tower or not enough bottom product is being removed. A tower with a high liquid level exhibits the same characteristics as a tower with flooded trays.

 There is very little temperature gradient from tray to tray. Fractionating efficiency decreases. Differential pressure is larger than normal and pressure throughout the column fluctuates. Another abnormal operating situation is dry trays. Dry trays sometimes occur because too much liquid is drawn off as side product. When this happens, the trays in a section of the tower immediately below the draw off point become dry. An interruption in the supply of reflux or insufficient reflux to cool the rising vapors can also cause dry trays. Reflux problems are usually responsible for dry trays at the top of a distillation column.

 Dry trays will also occur if the feed or re-boiler temperature is too high. The high temperatures produce an excessive amount of hot vapors. These vapors boil away all the liquid on the trays directly above the feed or re-boiler inlets.

 

You can identify a tower with dry trays by the following characteristics.

 First, the tower will produce poor quality products because no fractionation can occur on a tray without liquid.

 Second, the temperature on a series of trays without liquid will be about the same so the temperature gradient in this section of the tower is very reduced.

 And third, there will be a small differential pressure through a series of dry trays because there is no liquid to resist the flow of vapors.

 Abnormal operations can also be caused by water that inadvertently enters the system. The water may come in with the feed or accidentally through a pump line and gets trapped in an area of the tower where the temperature is below the boiling point of water. Most hydrocarbon streams are lighter than water so they float on top of it. When a section of trays becomes filled with trapped water, fractionation in this part of the tower stops because there are no hydrocarbons to make contact with the upward rising vapors. Since water is a relatively pure compound, trays with trapped water will exhibit very little temperature difference or gradient.

When trapped water vaporizes to steam, it can pose a grave safety hazard because it expands sixteen hundred times in volume. Sudden vaporization of water causes pressure surges that can damage internal tower hardware and in extreme cases can even rupture a tower.

 Another condition that can interfere with normal operations is upset or damaged trays. Upset trays are usually caused by excessive vapor velocities, by the sudden vaporization of water, or by liquid surges in the bottom of the tower. Upset trays do not fractionate properly because vapors are not forced to bubble through the liquid on the trays. As a result, the liquid on a series of upset trays will have the same composition and will exhibit little temperature difference. Since it is easier for vapors to flow around upset trays than through bubble caps, a tower with upset trays will usually exhibit a decrease in differential pressure.

 A tower will also operate abnormally if the cooling medium to a condenser is lost. When this happens, the amount of heat that can be removed from a tower is greatly reduced. The condenser will immediately start to overheat and may be damaged unless the heat input to the tower is quickly reduced. When there is not enough cooling water or air going to the condenser, tower top temperature and pressure will suddenly increase. The loss of the cooling medium will cause the liquid level in the accumulator to decrease because hot vapors can no longer be added to liquid. A higher temperature also means that heavier material is driven up the tower so the overhead product becomes heavier.

 A less severe but more common problem with overhead systems is fouling or scaling of condenser tubes. Deposits in the condenser tubes interfere with flow through the vessel and decrease the effectiveness of heat transfer from one fluid to another.

 Another situation that will cause a tower to operate abnormally is the loss of the heating medium to heat exchangers and reboilers or the loss of fuel supply to furnaces. When the heat supply for a column is cut off, temperature and pressure inside the tower will quickly start to decrease. Less heat means that fewer vapors will be formed so the liquid level in the bottom of the column will increase and the bottom product gets lighter. Instruments and or alarms will usually identify which piece of equipment has malfunctioned.

Fouling or scaling is a common problem in furnaces, reboilers and heat exchangers. As with a condenser, these deposits restrict flow through a vessel and reduce heat transfer efficiency. Solid deposits can collect and plug screens, valves and pipes that serve as outlets from a distillation column. Plugged outlets seriously restrict the amount of product that can be drawn from a tower and they must be cleared if the column is to operate normally.

 

The problems that occur in vacuum distillation columns frequently relate to the system's ability to maintain an adequate vacuum. Vacuum columns operate at very low pressures to reduce the amount of heat needed to separate the feedstock. In many towers the vacuum is maintained by passing high pressure steam through a series of ejectors. The movement of the steam pulls non condensable gases out of the system. After passing through the ejectors, the steam is condensed to water and collected in a vessel called a hot well.

A loss of vacuum in a vacuum distillation column is usually caused by one of the following problems.

 Improper steam pressure to ejectors, hot well flooding, the presence of too many non condensable gases, loss of cooling water to condensers or erosion or blockage of steam ejectors.

 Let's review the conditions that can cause a tower to operate abnormally and consider the solutions for each of these abnormal operating problems.

 One of the marks of a good operator is the ability to analyze various operating problems and control a tower that is operating abnormally. Abnormal operations present an operator with many different variables and alternate solutions. The operator must identify the problem, determine the cause and then implement a solution that will bring the column back to normal operation.

 When dealing with a tower that is operating abnormally, an operator should follow the same orderly pattern of thought that is used to control a tower during normal operations.

 The first thing an operator does is collect all data on current test results and instrument readings. Next, the operator compares current test results and instrument readings with previous data so he can correctly identify the problem.

For any given problem such as flooding there may be several different causes. So in addition to identifying the problem an operator must also determine what is causing it. Then the operator chooses the best way to solve the problem and makes an adjustment in tower operations to correct it. After an adjustment is made an operator continually monitors instrument readings to make sure the situation is not getting worse. The tower is given time to adjust to the operating changes and then new tests are run to see if the products are back on specification.

