Improving Oil Derivates Quality and Refining Margin – Catalytic Reforming Technologies
The continuous technological development, verified in the development of machines and equipment which need operates with higher quality fuels and lubricants ally to increasingly severe environmental regulations represents a great challenge to oil refining industry and process technology developers. Process improvements or new technologies capable of producing higher quality derivates with low contaminants content pass from competitive advantage to survival question in the market to the refiners.
One of the most consumed oil derivates in the global market is the gasoline. This derivate is, normally, produced through the mixture of process streams in the distillation range of naphtha, the streams normally involved in the gasoline production process are straight run naphtha, cracked naphtha, coke naphtha (after hydrotreatment) and reformate naphtha.
The reformate is produced in a processing unit called Catalytic Reforming. The main objective of the Catalytic Reforming unit is to produce a stream with high aromatics hydrocarbons content that can be directed to the gasoline pool or to produce petrochemical intermediates (benzene, toluene, and xylenes) according to the market served by the refiner, due the high content of aromatics compounds the reformate can raise significantly the octane number in the gasoline.
A typical feedstock to the catalytic reforming unit is the straight run naphtha, however, in the last decades due to the necessity to increasing the refining margin through the installation of bottom barrel units, hydrotreated coke naphtha stream have been consumed like feedstock in the catalytic reforming unit.
The catalyst generally employed in the catalytic reforming process is based on platinum (Pt) supported on alumina treated with chlorinated compounds to raise the support acidity. This catalyst has bifunctional characteristics once the alumina acid sites are actives to molecular restructuring and the metals sites are responsible for hydrogenation and dehydrogenation reactions.
The main chemical reactions involved in the catalytic reforming process are:
· Naphthene Compounds dehydrogenation;
· Paraffins Isomerization;
· Isomerization of Naphthene Compounds;
· Paraffins Dehydrocyclization;
Among the undesired reactions can be cited hydrocracking reactions and dealkylation of aromatics compounds.
Figure 1 present a basic process flow diagram for a typical semi-regenerative catalytic reforming unit.
Figure 1 – Typical arrangement to Semi-regenerative Catalytic Reforming Process Unit
The naphtha feed stream is blended with recycle hydrogen and heated at a temperature varying 500 to 550 oC before to enter in the first reactor, as the reactions are strongly endothermic the temperature fall quickly, so the mixture is heated and sent to the second reactor and so on. The effluent from the last reactor is sent to a separation drum where the phases liquid and gaseous are separated.
The gaseous stream with high hydrogen content is shared in two process streams, a part is recycled to the process to keep the ratio H2/Feed stream the other part is sent to a gas purification process plant (normally a Pressure Swing Adsorption unit) to raise the purity of the hydrogen that will be exported to others process plants in the refinery.
The liquid fraction obtained in the separation drum is pumped to a distillation column wherein the bottom is produced the reformate and in the top drum of the column is produced LPG and fuel gas.
The reformate have a high aromatics content and, according to the market supplied by the refinery, can be directed to the gasoline pool like a booster of octane number or, when the refinery has aromatics extraction plants is possible to produce benzene, toluene and xylenes in segregated streams, which can be directed to petrochemical process plants. The gas rich in hydrogen produced in the catalytic reforming unit is an important utility for the refinery, mainly when there is a deficit between the hydrogen production capacity and the hydrotreating installed capacity in the refinery, in some cases the catalytic reforming unit is operated with the principal objective to produce hydrogen.
The main process variables in the catalytic reforming process unit are pressure (3,5 – 30 bar), which normally is defined in the design step, in other words, the pressure normally is not an operational variable. The temperature can vary from 500 to 550 oC, the space velocity can be varied through feed stream flow rate control and the ratio H2/Feed stream that have a direct relation with the quantity of coke deposited on the catalyst during the process. To semi-regenerative units, the ratio H2/Feed stream can vary from 8 to 10, in units with continuous catalyst regeneration this variable can be significantly reduced.
Due to the process severity, the high coke deposition rate on the catalyst and consequently the quick deactivation leaves to short operational campaign periods to semi-regenerative units that employ fixed bed reactors.
To solve this problem some technology licensors developed catalytic reforming process with continuous catalyst regeneration steps.
The process Aromizing® developed by AXENS company apply side by side configurations to the reactors while the CCR Platforming® developed by UOP apply the configuration of stacked reactors to catalytic reforming process with continuous catalyst regeneration. Both technologies are commercial and some process plants with these technologies are in operation around the world.
Figure 2 presents a basic process flow diagram to a catalytic reforming unit with continuous catalyst regeneration step in stacked reactors configuration.
Figure 2 – Basic Process Scheme to Catalytic Reforming Units with continuous catalyst regeneration (Stacked Reactors Configuration)
In the regeneration section, the catalyst is submitted to processes to burn the coke deposited during the reactions and treated with chlorinated compounds to reactivate the acid function of the catalyst.
Despite the higher capital investment, the rise in the operational campaign and higher flexibility in relation of the feedstock to be processed in the processing unit can compensate the higher investment in relation to the semi-regenerative process.
The catalytic reforming technology gives a great flexibility to the refiners in the gasoline production process, however, in the last decades, there is a strong restriction on the use of reformate in the gasoline due to the control of benzene content in this derivate (due to the carcinogenic characteristics of this compound). This fact has been reduced the application of reformate in the gasoline formulation in some countries. Furthermore, the operational costs are high, mainly due to the catalyst replacement and additional security requirements linked to minimize leaks of aromatics compounds.
Face to the limitation of the aromatics content in the gasoline, mainly the benzene, the refiners has used alkylation or isomerization to produce streams capable of improving gasoline octane number in detriment of reformate naphtha.
Like aforementioned, some refiners have aromatics extraction plants in his refining scheme, in this case, the production can be directed to produce benzene, toluene, and xylenes as intermediates products to petrochemical industries, despite higher capital and operational investments this configuration can be economically attractive, these products have commercialization prices higher than gasoline and this fact can be potentialized in scenarios like saturation of gasoline market.
Reference:
MYERS, R.A. Handbook of Petroleum Refining Processes. 3a ed. McGraw-Hill, 2004.
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