Safety constraints

In the batch chemical reactor accident described in chapter 2, one safety constraint is a hmitation on the temperature of the contents of the reactor. 

The STAMP (System-Theoretic Accident Model and Processes) accident model is based on these principles. Three basic constructs underlie STAMP safety constraints, hierarchical safety control structures, and process models. 

The most basic concept in STAMP is not an event, but a constraint. Events leading to losses occur only because safety constraints were not successfully enforced. 

The difficulty in identifying and enforcing safety constraints in design and operations has increased from the past. In many of our older and less automated systems, physical and operational constraints were often imposed by the hmitations of technology and of the operational environments. Physical laws and the Umits of our materials imposed natural constraints on the complexity of physical designs and allowed the use of passive controls. 

In engineering,pflsvzvc controls are those that maintain safety by their presence— basically, the system fails into a safe state or simple interlocks are used to limit the interactions among system components to safe ones. Some examples of passive controls that maintain safety by their presence are shields or barriers such as containment vessels, safety harnesses, hardhats, passive restraint systems in vehicles, and fences. Passive controls may also rely on physical principles, such as gravity, to fail into a safe state. An example is an old railway semaphore that used weights 

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Special safety constraints apply to equipment selection, design, and operation in nuclear reprocessing (269). Equipment should be reHable and capable of remote control and operation for long periods with minimal maintenance. Pulsed columns and remotely operated mixer—settlers are commonly used (270). The control of criticaHty and extensive monitoring of contamination levels must be included in the process design. 

That work led to a routine 700 gram scale oxide reduction process that has been in use since that time. Recent development work at LANL has increased the batch size to one kilogram of oxide feed. It appears that the ultimate limitation on DOR batch size will be from criticality safety constraints. 

Large-scale reactors have low quantum yields as radiation does not penetrate deeply into the reaction vessel [72, 74]. As a consequence, high-power lamps have to be used causing a lot of excess heat and even posing safety constraints. These energy sources produce locally high quantities of radicals which may not mix thoroughly with the rest of the solution. Therefore, they may not find a second reaction partner, but instead react by themselves. This radical combination reduces selectivity and creates additional heat. 

Abel, O. A. Helbig W. Marquardt H. Zwick, et al. Productivity Optimization of an Industrial Semi-batch Polymerization Reactor Under Safety Constraints. J Proc Contr 10 351-362(2000). 

Process simulators contain the model of the process and thus contain the bulk of the constraints in an optimization problem. The equality constraints ( hard constraints ) include all the mathematical relations that constitute the material and energy balances, the rate equations, the phase relations, the controls, connecting variables, and methods of computing the physical properties used in any of the relations in the model. The inequality constraints ( soft constraints ) include material flow limits maximum heat exchanger areas pressure, temperature, and concentration upper and lower bounds environmental stipulations vessel hold-ups safety constraints and so on. A module is a model of an individual element in a flowsheet (e.g., a reactor) that can be coded, analyzed, debugged, and interpreted by itself. Examine Figure 15.3a and b. 

A good understanding of the process leads in most cases to a logical choice of what needs to be controlled. Considerations of economics, safety, constraints, availability and reliability of sensors, etc. must be factored into this decision. 

At the startup of the line, the extruder was operated at 91 rpm to produce the required rate of 148 kg/h for a specific rate of 1.63 kg/(h-rpm). The temperature of the extrudate was measured through the transfer line wall at 232 °C. Due to process safety constraints the extrudate temperature could not be measured using a handheld temperature sensor. The extrusion rate was required in order to maintain the downstream take-away equipment at its maximum rate. At first the extruder appeared to be operating well except that the specific rate was lower than predicted. That is, the screw was rotated at an rpm that was higher than expected to produce the 148 kg/h. At 91 rpm, the rotational flow rate was calculated at 228 kg/h the specific rotational flow rate was calculated at 2.51 kg/(h-rpm). Thus, the line was operating at only 65% of the rotational flow rate. A barrier design... 

Environmental/Health/Safety Constraints Combination effects include both synergism and antagonism. These constraints combine the following activities ... 

Equipment design, operating procedures, and operator training often address environmental, health, and safety constraints. Eliminating fugitive emissions, removing workers from potential chemical exposure, and ensuring that flammable concentrations never occur are just a few of the approaches used. 

This is the most common mode of addition. For safety or selectivity critical reactions, it is important to guarantee the feed rate by a control system. Here instruments such as orifice, volumetric pumps, control valves, and more sophisticated systems based on weight (of the reactor and/or of the feed tank) are commonly used. The feed rate is an essential parameter in the design of a semi-batch reactor. It may affect the chemical selectivity, and certainly affects the temperature control, the safety, and of course the economy of the process. The effect of feed rate on heat release rate and accumulation is shown in the example of an irreversible second-order reaction in Figure 7.8. The measurements made in a reaction calorimeter show the effect of three different feed rates on the heat release rate and on the accumulation of non-converted reactant computed on the basis of the thermal conversion. For such a case, the feed rate may be adapted to both safety constraints the maximum heat release rate must be lower than the cooling capacity of the industrial reactor and the maximum accumulation should remain below the maximum allowed accumulation with respect to MTSR. Thus, reaction calorimetry is a powerful tool for optimizing the feed rate for scale-up purposes [3, 11]. 

