.jpg)
Many industrial operations involve Mass Transfer and require intimate contact between phases, such as distillation and absorption, which involve exchange between the gas and liquid phases.
Mass transfer is by definition a non-equilibrium process characterized by the movement of molecules caused by a concentration gradient. Mass transfer is studied on a continuous scale, corresponding to the average motion of a very large number of molecules, so that the concentration profile of the solute within a phase is continuous.
In many cases the attention lies in the study of the mass transfer between two phases: liquid/liquid, gas/liquid, gas/solid or liquid/solid. These transfer phenomena occur at the interface between the phases, in which the chemical potential of the diffused solute must be equal in both phases. Due to the difficulty in measuring the chemical potential, a different approach is used in which its gradient is expressed in terms of concentration differences. However, at the interface between the phases, there is usually a discontinuity between the concentrations, which can be correlated by the partition coefficient or Henry’s constant, among others.
According to the image above, there is a concentration and chemical potential gradient between the bulk phase and the interface. This difference in chemical potential defines the magnitude of the molar flux. The chemical potential gradient (continuous at the interface) can be expressed in terms of the concentration gradient, which, although not continuous between phases, has a simpler and tangible measurement.
Thus, in order to have an efficient mass transfer between two phases, besides providing operating physical conditions, such as pressures and temperatures favoring the particular process, it is necessary to promote contact between the phases.
In columns designed for these conditions, the different devices that help or allow their operation are called tower internals. To promote contact between phases, there are two main types of internals: Plates and Packings.
Packed columns are commonly found in absorption, distillation, and stripping operations, among other applications. The mass transfer between phases is favored by the contact between them, promoted by the surface of the packing, which provides a large contact area for the phenomenon to occur.
The first use of packed towers probably dates back to 1820, when glass balls of an inch in diameter were used in alcohol stills. In 1907, in Germany, Dr. Raschig patented the first widely known packing, the Raschig Ring, in the form of cut tubes. However, at that time information on how to size these towers was still scarce, and packed towers were used only in pilot plants.
Although it is still very poor in performance, Dr. Raschig’s invention has shown that the geometry of the packing is a determining factor for its efficiency in phase contact. From that point on, his packing was modified giving rise to the Lessing Ring, Partition Ring and later the famous Pall Ring, developed in the 1950s in Germany, being the most popular packing in the 1960s and 1970s.
But the development of new packings did not stop at that stage. Different geometries may result in very different pressure drop values. Since the specific area of packing is not necessarily the effective specific area in mass transfer, many different packings are tested every year for better performance.
Packings may consist of a wide range of plastic, metal and ceramic materials and, with respect to other mass transfer devices, they may yield excellent results in terms of efficiency and capacity.
There are several advantages in using packings instead of trays for many systems. First, packings are considerably cheaper than trays, and in addition, they can be manufactured in a variety of materials resistant to corrosion and chemical attack. They have lower pressure drop when compared to trays and are excellent in supporting foaming operations.
The desirable parameters of packed columns are: uniform gas and liquid distribution, large surface area to promote inter-phase contact and packed structure which guarantees low resistance to gas flow. Packings are divided into random (distributed randomly) and structured types (distributed regularly), each with its advantages and disadvantages depending on the process.
This is because there are several factors that can affect the efficiency of the packing, such as: areas of liquid stagnation that hinder mass transfer; and geometric and surface factors that promote the breaking of the liquid film on the surface of the packing, providing constant renewing of the liquid film which increases its contact with the gas, thus increasing the effective exchange area of the packing.
In this sense, packings like CMTP® (metal), 3-Pack® (plastic), and MaxiSaddle BPC® (ceramic), are among the most modern and efficient commercially available packings, aiming to combine lower pressure drop with high mass transfer efficiency by renewing the liquid film on its surface and reducing the amount of liquid stagnant in the packing without, however, imposing great resistance to the gas flow.
.jpg)
The determination of the diameter and pressure drop in columns is directly affected by the choice of packing because of its intimate connection to the flooding of the column. The usual design of packed columns corresponds to 60 to 80 percent of the flooding velocity. However, the maximum operation velocity may be lower than this, especially for cases in which there is tendency for foaming. In operations at near-atmospheric pressure, the pressure drop must be minimized to reduce energy costs for inlet gas compression.
The determination of the amount of packing required to perform the Mass Transfer is a complex function involving the number of theoretical zones where the two phases of a substance establish equilibrium with each other within the column, the so-called number of equilibrium stages, or the change in mean composition with the average driving force for the mass transfer, the so-called number of transfer units, with the various operating parameters of the column, such as density, viscosity, temperature, pressure, diffusivity, flow rates, among others. These factors are often correlated with the thermodynamically calculated data for a given separation.
In spite of the complexity, these factors are correlated to the parameters of the effective interfacial area of mass exchange and the mass transfer coefficients of the packing, allowing for the determination of the amount of packing.
The Mass Transport Coefficient (k) represents a velocity constant related to the diffusion and convection of matter, which relates the mass transfer rate to the transfer area and the driving force, in terms of a concentration gradient. The coefficient can be expressed in different units, for example, when calculated in terms of partial pressures for cases involving transfer in the gas phase.
The coefficient can be calculated locally or globally, on each phase, and is a function of physical parameters such as temperature, concentration, flow and geometry. It is usual to compare different packings by the overall mass transfer coefficient in the gas phase multiplied by the effective interfacial area, the KGa, this being the constant of proportionality between the mass transfer rate and the expected concentration difference.
The above image shows the comparison between the KGa of the 3-Pack® and Pall Ring packings, both with nominal size of 2″ (two inches), tested under the same conditions. In this case, by taking the value found for the Pall Ring as base, and dividing the observed value for the 3-Pack® by this base value, it is observed that the KGa, and therefore the mass transfer for the 3-Pack® packing, is approximately 50% larger for almost the entire extent of liquid flow considered. This demonstrates the superior performance of the 3-Pack® packing in relation to mass transfer.
As the liquid moves down the column, it is evenly distributed over the surface of the packing, increasing the mass transfer coefficient. However, from a certain point in the column height, channeling effects begin to appear, and the distribution becomes irregular, that is, increasing the necessary height to perform the exchange.
Thus the liquid in the lower region of the packing can be found mostly in the vessel walls, or in concentrated regions (preferential paths). Hence the gas does not efficiently meet the liquid film at the base of the packing. Therefore optimization of packing size and determination of multiple bed sections are also important factors.
However, these preferential paths cause a significant decrease in mass transfer, especially when the ratio of liquid is below a certain critical value, called the minimum wetting ratio (MWR).
The wetting ratio is given by the ratio of the superficial velocity of the liquid phase to the specific area of the packing.
The packing wetting issue is critical as it can significantly affect the packing performance. The minimum ratio of packing is a function of its geometry, its size and especially its material and surface. Some materials in decreasing order of wettability are: Unglazed ceramics, Oxidized metal, Metals with treated surface, Glazed ceramics, Glass, Polished metal, PVC/CPVC, Polypropylene, PTFE. Therefore, in general, it is observed that ceramic materials are the most easily wetted and plastic materials are those in which wettability becomes a critical factor.
Therefore, different random packings may have the same function, but the behavior of the packed bed varies completely with the nature of the packing, which may vary in model and size, material, and the extent of the bed itself.
Thus, the challenge of continuous improvement and different or new versions of the same packings will come, allowing for debottlenecking and continuous improvement of industrial processes.