What is Distillation and Why is it Important?

 

Distillation is a core process that is fundamental to the economic operation of any process plant. Successful outcomes depend on quality distillation monitoring and troubleshooting, since just about every operation uses some form of distillation equipment or fractionation columns. Despite the need for quality, many operations have forgotten or ignored the importance of distillation. This practice can end up costing big in the long run.

The distillation process is used to separate components based on their boiling points. Chemicals like common gas, diesel, and jet fuel achieve their boiling points at different temperatures. Distillation columns are used to separate mixed feed streams into their own distinct products.

The last 50 years have shown tremendous improvement in our understanding of industrial distillation equipment and systems. Advanced technology is taking over in the design, control, and operation of towers. Tower internal innovation is working better than ever to improve tower efficiency and capacity. All of these innovations would seem to reduce the failure rate in distillation towers. However, the rate of failure in towers is actually on the rise and continuing to grow.

Distillation of Crude Oil

Crude oil has different components with their own sizes, weights and boiling temperatures, which can be separated easily by a process called fractional distillation. Following is the process for fractional distillation:

  1. Two or more liquids are heated with different boiling points to a high temperature. This is typically done with high pressure steam to temperatures of about 1112 degrees Fahrenheit / 600 degrees Celsius.
  2. As the mixture boils, it forms vapor (gases).
  3. As the vapor enters the distillation column, it passes through the column’s trays or plates. The vapor easily passes through the trays’ holes or bubbles (much like a loosened cap on a soda bottle). They increase the contact time between the vapor and the liquids in the column and help to collect liquids that form at various heights in the column. There is a temperature difference across the column (hot at the bottom, cool at the top).
  4. The vapor rises in the column.
  5. As the vapor rises through the trays in the column, it cools.
  6. When a substance in the vapor reaches a height where the temperature of the column is equal to that substance’s boiling point, it will condense to form a liquid. (The substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column).
  7. The trays collect the various liquid fractions.
  8. The collected liquid fractions may pass to condensers, which cool them further, and then go to storage tanks, or they may go to other areas for further chemical processing.

Problematic Issues With Distillation

Use of the wrong fuel/catalyst – Every plant focuses on items such as feed rates and temperatures. Even if these are on point, they don’t mean much if the product molecules are not recovered in the right streams. What good is there in using an expensive FCC catalyst if the extra distillate is being lost in your fractionators? Using the proper fuel is essential to any refinery distillation process, be it crude, propane, butane, isobutane, etc.

Improper pressure balance – Distillation columns must pressure balance. The fact is often overlooked by operators but remains a key principle. These pressure gradients are essential for separation quality and efficiency. Pressure is also one of the easiest metrics to monitor and is often done with a simple pressure gauge measuring pressure gradients between stages/trays.

Improper column mass balance – Operators should also routinely monitor the column mass balance. The practice can diagnose faulty level controllers, flow meters, and analyze results.  Good flow and level instruments are essential for these and other troubleshooting efforts.

Reflux ratio – Every column comes with a critical vapor and liquid ratio in its rectifying section. Operators should know the affects of a varying reflux ratio on the quality of the distillation. Knowing what this ratio should be, as well as monitoring it, can help resolve problems as they arise, or even before.

Regular analysis of the distillation column – Depending on its size, the distillation column may be analyzed via x-ray or gamma scan. The idea is to give operators a better understanding of the physical structure inside. This can be very helpful when diagnosing a mechanical defect involved with weeping, flooding, or other physical damage to the column.

Remember that distillation is one of the most important processes in a refinery or other process plant. AMACS is a trusted equipment contractor that replaces in kind distillation equipment and manufactures custom equipment. Our team of specialists are available 24/7! Please call or e-mail us with your requirements for responsive service.