Catalytic Reforming Processes
In 1940, The Universal Oil Products (UOP) introduced a catalytic reforming process by using catalyst which contain platinum for production of high octane naphtha from low octane naphtha, and the process became known as the Platforming according to Mayers (2004), Sutton et al (1973) and Dachos et al (1997). Furthermore, these authors point out that over the years other version of UOP original process have been developed by various oil companies but UOP process is still widely employed in the refineries. Over the years UOP process has been improved by introduction of bimetallic catalysts (Platinum and Rhenium on a silica or silica aluminium base) according to Mayers (2004). The greater activity and stability of this catalyst resulted in production of high octane naphtha on reduction of pressure. Bimetallic platinum-rhenium catalysts are known as R-56, which is a preferred catalyst to be used on naphtha platforming plants. The performance comparison of the UOP catalysts including R-56 is shown in Figure 2.1.
Sutton et al (1973) states that the UOP catalytic reforming technology has evolved from semi regenerative, fixed-bed processes to the more energy efficient, highly reliable, low operating cost, continuous catalyst reforming technology for naphtha reforming. Dachos et al (1997), described Stacked-type CCR process design, which is basically similar to UOP. The stack type CCR design can cope with longer regeneration time period associated with semi regenerative (SR) process. However, the short regeneration time period lead to a high rate of deactivation upon reducing the pressure which consequently give lower hydrogen (H2) to hydrocarbons (HCs) ratio as mentioned by Mayers (2004), Sutton et al (1973) and Dachos et al (1997) . The low H2 to HC ratio effectively leads to higher C5+ molecules and hydrogen according to Mayers (2004), Sutton et al (1973) and Dachos et al (1997).
The process selected in this project was very much influenced by above factors. The features of SR and CCR were incorporated in process system using same catalyst. Three catalyst beds were operated in series with same cycle length. Anotos et al (2004) has mentioned that if fourth reactor is added to the process then more catalyst would be required due to longer cycle length. However it is possible to improve the improvement CCR arrangement by addition of fourth reactor adjacent to other stacked reactors. And the system has a common catalyst bed that moves from top to bottom of reactor section.
According to Antos et al ( 2004 ), Mayers (2004), Sutton et al (1973), Dachos et al (1997) and Doolin et al ( 2001 ) UOP CCR Platforming Process considered to be the most successful reforming process available for the reforming of the naphtha because in the CCR Platforming unit, the regenerated catalyst is continuously replaced with spent catalyst of reactor.
Dachos et al (1997) described UOP platforming process which combines the concept of CCR system with SR system. This arrangement allows the fresh catalyst fed to the reactor while maintaining minimal recycle gas circulation rate and low pressure at the steady state operational conditions. In order to maintain a steady state reforming operation process uses stacked radial flow reactors and a CCR section at optimum process operating conditions according to Mayers (2004), Sutton et al (1973) and Dachos et al (1997).
A simplified schematic flow diagram of the CCR Platforming process is presented in Fig 2.2. The catalyst flows vertically down the stack due to gravity. The feed flows radially across the annular catalyst bed. The catalyst is continuously withdrawn from the last reactor and transferred to the regenerator. The catalyst which requires the regeneration flows down to the regenerator where the accumulated carbon is burned off from the catalyst surfaces. Regenerated catalyst with hydrogen is reintroduced into the top of stack reactor. By this way the quality of catalysts is kept as nearly fresh as possible. Due to separate reactor and regenerator sections allow each of them to operate at their own optimal conditions of operation.
Modelling and Designing of Catalytic Reforming by UOP (Importance)
The literature review suggests that The Universal Oil Products (UOP) has developed a number of Catalytic reforming Processes to upgrade or replace low octane naphtha to the high octane naphtha for the petrol blending.
SR units consist of three or four reactors generally loaded with bifunctional catalyst and operating in the temperature range of 460 0C to 540 0C whereas pressure range of 7 to 40 bars with hydrogen H2 to hydrocarbons (HCs) ratio of 2 to 3. However, high H2/hydrocarbons ratios may minimize coke deposition over the catalyst. The coke deposition on the catalyst surfaces decreases naphtha reforming activity and shortens the run length depending on operational severity. The reformer can be optimized for product yield, product quality and the catalyst cycle life time by changing the reactor inlet temperature. The values of reactor inlet temperatures are known from the reactor temperature profile.
Sowel (1998), points out that mathematical model sometimes do not match the plant which one is trying to simulate for optimum operational conditions. However, understanding the reasons can assist one in using the model to maximum the capabilities of the plant. The reasons why simulations do not match the plant characteristics fall into three main categories according to Sowel (1998):
* simulation effects or inherent errors
* sampling and analysis effects or measurement errors
* model misapplication effects
Sowel (1998) suggests that a model must predict behaviour not only within the reactor but in the auxiliary areas of the unit as well. It should also consider the complex nature of the process and the reaction that takes place during the process of reforming. According to Mayers (2004) UOP provides a detailed outline of the steps in catalytic reforming modelling, including the definition of the modeling objective, process identification, model selection, data collection and validation, and, finally, model calibration and verification. Beside that the Modelling of UOP process provides additional operational rules. The UOP model also allows to simulates how feed composition and operating conditions affect product compositions and yields.
Meyers (2004) suggested that most important part of a plant design concepts are as follow:
Feed Should Have
- Less Alkenes (olefins) because they are prone to coking of reactors.
- More Cycloalkanes (Naphthenes) because they are good for increasing the naphtha octane number.
- Less Alkanes (paraffinic) because they yield lower octane number naphtha.
