There are several automatic shutdown process interlocks in the design of this unit to protect the catalyst and the equipment during an incident or failure. Refer to the Process Design Book for the shutdown and safety interlocks description and the P&ID'S.
8.7.1. Hydrogenation reaction section
If the naphtha feed flow rate to the hydrogenation reaction section or if the recycle flow rate reduce the feed rate at the inlet of the reactor to 70% of design rate (or less), the low flow switch will automatically stop:
- naphtha feed,
- hydrogen,
- steam of the hydrogenation reactor feed heater,
- stop the feed to the isomerization section,
- close the bypass of the recycle air cooler.
This protects the catalyst from very low space velocity and potential hydrocracking leading to a temperature runaway.
In case of high high temperature in the hydrogenation reactor, the same procedure is followed.
8.7.2. Isomerization reaction section
If the naphtha feed flowrate to the isomerization reaction section reduces to 70% of design rate (or less), the low flow switch will automatically stop the feed to the section. If no problem is observed on the hydrogenation reaction section, the feed will bypass the two reactors to the stabilizer.
This protects the catalyst from very low space velocity and potential hydrocracking leading to a temperature runaway.
8.7.3. Stabilizer Reflux Drum High Liquid Level
The PSV protecting the stabilizer circuit is located on top of the Stabilizer Reflux Drum discharging to the Caustic Scrubber. The release of hydrocarbon liquid to the caustic scrubber causes foaming in the scrubber, and hence an HLL switch is added to the Stabilizer Reflux Drum to shut down the unit.
8.7.4. Regenerant Superheater Shutdown Activated by the following:
- a high liquid level in the Regenerant Vaporizer,
- a high coil skin temperature in the electrical Superheater,
- low regenerant feed flow.
8.7.5. Emergency Shutdown Hand Switch
This is not an automatic shutdown but a board mounted hand switch to be used at the discretion of the operator. Should an emergency situation occur such as a temperature
runaway in the reactors or warning of a potential slug of water in the feed, the operator can activate this switch which will:
- Shut off the fresh feed to the hydrogenation reactor.
- Shut off the recycle feed to the reactor.
- Stop the chloride injection pump.
- Shut off the hydrogen make-up.
- Shut off the isomerate to storage.
- Reroute scrubber off-gas to flare.
- Bypass LPG recovery section.
- Depressurize at a controlled rate the reaction section to the stabilizer. - Depressurize at a controlled rate the stabilizer to the scrubber.
At no time should reactor be allowed to set with liquid hydrocarbon inside and no flow. Under these conditions with no flow across, the thermocouples temperature measurement would be false and the temperature runaway could be occurring with no indication. Once depressurized reactor should be swept to remove any liquid hydrocarbon using cold hydrogen make-up or nitrogen.
8.7.6. LPG Recovery section bypass hand switch
The operator, for a particular emergency situation, could decide to activate the hand switch which will permit the bypass of the LPG recovery section on off gas side (UV 2501 closed and UV 2503 open).
It may occur during chloride breakthrough, water breakthrough, loss of make-up hydrogen, high reactor temperature, steam failure, cooling failure, power failure and major leak or fire.
SECTION-9 PROCESS VARIABLE
Process variables that affect the isomerization process are discussed below.
9.1 BENZENE SATURATION
There are six process variables which have an effect on the reactor performance: - Reactor temperature.
- Hydrogen make-up flow - Liquid diluents recycle - Space velocity.
- Feed composition 9.1.1 Reactor Temperature:
The high activity of the catalyst allows operation at moderate reactor inlet temperatures (100 to 140°C). Typical heat of reaction for the benzene hydrogenation reaction is 50,000 kcal/kmol.
9.1.2 Reactor Pressure:
A moderate pressure is sufficient to obtain the benzene hydrogenation. The pressure recommended in the heat balance leads to an almost liquid phase at the reactor outlet. 9.1.3 Hydrogen make-up flow
Hydrogen in small excess (5 mole%) is mixed with dry feed under flow control. Operator will adjust hydrogen flow set point according to amount of benzene measured by on-line analyzer.
9.1.4 Liquid diluents recycle
The exothermicity in the reactor is linked to the benzene content in the feed. With 7.5%
wt of benzene, the reactor delta T is estimated to be about 75-80oC. The control of
exotherm in then reactor, in smooth and safe conditions, is realised with the recycled hydrogenation reactor effluent, used as inert diluents recycle. The heat and material balances show the recommended recycle flow for the maximum benzene content in the reactor feed to maintain an acceptable delta T in the reactor. Temperature gradient should always be limited to 30oC.
9.1.5 Space Velocity:
Liquid Hourly Space Velocity (LHSV) is defined as the ratio of the hourly flowrate of liquid feed in m3 to the volume of catalyst in m3, i.e.
) ( ) / ( 15 3 3 m volume catalyst Total h m C @ rate feed Hourly LHSV 9.1.6 Feed Composition:
Feed composition is an indirect variable, a variable the operator reacts to, rather than adjusts for performance control. If the benzene content of the feedstock varies , the operator will have to:
- Adjust the hydrogen makeup to ensure excess of 5% vol.
- Modify the recycle flowrate in order to have an acceptable gradient of temperature in the
hydrogenation reactor (no more than 30 oC elevation
9.2 ISOMERIZATION
There are six primary process variables which have a direct effect on the reactor’s performances:
- Reactor temperature. - Space velocity.
