New solvent exceeds performance targets during testing at a Canadian sour gas treating facility.

A new solvent process for treating sour gas developed by Uhde GmbH(KU) and Gas Technology Institute (GTI) has been demonstrated for the first time on a commercial scale in Duke Energy Gas Transmission's (DEGT) Kwoen Gas Plant in northeastern British Columbia. The Morphysorb process was shown to be a cost-effective and environmentally beneficial method for debottlenecking the DEGT Pine River processing system. The process exceeded DEGT's performance targets, which were substantially above expectations for alternative solvents.

Upgrading plus injection alternative

The Kwoen Gas Plant is a new addition to the Pine River gathering and processing system. The system's primary gas treating facility, the 560 MMscf/d Pine River gas plant, employs Shell Sulfinol-D gas treating units, triethylene glycol dehydration and MCRC sulfur recovery units to treat a sour gas feed that can range from 5% to 40% acid gas. The Sulfinol-D system is designed to achieve a sales gas specification of less than 16 ppm total sulfur. The Pine River plant, however, was fully utilized due to higher-than-design acid gas in the feed gas (21.0% vs. 16.8%). In addition, new sources of sour gas totaling 130 MMscf/d were in need of processing. In order to meet these needs, DEGT looked at two alternatives: the addition of a fourth 130 MMscf/d train at the Pine River plant and the installation of a 300 MMscf/d sour gas upgrader (the Kwoen Gas Plant).

The company chose the Kwoen Plant because of significant economic advantages. As a ratio of capital cost over treated sour gas volume, the Kwoen Plant was installed at $0.8 million per MMscf/d vs. $2.0 million per MMscf/d for a new processing train. In addition, the Kwoen Plant design is expected to yield 3 MMscf/d more sales gas than a traditional gas treating/sulfur recovery design. No fuel is required to run compressor engines, large power generators or large heat medium systems. Partially offsetting these advantages are slightly higher operating costs because of purchased power requirements.

One of the fundamental reasons for this economic advantage is that the new plant is designed to dispose of more than 860 long tons/day of sulfur through injection of liquefied acid gas into a depleted gas reservoir. Sulfur disposal by injection will result in a cost savings for producers since the netback price for sulfur in the Pine River region is currently negative and the price is forecast to stay quite low for the foreseeable future. In addition, the Kwoen Plant is designed to produce 5,250 long tons/year less SO2 emissions and 105 long tons/day less NOx emissions. Reinjection of extracted CO2 equates to an emissions reduction of 176,000 long tons/day as compared to a new train.

Kwoen plant design

The Kwoen Plant, located 18 miles (29 km) upstream from the Pine River Plant, is sized to remove and inject 28 MMscf/d acid gas into depleted reservoirs and to permit an additional 130 MMscf/d gas to flow into the gathering system and the Pine River Plant. The plant is the first full-scale application of the new, patented gas treating solvent, Morphysorb. Co-developed by Gas Technology Institute (GTI) and KU, Morphysorb is a physical solvent that consists of a mixture of N-Formylmorpholine (NFM) and N-Acetylmorpholine. Morphysorb was selected for the Kwoen Plant based on KU's prediction of lower hydrocarbon losses to acid gas, larger capacity for acid gas removal, and lower pumping and recycle horsepower requirements. The benefits of Morphysorb were expected to be partially offset by higher solvent losses than could be expected with other physical solvents. However, actual Morphysorb losses measured by a variety of techniques were in line with losses expected with alternate solvents.

Like other physical solvents used for gas treating, Morphysorb has a strong selectivity for removing H2S over CO2. Despite the addition of 740 long tons/day of additional feed sulfur in the volume of sour gas undergoing processing, the selective nature of Morphysorb actually reduced the sulfur plant feed to the Pine River plant from 2,000 long tons/day to 1,900 long tons/day. At design flow rates, the Kwoen Plant will reduce the H2S/CO2 ratio of the Pine River Plant feed from 1.3 to 1.1.

