Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Patent
1996-03-21
1997-09-23
Phasge, Arun S.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
205615, 205763, C25B 124
Patent
active
056700369
DESCRIPTION:
BRIEF SUMMARY
The concentrations of ammonia, in both liquid and gaseous phases, that may be discharged into the environment are limited by legislation. The criteria may present severe operational problems in sewage treatment plants. The invention relates to a method for converting ammonia in a gas stream into nitrogen, which is, of course, a non-toxic and harmless product. Liquids may be subjected to conventional gas stripping to transfer the ammonia into the gaseous phase, thus enabling a common approach to both effluent types to be considered.
Many oxidising agents are known to convert ammonia to nitrogen, but a product mixture is often obtained. Resulting species such as nitrates, nitrites, and nitrogen oxides are undesirable. Apparently, the hypobromite ion (BrO.sup.-), in aqueous solution, gives only nitrogen, in accordance with equation (1), and is therefore a preferred reagent.
The bromide ion (Br.sup.-) so produced may be oxidised back to hypobromite at the anode of an electrochemical cell in accordance with equation (2)
These reactions have been used by Van Velzen et al (EP-A-0 412 175) in a method designed to remove nitrogen oxides or nitrates from an effluent. Ammonia which was formed by reducing such compounds cathodically, passed through a membrane dividing the electrochemical cell. The ammonia was then converted to nitrogen by hypobromite generated continually at the anode. In practice the scheme has fundamental drawbacks which lead to a reduction in efficiency. For example, it is not possible to balance the rate of ammonia transport across the membrane with that of hypobromite formation at the anode. In fact, hypobromite will always be present in considerable excess. This allows the unwanted side reaction of bromate formation in accordance with equation (3) to occur.
Reaction (3) reduces the overall current efficiency. At the same time, in a divided cell, the anolyte will become acidic, and bromide ions will be oxidised to elemental bromine:
Bromine does not react readily with ammonia, and may also be lost from the cell as vapour.
The present inventors have investigated this subject with a view to overcoming the above-mentioned disadvantages and have discovered that an efficient method can be operated in such a way that either the ammonia or the hypobromite in the solution is present in only a slight excess. Reaction (1) is sufficiently rapid for this to be feasible.
The present invention provides a process for converting ammonia in a gas stream to nitrogen, which process comprises contacting the said gas stream with an electrolyte containing bromide and hypobromite ions to dissolve ammonia and to cause oxidation of the dissolved ammonia to produce nitrogen by oxidation with hypobromite ions, and thereafter passing the said electrolyte through an electrochemical cell containing an anode and a cathode so that hypobromite ions are regenerated by the action of the electric current flowing across the cell, wherein said current is switched on or off according to whether the potential difference between a standard reference electrode and a sensing electrode which is immersed in the electrolyte at the exit to the cell or an electrolyte holding tank is greater than or less than a fixed value so that ammonium or hypobromite ions are present in only a slight excess.
Preferably the gaseous and electrolyte streams are contacted in co-current or counter-current flow in an absorption tower packed with inert material to provide a large contact area. It is preferred that the gaseous and electrolyte streams are contacted in co-current or counter-current flow in the electrochemical cell; and for this procedure the anode of said electrochemical cell may comprise a bed of conducting carbon or graphite chips, the said bed being in contact with an anode feeder plate, and the said bed being separated from the cathode by a porous cloth comprised of an inert non-conducting material, such as polypropylene.
In operating the process of the invention the electrolyte may be circulated by means of a pump in a loop comprising the cell,
REFERENCES:
patent: 4081345 (1978-03-01), Tolg et al.
patent: 5354436 (1994-10-01), Van Velzen et al. et al.
Campbell Duncan Alistair
Gilroy Daniel
EA Technology Limited
Phasge Arun S,.
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