NITROGEN-REJECTION PROCESS DEVELOPED FOR SMALL FIELDS

A nitrogen-rejection process for reservoirs producing 1-50 MMscfd with 10-60% inlet nitrogen content has a successful full-scale commercial installation at the Mist field in Columbia County, Oregon.

In the Nitech process, developed and installed by BCCK Engineering Inc., Midland, Tex., the product or sales-gas stream can be delivered at a purity of as low as 2% total inert content if required by local pipeline specifications.

MARGINAL RESERVOIRS

The Gas Research Institute has estimated that approximately 23 tcf or 16% of the raw 1991 Lower-48 U.S. nonassociated gas reserves has a nitrogen concentration exceeding 4%.'

With typical pipeline specifications calling for 24% total inert gas content, much of this low-quality gas cannot be sold as produced. Processing to remove nitrogen or blending with higher quality gas is required to upgrade the gas to pipeline quality.

Blending can only be considered where enough blend-gas of sufficient quality is available to produce an economical blended mixture of pipeline quality.

The processing option depends on the availability of a plant that can offer a technically and economically feasible nitrogen-rejection process for a given volume of gas.

Technology to remove nitrogen has been available for several years, but existing units have relied on technology borrowed from air-separation processes.

Because of the air-separation process complexity, utilization is only economical for larger volumes of gas, generally, gas streams of more than 30 MMscfd. A 1990 GRI report estimated the average size of nitrogen-rejection units in operation at that time to be 225 MMscfd per facility.'

High-nitrogen gas reserves located throughout the U.S. cannot, because of their location or daily throughput or both, be practically upgraded either with blending or with previously existing nitrogen-removal technology.

Producers have had no alternative but to leave these reserves shut-in.

FOR SMALLER STREAMS

The Nitech process is designed specifically for oil field installations and targeted to be used to process economically smaller high-nitrogen gas streams.

It employs a single column and cryogenic separation of methane and nitrogen as the basis of operation. The design keeps equipment requirements to a minimum.

The major equipment consists of a molecular-sieve gas dehydrator and associated regeneration gas heater, an aluminum plate-fin heat exchanger, and a single patented process column.

The process design is unique because of an internal reflux condenser located inside the process column that eliminates the need for several pieces of equipment and reduces the cost of the process.

The design of the internal reflux condenser and the effective integration of the condenser into the column represent an improvement over conventional cryogenic designs. Conventional designs require a reflux accumulator, reflux pump, reflux control equipment, and associated reflux piping. The Nitech process eliminates these.

The resulting design is less costly and is easier to control and operate. An internal reflux system also eliminates significant thermal loss that causes many nitrogen-rejection facilities to fail.

Also in the single column design, the operating pressure remains relatively high. The temperatures required for efficient separation of nitrogen from methane are therefore higher.

The higher separation temperature provides for a lower thermal gradient to the ambient. There is, therefore, less heat gain through insulation.

OREGON TEST

A full-scale test of the Nitech process took place at the nitrogen-recovery unit in the Mist field, Columbia County, Oregon.

The Mist plant was built to process gas from a field that had been shut-in since its completion. Nitrogen content from the three-well Mist field varies from 2045% but is generally close to the analysis shown in Table 1. (7197 bytes)

The production rate from these wells averages 7.25 MMscfd.

BCCK Engineering Inc., Midland, manages and owns the limited liability company that was formed to install and operate the Mist plant. A used cryogenic expander-style NGL recovery plant was located and modified to meet the nitrogen-recovery process needs.

Site construction began in April 1994; the plant came on-line in June 1994.

The plant consists of relatively few pieces of major equipment.

Inlet compression boosts the gathering system pressure to approximately 900 psig. Molecular-sieve beds dry the gas before a series of heat exchangers and expansion valves cool it.

A single packed column operating at 275 psig is used for separation. The process produces a high-pressure product stream (up to 325 psig), a low-pressure product stream (up to 10 psig), and a high-pressure nitrogen stream (up to 325 psig).

The product streams are compressed to the pipeline pressure of 275 psig. Total installed compression for the 7.5 MMscfd facility is 1,050 hp currently being supplied by one Worthington compressor driven by a Caterpillar engine.

The residue horsepower demand of nearly 91 bhp/1 MMscf inlet gas competes with larger, more expensive cryogenic facilities.

Mist-produced gas is essentially a binary mixture of methane and nitrogen. Nitrogen content variation of 20-45% depends on which wells are producing.

The Mist plant can deliver sales gas with as low as a 2% total inert content. And, because Mist sales gas is blended with gas from other sources, the BTU target changes frequently according to the purchaser's needs.

