Reforming and Isomerization
- The catalytic cracking involves enhancing the octane number of the product
- Heavy naphthas are used are typical feed stocks
- The reaction is carried out on a catalyst
- Reforming reaction produces hydrogen as a by product which is used elsewhere in the refinery
- Usually Platinum supported on porous alumina is used as a catalyst
- Catalyst activity enhanced using chloride
7.2 Cracking Chemistry
- Paraffin isomerisation takes place
- Naphthene isomerisation also takes place to produce cycloalkanes
- Cyclo alkanes undergo dehydrogenation to generate aromatics
- Dehydrocyclization takes places to convert side chained alkanes to cyclo alkanes and hydrogen
- In summary lower octane number feeds are converted to high octane products
- The reformate thus produced has high octane and aromatics (benzene, toluene and xylene) content.
- The reactions are endothermic. Therefore, heat needs to be supplied
7.3 Process technology (Figure 7.1)
- The feed is mixed with recycled hydrogen
- Subsequently, it is heated before sending to reactor
- Since the reactions are highly endothermic, several combinations of reactor + heaters are used.
- The products from the final reactor are cooled. Often this is carried out with heat recovery principle in which heat is recovered using the fresh feed to the first reactor.
- After this, the product mixture enters a phase separator which separates the hydrogen gas stream from the liquid stream.
- The liquid stream from the phase separator is sent to a debutanizer distillation column that separates butanes and lower alkanes from the reformate product.
- The hydrogen produced in the phase separator is compressed and sent back to the first reactor.
- Excess hydrogen generated in the reactions is taken out as a bleed stream
- Catalyst regeneration (not shown in the flow sheet) needs to be carried out to regain catalyst activity. This can be in different modes of operation namely cyclic, semi-regenerative or continuous. When continuous mode of catalyst regeneration is carried out (as in UOP continuous catalytic reforming process), the moving bed designs are used for the reforming reactor. Additional complexity in the moving bed reactors is to enable process intensification to club the sequence of three reactors + heaters into one single unit.
7.4 Process parameters
- Reactor pressure: 4 – 24 barg
- Reactor temperature: 500 – 525 oC
- H2/Hydrocarbon molar ratio: 2 – 3
Figure 7.2 Flow sheet of Isomerization of n-paraffin
- The basic principle of Isomerization is to straight chain alkanes to side chain paraffins. This enhances the octane number substantially
- For instance, n-pentane has an octane number of 61.7 where asiso-pentane has an octane number of 92.3
- Usually light naphtha is used as a feed stock
- Platinum base catalysts are used
- AlCl3 is used as a promoter for the catalyst
- During reaction, part of the AlCl3 gets converted to HCl
- Therefore, completely dry conditions shall be maintained to avoid catalyst deactivation and corrosion.
- Catalytic reaction takes place in the presence of hydrogen to suppress coke formation
7.7 Process technology (Figure 7.2)
- Light naphtha and hydrogen (make up) are totally dried and sent to an isomerisation reactor after pre-heating the feed mixture in a heat exchanger
- Reaction operating conditions: 150 – 200 oC and 17 – 28 barg
- Typical conversions: 75 – 80 % for pentanes.
- After reaction, AlCl3 is recovered from the product using condensation or distillation
- The basic principle for AlCl3 recovery is that at the reactor operating conditions, the AlCl3 is in volatile conditions and is soluble in hydrocarbons
- After AlCl3 is recovered from the product, it is sent back to the reactor along with the make- up AlCl3
- Eventually, the product enters a flash drum where bulkly light ends along with little quantities of HCl are separated from the liquid product.
- The light ends recovered from the flash drum are sent to aHCl absorber where HCl is absorbed into caustic solution to generate the light end gases. The light end gases can be further used for other processes in the refinery.
- The bottom product then enters aHCl stripper where most of the HCl is stripped from the isomerisation product rich stream. The HCl is recycled back to the reactor to ensure good catalyst activity. Make-up HCl is added to account for losses
- Subsequently, caustic wash is carried out to remove any trace quantities of HCl
- The isoermized product rich stream is then sent to a fractionators that separates the isomerized product from the unreacted feed.
- The unreacted feed from the fractionators is sent back to the reactor.
7.8 Technical questions
1. Why is hydrogen used in the reforming reaction?
Ans: Hydrogen reduces the coke formation on the catalyst and therefore increases the shell life of the highly expensive platinum catalyst.
2. Why is reforming a very important process in the refinery?
Ans: The reforming is one of the most important operation in the refinery to enhance product quality. Further, the total hydrogen requirement in the refinery for various hydrotreaters is produced in the reforming process. Therefore, the reformer contributes to both most important requirements of the refinery.
3. Looking at the process flow-sheet of the reforming and isomerisation process, what insights you gain towards the flowsheet evolution?
Ans: Both reforming and isomerisation flow sheets are typical examples of reactor-separator-recycle systems that are extensively studied in process synthesis and evaluation studies. These flow sheets are candidate examples to illustrate the process development required for other non-conventional and novel technologies such as bio-fuel processing etc.
4. What research contributions can be provided by chemical engineers to both reforming and isomerisation reactions?
Ans: Both reforming and isomerisation reactions are centrally built upon the catalyst. Therefore, catalyst engineering is the most important area where research contribution by chemical engineers will be of paramount significance. In fact, process modifications of the entire petroleum refinery are dictated by catalyst engineering and energy integration research.
5. Why is make up AlCl3 required in the isomerisation process?
Ans: A part of the AlCl3 gets converted to the HCl and therefore gets lost. The make up AlCl3 therefore replaces the lost HCl in the process.
6. Comment upon the HCl distribution between light ends and bottom liquid product in the flash drum?
Ans: Most of the HCl distributes towards the liquid phase only. Little quantites of HCl enter the light end stream (gas phase). The distribution of HCl is dictated by the operating conditions of the flash drum.