(18a) A New Nitrogen Rejection Technology from Produced Gas

A very significant fraction of world’s natural gas reserves are sub-quality (not meeting pipeline specifications), containing more than 2% vol. CO₂, 4% vol. N₂ and 4 parts per million (ppm) H₂S. For example, more than 50% of the volume of known gas resources in the U.S. is estimated to contain more than 2% vol. CO₂ and 3% vol. N₂. While nitrogen will not freeze or lead to corrosion of the equipment, it reduces the heating value of the pipeline sales gas. In addition, high N₂ concentrations in natural gas will increase the parasitic losses because a significant amount of energy is wasted in the cooling and compressing of the N₂.

Cryogenic nitrogen rejection units (NRUs) offer the highest methane recovery rates and is applied as standard practice for the monetization of low btu gas reserves for plants sized greater than 15-20 MMSCFD. But due to complexity of the hardware, the cryogenic separators do not scale down economically generating the need for new technologies for cost effective N₂/CO₂ rejection for the small distributed applications.

SulfaTrap LLC and TDA Research, Inc. is developing a new separation system that can selectively remove N₂ (and CO₂) from natural gas. The process is driven by a pressure swing cycle that delivers a high purity hydrocarbon stream (that meet the Btu requirement of the sale gas) and a nitrogen-rich reject stream that could either be flared or re-compressed to be re-utilized. The technology is based on a proprietary molecular sieve material with highly favorable pore size distribution to allow the removal of N₂ (and CO₂) based on size selection. Nitrogen and methane molecular diameters are approximately 3.6 and 3.8 angstroms, respectively and the sorbent pore size is 3.7 angstroms providing the basis for the separation. The new adsorbent permits the N₂ and CO₂ to enter the pores and adsorbed but excluding methane, which passes through the fixed bed of adsorbent at essentially the same pressure as the feed.

In this paper, we will provide the technical details on the sorbent and the modular Pressure Swing Adsorption (PSA) process for N₂ rejection from natural gas. The field tests results will be summarized indicating the new multi-stage PSA system can effectively reduce the total N₂ content of the gas to less 3% vol. The system also achieves hydrocarbon recovery efficiency in excess of 90%. The impact of moisture and heavy hydrocarbons were effectively managed in the process, ensuring the sorbent to maintain its durability over many cycles. In a high fidelity engineering and cost analysis, the cost of operation and capital requirement of the new system is also compared very favorable against a traditional cryogenic separation system.