Structured grid packing
Structured grid packings have traditionally been used for high severity services. Grids have a high open area, relatively low surface area, and a robust construction to withstand process upsets and to resist fouling. There are two main types of grids, categorized by structure - conventional grid and corrugated, high-efficiency grid. Developed first, conventional grids have a structure made from vertical members and angled flaps. The simple construct is fouling resistant, but has limited contact area for mass transfer. The newer corrugated, high-efficiency grids resemble a larger, more robust version of structured packing, which allows for improved mass transfer properties.
Invented in the 60’s, conventional grid was first used for vacuum column wash beds. Grid allowed faster installation when compared with ring packing, was easier to dismantle once it coked, and created a very low operating pressure drop that improved cutpoint.
The highly open nature of the conventional grids creates very low pressure drop, high hydraulic capacity, and resists fouling, but at the expense of liquid and vapor distribution ability. As such, conventional grids are seldom used in actual distillation applications, but rather for heat transfer, scrubbing, and de-entrainment purposes.
Non-Conventional or High Efficiency Grid
The increasing trend for plants to operate at higher severities created a market for a grid that could also provide better heat and mass transfer properties. Sulzer developed the MellagridTM series of grid, which has the highest capacity of any packing tested at Fractionation Research, Inc. (FRI).
The corrugated high-efficiency grids operate with the same principles as structured packing, allowing vapor and liquid to continuously mix. So, a corrugated grid packing retains the high capacity that is a characteristic of conventional grids, but can improve overall fractionation efficiency.
These benefits may allow for a reduction in bed height in some applications (like FCC slurry pumparound or Vacuum wash) that can reduce operating pressure drop or create additional headspace for inlet vapor distribution.
Grids are often used in combination with structured packing to optimize both de-entrainment and fractionation efficiency. Structured packing layers at the top of the bed provide good mass transfer and liquid distribution, while grid layers placed on the bottom provide de-entrainment to prevent the packing above from fouling, as well cooling the upflowing vapor to reduce coking tendency.
Some severe services with severe fouling use a mixed all-grid bed with high success. Systems subject to asphaltene deposition are often designed with several layers of conventional Nutter GridTM at the bottom of the bed, followed by layers of Mellagrid on the top.
Sulzer engineers can assist in providing an optimized, effective design for structured grid packing type and bed configuration according to a column’s specific operating conditions, like vapor mal-distribution, asphaltene precipitation, coking, pumparound, or wash sections.
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