Oct-2011

Tray revamp for demethaniser ethane recovery

As a first step in an ethane extraction plant’s operational improvement plan, a tray revamp was performed to improve both tray efficiency and ethane recovery

Darius Remesat, Koch-Glitsch Canada
Paul Wenger, Midstream Engineering Professional

Article Summary

There are many cryogenic light hydrocarbon processing units operating in Alberta, Canada. These facilities process pipeline-quality natural gas to remove natural gas liquids (NGL), primarily ethane, a valuable feedstock, for Alberta’s petrochemical and NGL industries. A study investigated whether any opportunities for operational improvement were available using the existing infrastructure at these locations. A list was generated with different cost levels and ethane recovery improvements. Operators decided first to pursue the lowest-cost, moderate-recovery improvement scenario, which was to replace the top trays in the column with high-capacity, high-efficiency trays.

Improving ethane recovery at a turbo-expander plant
The two primary factors for improving ethane recovery are:
• Equilibrium (thermodynamics)
• Energy (refrigeration for condensation).

The separation of molecules by distillation (primarily methane and ethane at these operating units) is limited by equilibrium conditions within the distillation column. To improve separation within the equilibrium constraints of the distillation tower, tower internals with higher mass transfer efficiency can be employed. In addition, if the tower internals can provide capacity gains, the overall performance of the ethane extraction can be improved tangibly.

For a turbo-expander plant, shown in Figure 1, providing additional energy through compression of the feed gas can supply further refrigeration via the Joule-Thomson (JT) effect. This added refrigeration translates to an increased top liquid feed, which serves as reflux in this scheme. If the mass transfer internals have the capacity to handle the extra liquid flow, improved ethane recovery results from the contact between the increased liquid and the upcoming vapour flow. 

Another related consideration for improving ethane recovery is the composition of the reflux, which impacts the equilibrium between methane and ethane. Ethane recovery improves as the amount of ethane in the top liquid feed is reduced (that is, it shifts the equilibrium point to allow greater ethane recovery overhead). During operation, improved ethane recovery is achieved and maintained by efficiently converting and using energy; for example, fouling of the heat exchangers limits the optimal use of energy.

Characteristics of the feed
The feed conditions of Alberta-based processing facilities when compared to feeds in the US Gulf Coast are lower in pressure (consistently below 800 psia), with a lot more CO2 (~1.1% vs 0.5%), more methane (~89% vs 84%) and fewer C3+ hydrocarbons (~2.6% vs 7%). As a consequence, the overall ethane recovery for these plants, with state-of-the-art technology, is lower than with feeds from the US Gulf Coast.

Process description
The key sections of a typical cryogenic light-ends recovery process unit are shown in Figure 1. A primary separator, expander, subcooler and demethaniser make up the cryogenic section of the unit. Specifically, the majority of these processes use the well-known gas sub-cooling process (GSP), in which a small portion of non-condensed vapour is used as the top reflux to the demethaniser after substantial condensation and sub-cooling. The main portion of the feed, typically in the range of 65–70%, is subjected to turbo expansion as usual.

A heat pump takes part of the feed stream and uses it as primary and intermediate reboil for the demethaniser column. A two-sided reboiler approach (heat pump) is used to reduce the need for external refrigeration. A heat pump design can be recognised by the use of a compressor, cooler for rejecting heat to a high-temperature sink, a JT valve or a second expander and, optionally, a second exchanger to take heat from the low-temperature source.

Revamp study
The first step in the revamp study was to develop a representative simulation of these plants based on a comprehensive set of test runs that provided the upper and lower limits of production. Table 1 illustrates the various options reviewed, categorises the revamp options as either low or high cost, and groups them according to expected incremental ethane recovery. Each plant processes slight different quantities, so specific returns on investment will differ but all remain positive.

The current maximum recovery of the Alberta-based units is limited due to the feed inlet pressure (<800 psia) and composition of the inlet feed from the local gas fields (high in methane and low in C3+ hydrocarbons). However, numerous solutions can be implemented to increase cumulatively the ethane recovery of the entire site. Overall economic evaluations indicated that the tray revamp provided the highest return on investment and so was approved.

Scope of project
The successful Inside-Out Design Approach2 used by Koch-Glitsch for revamps was followed to determine the benefits of an internals revamp in the column. Where trays and demisters were in operation, the project scope was to replace the top three trays in the demethaniser, add an inlet feed distributor and replace the existing Demister mist eliminator. The objective was to capture an additional 3% ethane, which translates into a payback of less than one year because of the low-cost nature of the revamp.

Existing internals arrangement
For four operating units, the existing top three trays were either standard-capacity trays or a previous-generation high-capacity tray. The old-style high-capacity tray has increased vapour handling capacity due to the truncated downcomer providing increased active area, and it offers the mass transfer efficiency of a conventional crossflow tray and other high-capacity crossflow trays. These two-pass trays on 24in tray spacing used either standard-diameter moving V-1 or caged type T valves.