Retired Belgian businessman Guy Ullens and his wife, Myriam, began collecting contemporary Chinese art 20 years ago — and never stopped. In November, they will open the Ullens Center for Contemporary Art in a 65,000-square-foot former factory in Beijing. The new institution will display both Chinese and Western art in rotating exhibitions. Ullens recently sold his collection of 14 J.M.W. Turner watercolors for a "painful" $21.7 million at auction in order to better focus on the burgeoning Chinese market.