High capacity trays have been the distillation success of the 90's. In spite of years of attempts, the 90's started with only one major choice available for high capacity tray operation. This has changed radically with every medium and larger sized equipment vendor offering its own proprietary high-capacity tray. Some vendors offer multiple types.
These offerings have revitalized the use of trays; especially in revamping processes for higher capacity. They have added low-cost revamping opportunities for many services not suitable for structured packing. However, relatively few high-capacity trays are in service compared to structured packing or more traditional trays. We are still in the learning stage of high-capacity tray implementation. As in every learning stage, failures occur as well as success.
In addition to the trays themselves, much of the information needed to design, apply, and evaluate high-capacity trays is proprietary. Considering competitive pressures, the high-capacity tray user can have many legitimate questions that cannot be easily answered in a public forum. Overall, the high-capacity tray consumer is at the mercy of the equipment supplier.
High capacity trays have been a great advance in moderate to high pressure distillation (above 100 psig). Until now, distillation at higher pressures has been limited to conventional trays or random packing. This has limited through-put for a fixed column to relatively lower rates than that in low pressure distillation where structured packing could be successfully used. Depending on the conditions, high capacity trays can increase unit throughput by 10% to 25% over a good standard tray design. Processes including propane-propylene separation, refinery gas plants, and NGL plants have all benefited from this.
Changing from normal to high capacity trays feels, to the process plant, like shifting from a station wagon to a roadster feels to an average driver. The performance (capacity) change is exhilarating, so too are the crashes at 150 mph versus 50 mph. Not everybody needs a sports car to solve their transport problem. The same way, high capacity trays are not for all distillation problems.
Our goal here is to give some general background by covering the basic concepts in high capacity trays; their application history; and pointing out advantages, disadvantages, and pitfalls. In addition, we will cover how to increase the capacity of the 'conventional' tray and how lessons learned with standard trays provide background in evaluating high-capacity trays. While many options have been offered for high-capacity trays, we will restrict this coverage to the currently commercially available options from the major tray vendors. The objective is to give the engineer the tools to sort out what is possible, what may be possible, and what is hype. With these tools, the tray consumer is not so absolutely dependent on equipment supplier's evaluations.
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