This article provides a systematic framework for the analysis and improvement of near-miss programs in the chemical process industries. Near-miss programs improve corporate environmental, health, and safety (EHS) performance through the identification and management of near misses. Based on more than 100 interviews at 20 chemical and pharmaceutical facilities, a seven-stage framework has been developed and is presented herein. The framework enables sites to analyze their own near-miss programs, identify weak management links, and implement systemwide improvements.

IntroductionTraditionally, distillation processes have been designed to operate in a batch column, in which a still is charged and distilled, or to operate continuously, in which a stream is fed to a column and distillate and bottoms products are withdrawn continuously. Batch distillation is usually preferred for low throughputs, when fine or specialty chemicals are produced, or when there is intermittent or seasonal chemicals production. In contrast, continuous distillation is favored for larger-scale throughputs and continuous upstream feeds.In this article, the design, control, and operation of a semi-Ž . continuous, middle-vessel column SCMVC to separate nearly-ideal ternary mixtures is investigated. The middle-vessel permits semicontinuous operation to be an attractive policy for the separation of multicomponent mixtures. The MVC is a distillation column with a large vessel between the rectifying and stripping sections. One MVC configuration is shown in Figure 1 Ž . In a brief companion article Phimister and Seider, 2000a , simulation studies of a SCMVC are presented for the separation of azeotropic mixtures in processes comparable to continuous extractive and azeotropic distillations. The MVC is shown to reduce capital investment, as compared with continuous processing. An MVC operated semicontinuously avoids column dumping and repeated startups, and allows for column automation, as compared with batch operation. In Table 1, several features of distillation processes are listed to compare a SCMVC with batch and continuous columns.In this article, recent work on middle-vessel columns is reviewed. Then, the semicontinuous process is reviewed for the separation of three alkanes. Next, shortcut design procedures are adapted for semicontinuous columns. Several PI control configurations are examined, and for a DB configuration, the decoupling effect of the PI-composition controllers is shown. Finally, the three-alkane separation is considered in greater detail, with a column designed using the shortcut procedures and a DB-control configuration. MV DistillationWithin the past decade, there has been much interest in the performance of alternative distillation configurations, including the MVC multivessel columns, series of heat-integrated, batch-distillation columns, and semicontinuous distillation configurations.Although the MVC was initially suggested by Robinson and Ž . Gilliland 1950, p. 388 , few papers appeared until the 1990s. Ž . One of the first articles, by Hasebe and coworkers 1992 , reintroduced the MVC, presenting a campaign policy with simulation results. The simulations involve ideal mixtures, with the heavy and light impurities removed in the bottoms product and distillate, respectively, while the desired product concentrates in the middle vessel. In a subsequent article, Ž . Hasebe and coworkers 1996 conclude that a batch MVC consistently provides superior performance compared to batch rectification followed by batch stripping. Meski and Ž . Morari 1995 also conclude that MVC performance is superi...

The merits of semicontinuous, pressure-swing, azeotropic distillation are examined. In continuous operation, two columns at different pressures are fed streams having compositions on opposite sides of the azeotropic compositions. The distillates, which approach the azeotropic compositions at high and low pressure, are cycled between the columns. In contrast, semicontinuous operation involves only a single distillation column, which has lower investment costs and, when the mixture to be separated is changed, shorter downtimes. An optimal-control algorithm is employed to determine desirable campaigns, and to schedule pressure switch-over policies. Simulation results for the dehydration of tetrahydrofuran, involving a pressure-sensitive azeotrope, indicate that switch-over between steady states occurs quickly with on-spec product removed during 93% of the campaign. The column achieves production rates near 89% of the maximum throughput of a single column in the continuous process and shows superior performance when compared to reverse-batch operation.

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