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How can the capacity and efficiency of an existing deisobutaniser be increased?
 
Answers
23/07/2007 A: George Dzyacky, 2ndpoint LLC, ged@2ndpoint.com
From an operations perspective, there are a few simple guidelines to follow for improving the efficiency and capacity of not only deisobutaniser columns, but virtually all separation processes. First and foremost is the maintenance of the assets: column instrumentation, heat exchange, condensing equipment and control valves. Ensure the repair and maintenance of these assets is a priority at daily or weekly work-planning meetings. There will inevitably be conflicts among work priorities. When this occurs, it is critical to demonstrate the economic penalty of operating without, or being limited by, a particular asset. As long as safety is not a concern, making a logical, economic case for repair should elevate its work priority.
Column efficiency can be improved by operating at minimum pressure. Establishing a routine back-flushing schedule may alleviate condenser limitations. However, something as simple as a condenser backflush request from the console operator may have negative human factor consequences if there is a perception that this unnecessarily increases the workload of outside operators. Once again, making the economic case for the activity is often all that is necessary to facilitate the backflush. Consider conducting brief crew meetings to educate the workforce on the economic benefits of reducing the column pressure constraint. Explain the negative economic consequences of high column pressure. Operators need to understand how their activities are tied directly to the plant’s profitability. Scheduling periodic educational sessions on the connection between routine activities and process economics can positively change crew behaviour.
23/07/2007 A: Steve Hyde, KBC Advanced Technologies Inc, shyde@kbcat.com
In cases where there is poor isobutane recovery in the deisobutaniser, the refiner should evaluate his deisobutaniser operating targets/conditions and verify that they are consistent with his objectives for running the column. When the refiner is purchasing isobutane to meet alkylation demands, isobutane in the bottoms product can be expensive. On the other hand, normal butane in the isobutane overhead product takes up space in the alkylation unit and can hinder alkylate RVP control. When there is no constraint in the alkylation unit on normal butane removal, loosening the deisobutansier overhead targets and driving the deisobutansier towards zero isobutane in the bottoms can be an effective strategy. Normally, the size of the isobutane makeup stream pales in comparison to the size of the isobutane recycle within the alkylation unit. Therefore, a decrease in makeup purity would not adversely impact the alky unit isobutane-to-olefin ratio. Reducing reflux on the deisobutaniser and allowing normal butane overhead debottlenecks the deisobutaniser when it is running at high loadings. Similarly, bypassing C4 streams relatively rich in iC4 around the column directly to the alkylation unit would debottleneck the deisobutaniser. If the isobutane losses would justify capital expenditures, the usual list of suspects must be evaluated: tower feedstock selection/optimisation, tower height/diameter/internals, tower feed location, reboiler capacity/feed preheat and overhead cooling capacity.
23/07/2007 A: Daniel Summers, Sulzer Chemtech U.S.A., Inc., dan.summers@sulzer.com
A deisobutaniser is a typical example of a moderate-to-high-pressure hydrocarbon distillation. Deisobutanisers have historically been “trayed,” and revamp opportunities would generally consist of the installation of high-capacity trays as well as possible modifications to the feeds, draws and ancillary equipment. There is a significant amount of experience with Sulzer V-Grid trays being installed in deisobutanisers, with proprietary SVG trays being the preferred tray for alkylation process licensors due to their fouling resistance. For moderate (15-40%) increases in performance, Sulzer VGPlus trays have been successfully used in several revamps of deisobutanisers. VGPlus trays have been tested at Fractionation Research Inc (FRI) in its 11 bar (165 psia) isobutane/normal butane test system, providing high capacity along with efficiencies in excess of 100%. This is an excellent representation of commercial deisobutaniser operations.
For ultra-high capacities (>40% increases in capacity), proprietary ConSep trays can also be used. ConSep trays use centrifugal forces to allow operation beyond the tower’s system limit. ConSep trays have also been tested at FRI in the butane system, showing good efficiency and a capacity Cs of 0.13m/s, which is well in excess of the FRI system limit correlation.
As with any tower, any time a revamp is conducted special attention is needed for the feeds and draws. This is especially true for deisobutanisers, which can frequently have large side recycle draws. The piping for these draws is often large in comparison with the tray spacing, and the higher flow rates generated from the new revamp conditions must be handled properly.