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Kep Operating Variables of Naptha Reforming Unit

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The major process variables that affect the performance of the catalytic naphtha reforming or platforming unit are reactor pressure, reactor temperature, space velocity, hydrogen to hydrocarbon (H2/HC) molar ratio, feedstock properties, catalyst selectivity, activity, and catalyst stability. The relationship between the variables and process performance is generally applicable to both Semi-regenerative (SR) and continuous regeneration modes of operation.

Details of the key operating parameters of the Naphtha Reforming process unit are described below;

1. Reactor Temperature

The primary control for product quality and conversion in the Platforming process is the temperature of the catalyst beds. By adjusting the reactor inlet temperatures, a refiner can change the octane number of the reformate and the quantity of aromatics produced. Typically, SR (Semi-regenerative) Platforming units have a WAIT range of 490 to 525°C (914 to 977°F). CCR Platforming units operate at a WAIT of 525 to 540°C (977 to 1004°F).

The reactor temperature is usually expressed as the weighted average inlet temperature (WAIT), which is the summation of the product of the fraction of catalyst in each reactor multiplied by the inlet temperature of the reactor, or as the weighted average bed temperature (WABT), which is the summation of the product of the fraction of catalyst in each reactor multiplied by the average of its inlet and outlet temperatures.

The obvious generalization about temperature is that the higher the temperature, the faster the rate of reaction and therefore, the higher the conversion of feedstock into the reformate, hydrogen and LPG products. On the other hand, very high temperatures above 540ºC may cause thermal reactions which will decrease the reformate and hydrogen yields, at the same time increasing the rate of coke laydown on the catalyst.

Considering the activation energy, highly active new or freshly regenerated catalysts and low severity permit lower temperature operation, which favours the desirable reactions over the side reactions. As the catalyst deactivates or severity increases, a higher operating temperature is required, which favours the undesirable reactions and yield selectivity to desired products declines.

2. Reactor Pressure

The average reactor pressure is generally considered as reactor pressure. For practical purposes, the closer application is the last reactor inlet pressure because it contains most of the catalysts in the multiple reactor systems. The reactor pressure affects reformer yields, reactor temperature requirements, and catalyst stability.

Reactor pressure is also a critical parameter to be controlled, although normally units operate at fixed pressure. Changes in pressure affect hydrogen partial pressure, hence affecting reformate yield, temperature requirements and catalyst coking rate. It is better to operate at designed operating conditions.

There is an incentive to operate at low pressure where the cracking rate is relatively low and the slower dehydrocyclization of the paraffin rate increases. Thermodynamics also favours low pressure for dehydrogenation and dehydrocyclization. Hence increase the hydrogen production, reformate yield and decrease the temperature requirements to meet the RON specifications. The only drawback of low pressure is the higher coking rate, which is the reason why low-pressure reformers are equipped with a CCR system. Further, the DP across the reactors is also motored to identify any blockages in the catalyst pathways or process fluids pathways.

3. Hydrogen to Hydrocarbon Molar Ratio (H2/HC)

The H2/HC ratio is the ratio of moles of hydrogen in the recycle gas to moles of naphtha charged to the unit. The recycle hydrogen is necessary to maintain catalyst-life stability by sweeping reaction products from the catalyst. The rate of coke formation on the catalyst is a function of the hydrogen partial pressure.

It also works as a heat sink in the high-temperature environment of the reactor. An increase in the H2/HC ratio increases the linear velocity of the combined feed and supplies a greater heat sink for the endothermic heat of the reaction. Increasing the ratio also increases the hydrogen partial pressure and reduces the coking rate, thereby increasing catalyst stability with little effect on product quality or yields.

Further, lower H2/HC ratios provide higher C5 and hydrogen yields, although this benefit is difficult to measure in commercially operating units. The effect of feed hydrogen is also added by the hydrogen produced in the reactors. The total H2/HC ratio is defined as the ratio in the combined feed plus the hydrogen produced in the reactor. If less hydrogen is produced in the reactor, more coke formation will be observed in the reactor.

4. Space Velocity

Space velocity is defined as the amount of naphtha processed over a given amount of catalyst over a given length of time. The space velocity is an indication of the residence time of contact between reactants and catalysts. Space velocity is expressed in LHSV in terms of volumes and WHSV in terms of weight.

