A nitrogen liquefier configured to be integrated with an argon and nitrogen producing cryogenic air separation unit and method of nitrogen liquefaction are provided. The integrated nitrogen liquefier and associated methods may be operated in at least three distinct modes including: (i) a nil liquid nitrogen mode; (ii) a low liquid nitrogen mode; and (iii) a high liquid nitrogen mode. The present systems and methods are further characterized in an oxygen enriched stream from the lower pressure column of the air separation unit is an oxygen enriched condensing medium used in the argon condenser.

An air separation unit and associated method for separating air by cryogenic distillation using a distillation column system including a higher pressure column, a lower pressure column, an intermediate pressure kettle column, and an argon column arrangement is provided. The present air separation unit and associated method employs a once-through kettle column reboiler, a once-through kettle column condenser, and a once-through argon condenser. The once through argon condenser is disposed within the lower pressure column where an argon-rich vapor stream is condensed against the descending liquid in the lower pressure column.

A process for cryogenic fractionation of air uses an air fractionation plant (100-400) comprises a rectification column system (10) having a high-pressure column (11) operated at a pressure level of 9 to 14.5 bar, a low-pressure column (12) operated at a pressure level of 2 to 5 bar, and an argon column (13). A recirculating stream is formed using a second top gas or a portion thereof, which is heated, compressed, cooled again, and after partial or complete liquefaction or in the unliquefied state is introduced partially or completely, or in fractions, into the first rectification column (11) and/or into the second rectification column (12).

A method for operating a process plant using a dynamic model of the process plant, the dynamic model being based on at least one of thermo fluidic correlations, thermo dynamic correlations, phenomenological correlations, and equations, and being based on geometry and/or topology of components of the process plant, the dynamic model receiving process parameters as input values, the dynamic model being adapted to represent a transition from one to another state of the process plant, wherein the dynamic model is used in an online mode, in which the dynamic model is used in parallel with the operation of the process plant, wherein signals from a control system of the process plant, the signals representing values of at least one first process parameter, are received and fed into the dynamic model.