Oct-2013

Retrofitting a glycol contactor to prevent carryover

A revamp of glycol contactor column internals, following CFD studies, resolved problems with gas production caused by glycol carryover

YANG QUAN and ANNE PHANIKUMAR
Sulzer Chemtech Switzerland

Article Summary

The gas production rate on an offshore platform was constrained due to a high level of glycol carryover from glycol contactors. The existing column internals were investigated and it was found that the area of the mesh pad in the top of the column was around half of the column area. In addition, the gas outlet location and associated piping arrangement appeared to create vapour maldistribution. A high-capacity Shell Swirltube separator and Sulzer structured packing internals were proposed in conjunction with a new gas outlet arrangement and the existing separator support system. CFD studies were carried out to evaluate the possible vapour distribution, based on which a revamp proposal was fine-tuned. New internals were manufactured and installed on site in 2012. Recently, the design was validated in high-rate trial runs, with significantly lower triethylene glycol (TEG) carryover.

All together, there are three trains on the platform. The facility dehydrates the gas and condensate separately, and then recombines the dried products before transportation via trunkline(s) to the gas plant. Subsequently, the gas plant processes these fluids to produce LNG, condensate and LPGs.

In 2006, a customer in the Asia Pacific region decided to proceed with the design of a new bridge-linked facility to its plant. The compression installed on the new plant would allow 20 000 t/d of gas throughput per train. Specification of the glycol content of the export gas plant must be less than 4 m3/d to prevent onshore upsets.

During a trial in 2007, dehydration capacity as well as TEG losses were evaluated at various flow rates. The gas rate was increased to around 18 000 t/d per train, at which point it was found that the glycol losses reached the maximum limit of 4m3/d (total of three trains). Dew points were found to increase and were worse at the higher rates. Condensate dehydration was established to be adequate. Overall, dehydration performance was found to be acceptable for the first and second trunklines, where the saturation specification had basically not been affected and remained at around 25% and 19%, respectively, which was less than the integrity limit of 75% and the operating limit of 65%. Therefore, it was concluded that the dehydration of both gas and condensate was sufficient at 18 000 t/d. This gas throughput became constrained by glycol carryover losses. It was decided by the customer to revamp the third train in early 2012 and leave the other trains to a later date. This would allow confidence and experience to be gained with minimum exposure (a single train rather than all three trains). Sulzer was selected to revamp the internals of the contactor to minimise the TEG losses at higher gas rates in the customer’s complex.

In the late 1980s, to increase throughput, the original bubble cap trays in the glycol contactor were upgraded to structured packing (GEMPAK 3A, which is comparable to Sulzer Mellapak 350Y). In the column top, a mesh pad was installed to prevent glycol losses. It covered about half of the column area (see Figure 1). A lean glycol distributor was installed above the structured packing, and in the bottom there was a chimney tray to collect the rich TEG before being directed to the glycol regeneration system (see Figure 2). This was the column internal configuration to be revamped. To avoid having to stress release the column after revamp, welding to the column wall was not permitted. The existing separator support system had to be reused for the various revamp options.

Gas-liquid separator in the column’s top section
The location and arrangement of the gas outlet was relatively unique, as the gas outlet nozzle is located laterally below the mesh pad. An elbow-shaped internal pipe passes through the centre of the mesh pad. This pad was supported by a shroud welded to the column head. Usually, the gas- loading capacity of structured packing in TEG contactors is higher than that of a wire mesh pad. Obviously, the excessive glycol entrainment observed during the trial in 2007 could immediately be attributed to the small mesh pad area, which was only half of the packing cross-sectional area. The arrangement of the existing mesh pad was perfectly symmetrical, but the space between the column head and mesh pad was very narrow, causing a poor inflow to the gas outlet piping above the mesh pad and an uneven vapour distribution through the mesh pad. This maldistribution also contributed to the observed excessive entrainment of TEG.

Any revamp proposal was limited by two key factors:
• The gas outlet could not be relocated
• Hot work to the column wall was prohibited.

Therefore, the existing shroud had to be reused, and it was necessary to install high-capacity demisting equipment to compensate for the limited area available for a separator.

There are several types of gas- liquid separators on the market, and they can usually be classified into three categories: mesh pad, vane pack and axial cyclone types. The selection of different types depends on required capacity, pressure drop and efficiency (indicated by cut-off size). Table 1 shows a simple summary of the selection criteria. It has to be noted that values in the table are indicative only. They are subject to individual separator products and operation systems. 

With increasing operating pressures, vane packs, even if they are equipped with a coalescer in the front, suffer higher efficiency loss than axial cyclones. The operating pressure of this TEG contactor is 108 bar. Axial cyclones are the best option for having up to three times higher capacity than mesh pads per unit area. They can compensate for the limitation caused by the smaller area available for separation. A Shell Swirltube separator was used in the investigation of various options.

Direct welding to the column wall was forbidden, so it was proposed that the existing shroud be extended downwards by welding additional parts onto it. To maximise the number of Swirltubes and to take into account the dimensions of the standardised cyclone modules, the outlet piping had to be shifted from the centre to the side of the shroud. Internal piping for the dry gas outlet nozzle needed to be cut and welded onto the extended shroud. A schematic of the proposed new arrangement is shown in Figure 2. This arrangement posed some uncertainty with regard to vapour distribution across the new Swirltube deck due to sudden changes in the vapour flow direction of 90 degrees after passing through the Swirltube separator and the asymmetry of the new arrangement in general. The typical pressure drop of a Swirltube separator is much higher than for a mesh pad (30 mbar versus 0.5 bar). This helps to improve vapour distribution but, because the concern for maldistribution could not be eliminated, CFD studies were conducted to understand and quantify the risk.