Startup and shutdown

The column s ability to weather foreseen and unforeseen difficulties is severely tested during startup and shutdown. Failure to recognize and eliminate pitfedls or hazards associated with these operations has caused catastrophic accidents, sometimes with injuries or loss of life damage to columns and their internals and/or prolonged periods of downtime. It is therefore imperative to recognize and eliminate (or at least minimize) the pitfalls associated with column startups and shutdowns. 

This chapter discusses several general pitfalls inherent in distillation startup and shutdown operations. Hazards and pitfalls specifically related to the nature of the chemicals processed (e.g., toxicity, flammability), the simultaneous startup/shutdown of the column with other units (e.g., reactors, furnaces), or to the overall startup/shutdown policy (e.g., operator training procedures) are outside the scope of this distillation text. The safe handling procedures of the relevant chemicals, as well as the overall startup/shutdovm policy, must be followed to cater for the latter hazards and pitfalls. 

This chapter reviews common column startup and shutdown practices, outlines those which are generally preferred, highlights the consequences of poor practices, and supplies general guidelines for avoiding the hazards and pitfalls generally inherent in column startup and shutdown. 

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Unsteady material and energy balances are formulated with the conservation law, Eq. (7-68). The sink term of a material balance is and the accumulation term is the time derivative of the content of reactant in the vessel, or 3(V C )/3t, where both and depend on the time. An unsteady condition in the sense used in this section always has an accumulation term. This sense of unsteadiness excludes the batch reactor where conditions do change with time but are taken account of in the sink term. Startup and shutdown periods of batch reactors, however, are classified as unsteady their equations are developed in the Batch Reactors subsection. For a semibatch operation in which some of the reactants are preloaded and the others are fed in gradually, equations are developed in Example 11, following. 

Other advantages of gravity beds include flexibility in gas and sohds flow rates and capacities, variable retention times from minutes to several hours, space economy, ease of startup and shutdown, the potentially large number of contacting stages, and ease of control by using the inlet- and exit-gas temperatures. 

Normal operations Any process operations intended to be performed between startup and shutdown to support continued operation within safe upper and lower operating limits. 

Process requirements. Are there speeial startup and shutdown eonsiderations sueh as eatalyst loading and unloading and minimum air requirements ... 

Ease of installation and commissioning is another reason for gas turbine use. A gas turbine unit can be tested and packaged at the factory. Use of a unit should be carefully planned so as to cause as few start cycles as possible. Frequent startups and shutdowns at commissioning greatly reduce the life of a unit. 

All gas turbines are provided with a eontrol system by the manufaeturer. The eontrol system has three fundamental funetions startup and shutdown sequeneing, steady-state eontrol when the unit is in operation, and protee-tion of the gas turbine. 

The startup and shutdown of a typical gas turbine is shown in figures 19-5 and 19-6, respectively. The time and percentages are approximate values and will vary depending upon the turbine design. 

If casing limitations are fixed by user-supplied relief valves, this information should be conveyed to keep the vendor from rating the compressors on other data. Evaluations can be more of a problem if the same design basis isn t universal with all vendors. Startup and shutdown consideration influence various components, shaft end seals, seal system pressures, and even thrust bearings in some instances. The use of an alternate startup gas, or the desire to operate a gas compressor on air to aid in plant piping dryout should be covered. 

Flashback protection is required for H2S flaring systems, either by water seal or continuous gas purge. If a water seal is used, special requirements apply to the disposal of the effluent seal water. In the case of an HjS flaring system handling a flow of HjS which in uninterrupted throughout the period that a plant is in operation, and which stops only when the producing plant is shutdown, then flashback protection is not required. However, steam or inert gas connections are required to permit purging the flare line startup and shutdown. 

I /ci sonnel. They include applicable safety precautions and warnings regarding pressure limits, temperature ranges, flow rates, and the meanuig and response to alamis and instruments. Procedures for startup and shutdown are included with the appropriate ranges for the process parameters. They include instructions and commands for computer process control, if used. 

