US Gulf Coast isobutane was bid at 5.25 cents/gal over normal butane Tuesday, the widest premium since June 20, according to Platts data.

Market sources attributed the strengthening premium to isobutane buying interest from a large Gulf Coast isobutane producer.

Normal butane was trading at $1.4435/gal, while there were no heard isobutane deals.

On Monday, non-LST Mont Belvieu isobutane was assessed at $1.5150/gal and non-LST Mt Belvieu normal butane was assessed at $1.4675/gal.

Isobutane's premium to normal butane has been depressed for most of 2013 and on September 12 the spread even reversed to a deficit. However, that was short-lived and the premium has slowly gained ground over the past six weeks.

That increase has coincided with Enterprise Products Partners restarting two isomerization units and a deisobutinizater at its Mont Belvieu NGL complex in early October.

Enterprise has three Mont Belvieu isomerization units with a total capacity of 116,000 b/d.

The units have been operating at lower rates than capacity, Enterprise said in its second quarter earnings call in July. The partnership's Q2 butane isomerization volumes were 97,000 b/d, down from 100,000 b/d in Q2 2012.

Merchant supplies from isomerization units, from sources such as Enterprise's Mont Belvieu complex, are being pushed out by more naturally occurring isobutane barrels as NGL production in the US continues to increase.

Bentek, a unit of Platts, estimates that gas plant production of isobutane is averaging 166,000 b/d while isomerization production is 106,000 b/d. By comparison, gas plant production of isobutane was 142,000 b/d in 2012 and isomerization production was 112,00 b/d, Bentek data showed.