HOUSTON, Jan 14 (Reuters) - Exxon Mobil Corp (XOM.N) plans to shut the second-largest crude distillation unit (CDU) at its 560,500 barrel-per-day (bpd) Baytown, Texas, refinery by late next week, sources familiar with plant operations said on Friday. The 135,000-bpd Pipestill-7 CDU is being shut for a planned overhaul, the sources said. An Exxon spokesperson was not available on Friday night to comment. The Baytown refinery, Exxon"s largest in the United States and the fourth largest in the nation, continues to operate at reduced production levels following a Dec. 23 explosion that forced the shutdown of the 70,000-bpd HDS-1 hydrotreater. The products normally sent to HDS-1 for removal of sulfur content in compliance with U.S. environmental rules have been routed to several smaller hydrotreaters, the sources said. The sources did not know the extent that production is reduced.