- Hydrogen / hydrocarbon ratio. - Feed composition.
- Reactor outlet pressure - Chloride Injection. 9.2.1 Reactor Temperature:
The high activity of the catalyst allows operating at moderate temperature (125-165 oC).
The inlet temperature to the reactors is the most direct variable the operator has available to control product quality. Higher temperatures enhance catalyst activity and approach to equilibrium, but it decreases the equilibrium amount of isomer in the reactor effluent. Higher temperatures also increase hydrocracking reactions, and consequently the coke deposits on the catalyst. An optimum temperature is determined which balances the potential slight exotherm, the approach to equilibrium and the equilibrium level to maximize the performance of the isomerization reactor. This optimum temperature varies with feedstock variations, catalyst activity levels and space velocities without forgetting that it must be adequate with the benzene saturation constraints.
9.2.2 Space Velocity:
Weight Hourly Space Velocity (WHSV) is defined as the ratio of the hourly flowrate of liquid feed in kg/h to the weight of catalyst, i.e.
WHSV = totalcatalyst weight (kg)
(kg/h) rate feed Hourly
This parameter is fixed by the feed rate.
The quantity of catalyst being a constant, the space velocity can only be changed by varying the liquid feed rate.
Decreasing the feed rate decreases the space velocity. At constant temperature this increases activity and therefore the approach to equilibrium. The operator can therefore reduce the reactor inlet temperature while maintaining catalyst optimum performance. 9.2.3 Hydrogen to Hydrocarbon Ratio:
This ratio is defined as the number of moles of hydrogen at the reactor outlet per mole of C5+ liquid feed passing over the catalyst. The design ratio for this unit is 0.06, to supply
saturate the benzene. Also, it avoids carbon deposition on the catalyst with sufficient hydrogen to saturate the hydrocracked components.
This ratio must at all times remain above 0.06. The operator should check this regularly and pay particular attention during feed changes and end of life conditions in the reformer. If necessary, reduce the reactor feed rate to maintain the ratio.
Hydrogen makeup is under ratio flow control according to the amount of the feed coming from separator drum and going to the isomerization section.
9.2.4 Feed Composition:
Feed composition is an indirect variable, a variable the operator reacts to rather than adjusts for performance control. As the feedstock compositions changes from processing different crudes, the operator must make the necessary temperature adjustments to maintain unit performance.
For example, when the cyclic content is high in the feed, the first stage isom reactor inlet temperature is increased to increase the weight average bed temperature. This increases ring opening in the first reactor and reduces the inhibition effect the cyclics have on the isomerization reactions in the second stage reactor.
9.2.5 Reactor outlet pressure :
At the outlet of the second reactor, pressure shall be above the minimum of 30 kg/cm2
9.2.6 Chloride Injection:
The chlorinated platinum catalyst loses a small amount of chloride continuously to the reactor effluent, and this is made-up by the continuous injection of a chloride agent to the isomerization reactors. Tetrachloroethylene (C2Cl4) not stabilized is the
recommended Chloriding agent. The isomerization reactors should not be operated for more than six hours without chloride injection, as the isomerization reactions slow down significantly with chloride deficiency.
ISOPENTANE/ISO + NORMAL PENTANE EQUILIBRIUM PLOT M O LE % IS O P E N TA N E /I S O + n -P E N TA N E 100 80 60 40 20 0 TEMPERATURE °C 100 150 200 250 300 Figure 2 iC5/C5 Paraffin equilibrium ratio
TEMPERATURE °C IC 5 /C 5 P 65 67 69 71 73 75 77 79 81 83 85 60 80 100 120 140 160 180 200
Vapor phase equilibrium Liquid phase equilibrium
Figure 3 C6 Fraction equilibrium Plot
TEMPERATURE °C 0 20 40 60 100 80 100 150 200 250 300 MCP/MCP + CH 2-MP+3MP/ C6 PARAFFINS n-HEXANE/ C6 PARAFFINS 2,2 DMB/ C6 PARAFFINS 2,3 DMB/ C6 PARAFFINS Figure 4 2.2 DMB / C6 equilibrium ratio TEMPERATURE °C 2 2 D M B /C 6 P 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 60 80 100 120 140 160 180 200
SECTION-10 TROUBLE SHOOTING
The main concern for the isomerization unit operation is water breakthrough through a malfunction of the dryers or major water upset in the upstream hydrotreater.
Two other concerns are:
- Sulfur poisoning which is reversible and poor de-nitrification in the upstream units
leading to chloride stripping off the catalyst.
- Chloride breakthrough in isomerate/LPG products.
The operator must be knowledgeable and experienced enough to make the proper corrective actions immediately and to know if he should cut feed to the unit before any catalyst activity loss occurs. The ultimate safe rule is to cut feed from the unit if a breakthrough is suspected and follow carefully the necessary procedures for total unit shutdown. Remember this unit produces a gasoline component for blending; it does not produce any feedstock for other units and can therefore be shutdown with relatively little disruption to the other refinery operations. The upset seriousness and the contaminant breakthrough level will dictate the necessary course of action to take.
In this chapter we will highlight some of the potential problems which the operator may have to face.