The Catalytic Reforming Process

Description

Catalytic reforming is a chemical process used to convert petroleum naphtha, particularly low – octane – number straight – run naphtha into high – octane gasoline called reformate. In addition to producing reformate, catalytic reforming is also a primary source of aromatics used in the petrochemical industry (BTX: benzene, toluene, and xylenes).

Straight-run naphtha obtained directly from the atmospheric crude oil distillation column is a mixture of paraffins (saturated aliphatic hydrocarbons), naphthenes (saturated cyclic hydrocarbons containing at least one ring structure), and aromatics (hydrocarbons with one or more polyunsaturated rings) in the C5 – C12 range with a boiling range between 30 and 200°C, constituting typically 15 to 30 wt% of the crude oil, with some sulfur and small amounts of nitrogen. The typical feed to catalytic reforming is a mixture of straight-run naphthas: 30 to 90°C light naphtha (C5 and C-), 90 to 150°C medium-weight naphtha (C- and C-), and 150 to 200°C heavy naphtha (C- and C- 2). These distillation ranges of naphthas differ slightly from those described in topic 1 (Table 1.5), but they are more commonly used in catalytic reforming operations rather than those employed for crude oil international assays. The properties of naphthas for various Mexican crude oils are reported in Table 4.1.

TABLE 4.1. Properties of Naphthas with International Assay Distillation Ranges from Various Crude Oils


Property

Crude Oil

10 ° API

13°API

Maya

Isthmus

Olmeca

Light Naphtha (TIE-71°C)

sg, 60°F/60°F

0.6689

0.6659

0.6686

0.6504

0.6503

Total sulfur (wt%)

0.064

0.062

0.050

0.020

0.020

PI ON A (vol%)

^-Paraffins

41.12

39.46

47.75

48.98

47.15

i-Paraffins

44.62

36.20

39.15

40.66

43.63

Oleffins

0.26

0.56

0.00

0.01

0.00

Naphthenes

10.22

18.14

10.89

8.45

8.45

Aromatics

3.69

5.14

2.21

1.90

1.81

Benzene (vol%)

3.28

0.89

2.21

1.90

1.81

Medium Naphtha

(7 1-177° C)

sg, 60°F/60°F

0.7550

0.7542

0.7512

0.7448

0.7408

Total sulfur (wt%)

0.432

0.412

0.200

0.030

0.030

PI ON A (vol%)

^-Paraffins

37.59

36.54

30.75

30.79

30.59

i-Paraffins

33.32

31.83

30.69

32.58

31.30

Oleffins

0.89

1.27

0.73

0.27

0.55

Naphthenes

16.65

17.03

19.35

19.74

18.41

Aromatics

10.54

11.18

18.48

16.62


19.15

Benzene (vol%)

0.52

0.55

0.32

0.38

0.46

Heavy Naphtha (177-204°C)

sg, 60 ° F/60 ° F

0.8012 0.8001 0.7928

0.7920

0.7912

Total sulfur (wt%)

1.589 1.511 0.600

0.100

0.050

Aromatics (vol%)

18.70 15.50 27.08

18.31

22.08

In commercial practice, the most preferred feed for catalytic reforming is naphtha with a boiling range of 85 to 165°C, since the light fraction (85°C-) is not a good feedstock, due to its composition (low-molecular-weight paraffins tending to crack to C- and to be a precursor in benzene formation, which is undesirable because of environmental regulations), and the heavy fraction (180°C+) hydrocracks to excessive carbon laydown on the reformer catalyst.

Prior to catalytic reforming, the naphtha feed needs to be hydrotreated to reduce the impurities content (sulfur, nitrogen, and oxygen compounds) to acceptable levels, which if not removed will poison the reforming catalysts. This pretreatment is mandatory since the catalyst is gradually poisoned, leading to excessive coking and rapid deactivation.

Apart from straight-run naphthas, the following are other streams usually fed to reformer units, which have a boiling range similar to that of typical catalytic reforming feed, and come from a visbreaking unit, coking unit, hydrocracking/HDT unit, or FCC unit. They generally contain high amounts of sulfur, nitrogen, and olefins, which are mostly aromatic and difficult to hydrotreat.

• Visbreaker naphtha, which requires severe hydrotreating in order to prepare a proper reformer feedstock. That is why visbreaker naphtha is usually limited to small percentages of the feed reformer.