Space velocity together with reactor temperature determines the octane of the product. The greater the space velocity, the higher the temperature required to produce a given product octane. If a refiner wishes to increase the severity of a reformer operation, she or he can either increase the reactor temperature or lower the space velocity.

A change in space velocity has a small impact on product yields when the WAIT is adjusted to maintain constant severity. Higher space velocities may lead to slightly higher yields as a result of less time available in the reactors for dealkylation reactions to take place. This advantage is partially offset by the higher rate of hydrocracking reactions at higher temperatures.

When the hourly volume charge rate of naphtha is divided by the volume of the catalyst in the reactors, the resulting quotient, expressed in units of h-1, is the liquid hourly space velocity (LHSV).  Alternatively, if the weight charge rate of naphtha is divided by the weight of the catalyst, the resulting quotient, also expressed is the weighted hourly space velocity (WHSV). Although both terms are expressed in the same units, the calculations yield different values.

Whether LHSV or WHSV is used depends on the customary way that feed rates are expressed in a given location. Where charge rates are normally expressed in barrels per stream day, LHSV is typically used. Where the rates are expressed in terms of metric tons per day, WHSV is preferred.

5. Feedstock Properties

The boiling range of Platforming feedstock is typically about 100°C to 180°C. Feedstocks with a low initial boiling point (IBP), less than 75°C generally contain a significant amount of C5 components which are not converted to valuable aromatics products. These components dilute the final product, thus requiring a higher severity to achieve an equivalent product octane. For this reason, feedstocks are generally C6+ naphthas.

The endpoint of the feed is normally set by the gasoline specifications for the refinery with the realization that a significant rise in endpoint, typically 15 to 25°C, takes place between the naphtha charge and reformate product.

The feedstock boiling range chosen covers lean through rich feeds. The aromatics plus cyclohexanes or naphthenes content is a measure of their ease of conversion, and the paraffins plus cyclopentanes content indicates the difficulty of reforming reactions.

The effect of feedstock composition on aromatics yield is shown in Fig below. Increasing conversion leads to an increase in the total yield of aromatics for each of the feedstocks. Feeds that are easier to process produce the highest yield of aromatics at any level of conversion. Charge feeds with high end points will cause coking on the catalyst. The end point of the product is also on the higher side with decreased hydrogen production. To compensate for this reactor severity must be increased, which further accelerates the coke rate.

6. Catalyst Selectivity

Catalyst Selectivity Catalyst selection is usually tailored to the refiner’s individual needs. A particular catalyst is typically chosen to meet the yield, activity, and stability requirements of the refiner. This customization is accomplished by varying basic catalyst formulation, chloride level, platinum content, and the choice and quantity of any additional metals.

Differences in catalyst types can affect other process variables. For example, the required temperature to produce a given octane is directly related to the type of catalyst. Catalyst selectivity can be easily described as the amount of desired product that can be yielded from a given feedstock. Usually, the selectivity of one catalyst is compared with that of another. At constant operating conditions and feedstock properties, the catalyst that can yield the greatest amount of reformate at a given octane in motor fuel applications or the greatest amount of aromatics in a BTX operation has the greatest selectivity.

7. Catalyst Activity and Stability

Activity is the ability of a catalyst to promote a desired reaction with respect to reaction rate, space velocity, or temperature. Activity is also expressed in a relative sense in that one catalyst is more active than another. In motor fuel applications, activity is generally expressed as the temperature required to produce Reformate at a given octane, space velocity, and pressure. A more active catalyst can produce reformate at the desired octane at a lower temperature.

Catalyst stability is a measure of the rate at which the catalyst deactivates over time. In semi-regenerative reforming, stability is an indication of how long the catalyst can remain in operation between regenerations. In CCR Platforming, stability is an indication of how much coke will be formed while processing a given feed at a given severity, which, in turn, determines the size of the catalyst regeneration section.

Top References

  1. Fundamentals of petroleum refining by M.A. Fahim, T.A. Alsahhaf, A.S. Elkelani
  2. Handbook of Petroleum Refining Processes by Rober A.Meyers
  3. Handbook of Petroleum refining, editors, Hsu Robinson

 

 

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