Methanol is commonly produced by steam reforming natural gas to produce a synthesis gas that is further converted to methanol. The crude methanol product from the methanol synthesis reactor is concentrated and purified in a two-step distillation train.
The first tower, commonly called the topping column, removes light ends. This distillation tower functions as the primary separation step to remove light-end impurities such as dissolved gasses (carbon dioxide and methane), dimethyl ether, acetone, lower hydrocarbons, and methyl formate from the methanol-water mixture that leaves the bottom of the tower along with trace quantities of higher alcohols. It is common to use 35 to 45 trays in this column.
The second tower, the refining column, removes intermediate components and water. This distillation tower functions to produce high-purity methanol as the top distillate product and to minimize the loss of methanol in the bottom product. Several locations near the bottom of the tower are typically selected to incorporate partial liquid draws to remove fusel oils (ethanol, propanol, butanol, higher alcohols, and alkanes) to avoid a buildup in the concentration of these intermediate boiling components. As the selectivity changes over the life of the reactor catalyst, so does the concentration and composition of the crude methanol. This results in a change to the optimum draw stage for the fusel oil. It is common to use 70 to 90 trays in this column.
Trays are most commonly used in the topping and refining columns. FLEXITRAY® valve trays combine high capacity and excellent efficiency with a wide operating range. A trayed column has the advantage over a packed tower in that it is relatively simple to include multiple draw nozzles at different trays to allow the draw stage to be varied as necessary. For this purpose, trays such as FLEXITRAY valve trays with type A or caged type T valves (for maximum flexibility), or MINIVALVE® tray decks with VG-0 valves are commonly used in the main two-step distillation train.
Depending on the specific plant design, packings such as INTALOX® ULTRA random packing, IMTP® random packing, and FLEXIPAC® structured packing may be used in various ancillary towers such as feed saturators, deaerators, and vent scrubbers.
Generally, there are no specific fouling problems, and the mass transfer equipment used is selected according to the required capacity, efficiency, pressure drop, and flexibility.
SUPERFRAC® high-performance trays are often used when revamping these towers to increase plant capacity. At high operating capacity, the lower surface tension of the liquid near the top of the tower can lead to unacceptable liquid entrainment before the tower reaches the hydraulic flood limit. In such cases, Koch-Glitsch has employed SUPERFRAC® trays with de-entrainment devices (PLUS technology) under the trays to help reduce entrainment and allow the tower to reach its maximum useful capacity. The SUPERFRAC® tray is a high-performance cross-flow tray that has the highest combined capacity and efficiency of all cross-flow trays tested at FRI.
The top of the refining column may also be revamped to increase capacity and/or efficiency using FLEXIPAC® HC® structured packing.
Certain impurities at concentration in the methanol-water mixture creates a potential for foaming. The Refining tower may also be characterized as a potential surface tension positive system where surface tension of the liquid increases as it travels down the tower due to mass transfer. This type of system has the potential to propagate a foaming tendency, and Koch-Glitsch typically considers a foam factor in the tray hydraulic calculations.
World scale methanol plants may employ very large towers that present additional challenges for the tray designer. Typical operating conditions have high vapor rates and moderate liquid rates, such that the resulting tray design may use only a limited number of downcomers and very large active areas (and long flow path lengths) to meet operating objectives. The tray design must address the tendency for vapor crossflow channeling that can limit capacity. The trays also need to maintain sufficient pressure drop to limit weeping at turndown conditions. Koch-Glitsch takes these into consideration during the tray design to provide optimal tray performance.
Large towers with few downcomers will typically require major beams for support. Koch-Glitsch can offer our patented Sectionalized Beams or Pinned Trusses to substantially reduce material weight and greatly reduce installation time compared to conventional welded trusses.