"Essential Shutdown and Startup Procedures for Distillation Columns: A Comprehensive Guide"

 

"Essential Shutdown and Startup Procedures for Distillation Columns: A Comprehensive Guide"

 

Distillation columns are periodically shut down for routine maintenance and inspection to ensure optimal performance and safety. In this section, we’ll explore some general procedures used during shutdown and startup operations of distillation columns.

 An operator should always read the specific shutdown and startup instructions for their unit before such operations. These procedures will vary from tower to tower and from plant to plant.

 The first step in most planned shutdown operations is to gradually reduce the feed rate into the column, ensuring the process remains stable. As we reduce the amount of material put in the tower, the product output also decreases. By making gradual feed rate changes, the tower instrumentation can maintain the design conditions inside the column. This allows us to produce on-specification products while the feed rate is being reduced. The feed rate is reduced until the tower is on total reflux. At this point, no feed is being added to the column and no products are being withdrawn. The material that remains in the tower continues to fractionate but is not removed as product.

 Next, gradually reduce the heat input to the tower to safely lower the temperature. In vacuum columns, the vacuum is taken off and the column is shut down under atmospheric conditions. As the temperature in the tower decreases, light components fall to the bottom of the column and mix with the heavier material. This liquid mixture is pumped out and saved for reprocessing. Because pressure decreases as temperature is reduced, there's always a possibility of a vacuum occurring in the tower while it is being shut down. If a vacuum occurs in a tower that is not designed for vacuum operations, the column could collapse. To prevent this from happening, an inert gas like nitrogen is introduced into the tower to maintain a positive pressure.

 After the tower has been shut down, drained, and pumped out, it must be cleaned of all hydrocarbons or other organic or chemical material. These substances can form potentially explosive mixtures when their vapors combine with air. For a mixture to be explosive, the percentage of vapor in air must come within certain limits.

 This chart shows the explosive range of some hydrocarbons when mixed with air.

For example, when an atmosphere contains between 5.3% and 14% methane, it forms an explosive mixture. Because it is difficult to tell precisely when a mixture of air and hydrocarbon vapor reaches the explosive range, it is best to completely avoid mixing hydrocarbon vapors with air.

 

During shutdown operations, all hydrocarbons must be removed from the tower before allowing any air in. And during startup, all the air must be removed from the tower before hydrocarbons are introduced into the column. One way to remove hydrocarbons from a tower is to purge the column with an inert gas like steam or nitrogen. Inert gases can strip or displace hydrocarbons off metal surfaces inside the tower and move these vapors out of the column. Water can also be used to clean out a tower. Some columns are flushed out with water, and the liquid containing hydrocarbons is drained out the bottom. Other towers are completely flooded with water. The flooding operation floats hydrocarbons to the top of the column where they are removed. In some instances, a combination of water and steam is used to clean out the tower.

 

If a tower is completely flooded with water, there will be no gas or oil left in the column. To drain the water, we vent the tower to the atmosphere. This prevents a vacuum from occurring above the liquid level while allowing the liquid to flow out the bottom. When a column is vented to the atmosphere, there is a possibility that hydrocarbon vapors from other parts of the unit will enter the tower and form an explosive mixture. To prevent this from happening, metal plates or blinds are placed in all tower inlet and outlet lines. Blinding a tower effectively isolates it from hydrocarbons that are present in other parts of the system. Purging with inert gases and water flooding or flushing should remove all hydrocarbons from the tower. But we can't assume the tower is safe until it is actually tested. Various types of hydrocarbon detectors and analyzers are available to ensure the tower is free from hydrocarbons. Before anyone enters a tower, it must be tested with one of these devices. After the column is purged and tested for hydrocarbons, one of the manways is opened, and an air mover is turned on. The air mover can be a fan or an air inductor.

 

In the event that any hydrocarbon vapors are released during the cleaning of the tower, the air mover will sweep these vapors out of the tower. Cleaning of the tower should be thorough and complete. Even after purging and water flooding, there are usually some stubborn deposits that remained in the column. These deposits are removed with special solvents or by manual scraping. After the tower has been thoroughly cleaned, it must be carefully checked for corrosion. We do this to make sure that all equipment is still strong enough for future use. A corrosion inspection is usually made by measuring and recording the thickness of various metal parts inside the column. While the tower is open for repairs, the vapor and liquid paths should be inspected to make sure they are free of obstructions. Downcomers and bubble caps should be free of solid deposits. Even a partial blockage of a vapor or liquid path will cause inefficient tower operation.

 

A list of scheduled repairs is normally compiled prior to shutdown. Any other problems that are discovered during the inspection of the tower are then added to this list. By keeping thorough written records, we make sure no necessary repairs are overlooked. After all repairs are made, the tower is given a final check to see that no tools, equipment, or clothing have been accidentally left in the column. A foreign object could plug an outlet after the tower is back on stream, making it necessary to once again shut down the process. After everything is checked and the tower is closed, all air in the column must be removed. This can be done by adding an inert gas like nitrogen or steam to the tower. An oxygen analyzer is then used to verify that no air remains in the system. All blinds installed during the shutdown process must be removed prior to startup.

 

Before the tower is brought back on stream, it is necessary to perform a tightness test to make sure there are no leaks in the column or in the connecting equipment. A tightness test is usually performed by running pressurized steam or nitrogen into the tower. A pressure buildup indicates there are no leaks. If the pressure continually bleeds off, there is a leak somewhere in the system. After the tower is tested for leaks, the inert gas or steam pressure is released. A hydrocarbon vapor is then backed into the column to avoid pulling a vacuum.

