PROCESS DESCRIPTION
3.4 PROCESS VARIABLES:
In the normal operation of a Penex Unit, having once set the operating pressure fresh feed rate and makeup hydrogen flows, it is usually only necessary to adjust the reactor inlet temperatures.
Once the catalyst has been loaded into the unit, the manner in which the catalyst it placed in seance and the treatment it receives when in service will to a large extent influence its effectiveness for making quality product as well as the length of service
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it will give. In making any changes to the operation, the welfare of the catalyst must be given prime consideration for it can be regarded as the heart of the operation or which the quality of the results obtained will depend.
3.4.1 Reactor Temperature:
In general, reactor temperature is the main process control. A definite upper limit exists for the amount of iso-paraffins which can exist in the reactor product at any given outlet temperature, as shown in Figures 3.3, 3.4 and 3.5. This is the equilibrium imposed by thermodynamics, and can be reached only after an infinity length of time, i.e. with an infinitely large reactor. In practice, therefore, the product will contain less than this equilibrium concentration of iso-paraffins. As reactor temperature is raised to increase the rate of isomerization, the equilibrium composition will be approached more closely. At excessively high temperatures, the concentration of iso-paraffins in the product will actually decrease because of the downward shift in equilibrium curve, even though the high temperature gives a high reaction rate.
The use of temperatures higher than necessary to achieve a reasonably close approach to equilibrium accomplishes nothing other than to increase the amount of hydrocracking. Extremely high temperatures may lead to an increased rate of carbon laydown on the catalyst; however, the carbon forming propensity of the catalyst is inherently so low that excessive hydrocracking would normally be encountered before carbon formation problems would develop. It is recommended, however, that UOP be consulted before temperatures above about 204°C (400°F) are employed.
A typical C5/C6 Penex Unit has two series reactors with provision for independent temperature control. The first reactor system effects the bulk of the isomerization so long as most of the catalyst therein is still active. Any benzene in the feed it hydrogenated in the first reactor, even when the catalyst therein has lost its activity with respect to paraffin isomerization. Some conversion, ring opening, of cyclohexane and methyl Cyclopentane to hexanes also occurs, as does some hydrocracking of C7 to C3 and C4. These three reactions (benzene hydrogenation, naphthene ring opening to hexane, and C7 hydrocracking) are exothermic and, for a typical feed stock contribute more to the temperature rise in the first reactor that does paraffin isomerization, which is also exothermic.
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Normally, the first reactor system will be operated at such a temperature as to maximize the concentration of isopentane and 2,2 dimethyl butane in its effluent. The concentrations attainable and the required outlet temperature will be influenced by the amount of active catalyst present and by the amount of C6 cyclic and C7 components present in the feed, higher the temperatures being required with high concentrations of these components in the feed. By this procedure, the required operating temperature on the second reactor system is reduced and it is possible to operate under conditions where the equilibrium is more favorable.
Figure 3.3 Iso-pentane equilibrium curve
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Figure 3.4 2-2Dimethyl butane Equilibrium curve
Figure 3.5 Equilibrium Curve
34 3.4.2 Liquid Hourly Space Velocity:
This term, commonly shortened to LHSV, is defined as the volumetric hourly flow of reactor charge divided by the volume of catalyst contained in the reactors in consistent units. The design LHSV for C5/C6 Penex operation is normally between 1 and 2. Increasing the LHSV beyond this will lead to lower product isomer ratios.
In order to avoid excessive reactor severity, the Penex reactor LHSV should always be maintained above 0.5 hr-1 overall or 1.0 LHSV minimum per reactor.
3.4.3 Hydrogen to Hydrocarbon Mole Ratio:
This ratio is defined as the number of moles hydrogen at the reactor outlet per mole of reactor charge passing over the catalyst, and is specified at 0.05 moles H2/mole HCBN. The primary purpose of maintaining the ratio at or above the design is to avoid carbon deposition on the catalyst and maintain enough H2 for the reactions to proceed. If necessary, the reactor charge rate is to be reduced to maintain the design hydrogen to hydrocarbon ratio. The H2/HCBN ratio is determined by measuring the total mole of hydrogen in the stabilizer overhead gas and dividing by the total moles of fresh feed feedstock.
3.4.4 Pressure:
C5/C6 Penex Units are normally designed to operate at 31.6 kg/cm2 gauge (450 psig) at the reactor outlet. Methylcyclopentane and cyclohexane appear to adsorb on the catalyst and reduce the rate of isomerization reactions. Higher pressure helps to offset this effect of the C6 cyclic compounds. Lowering the unit pressure or operating at a slightly lower level would not affect the catalyst life but the extent of isomerization decrease (although not necessarily instantaneously), other things being equal.
Restarting the injection of chloride will tend to return the activity of the catalyst to its previous level, but it is possible that full activity will not be restored if a decline in