Kwoen plant process description

The Kwoen Plant is a simple flash regeneration design (Figure 1). A total of 300 MMscf/d of sour gas is directed to the plant, which operates at 1,100 psia. The gas is absorbed by a Morphysorb solution in two parallel 150 MMscf/d packed column absorbers. The lean Morphysorb flow rate to each absorber is as high as 1,500 USgpm. Rich Morphysorb leaves the bottom of the absorbers loaded with extracted H2S and CO2. The rich Morphysorb is consecutively flashed at 425 psia and 185 psia. The off gas from these drums is recycled to the absorber feed via a 1,750-hp, two-stage reciprocating compressor. The recycle operation is necessary to minimize methane losses. The final two flash drums yield the gas feed to the acid gas compressors and produce a regenerated (lean) Morphysorb stream. The acid gas flash drums operate at 65 psia and 25 psia respectively. The lean Morphysorb flows from the final flash drum back to the absorbers via booster and high-pressure pumps. The plant also contains a mechanical and carbon filtration system. The filtration flow rate is 9% of the total Morphysorb circulation.

The Morphysorb solvent is designed to remove 33 MMscf/d of acid gas from the sour gas feed (actual conditions in the plant indicate more than 40 MMscf/d is possible), resulting in an upgrade from 22.1% acid gas to 12.5% (Table 1). The design acid gas quality is 78.7% H2S, 19.6% CO2 and 1.7% hydrocarbon and trace sulfur. Thirty percent of the inlet trace sulfur is removed by the Morphysorb solution.

The acid gas that flows from the final flash drum is compressed in the first stage of three 4,000-hp, reciprocating, four-stage acid gas compressors. This compressed stream is combined with 16.6 MMscf/d of 65-psia flash gas and compressed to a final discharge pressure of 1,100 psia. The compressor aftercooler liquefies the acid gas prior to it entering a 9-mile, 6-in. diameter pipeline. The liquefied acid gas arrives at the injection well as a liquid and flows down the tubing to a depth of more than 8,200 feet (2,501 m). The depleted reservoir will be filled with acid gas to a pressure less than 80% of the hydrostatic head of the tubing, at a sandface pressure less than fracture pressure. Even at this reduced volume, the estimated storage capacity is more than 200 Bcf, which should provide for 30 years at projected rates.

Plant performance testing

Construction of the Kwoen plant was completed in early August 2002. Morphysorb solvent was loaded to the process train and brought up to operating pressure with sweet gas in the system. Subsequent tests during the fall of 2002 were designed to demonstrate that the Morphysorb process is capable of achieving performance on several metrics: recycle gas flow rate, methane losses to injection well stream, solvent loss, specific circulation rate, total acid gas removal and operability. GTI was responsible for collecting and analyzing gas and liquid samples and developing a performance report, funded in part by the US Deptartment of Energy's National Energy Technology Laboratory (NETL). Gas and liquid samples collected during the test period (Nov. 12-15, 2002) were analyzed immediately after collection. The plant performance was consistent throughout this period, with some plant downtime discontinuities because of compressor and mechanical problems.

In all cases, the metrics satisfied the criteria established by Duke Energy. Measured solvent losses on product gas samples collected during the performance test are well within expected levels. No foaming incidents or upsets occurred prior to, during or after the performance test. (As of this writing, the plant has been in operation in excess of 4 months with approximately 8 Bcf of raw gas processed). This is interpreted as a positive indication of the solvent's low propensity for foaming, a problem often exacerbated in the early stages of plant operations by the presence of foreign contaminants in the system. Operations personnel reported that the Morphysorb unit was stable and required no special attention once it reached a steady state condition.

Application potential

Sour gas upgrading with Morphysorb shows great promise as a process for debottlenecking existing gas plants in situations where feed acid gas content climbs above the optimum host plant design. Preferentially, the feed gas should be hydrocarbon dry in order to reduce the impact of losing heavier hydrocarbons to the gas treating solvent. Additionally, acid gas injection can help maintain capital costs and keep emissions lower than with other sulfur recovery technology alternatives, provided that suitable reservoirs are accessible.

Acknowledgement

This article is summarized from a paper presented at the 2003 Laurance Reid Gas Conditioning Conference, Feb. 23-26, 2003.

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