The hazard and operability, or HAZOP, study is a prime method for the identification of hazards on process plants. This is the second in a series of papers which describes progress in the emulation of hazard identification in the style of HAZOP. The work reported is embodied in a computer aid for hazard identification, or HAZOP emulator, HAZID. The HAZID code is one of a suite of codes developed as part of the STOPHAZ project. The present paper describes the unit model system. Companion papers describe: an overview of HAZID, with an account of HAZOP and HAZOP emulation, and of the issues underlying it; the fluid model system and evaluation of consequences; the evaluation and improvement of HAZID using case studies and other methods; some development topics. Conclusions from the work are given in the final paper.