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.