Reactor Operating Procedures for Startup of Continuously-Operated Chemical Plants
Summary (5 min read)
- The chemical industry has put much effort on process optimization, resulting in increased process yields, reduced energy consumption, decreased environmental pollution, and improved product quality.
- To support these activities, extensive research has been carried out in the area of heat exchanger network design for energy integration.
- An actual industrial case according this strategy is reported by Bouwens and Kosters (1992).
- Unfortunately, the required dynamic process models describing a plant startup or shutdown are necessarily much more complex, and costly, than a model which covers the range of conditions encountered during normal operations only (Wolff et al., 1992) .
- Note that modern process control systems may perform several tasks, like analog control to maintain the process state variables at their target values, on/off control to establish material routing through the process, interlocking to ensure process safeguarding, and sequence control to guide the process through a series of operating phases (Slijk, 1985) .
- Operating Constraints termine intermediate operating states in between shutdown and the normal production state.
- A constraint guided strategy searches for a feasible sequence of actions to drive the process from shutdown to the production state.
- If no effective operating procedure can be generated, flowsheet structure modifications are proposed by the operating procedure planner.
- Also, almost no studies are published about process dynamics and operating procedures of industrial reactor systems during a plant startup.
- It is shown that for an exothermal adiabatic tubular reactor a much higher initial temperature is required, compared to the reactor inlet temperature at normal steady-state conditions, to ensure high reactant conversion levels during the startup.
- First, some axioms are defined to determine convenient startup criteria.
- Secondly, an entire plant has to be unraveled into smaller units, which can be operated stand-alone during the startup.
- Some rules for process decomposition are described, together with some guidelines for sequencing the startup of individual process units.
- Thirdly, the problem is addressed of how and when to put an exothermal adiabatic tubular reactor system into a production state.
- Industrial examples are used (a) to demonstrate some points regarding the response of operating personnel to dynamic reactor operations, and (b) to show some shortcomings about reactor conditioning operations which are incorporated in many industrial designs.
Deviations from Operating Targets
- In general, the steady-state conditions of a continuouslyoperated chemical plant have been proven to be safe, and the observation of a few key state variables is often sufficient for experienced operating personnel to control the process status.
- Ultimately, this may lead to operating errors, due to an improper understanding of the process dynamics (Stephanopoluos, 1988) .
- Hazardous situations may result from running a chemical plant outside the range of the normal operating conditions.
- The process state variables should be kept at the desired nominal targets, and their variability should be de-creased by making the appropriate changes in the operating conditions and/or strategies to improve process performance and product quality (Saraiva and Stephanopoulos, 1992) .
- It was proved quantitatively for cooled, fixed-bed, tubular reactors by Westerterp et al. (1984) , Westerterp and Ptasinsky (1984) , Westerterp and Overtoom (1985) , and Westerink and Westerterp (1988) .
- From an operational point of view, three categories of process constraints can be defined: Process safety constraints are set by the design pressure and temperature of equipment, the relief-pressure of safety devices, the maximum allowable temperature differences in heat exchanger equipment, the chemical and/or physical nature of process materials, and environmental safety.
- For a welldesigned process, the controllability constrained range of operating conditions is more confined than the range of possible operating conditions set by safety constraints.
- Process performance constraints are determined by product quality, product consistency and process efficiency requirements.
- Usually, these are tight operating constraints.
- To avoid these upsets, and to ensure a safe operation during startup, the process should be limited to the control-Iability constraint range of operating conditions:.
Convenient criteria for startup operations
- Generally, the most important controlled process variables are the temperature, pressure, inventory , material composition and flow.
- Fortunately, the flow and process inventory variables can be controlled usually at their intermediate targets during all phases of the plant startup operations (inventory control).
- First, the response time to a feed flow rate and/or composition change, is determined by the average residence time and the residence time distribution of the material in a process unit.
- Nevertheless, some key conclusions from studies on the relationship between the process structure and its dynamics are that (a) recycles increase theprocess sensitivity to disturbances, and (b) the response time of recycle processes is substantially longer than the response time of the forwardpath alone (Denn and Lavie, 1982; Kapoor et al., 1986) .
- On the other hand, a plant startup can be accelerated by restricting the material composition, as close as possible, to its (steady-state) target value when the process units are charged with material from storage.
Process Decomposition into Unit Operations
- Decomposition of an entire plant into smaller subsystems is necessary to decrease the complexity of the plant startup operations.
- The following sections describe some process decomposition rules in relationship with the startup of continuously-operated reactors.
- The nonheat-integrated process in Figure 2 will be used to illustrate the various steps of the analysis.
- Reactant A is fed from the buffer tank into the reactor, together with reactant B which is fed from storage.
- The finished products are routed to storage.
Reversible and irreversible unit operations
- The usual way to bring a process unit into operation is to charge the required amount of materials into the system, as soon as purging and leak testing are finished.
- Fusillo and Powers (1987) developed a so-called stationary state concept to determine stable intermediate operating states in between shutdown and the normal production state at which a process system can remain until the next operational action can be taken.
- A prerequisite for the simultaneous operation of the inverse process functions is that material can be recycled between them, like between the reboiler and condenser of a distillation column.
- Irreversible unit operations are started up by supplying the appropriate feed into the system, and production is started.
- Units with process independent heating or cooling sources, like aircoolers or steam reboilers, are usually started up first.
Startup of reversible unit operations
- Severa (1973) presented a simplified operating procedure for the startup of a crude and vacuum distillation unit of a refinery.
- Establish a cold product circulation from the distillation unit to the crude charge tank, with all streams being returned into the crude charge tank.
