A fundamental part of the alternative dehydration concept, is the injection and mixing of TEG with the feed gas. To minimize the circulation of TEG and enhance the overall unit efficiency, it is very important that the mixer creates an as near equilibrium mix as possible, at the same time conserving most of the pressure. There are several commercial mixing concepts available, but this chapter will concentrate on the mixing unit from ProPure, as it is one of the newest and most promising compact mixing units for use in the natural gas industry available today.
3.1 The ProPure compact mixer
Figure 3.1: Schematics for the ProPure compact mixer (adapted from Nilsen et al. (2006):p.4)
The ProPure compact mixer is an inline unit utilizing turbulence to mix a liquid with a gas. The gas enters from the left (See figure 3.1), and the pipe cross section area is reduced to increase the velocity of the gas. The liquid to be mixed with the gas enters through an annulus, and is evenly distributed around the perimeter. Before the smallest cross section of the pipe (point 3) the liquid is exposed to the gas, causing the gas to drag the liquid with it and form a liquid film on the pipe walls (point 11). At the smallest cross section area (point 6), the diameter is suddenly increased, creating a sharp edge. This causes the liquid to leave the pipe walls and form drops in the gas. The large
turbulence created by the sudden cross section expansion and the high velocity aids in breaking up the drops to small droplets, resulting in very effective contact between the liquid and the gas (Nilsen et al. (2006)). To reduce the permanent pressure drop over the unit, the gas is then expanded through a diffuser to the original pipe diameter.
The physics utilized in the mixing unit is surface forces, shear forces and drag forces. The liquid to be mixed with the gas is pumped through a small channel ending in an annulus, and is pushed into the gas stream. As the liquid is pushed into the stream adhered to the wall, the surface force between the liquid and the wall will keep the liquid adhered to the wall, but the shear forces transferred from the fast moving gas will pull the liquid forward, creating a moving film. When the liquid film reaches the sharp edge, the surface forces are reduced enough for the gas to launch the liquid into the gas stream creating liquid filaments. Once in the gas stream the filaments are exposed to drag forces from the faster moving gas, shattering them into smaller droplets. This process is governed by the Weber number (We) defined from formula 3.1 (Nilsen et al. (2006):p.13):
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Where ρG is the gas density, Urel the relative velocity between the gas and the liquid, dfil the characteristic filament dimension and σsurf the liquid-gas surface tension.
Figure 3.2: Drop shattering as We>Wecrit (adapted from ProPure (undated):p.2)
When the Weber number reaches a critical value, Wecrit, the surface tension is no longer strong enough to hold the filament together, causing it to shatter into smaller droplets (See figure 3.2).
From wind tunnel experiments it has been found that Wecrit has a value between 8 and 10 (Nilsen et al. (2006)). After the filaments have shattered, further mixing is provided by turbulence, governed by the Reynolds number (Nilsen et al. (2006):p.13).
Re mix mix i
≡ (Formula 3.2: Reynolds number)
It can be seen from formula 3.1 that the greater the difference in velocity between the gas and the liquid (Urel), the better the mixing, as smaller droplets (smaller values for dfil) will shatter because of reaching Wecrit. High gas velocities will also push the filaments forward, inertia at the same time causing the filaments to spread out perpendicular to the direction of flow, increasing the dfil and thereby also We (Illustrated in figure 3.2). As velocity is inversely proportional to the square of the pipe diameter, Re (and consequently also the turbulence) will increase when narrowing the cross section area. Accelerating the gas to greater velocities by narrowing the available cross section area will however increase the permanent pressure drop across the unit.
The design of the mixing unit is very strongly dependent on the operating conditions and the fluids involved, and must be reworked for each new project. Several parameters are adjustable including the smallest cross section, the rate of narrowing, the angle of the sharp edge, and the rate of expansion (Nilsen et al. (2006)).
Depending on the design criteria, the unit can thereby be optimized for best possible mixing, lowest possible pressure drop, or most compact
design, among others. The unit is installed into Figure 3.3: Assembled ProPure compact mixer (adapted from ProPure (undated):p.1)
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the same type of pipe used at the operating location, this to ensure compatibility and easily fulfill pressure/temperature requirements (See figure 3.3). Placement and inclination of the unit is arbitrary, but preferentially it is mounted horizontally with 15 to 20 pipe diameters of straight pipe downstream to provide further mixing (Nilsen et al. (2006)).
Although the ProPure compact mixer can be designed to mix any fluid with gas, it is presently marketed for injection of H2S scavenging chemicals in natural gas. Internationally this concept has been applied to more than 30 applications with up to 30% reduction in scavenger consumption compared to conventional systems (ProPure (2005)).
Advantages using the ProPure compact mixer include (ProPure (2005)):
- Large turndown ratio for both gas and liquid.
- Better mixing performance than conventional mixers.
- Robust towards plugging.
- Great mixing performance also for high viscosity liquids.
- Compact.
Disadvantages using the ProPure compact mixer include:
- Permanent pressure drop (approx 0.3 bar (Kalgraff (2008))).
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