 You should always be skeptical about tests and instruments. A situation that seems abnormal may only appear so because of faulty instruments or test results. Double check instrument readings and test results before making changes in tower operations.

 One of the difficulties of abnormal operations is that an operator often has a limited amount of time to discover the cause of a problem and remedy it.

Some situations like a rapid increase in tower pressure require quick action. Other situations like a gradual loss of cooling water can be approached and corrected more methodically.

 An operator should estimate the time he has available to correct a problem and then work within those time constraints while implementing a solution.

 When working with a tower that is operating abnormally you should have several alternate solutions. In the event that your first action does not solve the problem you will be able to make a second adjustment without delay.

Let's work through a hypothetical problem.

 IF chart gives the data for a tower that is operating abnormally.  The temperature gradient between the feed zone and the tower bottom has decreased from 200 degrees Fahrenheit to 50 degrees Fahrenheit. Specifications call for a bottom product with an initial boiling point of 400 degrees Fahrenheit. The IBP is down to 180 degrees Fahrenheit. Differential pressure between the feed zone and the bottom of the tower has increased from 10 pounds to 14. And finally, the tower bottom level is high.

 After collecting the data, your next task is to identify the problem.

 A decrease in temperature gradient and impaired products quality are found in all abnormal situations. An increase in differential pressure is found only in flooded trays and high levels, so one of these two conditions is probably causing the problem.

Since the decrease in temperature gradient is at the bottom of the column and the bottom level is high, you would assume the problem is due to high levels. You know that high levels are caused by either too much liquid entering the tower or not enough liquid being taken out.

 A check of the feed and reflux rates shows that these variables have not changed value, so the problem is probably not caused by too much liquid entering the column. Checking the flow leaving the tower, you find that the bottom product flow rate has decreased. The reduction in the bottom flow rate appears to be responsible for the high level.

 There are several conditions that could result in not enough liquid leaving the bottom of the tower.

 One possibility is malfunctioning instruments. A check of the control valve on the bottom product outlet line shows that this valve is wide open. Instruments do not appear to be causing the problem. Another possibility is a faulty pump. A pump check shows that it is working, but the pump has an erratic discharge pressure.

 You decide that the pump is causing the problem. In this situation, you would immediately switch to a standby pump to reduce the high level.

 The bad pump can either be repaired or replaced.

 

Let's work through some more hypothetical problems dealing with abnormal operations.

  Because the components in most liquid mixtures boil at different temperatures, we can usually separate these mixtures by normal distillation. But when the components have the same or nearly the same boiling points, separation by normal distillation can be difficult and in some cases may even be impossible.

 In these instances, it is necessary to use another type of distillation to bring about a separation.

 One distillation method that can separate substances with very close boiling points is called extractive distillation. In extractive distillation, a third component, usually a solvent, is added at the top of the column. As the solvent moves down the tower, it attaches itself to one of the components in the feed and carries it to the bottom of the tower. The component not absorbed by the solvent, in this case product A, is removed at the top of the tower. The solvents in product B are removed at the bottom of the column and are then sent to a second tower for separation.

The second tower, called a solvent recovery column, separates the absorbed component from the solvent by normal distillation. The product is taken off the top of this column and the solvent is recycled back to the first column to continue the separation process. The solvent used in extractive distillation has a boiling point that is considerably higher than either of the components being separated. Since it is heavier than both products, it falls to the bottom of the extractive column without being vaporized. The wide boiling point difference between the solvent and the component it absorbs allows us to separate this mixture by normal distillation. Some substances cannot be separated by normal distillation because they form a constant boiling point mixture when they are brought together. These constant boiling point mixtures are called azeotropes. azeotropes can boil at a temperature that is higher or lower than the boiling points of the components that combine to form them.

 Water, a byproduct in many petrochemical operations, and isopropyl alcohol form an azeotrope that boils at 165 degrees Fahrenheit.

 In this case, the boiling point of the azeotrope is lower than the individual boiling points of either of the two components. Note the composition of the azeotrope. It contains 88% alcohol and 12% water. If we attempt to separate water and isopropyl alcohol by normal distillation, the water not contained by the azeotrope will fall to the bottom of the column. The water moves downward because it has the highest boiling point. The product produced at the top of the column is the azeotrope because it has the lowest boiling point temperature. Since the azeotrope goes overhead, it is impossible to produce a dry alcohol product in this tower.

 In order to obtain a pure isopropyl alcohol product, the azeotropic mixture from the first column is sent to a second tower. In this example, a solvent is added to the system. The solvent joins with the other two components to form a three-way or ternary azeotrope. The major function of the solvent is to absorb all of the water that comes into the tower. The ternary azeotrope has a lower boiling point than the two component azeotrope that entered the column, so it goes overhead. Since all of the water is in the ternary azeotrope, a dry isopropyl alcohol product can be removed at the bottom of this tower. When the ternary azeotrope is condensed overhead, it forms two different liquid phases. The liquid phase that is rich in the solvents is recycled back to the azeotropic column. The other liquid phase contains mostly water along with some isopropyl alcohol. This phase is routed back to the first column. 

In choosing a course of action never compromise the safety of personnel or equipment. Always think through the consequences of any action you might take before making an actual change in tower operations.

 If this action risks the safety of personnel or equipment choose another solution.

By understanding the principles, controlling variables effectively, and developing troubleshooting skills, operators can ensure safe and efficient distillation processes. Continuous learning and vigilance are key to mastering distillation operations.

"Thanks for reading !"