Industrial Extractors

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 23  Characteristic curves of different types of pumps.

 

 

not applied to high-pressure technology show a reciprocal behavior of pressure with respect to mass flow: at higher pressure mass flow diminishes. The pump is actually operated at the point of intersection of the operating line and the characteristic line of the piping due to pressure losses caused by friction. On the other hand, piston pumps that are commonly used for high-pressure purposes ideally maintain a constant mass flow no matter the pressure at the outlet, as- suming a noncompressible liquid. Cavitations, i.e., evaporation due to pressure loss at the pump inlet, must be avoided in the case of both centrifugal and piston pumps. So-called net pressure suction height (NPSH) is a measure of the mini- mal inlet pressure that ensures complete liquid pumping, usually based on prop- erties of water.

Since piston pumps have a certain constant volume per stroke, it is very dangerous to shut valves right behind the pump. Pressure will increase until either tubes burst or major damages occurs, e.g., at the pump transmission. Therefore, security devices such as security valves and rupture discs are obliga- tory.

If the fluid to be circulated is highly compressible and the pressure is to be increased, compressors with multistage working principle need to be used. Pres- sure is increased stepwise with an intermediate cooling step. In each step pressure may be raised by a factor of 3–4. Compressors have a much lower volumetric efficiency than pumps but can work with almost any suction pressure.

Supercritical fluid extraction works with fluids under conditions ranging from the need of compressors to the possibility of applying pumps. Usually, the

 

 

 

 

 

 

 

 

 

 

 

circulating fluid enters the pump at elevated pressure. Whether or not a liquid state can be assured at this point depends on conditions of the previous separa- tion step and the chilling capacity. In the following we will assume that a pump is used. The two main different types of reciprocating pumps used for supercriti- cal fluids are plunger (piston) and membrane pumps. Membrane pumps have certain advantages, especially concerning sealing. While piston pumps need spe- cial dynamic sealing packing to guarantee hermetic operation, the membrane itself separates the pressure chamber from the pressure-transmitting liquid and the transmission of the pump (Fig. 24). For food and pharmaceutical applica- tions sealing toward, say, lubricants may be decisive for pump selection. On the other hand, the fact that the efficiency of piston pumps is higher than that of membrane pumps is important for realizing high mass flows.

For circulating fluids on a high-pressure level but relatively small pressure drops in the cycle, canned centrifugal pumps (hermetic) may be applied.

 

1.  Pump Efficiency (Volumetric Efficiency)

 

Multiplying the stroke volume and the pumping frequency gives a theoretical value of the volume flow of the pump. This volume flow is never reached because of various factors that are quantified by using the so-called pump effi- ciency, η_p. η_p  is composed of a systematic efficiency accounting for back flow through valves, fluid losses, and so forth, as well as an elastic efficiency taking into account elasticity of the pumping head, stagnant volumes, and compressibil- ity of the fluid. The elastic efficiency that usually comes close to the total pump efficiency is defined by (28):

 

_H

_{ηE  = 1 − (εTκ + λE)∆p}

^h

 

with H/h denominating the stroke ratio. In case of a nonelastic piston (λ_E  = 0) and a normal stagnant volume (ε_T = 1), the pump efficiency for a single acting piston pump comes out as:

 

η_E  = 1 − ∆<span class="dash041e_0431_044b_0447_043d_044b_0439__Char" style=" font-family: 'Times New Roman', 'Aria