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6.2. Experimental section 816.2.2 Core holder and fluid injection systemA custom-made core holder was designed which fitted in the insert of the NMR apparatus.The holder is made of poly-ether-ether-ketone (PEEK) and can withstand pressures up to23 bars. The temperature of the core holder was not controlled. During the experimentthe cores are oriented vertically, and the inlet is at the bottom. The fluids are injected bytwo continuous-injection pumps (P500, Amersham Bioscience) connected to a computer.The differential pressure over the core holder was monitored with a Deltabar S pressuretransducer (range: 0–1500 mbar). The tubing, between the core holder and the pressuretransducer, introduces an additional pressure loss at a given flow rate. This pressure losswas calibrated and taken into account in the permeability measurements. A schematicview of the set-up is given in Figure 6.3. For the shut-in step, during which the reactivemass transfer is monitored through the NMR measurements, the cores are taken out ofthe core holder, sealed and put in a second sample holder. In this sample holder, madeof polyvinylchloride (PVC), a fluorocarbon fluid (Galden TM HT135, manufactured bySolvay Solexis) with a regulated temperature is circulated so that the temperature of thesample can be controlled. The fluid is invisible to the NMR set-up and the temperaturecan be controlled between 10 and 65 ◦ C with an accuracy of about 1 degree.6.2.3 NMR apparatus and methodsA 0.95 Tesla NMR scanner was used to perform the NMR experiments. The scanner isequipped to measure both hydrogen, at a frequency of 40.5 MHz, as well as deuterium,at a frequency of 6.21 MHz. The set-up consists of an electromagnet with a vertical,narrow-bore insert with an inner diameter of 31 mm. A single rf coil is used with twodifferent front-end rf circuits for 1 H, respectively 2 H. Switching between the two circuits isautomated and takes less than a second, allowing for a fast sequential NMR measurementof hydrogen and deuterium. The insert is capable of producing a static magnetic gradientup to 0.2 T m −1 in the vertical direction. The position of the core holder and sampleholder is controlled by a stepper motor.The sequences used for the experiments and details about the methods were discussedin Chapter 2. T 1 of the hydrogen content in the oleic phase is measured with a saturationrecovery sequence to determine the TMOS concentration in oil, T 2 of the aqueous phaseis measured with a Carr-Purcell-Meiboom-Gill (CPMG) sequence to monitor the progressof gelation. In this case T 2 is measured for the deuterium content. A 1D static magneticgradient is used. Due to the relatively low signal to noise ratio for deuterium the signal isaveraged 64 times. The acquisition time for a T 2 measurement in a single slice is about 4minutes. The gradient strength was 0.030 T m −1 and 0.015 T m −1 for 1 H, respectively 2 H.In each experiment five equally-spaced slices were considered. The slice positions alongthe vertical direction were: -25 mm, -12.5 mm, 0 mm, 12.5 mm and 25 mm with respectto the center of the cores (see Figure 6.3). The relaxation times were acquired for eachslice by using and averaging the part of the signal profile corresponding to a thickness of3.1 mm. The saturation recovery data were analyzed and fitted with mono-exponentialdecay functions yielding the relaxation times T 1 . The CPMG data were analyzed withthe inversion routine CONTIN [66] to give quasi-continuous T 2 spectra, consisting of one