"Advanced Distillation Techniques: Design, Operation, and Control"

 

"Distillation Design and Control"

We will learn advanced distillation techniques with this comprehensive Blog. Understand the design and operation of distillation columns, the role of auxiliary equipment, and the importance of controlling temperature, pressure, liquid level and flow. Learn how to maintain product quality, make operational adjustments, and utilize control loops to ensure optimal performance in refineries and chemical plants.

In any refinery or chemical plant, you'll find that distillation columns are designed differently to produce different kinds of products. The column shown here separates two products that have a very small difference in their respective boiling points. This is a difficult separation that requires a large number of trays, so the tower is very tall.

This column is much shorter because the products it separates have relatively large boiling point differences and fewer trays are needed to make this type of separation.

 

Design and Operation of Distillation Columns:

Distillation columns are often classified according to the operating pressures they maintain.

Crude units, like above one, are often called atmospheric towers because they operate at or slightly above atmospheric pressure.

Other towers, which process very light materials, may operate under hundreds of pounds of pressure. The high pressures are needed to keep the products in a liquid state so they can be more easily stored and transported.

Many pressure towers are designed with narrow tops because there is less vapor-liquid traffic in this portion of the column and this design helps maintain the desired operating pressure.

And finally, there are columns like this one that operate below atmospheric pressure in a partial vacuum. 

Towers that operate below atmospheric pressure are called vacuum columns.     

When we reduce the pressure on a liquid, it boils at a lower temperature. Vacuum columns reduce the amount of heat that is needed to vaporize hydrocarbons because they operate at very low pressures. Vacuum columns are generally used to distill very heavy compounds that have very high boiling point temperatures. If we distill these compounds at atmospheric pressure, we'd need to use an excessive amount of heat in order to vaporize the large molecules. These high temperatures, in turn, could change the structure of some of these molecules, thus changing their characteristics.

We can obtain a better and a more economical separation if we moderately heat the liquid and then feed it into a column that is under a vacuum. The reduced pressure allows the lighter components to flash out or vaporize the moment the feed is introduced into the column.

Vacuum towers must be able to handle high vapor loads, so they are usually designed wider than other types of towers, and their trays are spaced further apart.

 

Internal Hardware and Efficiency:

Different types of internal hardware are used to control the downward flow of liquid and upward flow of vapors inside distillation columns.

The purpose of this hardware is to maximize the vapor-liquid contact, which is necessary for good fractionation to occur.

This bubble cap has teeth, or slots, that are set below the level of the liquid on the tray. Vapors rising through these slots are broken into small bubbles before they pass through the liquid.

By forcing the vapors to form small bubbles, the surface area for vapor-liquid contact is increased.

Other types of trays include the sieve deck, S-section, and valve tray.

They use different types of hardware to force vapors to bubble through the liquid on each tray. On each tray, there is an inlet and an outlet weir. These weirs act like dams to distribute liquid across the tray and maintain the proper liquid level.

Downcomers provide a passageway for liquid to flow down the tower. The downcomers can be designed to provide single or multiple flow patterns through the tower.

Some distillation columns use layers of packing instead of trays to promote vapor-liquid contact. The packing material breaks up the liquid so there is a large surface area for contact with the rising vapors.

 

Auxiliary Equipment:

The operation of a distillation column requires auxiliary equipment such as furnaces, reboilers and condensers.

This auxiliary equipment is needed to vaporize hydrocarbons and provide a cooled liquid reflux.

A furnace uses burners to heat the feed as it passes through a series of tubes. The tubes absorb heat in two different ways. First, the tubes at the bottom of the furnace receive direct rays of heat from the burners. Then the gases and vapors formed by burning fuel and air are channeled into the top section of the furnace to heat other tubes.

Reboilers and condensers are both heat exchangers.

A heat exchanger is a device in which the heat from one fluid flowing through a series of tubes is transferred to another fluid flowing across these tubes.

In this reboiler, the fluid to be heated is bottom product and the heating medium is steam. The steam flows through tubes inside the exchanger. The bottom product flows across these tubes, absorbs heat, and is vaporized. The purpose of heat exchange in this condenser is to remove heat.

The product being cooled is hot vapors and the cooling medium is water. The cooling water flows through tubes inside the exchanger. Hot vapors flowing across these tubes are cooled and condensed to liquid.

Let's take a closer look at how towers are designed as well as some of the equipment used in distillation columns.

 

Product Properties and Specifications:

The properties of a distillation product are determined by the type of hydrocarbons that are present in it. To ensure that distillation products have the desired properties, certain standards or specifications are set to control their composition. The products are then periodically tested to make sure that they are meeting these specifications.

These tests are sometimes run automatically by instrument analyzers. In other instances, an operator collects a sample and sends it to a laboratory for testing. If tests indicate a product is not meeting specifications, operating conditions inside the tower must be adjusted to change the composition of the product and bring it back on specification.

