Workbook for Chemical Reactor Relief System Sizing

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Predicting the physical properties of the reaction mix during various steps in the process due to composition and temperature changes can have a significant effect on the reactor performance. When processing flammable solvents it is standard practice to eliminate explosive mixtures by establishing and maintaining an inert atmosphere in the reactor. Inerting systems can be based on pressure balance or continuous flow. Some reactions involve the evolution of non condensable compounds which will become saturated with VOCs in proportion to the component vapour pressures at the condenser exit temperature and pressure.

The presence of non condensables in the condenser results in a significant increase in the thermal resistance which can result in up to half the heat transfer area being required for the last zone alone. The secondary loop thermal time constant must be less than the primary loop thermal time constant. The secondary controller is normally a Proportional only controller as Integral Action Time will slow the response.

Temperature measurement is invariably by resistance sensor in conjunction with Smart transmitters which allows considerable flexibility when setting ranges. There is hardly any thermal lag associated with the sensor, however, there can be significant thermal lags associated with the thermowell if incorrectly designed or installed which can lead to an uncontrollable system. Designs are available which ensure fast response and should be adopted.

The set up screen for the controller is shown below. The controller UnitOp provides the facility for proving the suitability of a particular control configuration and for optimisation of the controller settings dynamically. A valve has two characteristics namely the inherent characteristic relationship between flow and stroke at constant?

P and the operational characteristic where the inherent characteristic is modified by the process pressure conditions. P min increases. A linear valve operating characteristic tends towards a quick opening characteristic as? The operational characteristic of a valve can also be modified by controller output signal characterisation using various techniques.

Control valve actuators should be pneuma tic with positioners fitted. The calibration for split range operation of the valves should be achieved at the positioners, not with scaled multiple controller outputs, to ensure loop integrity is maintained. There are several methods available for control loop tuning. The Ziegler and Nichols 3 is commonly used and involves establishing the proportional band at which the process begins to oscillate at constant amplitude as shown below.

Control parameters are then set based on these values. Refer Appendix III for details of the method. Ethylene glycol mixes should not be used above their boiling point. The scheme below indicates the basic system with automatic control of the steam and cooling water services. The changeover between heating and cooling modes can be done manually or automatically involving a complex sequence of valve switching operations involving time.

The changeover is not seamless as with a single fluid system and requires careful consideration if used on exothermic reactions. The heat up and cool down curves demonstrate the fast response of these systems which can lead to thermal shock problems with glass line equipment. Operational problems associated with these systems include cross contamination of services, corrosion and the need for complex control routines when changing services from heat to cool involving time lags.

These systems are not always cheaper than other alternatives.

Relief valve - Wikiwand

The circulation rate is set depending on the number of mixing nozzles and achieving the recommended nozzle pressure drop. Changeover between heating and cooling modes is seamless using control valves in split range. The heat transfer fluid is on the plate side of the shell and plate heat exchanger, which provides a high film coefficient.

The heat transfer area is selected based on a reasonable LMTD at approach to service supply temperature and is sized to ensure that reactor heat transfer is limiting. Equipment in common use include sealless pumps and gasket free fully welded plate and shell heat exchangers. Thermal response on cool is excellent due to direct injection and the use of a three way valve on the heat exchanger minimises thermal lags on heating.

These systems require careful consideration to ensure thermal expansion is allowed throughout the loop. On initial commissioning these systems have to be thoroughly dried out to prevent operational problems and equipment damage. Water breakthrough due to contamination or equipment failure will result in considerable downtime.

The use of an additional heat exchanger for cooling allows the selection of a less expensive fluid for the cooling service which may provide cost benefits with a centralised refrigeration facility involving the use of significant volumes of heat transfer fluid. This system also allows for segregation of the reactor service system from other reactors which enables rapid identification of water breakthrough problems on a facility with several reactors.

The use of oversized reactors should be avoided and can lead to heat removal limitations. The density and specific heats differences are not significant for heat transfer but the thermal conductivity differences are. Glass lined carbon steel and Hastelloy C are similar with stainless steel a factor 1. Unbaffled jackets result in laminar flow and result in very poor therma l performance which is enhanced by the use of baffles, dimpled jackets and inlet mixing nozzles.

