Gasoline is a major product that refineries deliver to the market. The yield of gasoline originated directly from crude oils is insufficient answer to the demands of required volumes.
Crude distillation units produce heavy distillates, such as straight-run atmospheric gas oils, vacuum gas oils, and certain atmospheric residues in quantities that depend on the composition of the crude oil. Predominately enhanced consumption of heavy crude oils is the source of increased quantities of HVGO to be produced.
Heavy distillates are chemically composed of long paraffinic molecules, which are a very limited source for further application. However, these heavy molecules are susceptible to chemical catalytic cracking processes, which break up long molecular chains into small molecules that align chemically with naphtha and are utilized to produce gasoline. Economic and global political situations dictate many refineries to be flexible for processing a variety of crude oils, preferably crude oils of low cost in the international market. Moreover, various refineries produce more heavy distillates than they consume to run their own FCC unit. Excessive heavy material is further merchandized to other refineries that have enough capacity to process more heavy stocks in the FCC than produced by their own crude distillation units.
The fact that the FCC transforms low value products into a major high valued gasoline component makes the FCC one of the most important and most profitable units in any refinery.
The FCC is an integrated combination of different chemical and physical processes:
- Catalytic chemical reaction – long molecules are cracked into small naphtha molecules. It is a high-energy consuming endothermic reaction.
- Catalyst recycling – combination of chemical and physical processes to recover the spent catalyst for further use.
- Physical process, which separates catalytic reaction products into naphtha, light and heavy cycle oils.
It is up to the FCC unit to provide highest achievable quantities of light naphtha per ton of heavy feed. Processing of heavy feed stocks of different origin and of different chemical compositions requires adjustment of the process conditions to the composition of the consumed feed. Both the reaction process and the distillation process must each run in an efficient mode, to meet optimized performance of the overall FCC unit.
FCC optimization is a complicated task, since it involves many different variable process parameters, in different process units, where each of them influences the performance of the entire unit. Optimization of the catalytic reaction to control the following parameters:
- Reactor temperature
- Feed preheat temperature
- Catalyst circulation rate
- Catalyst activity
- Catalyst recycling rate
Optimization of the distillation unit includes:
- Cutting point shifting towards maximum naphtha yield
- Feed temperature.
- Reflux Ratio.
- Synchronization of the reactor production rate and the distillation unit feed rate.
Its main target of the FCC is the production of highest achievable volumes of naphtha for the production of gasoline. It is of highest priority to effectively operate FCC unit, full control of each feed an product streams. Optical spectroscopic based process analyses, such as the NIR process analyzers, are restricted to dry and transparent process streams, and are therefore in general only applicable to measure the quality of the naphtha distillate. However, measuring only one single stream is not adequate enough to control the chemical reaction and the distillation process alike. The primer condition for optimizing the FCC is to correlate between the physical properties of the HVGO feed stream, and the resulting product stream leaving the reactor. Process parameters, such as the feed temperature, the reaction temperature and the catalyst recycling, can only effectively be adjusted, when based on real-time physical properties of each process stream. NMR process analyzers have the feasibility to quantify physical properties in transparent and opaque process streams alike. It provides an excellent tool to present immediate feed-back about the behavior of process adjusting actions taken. This enables the operator to take the right actions, and to minimize damage in case of imbalance of the process. Chemical yields and production capacities of naphtha are maximized on the account of heavier products. The same concept is also applicable to the distillation device. By measuring feed and distillate streams, cut points between naphtha and light cycle oil can be fine tuned as such that the mainly naphtha distills.
Since gasoline is still a major product the refinery, the incorporation of NMR process analyzers to have power over the FCC will definitely contribute to a more economic management of the refinery.
The benefit of the NMR process analyzers is its feasibility to utilize the same analyzer for measuring each process stream, and to correlate between them. Major physical properties that are measurable by the NMR process analyzer include:
Distillation, API Gravity, Water, Viscosity, Sulfur, Aromaticity, Concarbons, Basic Nitrogen, Asphaltenes, Refractive Index, Parafins, Naphtalenes, Carbon Aromaticity, Carbon Naphthenicity and Carbon Parafinicity .
Without any doubt, combined optimization of both integrated FCC production units, increases, its production capacity. It definitely contributes to a more efficient utilization of the catalyst. By measuring the physical properties in FCC process streams, process conditions can be fine-tuned to optimize the hold-up time of the catalyst in the reactor, as well as in the regenerator, without losing its catalytic cracking capacity. It is the fundament to optimize the utilizing of the catalyst to its highest extent. This is especially of great influence when switching between different heavy feeds. On-line monitoring of all process streams by MOD 8000 NMR process analyzers enables to be aware of physical properties in each individual FCC process steams to achieve highest possible process yields and process capacity of naphtha. This also directly reduces the cost of energy and catalyst, and by that the production cost of naphtha, and the overall production cost of gasoline.