PTQ Q4 2024 Issue

Evaluating tray efficiency impacts on column design

Steps to avoid undersizing or oversizing of columns along with the selection of property packages essential for the model to generate reliable tray traffic data

Tek Sutikno Fluor Enterprises

C ommercial simulation programs are commonly used for generating tray traffic data for distillation col - umn design specifications. The model developed to simulate a column can generate tray traffic data, such as flow rates and physical properties of vapour-to-trays and liquid from trays, along with the number of stages needed to meet the product specifications and the heat and mass balance. The column diameter(s) and length are primarily specified or assessed based on the simulated tray traffic or loading data, which are also utilised to check hydraulic parameters, such as jet flood, downcomer flood, weir loading, and oth - ers resulting from the selected tray design and dimensions. To properly specify the distillation column design param - eters, tray traffic data from the simulation model need to realistically reflect the flow rates, phase equilibrium, and properties of the fluids in the column. Two simulation options, ideal (theoretical) stages and the actual number of trays, are generally available in com - mercial programs for column design simulation. In some, if not most instances, simulation models based on ideal stages are developed, and the simulated tray traffic data are directly utilised for specifying tray dimensions and the column diameter. The actual number of trays required to attain the simulated ideal stages is determined using a known or reference tray efficiency applicable to the par - ticular fluid system. Different efficiencies can be specified for a column with multiple sections. The column height can be specified mainly based on the actual number of trays and tray spacing. The other simulating option is specifying the actual number of trays and tray efficiency for generating the tray traffic data. The generated tray traffic data in this option, with reliable efficiency estimates, can be expected to be the same as those generated by the similar model using ideal or theoretical stages or tray efficiency of 100%. If the ideal stages (100% efficiency) and the actual stages with assigned efficiency provide significantly different tray traf - fic data, the specifications for tray and column will likely be underdesigned or overdesigned. This is especially the case when the traffic data selected for design are less or more than the real operation data. Three-column simulation models are presented to

evaluate the potential impacts of tray efficiency on column and tray design specifications. Property packages and tray efficiency For designing new columns, especially for first-of-its- kind types where design references and field data are not available, the designer will typically develop the simulation model to determine the number of ideal stages required to meet the product specification/separation requirements. One or more property packages available in the simulation program and applicable to the fluid system may be selected and used in the model. The actual number of trays required to attain the simulated ideal stages is determined using a known or reference tray efficiency applicable to the particular fluid system The simulated theoretical stages will likely be unreliable or erroneous if the property package selected does not real - istically represent the properties and vapour-liquid equi - librium of the fluid compositions. Property package usage guidelines are typically provided in commercial simulation programs and should be reviewed. When the fluid compositions are not explicitly described in the guidelines, but the components are all included in the property package available for selection, the ideal stages simulated from the selected package may be best checked against ideal stages estimated using the short-cut approxi - mation methods, the Fenske method as an example,1 espe- cially for first-of-its-kind type design. Tray efficiency is necessary to specify the number of actual trays from the simulated ideal stage model. For first- of-its-kind columns, without ‘goby’ or field data, tray effi - ciency inputs will need to be estimated and could involve uncertainty. In this case, the O’Connell correlation² may be considered for estimating the overall or sectional tray efficiencies.

PTQ Q4 2024