Paraxylene Feed/Effluent Vapourizer Increased Capacity 100% Improvement in tube side Heat Transfer Coefficient



ICI CHEMICALS (HUNTSMAN CHEMICALS)  Petrochemicals, Wilton, UK.

SERVICE Feed  vapouriser/reactor effluent interchanger.

 EXCHANGER Fixed tubesheet, TEMA type BEM. 1342 tubes: 25.4 x 2.03 x 3658 (1in. x 14swg x 20ft.)

PROBLEM

Plant capacity was limited by the feed vapouriser train performance. With a high shellside boiling coefficient, tubeside heat transfer limited overall performance. It was possible to install more tubes, but the poorer coefficient resulting from lower velocity, diminished the benefit. Other conventional improvement strategies were considered to be similarly ineffective. ICI also had concerns about the effect of fouling, which was known to occur in the bottom 10% of the tubes.

SOLUTION

In partnership, CALGAVIN and ICI, conducted a thermal test programme from which the inservice enhancement could be predicted with certainty. In parallel, mechanical trials proved that even under severe adhesive fouling conditions, the hiTRAN Matrix Elements would be removable. The trials confirmed that 100% improvement in tubeside heat transfer rate could be achieved, and engineered within the allowable pressure drop. A hiTRAN Thermal System was consequently retrofitted to each pair of vapourisers.

BENEFIT

The overall heat transfer rate was more than twice that of the original units offering a significant capacity increase. Also changing the tubeside heat transfer profile along the tubelength by installing hiTRAN lowered the tubewall temperatures. This in turn reduced the problem of asymmetric fouling on the outside of the tubes which had been causing very high local wall temperatures and subsequent damage of the tube to tubesheet joint. The replacement interval for these units is therefore lengthened significantly.

INSTALLATION

May 1986
Full Technical Paper Available

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