- In the third step, the unit is driven to the normal operating conditions, without being in a production state (simultaneous material and heat balance control).
- Nevertheless, it is profitable to put (some) additional time in running the particular process units strictly on the required operating targets before continuing with the operational integration with neighboring units.
- Also, the introduction of polymer or solid materials is postponed in many cases, until the process is running reasonably well at the target conditions.
Startup of irreversible unit operations
- Irreversible process units are started up by supplying the appropriate feed into the system, and by discharging almost simultaneously the effluent streams into the downstream process units.
- The startup of a sulfuric acid plant should ideally be fast and clean.
- The generated startup policies were experimentally verified in a laboratory reactor (Mann et al., 1986) .
- Significant differences are found on SO2 emission levels as a function of the initial reactor temperature, the SOz concentration at the reactor inlet, and the total feed rate into the reactor.
- At normal operating conditions, two reactants are fed via a mixer into the reactor, and the reactor effluent is discharged into a separator.
AIChE Journal January 1995
- Neously from bypassing to flowing directly into the cold fixedbed reactor, without controlling the initial reactor temperature.
- The effect of thermal shocks in process equipment was neglected too.
- The lines parallel to the u axis represent the response of the individual thermoelements at the dimensionless location z , and the lines parallel to the z axis connect the data at the same moment.
- The reactor is prepared for startup by filling the system via the normal flow route with a mixture of reactant A and product C. By starting up according to the policy just described, the required initial temperature profile over the entire reactor is difficult to establish, because the total amount of reactant A fed into the system should be minimized since the recovery system is not able to process the reactor effluent properly at this operating stage!.
Shutdown of irreversible unit operations
- As discussed above, intermediate stationary states are an important aspect of process safeguarding strategies, since process units can be put into these intermediate operating states in emergency or process wait situations.
- At these intermediate operating states production as such is stopped, but most of the process units remain at their normal operating conditions until production can be restarted.
- The data are expressed as a percentage of the range of the individual flow measurement devices.
- At time u = 1.45, the process control computer stopped the reactant B feed to the reactor, due to a reactor inlet temperature problem.
- As a result of all flow manipulations just described, the reactor contained too much unconverted reactant B, which resulted in a temperature runaway.
- The high heat integrated process section in Figure 7 will be used to demonstrate the derived heuristics for the startup of a continuously-operated, industrial reactor system.
- January 1995 ess and control strategy design of this system has been reported by Bouwens and Kosters (1992) .
- The process consists of a multifunctional heater H-1, a re- actor R-1, a reactor feed/effluent heat exchanger network, and a distillation column T-1. Stream A3 is mixed with stream B2, evaporated and superheated in the heat exchanger network, and fed into reactor R-1 as a second feed.
- The reactor effluent exchanges heat in the reboiler of column T-1 and subsequently in the heat exchanger network.
- Finally, this stream is fed into column T-1 and separated into bottom product C and distillate product A. Reactant A is supplied into the process via the suction line of the reflux pump of column T-1.
- Inert gases can be removed from the process via a purge system on the reflux drum of column T-1.
- The catalyst in reactor R-1 deactivates during run time, and can be regenerated, off-line, via the system shown in Figure 8 .
- The system is heated up to the required catalyst regeneration temperature by heater H-1.
- The reactor effluent is cooled, and liquid is knocked out in a flash drum.
- A stationary state can be maintained by simultaneous inverse operations with respect to thermal energy, those operations being the heater H-1 and the cooler in the regeneration system, until the reactor can be operationally integrated with the column T-1 subsystem.
- The column T-1 subsystem consists of reversible unit operations.
Choice of convenient startup criteria
- Now the authors have to focus on the choice of convenient operating criteria to demonstrate some aspects of startup operations with respect to material composition control.
- Hence, reactor R-1 is bypassed until the feed flows to reactor R-1 are completely vaporized, and the entire reactor is above a minimum temperature limit to avoid condensation of the feed in the catalyst bed.
- The heat exchangers are filled initially with liquid only, resulting in an excess amount of material in the system at reaching the required operating conditions, due to the liquid to vapor phase transition.
- To avoid reactor feed composition upsets during the startup operations, and to bypass a purification step for the material discharged from the process, virginal reactant A is used to drive the reactor to its startup conditions.
- At reaching these, product C is fed into column T-1 to bring this column at production state conditions.
- The various aspects of the derived heuristics can be identified easily in the following steps of the startup strategy: Inert gases, originating from purging and leak testing operations, have to be removed from the system.
- The reflux pump is started up, and also the column T-1 sump is filled up with reactant A. Before the heat carrier gas is supplied to heater H-1, the compressor and coolers in the reactor R-1 subsystem in Figure 8 should be first put into service.
- This subsystem should be operated at total recycle conditions also, without any heat supplied into the system.
- Subsequently, it should be stressed that the whole process, excluding the reactor, is running at production state conditions before reactant B is fed into the process.
- Some rules are presented for the startup of industrial adiabatic tubular reactor systems, based on a qualitative analysis of the dynamic behavior of continuously-operated vapor and liquid-phase processes.
- The rules can be extended potentially to polymer and solids processing units.
- First, the relationships between the process dynamics, operating criteria, and the operating constraints are investigated, because reactor startup cannot be studied effectively without taking into account the operational aspects of the entire plant section which includes the reactor.
- At this intermediate operating stage, the entire process system can be driven to the required operating conditions to start production safely.
- In practice, when operator interventions have a massive impact on the entire plant operation, the decision to stop some unit operations is delayed naturally.
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