 

Before starting the tower, make sure that all auxiliary equipment like reboilers and condensers are ready for operation. Also, ensure that any water that might have settled in low points has been drained out. The procedures for the actual tower startup will vary from column to column. You should always follow the guidelines for your particular tower. Remember to make gradual increases in temperature and pressure when starting up a column. Rapid temperature and pressure changes can stress and damage equipment.

 

Let's review the control procedures that are followed during startup and shutdown operations. 

 In recent years, computer control of distillation processes has become increasingly popular due to its efficiency and accuracy. A computer is a device that stores data and performs calculations. It is programmed with a number of mathematical equations which it uses to determine the ideal operating conditions for a tower.

 Information about a process variable is measured by traditional measuring instruments and is sent to the computer by process transmitters. The computer converts this data into mathematical calculations to determine if the process is operating at maximum efficiency. Based on the outcome of these calculations, the computer either sends a signal to position a control valve or signals a controller to position the valve. In some systems, the computer performs the necessary calculations, and an operator makes the actual adjustment on the controller. From this brief discussion, you can see that the computer occupies a position similar to the controller in conventional instrumentation. It receives and analyzes measurement information about a process variable. Then it provides an output signal that is used directly or indirectly to position a control valve. The computer, however, has many features that distinguish it from a typical controller. For example, a computer has a memory which allows us to store vast quantities of information about the process being controlled. This information can easily be reviewed for troubleshooting purposes or for checking historical trends in the values on specific variables. The computer has a very large capacity and can operate at very high speeds. Where a separate controller is needed for each control loop in conventional instrumentation, a single computer can operate the control loops for an entire plant. In a computerized control system, process data is displayed on a CRT console. These consoles organize and display process data in a variety of easy-to-read logical formats. In most systems, an operator can call up video displays ranging from a broad overview of the entire process plant to a detailed analysis of a single control loop. The majority of the computer systems on the market are equipped with alarm displays. These displays automatically flash to warn an operator about any abnormal operating situation. Other displays that can be shown on CRT consoles include checklists, flowcharts, and full-color schematics of process equipment. Any of these displays can be called up instantly with a touch of the finger. This quick and orderly display of information makes it easier for an operator to understand the dynamics of the process and gives him more time to devote to controlling the operation. A keyboard attached to the video console is the interface between the operator and the process. Setpoint adjustments and requests for display information are made by pressing the appropriate keys on the board. Many computer systems are equipped with printers that provide written records about the condition of variables being controlled. The ultimate justification for using the computer is based on economics. Results from plants and refineries that have switched to computer control indicate considerable savings in the areas of energy conservation, throughput capacity, and recovery of valuable components.

 

Let's take a look at how computer control can improve the operational efficiency of a distillation column. Measurement data on process variables is continuously fed into the computer's memory. The computer uses this data to calculate the actual liquid and vapor flow rates inside the tower. This information on internal liquid and vapor rates, along with data on other relevant variables, allows the computer to accurately predict the composition of products before they are actually produced. The computer continuously compares its product composition estimates with the current operating conditions to determine if ideal conditions are being maintained in the tower. Based on this comparison, the computer adjusts tower variables so that the products will meet specifications but not greatly exceed them. This saves energy and maximizes capacity. If there is a change in the condition of a process variable like reflux temperature, the computer immediately calculates how this change will affect product composition. The computer can then adjust the tower operation to keep the column producing on-specification products at the lowest possible cost. Most computers rely on product composition analyzers to verify the accuracy of their predictions. The most common composition analyzer is the gas chromatograph. A gas chromatograph can analyze the composition of a product stream just as the plant laboratory would in as little as 30 seconds. This information is fed into the computer's memory to trim and adjust computer calculations and the output signals that are sent to control values. A computer-assisted on-stream process analyzer is often used to control a chain of distillation columns. When working with a group of towers, a control strategy that considers the entire chain as well as the individual towers is needed. This is because products often have specifications limiting the amount of both heavy and light impurities.

 Let's consider an example. The overhead product in this depropanizer is propane. Specifications will allow up to 3% ethane in the overhead product. Can the amount of ethane in the propane product be controlled by the depropanizer? If you answered no, you are correct. The only way to keep ethane out of the propane product is to limit the amount of ethane that leaves the depropanizer. Ethane has a lower boiling point than propane, so all of the ethane entering the depropanizer will rise to the top of this column and become part of the propane product. When an operator is controlling a series of towers with conventional instrumentation, he must balance out the specification requirements for a number of different products. In some instances, he may have to temporarily relax specifications on one product to avoid changing the composition of a more valuable product. During a process upset, it becomes especially difficult to maintain product specifications because an operator has to simultaneously make adjustments to several towers. More effective control of a series of towers is realized by using chromatograph analyzers in conjunction with a computer. During normal operations, a computer-based system makes closer cut-point divisions than conventional instrumentation. This saves energy and allows us to recover more of the valuable components. During process upsets, the computer quickly gathers and analyzes all the important data. The computer then advises the operator on what operational changes to make or makes the changes automatically. These adjustments keep the entire series of towers operating at maximum efficiency until the problem is resolved.

 In this blog, we also outlined some general control procedures that are followed during shutdown and startup operations and included a basic introduction to computer control of distillation processes. In addition to making an operator's job less complex, computer control saves energy, increases capacity, and allows us to produce more of the valuable components.

 This concludes our comprehensive guide on shutdown and startup procedures for distillation columns. Thank you for reading!