Optimal feed profile for a second order reaction in a semi-batch reactor under safety constraints, Experimental study. Journal of Loss Prevention in the Process Industries, 12 (11), 485-93. 

Ubrich, O., Srinivasan, B., Stoessel, F. and Bonvin, D. (1999) Optimization of semi-batch reaction system under safety constraint, in European Control Conference, European Union Control Association, Karlsruhe. 

Toulouse, C., Cezerac, J., Cabassud, M., Lann, M.V.L. and Casamatta, G. (1996) Optimization and scale-up of batch chemical reactors impact of safety constraints. Chemical Engineering Science, 51 (10), 2243-52. 

The control structure developed to enforcing the system safety constraints usually presents slow variation or degradation over time, particularly in the physical system, human operator behaviour, or the environment. One should be prepared for unexpected classifications applied may lead to results one expected. 

Multivariable controls (MVCs) are particularly well suited for controlling highly interactive fractionators where several control loops need to be simultaneously decoupled. MVCs can simultaneously consider all the process lags, and apply safety constraints and economic optimization factors in determining the required manipulations to the process. The technique of multivariable control requires the development of dynamic models based on fractionator testing and data collection. Multivariable control applies the dynamic models and historical information to predict future fractionator characteristics. For towers that are subject to many constraints, towers that have severe interactions, and towers with complex configurations, multivariable control can be a valuable tool. 

Several schemes suggest themselves for using these bounds. Obviously one can develop any network he or she chooses subject to safety constraints, controllability constraints, etc., and... 

The case study on Vinyl Acetate Process, developed in Chapter 10, demonstrates the benefit of solving a process design and plantwide control problem based on the analysis of the reactor/separation/recycles structure. In particular, it is demonstrated that the dynamic behavior of the chemical reactor and the recycle policy depend on the mechanism of the catalytic process, as well as on the safety constraints. Because low per pass conversion of both ethylene and acetic acid is needed, the temperature profile in the chemical reactor becomes the most important means for manipulating the reaction rate and hence ensuring the plant flexibility. The inventory of reactants is adapted accordingly by fresh reactant make-up directly in recycles. 

To avoid the high-pressure safety constraint, we must control reactor pressure. We can use the distillate valve from the purge column, the flooded condenser cooling water valve, or the DIB column feed valve. The most logical variable to use for control of the flooded condenser (.reactor) pressure is the DIB column feed valve (as shown in Fig. 5.5). Based upon the discussion in Step 3, we would then use the flooded condenser cooling water valve to keep the liquid level in a good control range. 

The overriding safety constraint in this process involves oxygen concentration in the gas loop, which must remain below 8 mole % to remain outside the explosivity envelope for ethylene mixtures at process conditions. The most direct manipulated variable to control oxygen composition at the reactor feed is the fresh oxygen feed flow. ... 

The critical product-quality and safety-constraint loops were tuned by using a relay -feedback test to determine ultimate gains and periods. The Tyreus-Luyben PI controller tuning constants were then implemented. Table 11.12 summarizes transmitter and valve spans and gives controller tuning constants for the important loops. Proportional control was used for all liquid levels and pressure loops. 

L. L. Simon, M. Introvigne, U. Fischer, K. Hungerbuhler, (2008), Batch reactor optimization under liquid swelling safety constraint, Chemical Engineering Science, 63, 770. 

The Pressurized Water Reactor (PWR) reload core optimization problem, though easily stated, is far from easily solved. The designer s task is to identify the arrangement of fresh and partially burnt fuel (fissile material) and burnable poisons (BPs) (control material) within the core which optimizes the performance of the reactor over that operating cycle (until it again requires refueling), while ensuring that various operational (safety) constraints are always satisfied. 

The Hansen method is very valnable. It has fonnd widespread use particularly in the paints and coatings indnstry, where the choice of solvents to meet economical, ecological, and safety constraints is of critical importance. It can explain cases in which polymer and solvent solubility parameters are almost perfectly matched, yet the polymer will not dissolve. The Hansen method can also predict cases where two nonsolvents can be mixed to form a solvent. Still, the method is approximate, it lacks the generality of a Ml thermodynamic model for assessing miscibility, and it requires some experimental measnrements. The determination of R is typically based on visnal observation of solubility (or not) of 0.5 g polymer in 5 cm solvent at room temperature. Given the concentration and the temperature dependence of phase boundaries, such a determination may seem a bit arbitrary. Still the method works out pretty well in practice, probably because the liquid-liquid boundaries for most polymer-solvent systems are fairly flat. ... 

The safety constraint has been defined in the brief and can be checked by computer analysis or by the safety officer at any time. 

Not only do safety constraints sometimes conflict with mission goals, but the safety requirements may even conflict among themselves. One safety constraint on an automated train door system, for example, is that the doors must not open unless the train is stopped and properly aligned with a station platform. Another safety constraint is that the doors must open anywhere for emergency evacuation. Resolving these conflicts is one of the important steps in safety and system engineering. 

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