Apr-2006

Comparing packing efficiencies

In mechanically demanding applications, grid packing can help control costs and product degradation. Important parameters such as grid surface area and its effect on efficiency are discussed

Mark Pilling and Nina Prohorenko-Johnson
Sulzer Chemtech USA Inc

Article Summary

Grid packing is a specialised form of structured packing commonly used in refinery de-entrainment and heat-transfer applications. Conventional grids have been in use since the 1980s, with most types being fairly similar from a mechanical standpoint. During the last few years, however, a new type of proprietary grid, Mellagrid, has started to be used in the industry. Developed in 1991, its first installation was in an FCC main fractionator in Finland. There are now nearly 100 Mellagrid applications worldwide, including FCC main fractionators, crude atmospheric and vacuum columns, coker and visbreaker fractionators, and quench columns. Fractionation Research Inc (FRI) has conducted tests at its independent testing facilities to evaluate Mellagrid.

Process considerations
Grid packing is truly an application-driven product. It is used in services where older technologies such as trays, structured packing and random packing have been tried and have either failed or fallen short of process needs. Services using grids usually require very high capacities. Vacuum columns with low vapour densities have high vapour velocities with relatively low liquid flux rates. Another requirement for vacuum services is a low pressure drop. There are two main factors driving this: cost and product degradation. Energy is required to generate high vacuums, so a lower pressure drops result in lower operating costs. A lower pressure drop also allows lower operating pressures and temperatures in flash zones, which decreases the thermal degradation processes such as coking and cracking.

In order to achieve low pressure drops in packings, the effective packing channel size is increased. With conventional grids, this is done by increasing the blade spacing, but with structured packings and Mellagrid it is done by increasing the crimp height and spacing. The larger channel serves to reduce the surface area of the packing and makes it more difficult for fluids to bridge the gap between the channels. These factors combine to reduce the frictional losses within the packing and decrease the pressure drop. Increased channel size also has the benefit of improved fouling resistance.

One penalty for increased channel size is reduced mechanical strength, so grid packings are always made of thicker material. In less mechanically demanding applications, a standard structured packing would be an ideal solution. However, applications that use grid packings invariably require additional strength and fouling resistance beyond that provided by structured packings.

In typical grid applications, the tower in question is central to the process unit operation, if not the entire refinery operation. Any downtime or malfunction of these towers is a major expense that must be avoided if possible. As the typical periods between refinery turnarounds continue to increase, any device that can provide longer run times for the major columns is a valued commodity. Therefore, fouling resistance and the mechanical strength to withstand moderate to severe process upsets are fundamental requirements for grid applications.

In order to meet these needs, the grid must be stronger than other common heat and mass-transfer devices. Although constructed of much thinner materials, packings are typically more resistant to process upsets than trays. Since trays are generally limited to an open area of 15% of the column cross-sectional area, they are usually displaced or damaged during a violent process upset. Packings, however, have a much higher open area and so are often able to withstand upsets with little, if any, damage. As a result, a strong packing structure will provide the best solution to withstand these upsets. While structured packings are made from materials that are typically 0.1mm, a grid packing is ten or even 20 times thicker than this in order to provide the exceptional mechanical strength.

Fouling and coking are similar, but process concerns are different. Fouling resistance is generally a function of a device being able to pass the fouling materials through it. With packings, fouling resistance is a function of channel size, channel angle and surface treatment. As mentioned earlier, larger channels mean increased fouling resistance. This is due to a larger channel opening size, as well as fewer contact points between adjacent sheets. Channel angle is important, because this slope promotes the passage of liquid and solids downward through the packed bed. Since structured packings have no horizontal surfaces, they are more fouling resistant. While conventional structured packings are textured to promote surface spreading, grid packings, including Mellagrid, are manufactured with smooth surfaces in order to further reduce the hold-up of fouling materials. Coking is strongly a function of residence time within the packing. Therefore, anything that holds up liquid within the device decreases coking resistance. The slope of structured packings also helps with coking resistance by decreasing the liquid residence time.

The fundamental difference in hold-up between conventional grids and Mellagrid is the body style. Conventional grid packings are constructed from thick metal strips positioned vertically and spaced parallel to each other, forming a grid; hence, the name “grid”. Mellagrid, however, is a hybrid between a traditional grid packing and a standard structured packing. It has the corrugated form of a structured packing, but is manufactured from a thicker untextured sheet metal for enhanced mechanical strength. Conventional grids have multiple tabs and breaks cut from short vertically standing strips within the structure of the packing. At each of these physical disruptions, droplets form and create highly localised residence times. Since Mellagrid has a smooth, continuous corrugated structure, these breaks are not present, so this method of hold-up is not possible. Another issue is the contact points between the pieces of grid in the bed. Conventional grids have short layer heights (1–3in or 25–75mm) and the blade spacings are quite close (usually 2in or 50mm). As these multiple layers stack upon each other, many contact points are formed that will likely accumulate either a fouling material or droplets with long residence times, which may lead to coking. Mellagrid has similar contact points between the peaks of the corrugated sheets. However, the larger crimps and spacings of Mellagrid cause it to have only 5–10% of the contact and drip formation points found with a conventional structured packing and approximately half that of a conventional grid packing. 

Testing and evaluation
Industrial-scale testing of Mellagrid was conducted at the Fractionation Research Inc (FRI) facility in Stillwater, Oklahoma, USA. FRI is a consortium of engineering and construction companies consisting of chemical, petrochemical and refining companies, as well as equipment manufacturers whose charter is to conduct testing to determine the operating characteristics of mass transfer internals.