• Coker naphtha, whose properties are more or less the same as those of visbreaker naphtha but whose amount available from refineries is higher.

• Hydrocracked and hydrotreated naphtha, which is produced by hydro-cracking and hydrotreating of heavier petroleum fractions. This naphtha is a suitable reformer feedstock since it is rich in naphthenes.

• FCC naphtha, which is produced by catalytic cracking of gas oils. Although not being a viable feed to catalytic reforming, some refineries use it, particularly the 75 to 150 ° C fraction.

The distribution of paraffins, olefins, naphthenes, and aromatics in the feed to catalytic reforming determines the richness of the feedstock, which is normally rated by its naphthenes + aromatics or naphthenes + 2 aromatics value. To convert low-quality naphthas, the catalytic reforming process rearranges (or restructures or reconstructs) the hydrocarbon molecules to form more complex molecular-shaped hydrocarbons with improved octane values. Although a certain degree of cracking occurs, the conversion is done without changing the boiling-point range of the feed. During this transformation, catalytic reforming produces significant amounts of hydrogen, which is used in other processes, such as hydrotreating and hydrocracking, as well as small amounts of methane, ethane, propane, and butanes.

A typical catalytic reforming unit consists of a feed system, several heaters, reactors in series, and a flash drum. Part of the flashed hydrogen is recycled to the feed before it enters the first heater, while the liquid is sent to the frac-tionation section (stabilizer). The reformate is obtained as a bottoms product from the stabilizer. Off-gas and liquefied petroleum gas (LPG) are recovered from the top of the stabilizer.

Since most of the reforming reactions are endothermic, several heaters are used to maintain the reactor temperature at the desired levels (400 to 500°C). As the feed flows through the catalytic bed in the reactor, the major reaction is the dehydrogenation of naphthenes to aromatics, which is fast and highly endothermic, resulting in a large decrease of temperature within the reactor. The product from the first reactor is reheated and fed to the following reactor. As the feed passes through the reactors in series, the reaction rates decrease and the reactors become larger, the reaction becomes less endothermic, and the temperature differential across them decreases, while the amount of heat required between the reactors also decreases.

Types of Catalytic Reforming Processes

Catalytic reforming processes are commonly classified according to the frequency and mode of catalyst regeneration, into (1) semiregenerative, (2) cyclic regeneration, and (3) continuous regeneration. The main difference among the three types of processes is the need of unit shutdown for catalyst regeneration in the case of a semiregenerative process, the use of an additional swing or spare reactor for catalyst regeneration for the cyclic process, and catalyst replacement during normal operation for the continuous regeneration type. Figure 4.1 illustrates the reaction section of the three types of catalytic reforming processes.

The most used process worldwide is the semiregenerative type, followed by continuous regeneration and by the less common cyclic regeneration. Currently, most catalytic reformers are designed with continuous regeneration, and the former semiregenerative plants are being revamped to operate as continuous regeneration.

Semiregenerative A semiregenerative catalytic reforming process usually has three or four reactors in series with a fixed-bed catalyst system and operates continuously (cycle length) from six months to one year. During this period, the activity of the catalyst diminishes due to coke deposition, provoking a decrease in aromatics yield and in hydrogen gas purity. To minimize the catalyst deactivation rate, the semiregenerative units operate at high pressure (200 to 300 psig). To compensate for catalyst activity decline and to keep conversion more or less constant, the reactor temperatures are increased continuously. When the end- of-cycle reactor temperatures are reached, the unit is shutdown and the catalyst is in situ regenerated. A catalyst cycle ends when the reforming unit is unable to meet its process objectives: octane and yield reformate. Catalyst regeneration is carried out with air as the source of oxygen. A catalyst can be regenerated five to ten times before it is removed and replaced.

Cyclic Regeneration Apart from the catalytic reforming reactors, the cyclic regeneration process has an additional swing reactor, which is used when the fixed-bed catalyst of any of the regular reactors needs regeneration. The reactor with the regenerated catalyst then becomes the spare reactor. By this means, the reforming process maintains continuous operation. Operating at lower pressure (-200 psig) allows the cyclic regeneration process to achieve higher reformate yield and hydrogen production. Compared with the semire-generative type, in the cyclic regeneration process the overall catalyst activity varies much less with time, so that conversion and hydrogen purity are kept more or less constant during the entire operation. The main disadvantage of this type of catalytic reforming is the complex nature of the reactor switching policy, requiring high safety precautions. Also, to make switches between reactors possible, they need to be of the same maximal size.

