The light naphtha isomerization process is more complex than the description given in Example 4.7. 

1. Hydrogen is flowed through the plant to reduce catalyst deactivation. The hydrogen flow rate is typically about 2 moles per mole of hydrocarbon on a pure hydrogen basis. The hydrogen make-up gas is typically about 90 mol% hydrogen, with the balance methane. 

2. Light hydrocarbon compounds are formed by cracking reactions. These compounds accumulate in the hydrogen recycle and are controlled by taking a purge stream. A stabilizer column is also required, upstream of the distillation column, to remove light hydrocarbons and hydrogen before the distillation. 

3. Each of the C6 isomers has a different blending octane value. The total octane value of the product can be found by summing the products of the mole fraction of each component and the component blending value. The blending values are: n-hexane 60; 2-methyl pentane 78.5; 3-methyl pentane 79.5; 2,2-dinethyl butane 86.3; 2,3-dimethyl butane 93. 

Optimize the design of Example 4.7, subject to the following: 

1. The selectivity loss due to cracking reactions can be approximated as 1% conversion of C6 compounds to propane per reactor pass. 

2. The wholesale value of gasoline can be assumed to be 2.0 + 0.05 (octane number – 87) $/US gal. 

3. The cost of hydrogen is $6/1000 scf, and the fuel value of the hydrogen and propane purge stream is $5/MMBtu. 

4. The reactor plus catalyst total installed cost can be taken as $0.5 MM per 1000 bpd of liquids processed. 

5. Other costs can be estimated using the cost correlations given in Section 6.3. Additional flow-sheeting problems are given in the form of design projects in Appendices E and F.

Data from example 4.7

Light naphtha is a mixture produced by distillation of crude oil. Light naphtha primarily contains alkane compounds (paraffins) and it can be blended into gasoline. The octane value of methyl-substituted alkanes (iso-paraffins) is higher than that of straight-chain compounds (normal paraffins), so it is often advantageous to isomerize the light naphtha to increase the proportion of branched compounds.

A simple naphtha isomerization process has a feed of 10,000 barrels per day (bpd) of a 50 wt% mixture of n-hexane and methyl pentane. The feed is heated and sent to a reactor where it is brought to equilibrium at 1300 kPa and 250 °C. The reactor products are cooled to the dew point and fed to a distillation column operated at 300 kPa. The bottoms product of the distillation is rich in n-hexane and is recycled to the reactor feed. An overall conversion of n-hexane of 95% is achieved. Simulate the process to determine the recycle flow rate and composition.