BIO GAS PURIFY

please tell the purifier

It actually depends on a number of factors. Could you describe your application, bio gas source and expected gas flow please.

Is this for an experiment, small scale residential biogas plant or large scale waste dump with multi MW sized generators?

Check out Perry, The "Chemical Engineer's Handbook" for design protocols for countercurrent packed towers. CO₂ can be scrubbed from gases by passing the mixture through a tower with the gas going in one direction and the liquor going in the other driven by gravity and lifted to the top of the tower from tankage with a pump.

Sulphur [S] does not remain an element in the gaseous phase for very long as oxygen [O] is usually present, and usually oxidises readily to SO₂ and SO₃ [SO_x^]. Scrubbers will take this out too using the same liquor, an alkaline solution. Sodium hydroxide [NaOH] solution is recommended as sodium carbonate, sulphite and sulphate are readily soluble, whereas the alternative calcium hydroxide would form insoluble precipitates after reaction that would eventually clog-up the voids in the tower.

NaOH is relatively inexpensive as a basic industrial chemical.

There is nothing much of a reaction between NaOH and the hydrogen sulphide [H₂S] that is inevitably present in the biogas, nor any ammonia [NH₃], although NH₃ dissolves readily in water and will do so in the aqueous part of the NaOH solution in the tower loop.

There are, as usual, a number of options on the flowsheet. Is H₂S tolerable in the first-stage scrubbed-up biogas [SUB]? If so, then the SUB could be fed as a fuel to generator sets or simply flared-off, if the installation is small enough. H₂S when burnt forms SO_x, so the exhaust from these might be routed through a second tower if an odour-free exhaust gas is required. Or, the raw biogas might be fed to these directly if the wetted materials can withstand the SO_x, thereby saving energy and the installation and maintenance costs of the first tower.

The aqueous stream leaving the tower needs monitoring for both NH₃ (for Chemical Oxygen Demand limits) and pH, either to send it round again or to send it to effluent, correcting if required before passing to sewer, again depending on local discharge consent limits. So that might need the installation of some more tanks, pumps, valves, instruments and pipework.

Exactly what to do depends upon the size of the installation and the emission and discharge consents applicable in this case, which in turn depend upon the composition and flowrate of the raw biogas. So, first step:

• how big is the flowrate?
• what is its composition?
• what are the local emission constraints?

After that, a local Process Engineer can work-up an applicable flowsheet before detailed civil, mechanical and instrumentation design can take place.

Hydrogen sulfide is absorbed by solutions of alkali hydroxides, forming sulfides and hydrosulfides.

2 NaOH + H2S = Na2S + 2H2O

Na2S + H2S = 2NaHS

So, the scrubbing column should remove H2S as well as CO2 I think. That just leaves the ammonia to deal with.

That's useful additional information. Thanks!

There are a number of companies that sell PSA (Pressure Swing Adsorption) systems that can be used to purify biogas to pipeline grade natural gas. These systems come skid mounted and are easy to install.