Processes involving mass transfer are among the main unit operations in the industry today. Due to the ever-increasing need to produce chemical commodities, distillation and absorption are still among the most widely used techniques for producing and/or separating components.
The main consequence of this is that there is constant effort to make significant improvements in column internals, in order to increase either the production capacity of the column or its efficiency. In this sense, structured packings represent a significant technological evolution in column internals.
| Despite the reduction in pressure drop, these packings had poor performance in mass transfer efficiency, and for this reason its current use is practically restricted to applications in heat transfer.
The first two generations of structured packings later developed consisted of metallic screens. Despite their good efficiency, they had limited application to vacuum distillation towers. However, it was the advent of structured packings consisting of corrugated metal plates that came to revolutionize the technology of internals for mass transfer. |
Schematic drawing of an Absorption Tower |
The need for packings with greater efficiency combined with extremely low pressure drop led to the development of packings with regular structure.
The concept of structured packings arose ideally from tests with random packings arranged neatly in columns, the so-called ordered packings. Subsequently, grids were developed for the same purpose.
Cross-Partition Ring Random Packing still currently used as Ordered Packing
Corrugated Plate Metallic Structured Packing – Clark Solutions MaxiPack® 250 |
Firstly, these packings were significantly cheaper than those of metal screens, although at the time they were still much more expensive than random packings (in the 1970s, their value could be up to 100 times more expensive than random packings. However, its technology cost has been greatly reduced, and there are cases in which the prices of the packings are very similar). Nonetheless, the gains from the process were highly worth the costs. To start with, the pressure drop of this packing type is significantly lower than that of any other high performance internals developed until then, comparable to the low pressure drop observed in the grids already described. Also, the mass transfer efficiency in several cases proved to be much higher in structured packings and trays.
The lower pressure drop of structured packings is due to the almost complete absence of entrainment of liquid by the gas and the form factor of these packings. The existing pressure drop can be attributed almost completely to the viscous drag that occurs by the presence of the liquid film formed on the wall of the internal. This phenomenon contributes to its efficiency in mass transport, as it increases the interfacial turbulence and the renewal of the liquid film on the surface of the packing. |
The high efficiency of structured packings is due to the almost complete wetting of its surface, thus increasing the effective area of mass exchange. Nevertheless, the already mentioned viscous drag promotes turbulence in the liquid film on the surface of the packing, increasing the mass transfer. To aid the formation of this turbulent region, the surface of the packing is often textured. The surface of the packings may also be perforated, promoting mixing between different parts of the packing, causing both sides of the sheet to be wetted even at high liquid flows (avoiding the opposite effect where only one side of the sheet is preferably wetted, known as blanking).
Surface detail of the MaxiPack® 250 structured packing, highlighting the Corrugation, Texture and Holes on its surface.
By associating a lower pressure drop and high efficiency, today’s structured packings are the first choice for vacuum column design or revamps or at moderate pressures, for increased capacity or efficiency especially in situations free of severe fouling and polymerization risks. There are, however, limitations on their use in cases of high pressure, because of detachment of the wall fluid, backmixing and other effects. Nevertheless, there are cases where even at high pressures these packings can be considered.
Briefly, the most relevant concepts when considering these packings are:
Pressure Drop – Its value by theoretical stage is much smaller than other internals, increasing as the vapor or liquid flow is increased.
Capacity Limit – It is defined as the point at which column operation stops being controllable. As the packing approaches flooding, small increments in the ratio of liquid or vapor can lead to a sharp increase of pressure drop and liquid holdup, causing excessive retention in the column. Above this value, the transfer efficiency is also hindered.
The Capacity Limit of structured packings is highly dependent on its model and geometry, and is often far superior to that of other internals.
Liquid Holdup – Liquid retention in the column promoted by the presence of the packing. Its value increases with increasing liquid flow.
HETP – Height Equivalent to a Theoretical Plate, or equilibrium stage in mass transfer. In structured packings, it is not very sensitive to load conditions in the column over normal operation, and it is practically the same for different operating pressures. However, it may be hindered by poor distribution of liquid.
The geometry of Structured Packings is an essential factor for its performance. Metal plate structured packings are fabricated from corrugated (crimped) metal sheets arranged parallel to each other. The blades are grouped into standard height elements (or blocks), which are mounted to fill the horizontal section of the columns. Each blade is crimped with a fixed angle relative to the vertical axis, originally conceived as 45°. However, aiming to increase the capacity of the packings, internals were created with a crimp direction of 60°.
Commonly, models with a 45° angle are known as “Y” models, and packings with a 60° angle are known as “X” models. Y model packings allow for longer liquid residence time in the tower, thus increasing the efficiency of the packing. However, a greater verticalization of the channels gives the model X greater efficiency.