One way to analyze the composition of a product is to perform a laboratory distillation test. This test is performed by heating a liquid sample and then collecting the condensed vapors. The temperature at which the first drop of condensate is collected is called the initial boiling point. The end boiling point or end point is the temperature recorded when the last drop of liquid in the container vaporizes.

A range of temperatures between the initial boiling point and the end point determines the product's boiling range.

This sample carries an initial boiling point specification of 150 degrees Fahrenheit. If the product contains the desired proportion of light hydrocarbons, the sample should begin boiling at this temperature. Lab tests show the sample actually begins boiling around 125 degrees Fahrenheit. Since light components boil at low temperatures, this test indicates the product contains too many light fractions.

Here is a sample with an end point specification of 475 degrees Fahrenheit. If the sample boils away at this temperature, the product contains the right proportion of heavy components. The actual temperature, however, at which the last of the sample boils is 500 degrees Fahrenheit. Since heavy material boils at high temperatures, the product in this example contains too many heavy fractions.

 

Some distillation products are given a boiling range test. A boiling range test identifies both light and heavy components over a product's entire boiling range.

In other process streams, there may be very small temperature differences between the initial and end boiling points. These streams generally do not contain a wide range of different molecular components. So the best way to measure the product purity or specification is to identify the actual light or heavy components in the product instead of measuring boiling points.

For example, this column is separating propylene and ethylene, two hydrocarbons that boil at nearly the same temperature. The lighter component ethylene is going overhead. Specifications call for a maximum of 3% propylene in the overhead product, but lab tests indicate the top product contains 4% of this heavy key component. So the top product is off specification because it contains too much heavy material.

 

Most petroleum-related products release flammable vapors. The lowest temperature at which these vapors ignite is known as the flashpoint of a product. Flashpoint is an important specification for some distillation products.

Products that contain mostly light components release flammable vapors at a lower temperature than heavy hydrocarbon products. Therefore, a relatively light product like kerosene will have a lower flashpoint temperature than a heavy product like reduced crude.

Specifications on this product call for a flashpoint between 145 and 150 degrees Fahrenheit. The actual flashpoint is 125 degrees. This product is off specification because it contains too much light material.

 

Many products must meet certain weight specifications. The lightness or heaviness of petroleum products is usually measured by comparing their weight to the weight of an equal volume of water. Two different scales are used to do this.

The API gravity scale uses a reference in which 10 degrees API gravity is the same weight as water. On this scale, the lighter the product, the higher its API gravity. So relatively light products like gasoline and naphtha have a higher API gravity reading than heavy products like lube oil and fuel oil.

Another way to gauge the relative weight of a product is by specific gravity. On this scale, water is given a value of 1, and products lighter than water have a reading below 1. When measured on a specific gravity scale, you can see that the lightest products, gasoline and naphtha have the lowest readings.

The color of a product will often indicate whether that product contains the hydrocarbon molecules that specifications call for.

Heavy hydrocarbons like lube oil are dark, while light hydrocarbons like methanol are very light in color. Color tests are generally used to check for contamination of light products by heavy hydrocarbon molecules.

Setting specifications for products allows us to control the type or range of molecules that are included in any one product.

This is important because the properties of a distillation product, such as its boiling range, purity, color or flammability are determined by the type of molecules that are present in it.

 

Adjusting Tower Operations for Product Specifications:

When a product is off specification, a change in tower operations must be made to change the composition of the product and bring it back on spec. This is usually done by adjusting the heat balance inside a column.

This column is producing propane as the overhead product and butane as the bottom product. Lab tests indicate there is too much butane going overhead with the top product. The top product is off specification.

One way to reduce the amount of butane going overhead is to reduce the heat input to the tower. This can be accomplished by restricting the flow of steam to the reboiler. Because butane requires more heat to vaporize than propane, reducing the heat input to the tower decreases the amount of butane going overhead.

Another way to remove butane from the top product is to increase the amount of heat removed from the tower. We can do this by increasing the reflux rate. The downward flow of reflux condenses the heavier components in the upward flowing vapors.

In this column, butane is the heavy component, so increasing the reflux rate results in a top product containing less butane.

To summarizes what happens when the reboiler temperature is decreased or the reflux rate is increased. Either of these adjustments will reduce the temperature inside the column. When the temperature falls, less heavy material rises up the tower. As a result, there is a decrease in the amount of overhead product produced as well as a decrease in the percentage of butane going overhead.

We get just the opposite effect if we increase the reboiler temperature or decrease the reflux rate. Either of these adjustments will cause the temperature to rise. As the temperature rises, more heavy material is driven up the column. This increases the amount of overhead product produced and increases the percentage of butane going overhead.

