Comparison between N-pentane and the Mixed RefrigerantsRefrigerants

Definition of the Mixed Refrigerants

Two sets of mixed refrigerants, MR1 and MR2, were defined as the basis for comparison. MR1 and MR2 consist of typical mole-concentrations of refrigerants which have been commonly used in LNG processes. Table 3.3 shows information on the specified mixed refrigerants.

3.Experimental Methods

Table 3.3: Specified general mixed refrigerants and corresponding condi-tions (pressure assumed 4.5 bar).

Refrigerant(mole%) MR1 MR2

Nitrogen 0 10

Methane 5 55

Ethane 90 35

Propane 5 0

Sum 100 100

Comparison between Physical Properties of N-pentane and the Mixed Refrigerants

Lex et al. (2007) used 80 mol% n-pentane and 20 mol% iso-octane as the model fluid for mixed refrigerant. A possible problem of using a mixture was that the composition could vary as there might be temperature gradients in the test cell. Pure n-pentane was therefore selected to avoid composition change in the present experiments. An attempt was made to compare the physical properties of the test fluid with those of the mixed refrigerants, and the data of water and air was also investigated to serve as a reference.

The chosen physical properties are liquid viscosity, surface tension, liquid density, vapor viscosity and vapor density.

Fluid properties were taken from Aspen HYSYS 2006. The Peng-Robinson correlation was suggested to be used as the thermodynamic method for both the test fluid and the mixed refrigerants by ThermSel V1.0 which was a thermodynamic package selector for HYSYS, and the SRK correlation was recommended for air and water system. Detailed description of methods, results and discussion is given in the following sections.

The temperature conditions for comparing n-pentane and the mixed re-frigerants are listed in Table 3.4, and water was chosen as a reference.

Figure 3.18(a) to (e) compare n-pentane (black curve) and two sets of mixed refrigerants (green and blue curves) regarding liquid viscosity, surface tension, liquid density, vapor viscosity and vapor density, respectively. The properties of water (red curve) were also plotted in the figures as a reference.

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3.5Experimental Fluids

Table 3.4: General mixed refrigerants and corresponding operational con-ditions

Temperature points selected for comparison of properties (^◦C) n-pentane 20 25 30 35 40 45

(a) Comparison of liquid viscosity.

Figure 3.18: Comparison between the physical properties of n-pentane and the mixed refrigerants.

3.Experimental Methods

(b) Comparison of surface tension.

20 25 30 35 40 45

(c) Comparison of liquid density.

Figure 3.18: Comparison between the physical properties of n-pentane and the mixed refrigerants.

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3.5Experimental Fluids

(d) Comparison of vapor viscosity.

20 25 30 35 40 45

(e) Comparison of vapor density.

Figure 3.18: Comparison between the physical properties of n-pentane and the mixed refrigerants. (Cont.)

3.Experimental Methods

Figure 3.18(a) to (e) show that, compared to water, n-pentane shows much similar physical properties to the typical mixed refrigerants, MR1 and MR2. To make a quantitative evaluation of the properties, the following parameters are defined.

Π =¯ Pn

1Πn

n (3.1)

Ω1 = |Π¯−Π¯MR1| Π¯MR1

(3.2)

Ω2 = |Π¯−Π¯MR2| Π¯MR2

(3.3) Ω =¯ Ω1+ Ω2

2 (3.4)

where

• Π denotes a property of a fluid, and ¯Π denotes the averaged property of a fluid over different temperatures;

• Ω denotes the “difference” between a fluid and the mixed refrigerants.

Ω1 and Ω2 denote the differences to MR1 and MR2 respectively, and Ω denotes the mean value of Ω¯ 1 and Ω2.

As can be seen from the equations, Ω = 0 means no difference between a fluid and a set of MR, and thus lower value of Ω means higher similarity of a fluid to a set of MR.

Table 3.5 shows the physical property differences of n-pentane and water to MR1 and MR2. For most of the properties, n-pentane was quite similar to the two sets of mixed refrigerants, and in most of the cases the differences are within 20%. Comparing between n-pentane and water, it can be seen that, in most of the cases, the properties of n-pentane are much closer to the properties of the mixed refrigerants. In the comparison of vapor density, both n-pentane and water are not very close since the differences are above 50%, but n-pentane was still closer.

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3.5Experimental Fluids

Table 3.5: Physical property differences of n-pentane (pt) and water (wt) to MR1 and MR2.

Property Diff. to MR1 (Ω1) Diff. to MR2 (Ω2) Mean diff. to MR (¯Ω)

µpt 0.501 0.187 0.344

µwt 4.617 3.436 4.024

σpt 0.096 0.084 0.090

σwt 4.322 3.447 3.855

ρpt 0.167 0.155 0.161

ρwt 0.907 0.888 0.897

µpt−gas 0.093 0.047 0.070

µwt−gas 1.631 1.764 1.698

ρpt−gas 0.606 0.685 0.645

ρwt−gas 0.831 0.864 0.848

Determination of the Experimental Condition for N-pentane For determining the experimental temperature, two factors were consid-ered. The first factor was the similarity of the physical properties of the test fluid to those of the mixed refrigerants, and it can be seen from the above figures that as the experimental temperature increases from 20^◦C to 45^◦C the physical properties of n-pentane get closer with the properties of the mixed refrigerants.

The second factor was the saturation pressure which should be prefer-ably close to and slightly higher than the atmospheric pressure so that the danger of overpressure and air suction can be avoided. Figure 3.19 shows the saturation pressures of n-pentane at different temperatures. It can be seen from the figure that when the experimental temperature is higher than 36^◦C the saturation pressure becomes higher than the atmospheric pressure. The physical properties of n-pentane at 45^◦C are the closest to those of the mixed refrigerants among the chosen temperatures from 20^◦C to 45^◦C. The saturation pressure at 45^◦C is 136·10³Pa, and the design pressure will be exceeded if a safety factor of 1.3 is multiplied with this pressure. Due to the above arguments, the experimental temperature were chosen to be 40^◦C at which the saturation pressure was 115·10³Pa.

3.Experimental Methods Saturation pressure of N-pentane

Figure 3.19: Saturation pressure of the test fluid at different tempratures