Effect of feed temperature and pressure
The membranes were tested at different operating temperatures and a constant feed pressure of 1.5 bar. Figure 4 (a and b) showing the effect of temperature on the permeation and ideal selectivity (BD/N2) in pure PDMS and Ni-ZIF-8 MMMs. The temperature range was set according to the actual emission conditions of synthetic rubber plant tail gas. The permeance of butadiene decreased in pure PDMS and 15% Ni-ZIF-8 MMM with respect to the increase in operating temperature, which confirmed that the solubility phenomenon is dominant in the transportation of butadiene across the membranes. The BD permeance decreased from 323 to 284 GPU in 15% Ni-ZIF-8 MMM from 40 to 50°C temperature increase. While the permeance in pure PDMS was reduced from 202 to 187 GPU at the same temperature change. It was worth noticing that the effect of temperature on Ni-ZIF-8/PDMS MMMs permeance is more prominent compared with pure PDMS, which can be explained due to the less adsorption of BD over Ni-ZIF-8 at high temperature as shown in Figure S10, and the same trend existed in its MMMs.
In the case of nitrogen, the permeance enhanced at elevated temperature, which suggested that the diffusion phenomenon is responsible for the transport of nitrogen gas molecules through the membranes. In this work, the nitrogen permeance improved from 18.8 to 19.6 GPU in PDMS and from 16.6 to 17.9 GPU in 15% Ni-ZIF-8 MMM, respectively, by rising the temperature from 40 to 50°C. The BD/N2 ideal selectivities decreased at elevated temperatures; the pure PDMS selectivity decreased 12.5%, while15% Ni-ZIF-8 MMM selectivity decreased 20% from 40 to 50°C, respectively. The decrement of selectivity was higher in Ni-ZIF-8 MMM, which could be ascribed to the low sorption of BD at high temperatures. The low selectivity at high temperature indicates that the temperature should be maintained at 40°C for further membranes testing. From the previously reported works [26,39], it was concluded that the separation of the hydrocarbons through the gas separation system was affected by the high temperature, so that the low operating temperature favored the recovery of hydrocarbons from the mixed nitrogen stream. Consequently, 40°C temperature was selected for further testing of these membranes.
The effect of feed pressure was tested on pure PDMS, and all Ni-ZIF-8/MMMs at a constant temperature of 313 K. Figure 4 (c, d) presenting the effect of pressure on separation properties of membranes, in which the permeance enhanced with respect to the feed pressure. By increasing the pressure from 1.5 to 2.5 bar, the BD permeance improved by 10% in pure PDMS and improved by 14% in 15% Ni-ZIF-8 MMM, respectively. Nitrogen permeance increased from 18.8 GPU to 22 GPU in pure PDMS, and 16.6 GPU to 20 GPU in 15% Ni-ZIF-8 MMM, respectively. The increase in permeability was mainly due to the high trans-membrane pressure, which is the driving force of the gas molecules transportation across the membrane.
The ideal selectivity has decreased slightly (from 10.78 to 10.15) in pure PDMS, and from 19.5 to 18.3 in 15% Ni-ZIF-8 MMM by increasing the feed pressure from 1.5 to 2.5 bar, respectively. The reduction of selectivity was mainly due to the increment in nitrogen permeance by elevating the feed pressure. The details about the permeation mechanism, which includes the sorption and diffusion of both gases, will be discussed in a later section. As the performance of the membrane considered better at low transmembrane pressure, so that 1.5 bar feed pressure was selected for the further analysis of these membranes.