Processes for producing citraconic anhydride and citraconic acid using porous materials with specific surface acidities and surface areas are described. Each of the drive units includes a generally transversely extending base beam having first and second ends. It solves a problem of a conventional optical distribution network system in that the maximum bandwidth available by DBA is equal to the total transmission bandwidth of working side minus the sum total of the minimum cell rates of the ONUs, and hence it cannot secure a large usable bandwidth for DBA, when congestion of bandwidth increase takes place among the plurality of ONUs. In addition, as components of the apparatus are self-contained within modules, the risk of contamination of the internal components is reduced, thus improving control of the quality of the construction.