Control method and apparatus

An apparatus for applying electrical power in an electrically driven oxygen separation device, said apparatus comprising:

• at least one separation zone having a DC power source for generating part of the electrical power and at least one composite membrane element housed within an electrically heated enclosure of the electrically driven oxygen separation device and connected to the DC power source such that an electrical potential difference is applied to anode and cathode electrodes of the at least one composite membrane element, thereby to separate the oxygen from the oxygen containing feed stream by inducing oxygen ion transport in an electrolyte situated between the anode and cathode electrodes through the application of the electrical potential difference;
• an AC power supply connected to the electrically heated enclosure for generating a further part of the electrical power and supplying the further part of the electrical power to the electrically heated enclosure to maintain the at least one composite membrane element at an operational temperature at which the electrolyte is capable of oxygen ion transport;
• the at least one composite membrane element having a resistance increasing over time and during operation of the at least one composite membrane element;
• and
• means for controlling the electrical potential difference applied to the at least one composite membrane element such that the voltage of the electrical potential increases to maintain an electric current drawn by the at least one composite membrane element at a substantially constant, predetermined electrical current level as the resistance increases until the electrical potential difference reaches a predetermined voltage level and thereafter, to maintain the electrical potential difference at a constant voltage level when the electrical potential difference reaches the predetermined voltage level, the constant voltage level being below a maximum voltage level at which the at least one composite membrane element will fail.