HOW TO CALCULATE THE HEAT TRANSFER AREA OF BATCH REACTOR

When using heat transfer fluids that may have high viscosities within the operating temperature range, mixing nozzle pressure drops may become limiting and ha lf pipe coil constructions should be considered. The lags associated with the utility side using thermal fluids are minimised by using forced circulation and plate heat exchangers for fast response. For a given set of process conditions the reactor UA is predetermined and it is important that the external heat exchanger UA does not become limiting at approach temperature differences.


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Dynamic modelling confirms the suitability of the design under all conditions. The changeover to cooling water based fluid systems requires a control sequence.

CHEP8024 - Chemical Safety Applications

Direct steam injected circulating liquid systems avoid the thermal lags associated with external heat exchangers and hence have superior heat input characteristics. The reduced heat input associated with single fluid systems can lead to difficulties in achieving required boil up rates when doing batch distillations. Single fluid systems using heat transfer fluids and external heat exchangers have the lowest heat input capability for a given temperature difference of the systems considered.

The heat input is achieved by increasing the temperature difference at a rate consistent with thermal shock considerations. Heat Removal Heat removal is normally achieved using single fluid liquid systems which will determine the UA achievable for a given reactor system.

The use of partially filled reactors is to be avoided if possible. The heat removal is therefore determined by the operating temperature difference which can be enhanced by operating at higher reaction temperatures or under reflux conditions consistent with reaction kinetics. The heat removal capability of a facility is limited by the temperature of the utility fluids available. The relief valve RV is a type of valve used to control or limit the pressure in a system or vessel which can build up for a process upset, instrument or equipment failure, or fire.

The pressure is relieved by allowing the pressurised fluid to flow from an auxiliary passage out of the system. The relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. When the set pressure is exceeded, the relief valve becomes the " path of least resistance " as the valve is forced open and a portion of the fluid is diverted through the auxiliary route.

Workbook for Chemical Reactor Relief System Sizing (Research Reports)

The diverted fluid liquid, gas or liquid—gas mixture is usually routed through a piping system known as a flare header or relief header to a central, elevated gas flare where it is usually burned and the resulting combustion gases are released to the atmosphere. Once it reaches the valve's reseating pressure, the valve will close.

The blowdown is usually stated as a percentage of set pressure and refers to how much the pressure needs to drop before the valve reseats. In high-pressure gas systems, it is recommended that the outlet of the relief valve is in the open air.

In systems where the outlet is connected to piping, the opening of a relief valve will give a pressure build up in the piping system downstream of the relief valve. This often means that the relief valve will not re-seat once the set pressure is reached. For these systems often so called "differential" relief valves are used. This means that the pressure is only working on an area that is much smaller than the openings area of the valve.

If the valve is opened the pressure has to decrease enormously before the valve closes and also the outlet pressure of the valve can easily keep the valve open. Another consideration is that if other relief valves are connected to the outlet pipe system, they may open as the pressure in exhaust pipe system increases. This may cause undesired operation. In some cases, a so-called bypass valve acts as a relief valve by being used to return all or part of the fluid discharged by a pump or gas compressor back to either a storage reservoir or the inlet of the pump or gas compressor.

Formed in , the Design Institute for Emergency Relief Systems [4] was a consortium of 29 companies under the auspices of the American Institute of Chemical Engineers AIChE that developed methods for the design of emergency relief systems to handle runaway reactions. Its purpose was to develop the technology and methods needed for sizing pressure relief systems for chemical reactors, particularly those in which exothermic reactions are carried out. Such reactions include many classes of industrially important processes including polymerizations, nitrations, diazotizations, sulphonations, epoxidations, aminations, esterifications, neutralizations and many others.

For chemical reactions, it requires extensive knowledge of both chemical reaction hazards and fluid flow. DIERS became a user's group in The EDUG started in the late s and has an annual meeting. From Wikipedia, the free encyclopedia.

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Workbook for Chemical Reactor Relief System Sizing

Fundamentals Of Stack Gas Dispersion 4th ed. Retrieved Fisher; H. Forrest; Stanley S. Grossel; J. Huff; A.


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