New Multi-Sample Volumetric Instrument for High Throughput Adsorption Measurements

High throughput adsorption measuring methods are a requirement for fast screening or characterization of new materials (e.g. MOF). Also, routine quality control measurements of fresh or reactivated adsorbents prior to their application need fast, reliable, and high throughput methods. Today multi-sample volumetric instruments are state of the art for BET and porosity measurement in the sub-atmospheric pressure range. However, no high throughput instrument is available for measuring data at application relevant higher pressures and temperatures.

Therefore, we designed a new multi-sample volumetric instrument (MSVI) for measuring the ad- and desorption of gases in a technically relevant pressure and temperature range. The development and test of the instrument is carried in a co-operation between Rubotherm and the Ruhr-University within a national research project for biogas purification. One task of the project is to screen many different adsorbents and solvents and identify the most promising substances.

The modular design of the MSVI allows combining the volumetric gas dosing manifold with different numbers of sample modules. A sample module consists of two independent sample ports, each equipped with its own pressure sensor and a valve. The manifold and the sample modules are designed as thermostated stainless steel blocks in which the valves and the P-sensors are installed. The blocks are connected by means of metal gaskets without leakage. The first prototype consists of 6 sample ports; the flow scheme is shown in figure 1.

Figure 1:   Flow schematics of the multi-sample volumetric instrument for high pressure adsorption measurements.

The dosing manifold can be operated in the pressure range up to 100 bar and is equipped with two different pressure sensors (100 bar and 20 bar full scale) for good accuracy at all pressure levels. The sample modules are equipped with 50 bar P-sensors. Thus, in the current instrument measurements can be performed from vacuum up to 50 bar. The dosing manifold and valve blocks of the sample modules are thermostated to a constant temperature for best accuracy. This part of the volumetric instrument can be heated up to 70°C, thus allowing performing high pressure measurements with CO₂ without condensation problems.

The samples are located in pressure and temperature resistant SS tubes connected to the valve blocks by means leak free of Swagelok VCR connections. All samples can be thermostated to the same temperature either by liquid thermostation or electrical heating. The current design allows cooling / heating the samples in the temperature range between -30°C…400°C. Other thermostat devices (e. g. cold gas or LN2 cooling) can easily be adopted thanks to the modular design for increased sample temperature ranges.

We will briefly present the design and working principle of the new MSVI. Further the performance of the instrument will be demonstrated by presenting results of measurements performed with the prototype. So far adsorption measurements with N₂ and CH₄ were performed. The instrument was checked first by performing adsorption measurements with the same sample at all ports and comparing the results. No unexpected deviations between the different ports were found. Then adsorption measurements with different activated carbons were performed and compared to literature data and the results of simultaneously performed gravimetric measurements. Results for CH₄ adsorption measurement on activated carbon at 25°C and 50°C are shown in figure 2 and are in good agreement.

Figure 2:   Comparison of adsorption isotherm data measured with the new MSVI and a gravimetric instrument on activated carbon at 25°C and 50°C.

These results prove the high resolution, reproducibility and accuracy of the new MSVI instrument for high throughput adsorption measurements in application relevant pressure and temperature ranges.