Confirming Repeatability of Results with the New Approach Using Different

As described in Section 2.3 experimental set-ups used to develop the new CGI approach and employ to assess the effects of various parameters on KHI performance have been high-pressure stirred autoclaves. However, previous literature studies have reported quite different KHI performance results for different set-ups at least in terms of induction times (e.g. Klomp, 2008).

However, using the new approach it was found that results were completely repeatable in different autoclave set-ups. Also, based on the current understanding that crystal growth inhibition (CGI) patterns are thermodynamically driven (by polymer crystal surface adsorption), it was assumed that results using the new method should be no different in totally different set-ups like rocking cells than they are in autoclaves so long as the correct methodology is followed.

Therefore, to confirm suitability of other set –ups such as rocking cells for applying the new CGI approach, CGI studies for methane with aqueous PVCap were carried out in a standard rocking cell to compare with autoclave results.

Development of the CGI Technique

44 2.4.1. Experimental equipment and set-up

Experiments were carried out on a 300 ml, high pressure (up to 690 bar), titanium rocking cell illustrated in Figure 2-15. Temperature control and pressure measurements were done using very similar apparatuses and techniques to high pressure stirred autoclaves explained in Section 2.3.1. Similar to an autoclave system temperature measurements were determined by a platinum resistance thermometer (PRT, ± 0.1 °C) which was connected to a computer for direct acquisition, but this time due to the equipment design could not be in direct contact with cell fluid and was placed inside the cooling jacket surrounding the cell.

Figure ‎2-15 Schematic Illustration of the high pressure (max 690 bar), 300 ml (when piston fully retracted) high pressure rocking cell used in experiments. The cell is rocked fully through 180°,

typically once every 30 seconds or more.

The main difference between a high pressure autoclave and rocking cell is the mixing.

In a rocking cell, mixing is done using mixing balls with the cell rocked fully through 180°, typically once every 30 seconds or more.

As mentioned earlier in this chapter ultra high purity grade methane gas used was 99.995%. Also deionised water was used in all tests. The PVCap used was a lower molecular weight Luvicap-MEG base polymer (average MW = 4000-8000) supplied by Champion Technologies, with the mono-ethylene-glycol solvent removed by oven drying. For these experiments, 0.5 mass% PVCap aqueous was prepared in deionised water. In line with the autoclave CGI methodology, aqueous phase was ~80% of cell volume.

Development of the CGI Technique

45

The experimental procedure explained as the new CGI technique was employed in these tests. Hence after determining the KHI free phase boundary, each step was performed to establish different CGI regions for this system. Results for this system are then compared to the same system tested in an autoclave to conclude on the repeatability of results in different experimental set-ups using this technique. Results presented here for 0.5 mass% PVCap aqueous tested in an autoclave are from experiments described in detail in Chapter 3.

2.4.2. Results and Discussion

Figure 2-16 shows example CGI method cooling curves and interpreted CGI regions for 0.5 mass% PVCap aqueous with methane in the rocking cell in comparison with autoclave data for the same pressure. Also, Table 2-2 details average PVCap induced inhibition region subcooling extents for 0.5 mass% PVCap (same PVCap sample / AMW) with methane systems in the rocking cell and stirred autoclave at ~85 bar (average of more than 5 runs for measuring CGI regions in each set-up with ±0.5°C deviation). As can be seen from both the figure and data in the table, the results for the rocking cell and autoclave are almost identical within experimental error (±0.5 °C). In both systems, CIR which is the key for evaluating KHI properties in CGI studies is

~5.2 °C. Also for both systems, for RGR(S-M) ΔTsub= ~ 7.3°C , for RGR(M)-RFR ΔTsub = ~ 9.6°C and for SDR ΔTsub = ~ 4.0 °C.

Table ‎2-2 Comparison of experimentally determined CGI region ΔT extents for methane with 0.5 mass% PVCap aqueous at ~85 bar in autoclave and rocking cell experiments. Regions are

indistinguishable within experimental error.

ΔTsub RGR(M)

‐RFR / °C

ΔTsub RGR(S-M) / °C

ΔTsub CIR

‐RGR(S) / °C

ΔTsub SDR /

°C

Rocking Cell −9.5 −7.9 −5.2 4.2

Autoclave −9.6 −7.3 −5.2 3.9

The only difference observed in data is due to an experimental artefact; namely that the rocking cell probe is in the cell jacket, rather than fluids. This means that the recorded temperature can deviate from actual fluid temperature during fast heating/cooling. This is evident for example at the beginning of rocking cell heating steps in Figure 2-16;

while they show slow dissociation patterns just like the autoclave data; the latter track the SDR boundary more closely as ‘true’ cell temperature is being recorded. In the rocking cell case, the jacket temperature outpaces cell temperature, resulting in the

Development of the CGI Technique

46

observed lag. This also can occur on rapid cooling, giving an apparent non-linear convex liquid + gas relationship, but is wholly equipment related. Such an issue is generally eliminated simply by lowering heating/cooling rates if required.

20 30 40 50 60 70 80 90 100

-4 -2 0 2 4 6 8 10 12 14 16 18

T / °C

P / bar

Rapid cooling, no history Cooling, with hydrate, 1C/hr Step heating

CIR SDR

Grey points = Autoclave RGR(S)

RGR(M)

Figure ‎2-16 Comparison of example CGI cooling and heating runs for 0.5 mass% PVCap aqueous with methane in the rocking cell and standard autoclave.

Results are identical with the exception of some temperature probe positioning artefacts.This result supports the theory that CGI regions are primarily thermodynamically driven and thus results are fully repeatable and transferrable between different types of equipment assuming good mixing and appropriate methodology as outlined in Section 2.3.

2.5. Correlation of Crystal Growth Inhibition Patterns with