- High hydrogen to hydrocarbon ratio because this results a decrease in coking severity.
- No nitrogen because nitrogen would produce ammonia and which will subsequently convert to ammonium chloride. This may decrease the chloride level in the reactor and cause scaling or physical deposits to accumulate.
- About 20 parts per million (ppm) of water, which is required for the acid in the reactor to become active in ionizing. Acids are not effective in ionizing without the presence of
some form of ion transfer (i.e. water or water vapour).
The catalyst activity is the most important consideration in naphtha reforming operations. If the catalyst activity is high say 1.0 for brand new or freshly generated catalyst, the time required for the conversion is significantly small, therefore, conversion of naphthenes to aromatics is very quick with lesser amount of catalyst or segment of reactor required. If the activity of the catalyst is low, there would be a lower conversion of naphthenes to aromatics.
The yield of reformer is directly linked with Pressure effect, temperature and Catalyst stability. However, there is no theoretical limitations for reactor pressure, although practical operating constraints have led to a historical range of operating pressures from 345 to 4830 kPa (50 to 700 psig) according to Mayers (2004), Sutton et al (1973), Dachos et al (1997) and Doolin et al (2001). By decreasing the reactor pressure hydrogen production increases along with octane number naphtha (reformate) yield. However, at low pressures coke builds up quickly on the Platforming catalyst at reaction conditions. The high coking of catalyst associated with lower operating pressures require continuous catalyst regeneration.
Temperature of the catalyst beds give the primary control of Product quality in the Platforming process. There are two ways to calculate the reactor temperature. First way is (WAIT) means Weighted Average Inlet Temperature, which can be obtained by summing the product of fraction of catalyst in each reactor. The result then multiplied with inlet temperature. Second way s (WABT) means Weighted Average Bed Temperature, which can be obtained by summing the product of each reactor, then result multiplied with average of its outlet and inlet temperature. Generally, it is referred to have WAIT calculations for temperature. For SR Platforming, WAIT ranges from 4900C to 5250C whereas for CCR Platforming, it is ranges from 5250C to 5400C.
For lean naphtha [i.e. high Alkanes (paraffin) content] requires the feed to be operated at a higher LHSV. Otherwise Cycloalkanes (naphthenes) would convert rapidly, whereas the Alkanes (paraffins) could crack if there residence in the reactor is prolonged. Furthermore, the temperature drop would be lower and the furnace firing requirements would not be that efficient. The yield is related to the residence of the feed. This important aspect made us adopt changes for the simulator.
The role of catalyst is to decrease the activation energy required for a particular reaction. The kinetics are usually similar but the temperature requirements, physical requirements (attrition, surface etc), and selectivity are used in determination of catalyst selection. In naphtha platforming processes catalyst mostly contains platinum or rhenium on a silica-aluminium base or some contain both platinum and rhenium as stated by to Antos et al (2004) , Mayers (2004), Sutton et al (1973), Dachos et al (1997) and Doolin et al (2001).
The rate equations for platforming reactions are as follow:
r1 = ηkp1(PE –PAPH3 /Kp1)
Kp1 = 9.87 exp (23.21-34750/1.8T)
r2 = ηkp2(PEPH –Pp/Kp2)
kp2 = 9.87 exp (35.98- 59600/1.8T)
r3 = ηkp3 Pp/P
kp3 = exp (42.97- 62300/1.8T)
r4 = ηkp4 PE/P
kp4 = exp (42.97- 62300/1.8T)
Here subscripts A, N, P and H means aromatics, naphthenes (cycloalkanes), paraffins (alkanes) and hydrogen respectively. P represents system pressure and p represents the partial pressure of the components. T is the absolute temperature in Kelvin.
The physical model of catalytic reforming radial flow is shown in Figure 2.3. Ro and Ri represent the outer and inner radii of catalyst bed. For an element dR on bed at (R, R+dR), the material stream for inlet is Ni and temperature T. then outlet stream is N+dNi and temperature T + dt. From the mass and enthalpy balance five different differential equations are derived which is described by Smith (1959) as:-
dNA / dR = 2 x π x R x L x ρC x (r1)
dNN / dR = 2 x π x R x L x ρC x (-r1 – r2 – r4)
dNP / dR = 2 x π x R x L x ρC x ( r2 – r3)
dNH / dR = 2 x π x R x L x ρC x [3 r1– r2-(n-3)/3 x r3 – (n/3) x r4 ]
dNT / dR = (2 x π x R x L x ρC) / Ʃ C pi Fi x -[(3 r1ΔH1 + r2 ΔH2 +(n-3)/3 x ΔH3 +(n/3) x ΔH4 ]
It is required to calculate first the rate by calculating partial pressure assuming ideal gas given to the system pressure and then calculate the rates at a particular temperature.
Dachos ,N., Kelly,A., Felch, D.,Reis,E. (1997) UOP platforming process. Handbook of Petroleum Refining Processes; Meyers, (R. Ed)., McGraw-Hill: New York, 2nd ed. . Section 4.3.
Doolin, P.K., Zalewski, D.J. , Oyekan, S.O. (2001) Meeting the challenges of cleaner fuels, World Refining; Chemical Week Associates Production; pp.12
Smith, R.B. (1959) Kinetic analysis of naphtha reforming with platinum catalyst. Chem Eng Progr; 55(6):76–80.
Sutton, E.A.,Greenwood, A.R., Adams, F.H. (1973) A new processing concept for continuous Platforming. Oil Gas J. 71 (20), pp 136.
Copyright of this article belongs to Research Prospect Ltd, UK’s leading dissertation writing service. It has been published on www.academiaservice.com after obtaining their approval.