PrH A improves the safety, reliability and quality of the design and construction of a new or old process. P IDs must be correct as constructed operating, startup and shutdown procedures must be validated, and the operating staff must be trained before startup. Incident inr estigation recommendations, compliance audits or PrHA recommendations need resolution before startup. 

Apart from the degree of novelty of a process event, its complexity (e.g., the range of operations to be carried out), the interrelationships of the process variables involved and the required accuracy, will affect performance. Startup and shutdown operations are examples of tasks which, although are not entirely unfamiliar, involve a high degree of complexity. 

Most state laws and safe practice require a safety relief valve ahead of the first stop valve in every positive displacement compressed air system. It is set to release at 1.25 times the normal discharge pressure of the compressor or at the maximum working pressure of the system, whichever is lower. The relief valve piping system sometimes includes a manual vent valve and/or a bypass valve to the suction to facilitate startup and shutdown operations. Quick line sizing equations are (1) line connection, (i/1.75 (2) bypass, ii/4.5 (3) vent, dl63 and (4) relief valve port, cU9. 

In addition to the product, the designer, equipment installer, user, and all others involved in production should all consider performing a risk assessment and target in the direction of perfection. The production is reviewed for hazards created by each part of the line when operating as well as when equipment fails to perform or complete its task. This action includes startups and shutdowns, preventative maintenance, QC/inspection, repair, etc. 

All process equipment has procedures to operate and meet safety requirements. They include a checklist that includes preparation (moving material, etc.), startup and shutdown procedures, tooling changes, and to cleanup of all equipment. Most equipment generates high heats and pressures. They are built to run safely, but they must be treated with respect . 

Inspecting and maintaining the interlock system is a vital function. It is required to ensure general safety and to avoid risks of boiler explosion. Interlocks provide for the safe sequence of boiler startup and shutdown procedures. 

Device used to vent noncondensable gases from a boiler during startup and shutdown procedures. 

While startup and shutdown occur relatively infrequently in large continuous plants, they are inherent in batch plant operation. Most startup and shutdown procedures, whether devised empirically or theoretically, are designed to follow a recipe of actions with no feedback. Thus, if upsets occur, there is often no way to change the startup or shutdown in time to avoid imwanted process exclusions. Procedures are needed that incorporate feedback and adaptive techniques to the problem of plant startup and shutdown. 

The general case treats time-dependent volumes, flow rates, and inlet concentrations. The general case must be used to for most startup and shutdown transients, but some dynamic behavior can be effectively analyzed with the constant-volume, constant-flow rate version of Equation (14.2) ... 

Startup and Shutdown Strategies. In addition to safe operation, the usual goal of a reactor startup is to minimize production of off-specification material. This can sometimes be accomplished perfectly. 

Special efforts are needed to ensure safety at this stage of the development. Since startup and shutdown procedures are responsible for many accidents, these procedures merit special attention. 

In addition, during the startup and shutdown of an incinerator, hazardous waste must not be fed into the unit unless it is operating within the conditions specified in the permit. An incinerator must cease operations when changes in waste feed, incinerator design, or operating conditions exceed the limits designated in its permit. 

Yang, W. C., Margaritis, P. J., and Keaims, D. L., Simulation and Modeling of Startup and Shutdown in a Pilot-Scale Recirculating Fluidized Bed Coal Devolatilizer, AIChE Symp. Ser., 74(176) 87 (1978)... 

In this chapter, we develop the basis for design and performance analysis for a plug flow reactor (PFR). Like a CSTR. a PFR is usually operated continuously at steady-state, apart from startup and shutdown periods. Unlike a CSTR, which is used primarily for liquid-phase reactions, a PFR may be used for either gas-phase or liquid-phase reactions. 

There are situations where the control loop should be aware of more than just one controlled variable. This is particularly true in highly automated plants where the operator cannot be expected to make all the decisions that must be made under abnormal conditions. This includes the startup and shutdown of the process. 

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