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What is an Oldershaw column?
Simply put, an Oldershaw column is a sieve-tray unit with downcomers. Originally patented by Shell Development Co. more than six decades ago, its design has remained unchanged, and it has become a non-proprietary item available in laboratory supply-house catalogs. Early models were fabricated of glass, and later, bundle-type counterpart tray sets were constructed from metal. Column diameters range from 25 mm to 100 mm, and accessories typically include a condenser, a reboiler, and a reflux splitter, as shown in Figure 1 .
Because of the large surface-to- volume ratio inherent in small columns, it is common to specify that tray assemblies have vacuum jackets. The ground glass joints make the assemblies adaptable so that the number of trays used to study a problem can be changed quickly. Typical characteristics of Oldershaw trays are given in Table 1 (p. 28).
What is it used for?
The Oldershaw column performs bench-scale simulation of proposed chemical processes to demonstrate the feasibility of separation by distillation, and to indicate the level of efficiency expected upon scale-up. Even though the actual mechanisms of vapor-liquid contacting in the Oldershaw are poorly understood, the device is quite useful for many applications - to the extent that every laboratory involved with distillation studies should stock a full complement of Oldershaw equipment.
How does an Oldershaw predict scale-up efficiency?
Large trays. A 1982 study by Fair, et al., (1) concluded that the Oldershaw provides a close approximation of the point efficiency (well-mixed vapor and liquid) of larger trays. The efficiency of those trays can be estimated from Oldershaw measurements together with a model representing efficiency enhancement from liquid concentration gradients. Since the objective of determining the crossflow concentration gradient is to ultimately provide a tray efficiency greater than the point efficiency, some designers use the Oldershaw measurement for design, recognizing that the conservatism provides for future increases in throughput (reducing the reflux ratio) or tightening of product-purity specifications.
Vacuum stripping. In a 1964 paper, Martin (2) described scale-up problems in a large 20-tray vacuum stripper serving to remove water from a heavier organic solvent, ?-butyrolactone. Operating head pressure was 112 mm Hg and startup revealed the overall tray efficiency to be 28% - much lower than predicted during design....