Simulation and Experimental Analysis of Operational Failures in a Methanol - Water Distillation Column

A rigorous model for the simulation of the dynamic behaviour of distillation column including pressure relief has been developed. This model allows the analysis of the influence of different disturbances (failures) on the dynamic behaviour of a methanol/water column. Disturbances of the cooling medium supply to the condenser and the reflux stream have been examined. In this paper the results of the simulation for a failure of the cooling water supply are presented and discussed. Experimental results, which should help to corroborate the simulation results as well as visual observations of the hydrodynamics on one of the distillation trays of the column during relief are also presented. INTRODUCTION Distillation columns are the most widely used separation operation in the process industry. Distillation columns present a special safety hazard because of their large inventories, which often involve flammable solvents. Disturbances during operation such as the failures of individual components can result in a deviation from the normal operation and could in the long run lead to a situation were the unit has to be depressurised (relief). That is, conditions which are not within the defined operating procedure may occur. The new European control of major accident directive Seveso II will in its implementation make it necessary that plant operators deliver detailed evidence ( i.e. Safety Report) that the plant design conforms to the required safety standard [1]. Although simulation studies are widely used to examine the operating behaviour of distillation columns, they are seldom used to carry out systematic studies of failures during column operation. Deerberg et al [2] state that in addition to the conventional methods, dynamic simulation can be a powerful tool for safety examinations in chemical engineering. Detailed dynamic simulation of operational failures gives a wide range of information about unknown internal process quantities. This leads to a deeper understanding of the process behaviour. With this instrument at hand risks can be assessed for normal operation states as well as in the case of operational failures so that applicable preventive measures can be defined. Testing improvements of the constructive and operational conditions with respect to important aspects of production processes is possible. It is necessary to consider not only the result obtained within the column but also that of all other important components of the complete plant (valves, pumps, reboiler, condenser, steam supply, cooling medium supply etc.) as well as the important components of the process control system including the installed protective systems. By considering these influences on the process dynamics, many safety related questions can be answered. The most important physical effects which must be investigated in a distillation column are: ⇒ Influence of the hydrodynamics and mass transfer ⇒ Stability of the control loop during non-standard operating conditions ⇒ Effects of operational conditions on process safety ⇒ Effectiveness of the protective system To develop simulation tools for safety analysis, the characteristics of the process must be known. The effects most important for safety have to be detected and integrated into the process model. DISTURBANCES IN DISTILLATION COLUMNS A study carried out by Kister [3] showed that safety related incidences do occur in distillation columns and have in fact increased in recent years. Many different causes are reported, but imprecise design, operational errors and failure of plant components are mentioned quite often. The study also found that almost half of the failures could have been avoided, if better education and experience were available during design and operation. The ZEMAReport [4] also relates incidences of accidents in which distillation column are involved. Depending on the type of distillation column, different sources of hazard can be identified. Firstly there is the hazard of fire or explosion due to emission of flammable material from the column. Secondly, unwanted reactions such as Polymerisation or decomposition may result when there is an unwanted temperature increase due to deviation from the standard operating conditions. It is the job of safety technology to analyse all the hazard potentials as well as determine suitable preventive and operative measures. This includes recommendations on dimensioning and fail-safe design. A systematic examination of the causes that could lead to operational failures and the resulting dynamic behaviour in distillation column has not been done till now. Kister [5] has listed some of the causes that could result in overpressure in a column. These are : Utility failure such as • Loss of coolant • Loss of electric power • Loss of steam • Loss of instrument air Controller failure • Failure of steam controller • Failure of pressure controller • Failure of feed controller • Failure of reflux controller Extraneous sources • Valve opening to external pressure source • Loss of heating in an upstream column (dumping) • Failure of an exchanger tube • Exterior fire Internal sources • Accumulation of non-condensables • Chemical reaction • Closed column outlets • Internal explosion DYNAMIC MODELLING OF A DISTILLATION COLUMN WITH RELIEF STREAM A rigorous dynamic column model was developed under gPROMS which can describe the dynamic behaviour of a distillation column. Figure 1 shows the scheme of a separation stage.