Adjusting the heat balance so that a column will make more or less of a particular product is called changing the cut point. Although cut point changes are frequently made by changing the column temperature profile, they can also be made by adjusting the column pressure.

Recall how pressure affects the boiling point of a liquid.

In a vacuum column, for example, the pressure is reduced so that more vaporization will occur at moderate temperatures. So reducing the pressure has the same effect as increasing temperature.

Consider what happens in the tower that is separating butane and propane when the pressure is reduced. The lower pressure allows more vaporization to take place, which increases the amount of butane driven overhead. The overhead product gets heavier and more of it is produced. So when we reduce the operating pressure, we get the same effect as we did by reducing reflux or increasing reboiler temperature.

Suppose we increase the system operating pressure without adjusting any other variables. The increased pressure causes less vaporization to occur, so less butane will enter the top product. The overhead product becomes lighter and less of it is produced.

We get the same effect as we would by decreasing the column temperature.

Normally, we try to maintain the column pressure as low as possible within the design limits because this reduces the amount of fuel needed to vaporize the feed components. Let's take a closer look at how a distillation unit is operated to make cut point changes.

 

Control Loop Instruments and Automation:

In modern processing industries, instruments are used to automatically sense, measure, and control operating conditions or variables in distillation columns.

The use of automatic instruments allows us to continuously produce products that are on specification and at the same time makes it easier to monitor and adjust the distillation operation.

There are four major variables or process conditions that must be controlled in any distillation process. These variables are temperature, pressure, liquid level, and flow.

Suppose we want to control the temperature at the top of this distillation column. The first thing we need is a measuring instrument that can sense the temperature at the top of the column. Next, there is a transmitter that interprets this measurement information and sends a signal to a controller. The controller is programmed to maintain tower top temperature at a desired value called set point. If the temperature has deviated from set point, the controller sends a signal to a valve telling it either to open or close. This changes the reflux rate to the top of the tower and brings the temperature back to the desired value or set point.

 

In any control situation for any type of process, four things are necessary.

First, there is a process variable that needs measurement and control. For each process variable that must be controlled, there is a measuring means, one or more instruments that can measure the process variable.

There must be some kind of control mechanism to receive the measurement information and determine how it compares with the desired value or set of values.

The control mechanism must also tell the valve what action if any, it should take.

And there is a final control element which is usually a valve.

The final control element makes the actual process change that keeps the process variable at set point.

 

So four things are necessary in any control situation.

These four elements are called a control loop.

 

Let's look at some actual instruments to review how the parts of the control loop work together.

Assume that the variable we want to control is pressure inside this distillation column. A pressure tap connected to the overhead vapor line is used to gather measurement information. This information is then sent to a transmitter. The transmitter interprets the measurement information on tower pressure and signals a controller. A controller compares this measurement signal with a set point signal that represents the desired tower pressure. A valve is the final control device in the loop. This valve regulates the flow of vent gas out of the accumulator. If pressure in the column rises above set point, the valve opens to decrease pressure in the column. If pressure in the column falls below set point, the valve closes and pressure in the tower will build up.

 Let's take a look at some other common control loops in distillation towers and see how they work together to control tower operating conditions.

 In this program, you learned that many common raw materials are made up of a mixture of different molecules.

In order to produce useful products, we must separate these various molecules into cuts or fractions that contain the same types of molecules.

This is the function of the distillation process.

We used a series of stills to demonstrate how a mixture can be separated by distillation. The top stills operate at progressively lower temperatures, so fewer heavy components are vaporized as we progress up from one still to the next. This eventually isolates a light fraction in the top still. The bottom stills operate at increasingly higher temperatures, so more light components are vaporized out of the liquid as it is piped down to each lower still. This forms a heavy fraction in the bottom vessel.

In a distillation column, hot rising vapors contact a cooler descending liquid on a series of trays. As a result of this contact, 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 and the column pressure.

Since the lowest temperature occurs at the top of the column, a light fraction is formed here. A heavy fraction is isolated at the bottom of the tower where the temperature is highest.

In any refinery or chemical plant, there are many different kinds of distillation columns that produce a variety of products. While these towers may be designed differently and may operate under different conditions, the purpose of each tower is the same, to produce quality products that meet specifications.

 

You learned in the program that when products are off specification, an operating change must be made to bring the products back on spec. This is usually done by adjusting the heat balance inside the column. If we change the amount of heat that is put into or taken out of a distillation column, both product composition and quantity are affected.

You also learned that most tower operations are controlled automatically by control loop instruments. Temperature, pressure, liquid level, and flow are the four primary variables that affect the heat balance inside a distillation column. Control loop instruments function to keep these variables at a desired value or set point.

This concludes our program on distillation principles and practices.                                                 Do feel free to share this post or leave your comments below!

“Thank you for Reading!”.