Natural gas as the most clean-burning fossil fuel, has an important role in power generation, heating and as a petrochemical feedstock [1–4]. It is mainly composed of methane (CH4), typically more than 80%; however, there are significant amounts of pollutants ubiquitous in the raw natural gas that must be separated to prevent a variety of operational and environmental problems. Hydrogen sulfide (H2S) is an acid gas and the most notable impurity in the natural gas that loss fuel value and cause corrosion in pipelines and equipments. Moreover, H2S is a highly poisonous and perilous gas that threaten human health even at ultra-low concentrations [5, 6]. Hence, there are some regulations were issued to restrict H2S concentrations in natural gas below 4ppm to prevent corrosion and touch the product gas specification [7]. In order to remove acidic gases, sweetening process is carried out in gas treatment plants. Actually, absorption and adsorption are the two traditional technologies to purify CH4 from acid gases. Unfortunately, in regards to energy and environmental issues, gas sorption plants have some deficacies [8–10]. Since membrane technology is simple, affordable and has no chemical pollution, it is attracting remarkable interests as an alternative candidate to substitute common upgrading units [11–13].
Membrane gas sweetening has thoroughly developed to separate CO2 from natural gas. There are several industrial plants in some countries that reduce CO2 concentration in the natural gas. For example, a plant was initiated in Malaysia that processed 680 million scfd of raw gas to diminish CO2 percentage from 45 to 6 in 2007 [14]. In such operational units, the CO2 concentration in the feed stream is up to 50 percent and the CO2 content of permeate set to be between 2 to 6 percent. But in the case of H2S, the feed concentration may be as low as 20 ppm and the permeate concentration must be less than 4ppm. That is why those plants do not process H2S as well as CO2. Principally, membranes are more appropriate to operate at higher concentrations and almost all well-developed industrial application of membranes separate the species at a significant concentration [15, 16]. In order to have an efficient industrial H2S membrane gas separation, there are two main approaches in the investigations. The first one is to make membranes by bottom-up methods and fabricate a precise and perfect membrane by molecular tailoring [17]. The hugest detriment to this approach is the expensive cost. So, there is little chance for it to compete with the common sweetening plants. The last method is making MMMs by cheap and abundant organic polymeric matters that present great perspective to mitigate H2S with the least environmental footprint [18–20].
In the recent decades, there have been some researches on H2S/CH4 separation via MMMs. Latest developments on polymeric membranes have provided high susceptibility to remove H2S from the H2S containing gas (beyond 0.5%) [21, 22]. But, despite its importance, there are less works on H2S/CH4 separation at lower H2S concentrations. The MMMs have illustrated very high sour gas permeabilities compared to the neat polymeric membranes [23]. Coincidently, a H2S porter compound accommodated in the MMM is able to adsorb H2S via physical adsorption or chemical reaction, even at ultra-low concentrations of H2S. These reversible interactions between H2S and the carrier transfer H2S molecules through membrane, whenever CH4 and inert gases such as nitrogen molecules transfer via a solution-diffusion mechanism. So, MMMs represent high permeability and high selectivity simultaneously [24, 25].
Polycarbonate (PC) as a thermoplastic glassy polymer is supposed to be an excellent candidate for the matrix to fabricate MMMs because it is a cheaper material than other alternatives such as polysulfone [26], polymers of intrinsic porosity (PIMs) [27], polyimides [28] and polyetherimides [29]. Also, many PC MMMs have been prepared including a variety of fillers such as silica nanoparticles, polypyrrole [30], zeolites [31], and carbon nanotubes [32]. Moreover, the PC matrix is tough, strong and has good mechanical properties such as impact resistance [33]. However, it has been illustrated that silica nanoparticle loading developed the gas transport performances through the membrane matrices [34].
In this regard, polycarbonate-silica (PC-Silica) mixed matrix membranes have exhibited good results for CO2 removal from natural gas [35]; however, their capacity for H2S removal has not been investigated in details. Herein, we reported application of PC membranes for natural gas upgrading under mixed gas operational conditions. The membranes were tested under two feeds by H2S levels of 24 and 180 ppm with CH4 comprising as the balance gas. The effects of operational parameters including pressure up to 20bar, H2S concentration, and temperature (30°C and 45°C) were studied on the membrane performance. The role of nano silica type was also considered. Finally, aging was studied for the membranes through more than two months. It’s worth mentioning that a membrane has to show good physical aging resistant to be an appropriate candidate for industrial utilizations.