Table 2 (7092 bytes) represents a typical sales-gas composition. The process can change sales-gas BTU quickly and with relatively little disturbance to the process.

The vented gas is primarily nitrogen with a small amount of entrained methane. Table 3 (7023 bytes) represents an average composition of the vented stream.

This stream is removed from the process at a relatively high pressure (275 psig) and is used as regeneration gas for the molecularsieve dehydration system and for instrument gas.

The plant site is remote so that power must be generated on site by a small natural gas-driven generator. An Allen-Bradley programmable logic controller (PLC) controls the process; Wonderware software provides operator interface.

One part-time operator oversees the operation and maintenance of the compressor, generator, and process area. A modem link allows for remote monitoring and operation from Midland.

APPLICATIONS

Completion of the Mist plant has led to additional development of the Nitech process technology.

Actual plant data have confirmed simulations of the new technology and use of an internal reflux condenser for nitrogen-rejection service.

Mist has also proven that the process will provide successful operation over a wide operating range reflecting various operating conditions.

The design can deliver a sales-gas product with a nitrogen concentration as low as 1-2%.

In addition, inlet gas streams with an NGL content can be processed without significant modifications. NGL recovery typically enhances overall project economics and if the C2+ content is 3 gal/Mcf or greater, NGL recovery is recommended.

If the feed stream has 4 gal/Mcf or more, NGL recovery is most likely required to guarantee satisfactory nitrogen rejection. NGL recovery requires the addition of a conventional top-feed stabilizer and the associated stabilizer reboiler.

For higher capacity nitrogen-rejection facilities and where a deeper NGL recovery is desired, an auxiliary refrigeration system can be added into the process to reduce the overall horsepower demands of NGL extraction and nitrogen rejection.

The equipment and technology used in the Nitech process are common to the gas-processing industry.

An estimated 10-15 kwh are required to operate equipment critical to the process. Electrical power is consumed only by the regeneration heater blower motor and the process control system.

The process allows elimination of equipment common to conventional cryogenic nitrogen removal. No external reflux equipment is needed. The process also uses no cryogenic rotating equipment.

Conventional cryogenic NRU processes, single or dual column design, use a cryogenic (liquid methane) pump with an appreciable recompression-horsepower requirement.

Cryogenic pumps are expensive to purchase and install and can be difficult to operate. Nitrogen-rejection designs which use cryogenic pumps are often prohibitively expensive for small applications.

Additionally, the process' ability to handle a wide range of inlet-nitrogen concentrations comes without manual or operator intervention. The amount of nitrogen in the sales gas stream can be varied to meet various pipeline specifications.

The unit design is mostly skid-mounted.

Although the Nitech process is cryogenically based, the permissible CO2 content, compared to other cryogenic processes, is extremely lenient. Conventional nitrogen-rejection processes require the inlet CO2 content to be below 25 ppm.

The process allows for the CO2 content to exceed 1,500 ppm depending on the NGL content present in the inlet gas stream. If pretreatment of the inlet gas is required, the inlet-gas treatment uses conventional amine removal techniques.

WHAT IT COSTS

In evaluations of nitrogen-rejection technologies, one of the most important considerations is the horsepower requirement for that process.

Fig. 1 (40779 bytes) provides an estimate of the recompression horsepower required for a typical Nitech nitrogen-rejection process. This case reflects a sales-line pressure of 900 psig and inlet gas with a 1 gal/Mcf C2+ content.

The curves represent horsepower requirements as the percent of nitrogen in the sales stream decreases. Recompression horsepower increases as the concentration of nitrogen in the sales gas decreases.

BCCK's NRU cost, uninstalled, for a 10-MMscfd gas stream is approximately $0.18/1 Mscf inlet. When costs are also included for installation and compression, the total cost for the installed facility is between $0.30-0.45/1 Mscf inlet of processed gas.

THE AUTHORS

Barbara Slaton is a senior process engineer for BCCK and responsible for operations at the Mist field. Previously, she was advanced process and construction engineer for Marathon Oil Co. Slaton holds a BA (1981) in chemical engineering from the University of Pittsburgh

REFERENCES

1. "Chemical Composition of Discovered and undiscovered Natural Gas in the United States," Energy and Environmental Analysis Inc., GRI Report 93/0456.1, December 1993.
2. "Business Characteristics of the Natural Gas Conditioning Industry," Purvin & Gertz Inc. GRI Report No. 93/0342,1993.

Copyright 1995 Oil & Gas Journal. All Rights Reserved.