Pressure exchanger

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3D schematic drawing of a rotary pressure exchanger for liquids

Pressure exchangers are used for isobaric energy transfer in pressure-driven processes . In process engineering , pressure exchangers can be used to transfer the pressure energy of a liquid or gas stream that has remained after the processing process has passed through to the inlet and thus be recovered ( energy return ).

Pressure exchangers are often used in seawater desalination plants, in which salt water is forced through a membrane at high pressure ( reverse osmosis ). The turbocharger is also widespread in internal combustion engines .

function

Scheme drawing of a rotary pressure exchanger for use in a seawater desalination plant. A : High pressure, B : Low pressure, C : Direction of rotation, D : Currently disconnected pipe, 1 : Inlet line of the high pressure drain , 2 : Seawater (high pressure), 3 : Inlet seawater at low pressure, 4 : Drain / Low pressure retentate,:    spent water / concentrate    ,: piston / barrier    ,: seawater
Scheme drawing of a reverse osmosis system (seawater desalination) with a pressure exchanger. 1 : sea water inflow , 2 : fresh water flow (40%), 3 : salt water concentrate (60%), 4 : sea water inflow (60%), 5 : drainage of the salt water concentrate, A : inflow by high pressure pump (40%), B : circulation pump , C : Osmosis unit with membrane, D : pressure exchanger

Principle of pressure transmission

While fluid energy machines such as turbines convert kinetic energy into another form of energy, with pressure exchangers the energy is transferred while maintaining the hydraulic pressure (isobaric). Technically, this is done through spatial displacement in pressure pipes.

In membrane processes , pressure exchangers transfer the energy from the retentate or effluent stream directly to the process inlet ( feed ). They are made up of several pressure pipes, at the ends of which the feed and discharge lines are connected via pressure valves .

Work steps:

  1. Pressure build-up : A pipe filled with feed at low pressure is brought to high pressure by opening the high-pressure side retentate valve.
  2. Displacement : After closing the retentate valve and opening the high-pressure feed valve, freshly run feed is displaced by the retentate.
  3. Filling : The retentate is removed from the next pipe and refilled with feed from the inlet.
  4. Transport : Continuous transport and pressure equalization when switching from high to lower pressure can be optimized by means of additional pressure pipes .

A pump is used to maintain the working pressure in the system.

Rotary pressure exchanger

One type of exchanger with particularly good energy transfer is the rotary pressure exchanger ; it works with a cylindrical rotor in which bushings are bored parallel to the rotor axis. The rotor runs in a housing between two end plates, to each of which two pipes for the supply of liquid are connected, and thus takes on the function of a pump. High pressure liquid enters at one end and transfers the pressure immediately to the lower pressure liquid flow at the other end of the rotor. A movable barrier (sealing liquid or piston ) in the bushing prevents the two liquids from mixing. As the rotor rotates, each feedthrough reaches the respective connection lines and thus alternately connects the high-pressure (discharge or feed) and low-pressure side (feed or discharge), with the end plates in the intermediate positions closing the through-holes at both ends and thus taking on the function of pressure valves.

Energy recovery

The use of pressure exchangers in reverse osmosis systems saves energy by relieving the high pressure pump. If a desalination plant has a fresh water output of 40% of the seawater inflow, 60% retentate remains, the expansion of which is coupled via the pressure exchanger to the pressure build-up of 60% of the seawater inflow; therefore only 40% of the inflow must be conveyed via a high pressure pump. A circulation pump is only required to compensate for the friction losses in the circuit.

Compared to energy recovery using turbines, pressure exchangers have the advantage of particularly low losses. In processes like reverse osmosis, the recovery rate can be increased from approx. 65% to up to 98%.

Individual evidence

  1. ↑ Max Berchtold: On the development of interstationary gas dynamics . Swiss construction newspaper; 78, Issue 28, 1960. pp. 464-469.
  2. ↑ a b Melin, Thomas and Rautenbach, Robert: Membranverfahren , page 264f. Springer, 2007, ISBN 3-540-00071-2 .

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