Schematic drawings for the angle of corrugation. Model Y (45°) on the left and Model X (60°) on the right.
The geometry of the structured packing has a direct impact on its performance. Due to the corrugation of its surface, two adjacent blades form channels for fluid flow, with the flow area being directly influenced by the dimensions of the corrugation. Smaller corrugation dimensions make the metal blades more compact and close together, thus increasing the specific transfer area. However, by reducing the channel area, the gas finds greater resistance as it flows up the column, increasing the pressure drop and reducing the capacity of the packing, in addition to reducing its resistance to incrustation.
The drawing below exemplifies this fact by schematizing the top view of a structured packing. For an equal area selected, the reduction of the dimensions of the crimp (drawing on the right), increases the number of blades that fit in the same space, increasing the packing mass transfer area. However, the reduction of the channels imposes greater hydrodynamic limitations on the packing. In spite of this fact, these are usually smaller than those of random packings and plates. These high mass transfer efficiency models are responsible for reducing the heights of towers dimensioned for them, since other internals with lower efficiency will require larger volumes or internals spacing.
Top view of a structured packing showing the channels formed by the blades |
Illustrative schematic drawing comparing the area of the channels seen from above varying the size of the crimp (on the left a greater corrugation height and to the right a smaller one, showing the difference in area and channels) |
To ensure uniform gas and liquid distribution, each metal sheet is grouped consecutively with a 90° rotation in the radial direction.
Schematic drawing showing channels formed between two plates rotated by 90°
In addition to the differences between the X and Y models already mentioned, another geometric element has been employed in structured packings more and more. This element means verticalizing the slope of the crimp near the top and bottom ends of each packing block. The accumulation of liquid and the flooding of columns packed with structured packings occur in the transition regions between elements.
In this new configuration, where in certain cases the metal sheet of the packing acquires an “S” shape, the drainage of liquid accumulation in the transition region between blocks is facilitated. Therefore the packing has a huge increase in capacity, with no substantial difference in its mass transfer efficiency. Thus, they are named as High Capacity X or Y models.
Schematic drawing of the conventional profile (left) and the S profile for High Capacity (right)
The consolidation of metallic structured packings has led to the development of techniques so that they can be assembled in different materials. Today there are also structured ceramic and plastic packings, which may be applied in cases in which metal would be inoperative.
The distribution of liquid is a special matter that must be well taken care of in structured packings, as a poor initial distribution tends to extend along the packing and can cause a decrease in efficiency. For this purpose it is important to assess whether the number of distribution points is adequate, to ensure that the distributor is properly installed and leveled and, in particular, to ensure that all distribution points are functional and unblocked.
Drawing of a Distributor Example used in Structured Packing Towers
In order to assure a proper redistribution of the liquid between the structured packing blocks, during their assembly each layer of blocks is rotated 90° perpendicular to the next layer, in order to break the continuity of the flux direction in the channels, forcing the liquid to change direction. It is important that the layers are always rotated at the same angle and always maintain the same rotation direction.
Because of their highly organized nature, the procedures for installing structured packings are more specific than those of random packings, since poor installation can cause a decrease in the performance of these packings.
| Structured packings come in standard-sized blocks so that they can easily enter the column through a manway. It is recommended that each layer of structured packing (or part of it in the case of large diameter towers) is previously assembled outside the column to make sure that the packing configuration has been understood by the assembler, assisting the assembler to understand how each layer of blocks must be rotated. Special care should be taken on the first layer, to ensure that the packing is properly adjusted over the packing support.
Packings are relatively fragile, so one should not walk directly over the structured packing. It is recommended to use plywood boards or other shapes to distribute the weight of the assembler. During the assembly of the first section, the assembler must be on the packing support or suspended during installation. Care must be taken so that none of these boards used for mobility are left in the tower or between blocks. |
Schematic drawing where different block patterns used to fill the cross-sectional area of the tower are identified by numbers |
The packing blocks must be properly arranged so that there is no free space between them and they fit the vessel wall. In some cases it may be necessary to fill the spaces between blocks with individual sheets or metal strips so that it fits adequately into the vessel. Therefore, it must be ensured that there is not enough space to form preferential paths.
Properly installed, structured packings provide extremely low pressure drop values and high mass transfer efficiency, factors that often compete with each other.
Whether in revamps or new designs, structured packings are present in cases where smaller column sizes, more efficient columns, or larger capacities are desired. The technology of internals for mass transfer continues to be constantly developed, both to debottleneck processes and to enable new ones. Structured packings are a milestone in this technology, which has been evolving in order to find new, cheaper and more efficient solutions.
Clark Solutions manufactures, in Brazil, a complete line of structured and random packings and other internals for distillation, absorption, and stripping columns and air and gas treatment.