  A high-flow radon removal system based on cryogenic distillation was
developed and constructed to reduce radon-induced backgrounds in liquid xenon
detectors for rare event searches such as XENONnT. A continuous purification of
the XENONnT liquid xenon inventory of 8.4 tonnes at process flows up to 71 kg/h
(200 slpm) is required to achieve a radon reduction by a factor two for radon
sources inside the detector. To reach such high flows, the distillation
column's design features liquid xenon inlet and outlets along with novel
custom-made bath-type heat exchangers with high liquefaction capabilities. The
distillation process was designed using a modification of the McCabe-Thiele
approach without a bottom product extraction. The thermodynamic concept is
based on a Clausius-Rankine cooling cycle with phase-changing medium, in this
case the xenon itself. To drastically reduce the external cooling power
requirements, an energy efficient heat pump concept was developed applying a
custom-made four cylinder magnetically-coupled piston pump as compressor. The
distillation system was operated at thermodynamically stable conditions at a
process flow of (91$\pm$2) kg/h ((258$\pm$6) slpm), 30 % over design. With this
flow, a $^{222}$Rn activity concentration <1 $\mu$Bq/kg is expected inside the
XENONnT detector given the measured radon source distribution.
