What turns a good engineer into a great engineer?

The same thing that turns a good doctor into a great doctor: experience.

Greatness first manifests itself in an uncanny diagnostic capability. A great engineer can look at a piece of equipment, listen to it, smell it, feel it, and can arrive unfailingly at the identification of the problem. Similarly, a great engineer can always find a better way to do what others have accepted as impossible to improve upon. The same holds for the age-old process of distillation. One of the most popular books ever written was Rules of Thumb for Chemical Engineers. Not that these rules take the place of rigorous calculations, but when memorized can save many hours of analysis and design.

Case in point is distillation and in particular vacuum distillation as it is used at Allen Filters for the purification of fluids.

Distillation is the process used for separating multi-component fluids into high purity products. A designer is required to have a thorough understanding of mass transfer, phase change, and pressure-drop fundamentals before he or she can successfully optimize the process design. Most engineers rely heavily on vendor recommendations, thereby absolving themselves of the fundamental function of the creative application of the basic principles of chemical engineering.

The purification of fluids contaminated by multi-component mixtures with different solubilities and partial pressures, involves the separation of these contaminants from the base fluid.

Allen Filters technology originally applied by founder Albert Allen is based on the design of distillation columns similar to those used in refineries. A distillation column can use either trays or packing. The mechanisms of mass transfer differ but both are based on achieving a steady-state equilibrium by using large amounts of interfacial area. This interfacial area is the result of the passage of vapor through the open spaces in the trays or between the packing and the spreading of the liquid in a very thin layer on the surface of the packing. While at first we used interlocking trays in our vacuum vessels, after several years of research using pilot systems and computer modeling, verified by countless tests, we found that for the optimum mass transfer a certain combination of packing type, heat, and vacuum produced the best separation efficiency. Serious consideration was given to the influence of the shape of the packing on the liquid flow behavior as a result of fluid dynamics studies in our pilot systems.

The variety of shapes and types of packing and the variations in geometry that are available tend to empiricize the design of packed columns and we had to limit the choices for our research into the optimum performance to just a few packing types. We concentrated on obtaining an even distribution of liquid in the packed bed, in addition to providing the shortest diffusion pathway between vapor and liquid flow

Packing can be divided in two groups, depending on the type of packing used: random (dumped) or structured (ordered). Each type of packing is also available in a variety of sizes. This means they are geometrically similar but vary only by a characteristic dimension.

Mass transfer in the distillation column occurs at the interface between the liquid film on the surface of the packing and the vapor. The efficiency of mass transfer depends on the specific area of the packing, which is controlled by the packing's nominal size. We found after many experiments that random packing provided the best results. Packing installed in a random fashion orients itself in an irregular fashion and this influences the relationship between the characteristic dimension and the specific area of the packing. The vertical height of the packing column also has an influence on the mass transfer efficiency.

Mass transfer in a packed column is typically described in terms of HETP, the height equivalent of a theoretical plate—that is the height of packing required for a theoretical stage of separation.

Performance remains relatively constant and stable, except for at high or low rates. At low liquid throughput, sheets of liquid are not stable and rivulets do not spread evenly to wet the entire surface of the packing, so mass transfer is poor. As the column throughput increases, there is a large stable area of the curve where packing is properly wetted and the HETP is relatively stable.

At the higher rate portion of the stable region, mass transfer improves. This improvement is caused by an increase in interfacial area due to liquid turbulence and entrainment. At higher rates, performance decreases rapidly due to increased liquid entrainment in the vapor that is being carried upwards through the column. This entrainment degrades the composition of the vapor and can result in liquid carryover and vapor redistribution.

Poor Wetting of Packing

In the distillation column, liquid flows from the distributor onto the packed column in fine streams. Good wetting requires that the packing elements distribute these streams evenly throughout the column's cross-section. The liquid flow rate controls the hydrodynamic behavior of the liquid film.

Thus for continuous distillation, the downward flowing liquid phase is in contact with the upward flowing vapor phase.

A significant amount of data was also generated on behavior of pressure drop in the column. Liquid as it flows down the packed column raises pressure drop by occupying space, increasing turbulence related eddies and providing liquid droplet entrained in the vapor phase.

Flooding

As the column throughput increases beyond the stable region, the pressure drop rises more quickly due to more liquid entrainment in the vapor and greater vapor velocity due to the reduced area of escape. Substantial quantities of liquid flow upwards through sections of the column and because the packing is relatively open, liquid and vapor tend to redistribute themselves, resulting in a sharp drop-off of mass transfer. Ultimately the column will flood and in the case of vacuum distillation, liquid and vapor will carry over into the condensate tank.

Optimization Rules

1. Allen Filters uses random packing elements with a small characteristic dimension.
2. We run the column near the upper range of the stable region.
3. We use packing with large specific area which tend to be more efficient.
4. Because pressure drop tends to improve mass transfer, we use packing types with a relatively higher pressure drop, although this may sometimes require a larger column diameter.
5. The operating range of internals, especially the liquid distributor is more often than not the controlling factor. Allen Filters uses a proprietary (patent pending) fluid distributor on our vacuum distillation columns.
6. Allen Filters process design always places a solids filter before the vacuum distillation column to prevent fouling of the packing. For that reason, under normal operation, the vacuum vessel never needs to be opened for cleaning.
7. Packed columns can be designed with great accuracy, using the empirical equations generated by research.

Vacuum Distillation

Vacuum lowers the boiling points of individual components in a multi-component mixture by reducing the vapor pressure, causing evaporation beginning with the compounds with the highest boiling points. As such it facilitates mass transfer at relatively lower temperatures. Numerous pilot plant tests have shown that between vacuum and heat, the latter is the more important variable. These tests in addition to extensive proprietary computer modeling have allowed us to optimize the combination of the right amount of heat and vacuum to achieve the most efficient mass transfer at the highest capacity and at the lowest pressure drop.

Raschig Super-Rings

Raschig Super-Rings are a fourth generation random packing made by Raschig GmbH. They were first introduced in 1995 and have since been widely used in the refinery and chemical process industry. The distinct characteristic of Raschig Rings is that the emphasis was on producing liquid films as much as possible and avoid droplet formation. The rings have no droplet promoting edges in their geometry and as a consequence achieve a very even distribution of liquid over the packed bed. This leads to a highly homogeneous distribution and optimal mass transfer.

References

1. Pd Dr. –Ing. M Schultes. “Raschig Super-rings-A New Fourth Generation Random Packing.” February 2001.
2. American Institute of Chemical Engineers. “Optimize Distillation Columns.” 2000.
3. Koshy, T. D. and Rukovena, F. “Distillation Pilot Plant Design, Operating Parameters, and Scale-up Considerations.” The Chemical Engineers Resource Page.
4. Seader, J.D. and Henley, E. J. “Separation Process Principles.” John Wiley & Sons, Inc., 1998.
5. Perry, R.H. “Chemical Engineers’ Handbook.” McGraw-Hill.
6. Simon, R.J. “Computational Hydrodynamics and Distillation Column Design.” Allen Filters, Inc. Internal Publication, 2006.