Reaction section of the catalytic reforming processes.

Figure 4.1. Reaction section of the catalytic reforming processes.

Continuous Regeneration The deficiencies in cyclic regeneration reforming are solved by a low-pressure (50 psig) continuous regeneration process, which is characterized by high catalyst activity with reduced catalyst requirements, producing more uniform reformate of higher aromatic content and high hydrogen purity. This type of process uses moving-bed reactor design, in which the reactors are stacked. The catalyst bed moves by gravity flow from top to bottom of the stacked reactors. The spent catalyst is withdrawn from the last reactor and sent to the top of the regenerator to burn off the coke. The transport of catalyst between reactors and regenerator is done by the gas lift method. During normal operation, small quantities of catalyst are withdrawn continuously. Fresh or regenerated catalysts are added to the top of the first reactor to maintain a constant inventory of catalyst.

Process Variables

Similar to the hydrotreating process described in topic 3, in the catalytic reforming process there are four principal variables that affect the performance of the unit, either semiregenerative or continuously regenerative: reactor pressure, reactor temperature, space velocity, and H2/oil molar ratio.

Pressure A reduction in the reactor pressure increases the hydrogen and reformate yield, decreases the required reactor temperature to achieve a constant product quality, and shortens the catalyst cycle by increasing the catalyst coking rate. Due to the pressure drop, the reactor pressure declines across the various reaction stages. The average pressure of the various reactors is generally referred to as the reactor pressure. Typical reactor pressures are 200 to 500psig (semiregenerative and cyclic regeneration) and 60 to 150 psig (continuous regeneration).

Temperature The reaction temperature is the most important variable in catalytic reforming, since the product quality and yields are highly dependent on it. WABT (weighted-average bed temperature) and WAIT (weighted-average inlet temperature) are the two main parameters to express reforming reactor average temperature. The difference between WABT and WAIT is that the former represents the integrated temperature along the catalyst bed, and the latter is calculated with the inlet temperature of each reactor.

WABT is calculated as indicated in topic 3 [Eqs. (3.1) and (3.2)], and WAIT is determined as follows:

tmp6C5658_thumb

where WAIT; is the inlet temperature of each reactor, N the number of reactors, and Wci is the weight fraction of catalyst in each reactor bed with respect to the total. Semiregenerative units operate at a higher reactor temperature (450 to 525°C) than that of continuous regeneration units (525 to 540°C).

All reaction rates are increased when operating at high temperature. Hydrocracking, which is not desirable in catalytic reforming, occurs to a greater extent at high temperatures. Therefore, to obtain high product quality and yields, it is necessary to control the hydrocracking and aromatization reactions carefully. Reactor temperatures are monitored constantly to observe the extent of each of these reactions.

Space Velocity Both LHSV and WHSV are of typical use in catalytic reforming units to express space velocity. Space velocity and reactor temperature are commonly employed to set the octane of a product. The greater the space velocity, the higher the temperature required to produce a given product octane. The severity of the catalytic reforming unit can be increased either by increasing reactor temperature or by lowering the space velocity. Since the amount of catalysts loaded to the reactors is constant, the reduction of space velocity during operation can be reduced only by decreasing the feed flow rate.

H2/Oil Ratio In contrast to the catalytic hydrotreating process, in which the H2/oil ratio is reported in volumetric units [e.g., standard cubic feet of hydrogen per barrel of liquid feed (ft1/bbl)], in the catalytic reforming process, this ratio is stated on a molar basis [i.e., moles of hydrogen in the recycle gas (a mixture of hydrogen and light gases) per mole of naphtha feed (mol/mol)]. Values of 4 to 6 mol/mol are typical in commercial reforming units. An increase in H2/oil ratio causes an increase in the hydrogen partial pressure and removes coke precursors from the metal sites. The global effect of this is increased catalyst life. In other words, the rate of coke formation on the catalyst and thus catalyst stability and life is a function of the H2/oil ratio and hydrogen partial pressure present in the reactor system.

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