Article pubs.acs.org/EF
Rational Formulation Design and Commercial Application of a New Hybrid Solvent for Selectively Removing H2S and Organosulfurs from Sour Natural Gas Feng Zhang, Benxian Shen,* Hui Sun,* Jichang Liu, and Lu Liu State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *
ABSTRACT: Based on the investigation on nonbonded interactions between solvent and organosulfur molecules, the reaction kinetics of carbonyl sulfide (COS) with chemical solvents in aqueous solutions, and experimental validation, a hybrid solvent (named UDS-2) was designed to simultaneously remove H2S and organosulfurs from sour natural gas at high removal efficiency. The solvent components, which could enhance physical and chemical absorption of methyl mercaptan and COS, were screened using quantum chemistry method coupled with kinetics analysis. The results show that the five-membered sulfur heterocyclic compound (SUL) exhibits significant advantage of physical solubility of methyl mercaptan and COS. Meanwhile, the cyclic amine (CA) with weak steric hindrance effect as well as moderate basicity could enhance chemical removal of COS. UDS-2 solvent was obtained by blending SUL and CA with N-methyldiethanolamine (MDEA) at optimal proportion, and the removal efficiencies for methyl mercaptan, COS, and total organosulfur of UDS-2 were 53.1, 23.9, and 42.4 percentage points higher than those of MDEA. UDS-2 was successfully applied in a natural gas purification plant. Under the operation conditions of absorption pressure of 5.5 MPa, gas flow rate of 8.48 × 103 N m3/h, gas−liquid ratios of 240 in absorber-I and 471 in absorber-II, the removal efficiencies for methyl mercaptan, COS, and total organosulfur are 77.6%, 74.5% and 75.6%, respectively. The contents of H2S, CO2, and total sulfur in purified gas can be reduced below 0.7 mg/(N m3), 0.16 vol % and 43.5 mg/(N m3), respectively, which all met the corresponding specification for Chinese first-grade commercial natural gas. Additionally, the low hydrocarbon loss of 1.34 N m3/(N m3 solution) indicates UDS-2 solvent has good selectivity for sulfur compounds over hydrocarbons.
1. INTRODUCTION Sour natural gas usually contains varying amounts of impurities, such as H2S, CO2, and various organosulfurs, the majority of which exist in the forms of carbonyl sulfide (COS) and mercaptans. Prior to commercial application of natural gas, these impurities should be removed efficiently in order to meet the requirements of commercial application and environment. At present, owing to the advantages of large treatment capability and low operating cost, the chemical absorption process employing alkanolamine solvents dominates the natural gas purification market. Several alkanolamines including Nmethyldiethanolamine (MDEA), monoethanolamine (MEA), diethanolamine (DEA), and diisopropanolamine (DIPA) have been widely used in commercial plants and exhibit high performance for the removal of H2S and CO2. However, all alkanolamines have been proven to be less effective for removing organosulfurs.1−4 As a result, it is difficult to satisfy the purification requirement while treating sour natural gas with high organosulfur content by using an alkanolamine aqueous solution. Some physical solvents (such as N-formylmorpholine, polyethylene glycol dimethyl ethers, N-methylpyrrolidone, and propylene carbonate) show higher efficiency for organosulfur removal5,6 but lead to large solution loss of hydrocarbons. The development of functional solvents allowing simultaneous removal of H2S and organosulfurs, along with a low solubility of hydrocarbons, therefore, is considered a challenge in the natural gas purification field. Hybrid solvents, which are composed of chemical and physical solvent components and therefore combine their © 2015 American Chemical Society
advantages, possess the performance for selectively removing H2S and organosulfurs. Although some proprietary hybrid solvents (Sulfinol, UCARSOL LE series, and so on) have been developed and are commercially available,7−9the theory of formulation design based on intermolecular interactions is rarely expounded in the literature. In this work, the formulation design of a new hybrid solvent (named as UDS-2 solvent) was expounded through the investigation on nonbonded interactions between solvent and organosulfur molecules (methyl mercaptan and COS), the reaction kinetics of COS with chemical solvents in aqueous solutions, and experimental validation. The sour natural gas with high organosulfur content was treated to satisfy purification requirements by applying UDS-2 solvent at an industrial purification plant in southwest China. This current study provides reliable guidance for designing hybrid solvent formulations, therefore accelerating the development of natural gas fields with high organosulfur content in order to meet increasing clean energy demands.
2. CALCULATIONAL AND EXPERIMENTAL SECTION 2.1. Quantum Chemistry Calculation. The initial molecular geometries of organosulfurs and solvents were defined using the Materials Studio 5.0 DMol3 program Received: May 26, 2015 Revised: November 7, 2015 Published: November 25, 2015 12
DOI: 10.1021/acs.energyfuels.5b02214 Energy Fuels 2016, 30, 12−19
Article
Energy & Fuels package. Then their molecular geometries were optimized with the Gaussian 03w program package at the B3LYP/6-31+ +G(d,p) theory level.10 The total electrostatic contributions to solvation free energies and the polarizabilities of organosulfurs in different solvents were calculated using the polarizable continuum model in Gaussian 03w at the B3LYP/6-31+ +G(d,p) theory level. 2.2. Materials. The compositions of sour natural gases (feed gas-1 and feed gas-2) are listed in Table 1. Feed gas-1 is Table 1. Average Composition of Sour Natural Gas content component
feed gas-1
CH4 (vol %) C2H6 (vol %) N2 (vol %) H2S (vol %) CO2 (vol %) COS (as S, mg/(N m3)) methyl mercaptan (as S, mg/(N m3)) total organosulfur (as S, mg/(N m3))
88.4
a
Figure 1. Flow diagram of experimental apparatus for natural gas desulfurization.
feed gas-2b 89.5
