15.25 Hot vapor assist

As the temperature of LPG drops, so does its pressure.  The Engineering Tool Box offers a handy reference showing the pressure of different mixes at different temperatures:

http://www.engineeringtoolbox.com/propane-butane-mix-d_1043.html

Here is a summary of its contents:

Vapor Pressure in PSIG

 

Mixture

Propane %

100

70

50

30

0

Butane %

0

30

50

70

100

 

 

 

 

 

 

Temperature

      (F)

-44F

0

0

0

0

0

-30F

6.8

0

0

0

0

-20F

11.5

4.7

0

0

0

-10F

17.5

9

3.5

0

0

0F

24.5

15

7.6

2.3

0

10F

34

20.5

12.3

5.9

0

20F

42

28

17.6

10.2

0

30F

53

36.5

24.5

15.4

0

40F

65

48

32.4

21.5

3.1

50F

78

56

41

28.5

6.9

60F

93

68

50

36.5

11.5

70F

110

82

61

45

17

80F

128

96

74

54

23

90F

150

114

88

66

30

100F

177

134

104

79

38

110F

204

158

122

93

47

When running subzero temperatures for extraction, tank pressure nor the push pull from the pump are adequate for injection at some point, because of the dramatic increase in viscosity accompanying the drop in pressure.

You can increase the Propane content of the LPG to lower its viscosity at subzero temperatures, but alas a 100% propane mix has not only a potential for high pressures if something goes wrong, but by itself doesn't produce as flavorful a product as a mix with Butane.

We typically use a 70% n-Butane/30% n-Propane or a 50/50% blend at -50C and below, but even that mix can need help as the temperature falls, so we typically use a hot LPG vapor assist to add the required boost. 

To that end, we set a 50# refrigerant tank of the same LPG mixture that we are using into a pot of hot water, and connect its vapor port to the vapor port of the injection tank. 

The extra pressure head, in conjunction with the pumps push pull, is enough to force the liquid in the tank through the heat exchangers and subsequently through the column, after which it is easily recovered. 

That contrasts with an alternate process using N2 to pressurize the tank head space, which must be burped off before recovery, lowering the LPG recovery percent, so we don't typically use it.

 

Hot vapor assist diagram


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