Fcc process for maximizing diesel

Abstract

A process is described for maximizing the FCC middle distillates comprising the use of two different converters, operating in a coordinated manner that seeks to maximize the production of LCO for diesel, generating a specified gasoline and reducing fuel oil production. Converter “A” operates with a low contact time in the riser, of 0.2 to 1.5 sec. (preferably from 0.5 to 1.0 sec.) making a higher reaction temperature possible even at low severity, from 510° C. to 560° C. (preferably from 530° C. to 550° C.) and with a catalyst suitable to the maximization of LCO. Converter “B” possesses a high activity catalytic system, suited to cracking naphtha and DO generated in the first converter. Preferably, converter “B” has two separate risers, allowing the reaction temperatures of each to be adjusted independently according to the range most recommended for maximizing the cracking of each of the streams: 530° C. to 560° C. for the DO riser and 540° C. to 600° C. for the naphtha riser. The high-quality LCO stream generated by cracking at low severity in converter “A” is not contaminated by the poorer quality LCO generated by re-cracking the DO in converter “B,” since each converter has its own fractionating tower. The use of low contact time as a route for reducing severity in converter “A” geared towards the production of better quality LCO allows it to operate with a higher reaction temperature for the same LCO conversion and quality level, entailing greater operating reliability for the unit, and providing benefits for the heat balance of the converter. In existing units, the improvement in the heat balance provides leeway to the air blower via increased batch temperature, and makes room for processing more residual batches.

Description

FIELD OF THE INVENTION[0001]The present invention relates to the field of fluid catalytic cracking processes (FCC) and concerns the maximization of middle distillates. More specifically, this invention describes an FCC process using heavy hydrocarbons or mixture of hydrocarbons as feedstock, which proposes the use of two different converters operating in a coordinated manner, one of the converters being dedicated to the production of top quality LCO (Light Cycle Oil) and another converter dedicated to “re-cracking” unwanted (decanted oil) or non-specified (naphtha) streams generated in the first converter. Moreover, the invention describes the benefit of using low contact time as a method for reducing the severity of the reaction in the converter, so as to produce quality LCO. The process seeks both to maximize the production of LCO and to improve its quality, as it makes it possible to produce gasoline with the required quality and to reduce fuel oil production, eliminating the typ...

AI Technical Summary

Benefits of technology

The patent is about a way to make gasoline and reduce fuel oil production by cracking different streams of material at specific temperatures. The process aims to produce the best quality gasoline and maximize the production of LCO. By using lower contact time in the first converter and higher temperatures, the process can achieve better quality LCO even when using higher temperatures. The result is increased operating reliability, improved thermal balance, and more flexibility in processing residual feedstock.

Problems solved by technology

Its addition to the diesel pool is primarily limited by its high aromaticity and density.

The problem is that unwanted factors are associated with this benefit, such as increased production of Decanted Oil (DO) and lower quality cracked naphtha, thus preventing its introduction into the gasoline pool.

Implementing the FCC operation at very low reaction temperatures likewise reduces the operating reliability of the unit, in addition to its negative impacts on the converter heat balance.

Catalytic cracking in a fluidized bed plays vital role in oil refining, particularly when heavy, therefore, difficult to distill hydrocarbons must be processed.

Normally the LCO has a high concentration of aromatics, reaching over 80% of the total composition, a fact that makes its incorporation into the diesel oil pool difficult.

The aromatics in the LCO range impair its quality, reducing the cetane number (index based on the linear paraffin hydrocarbons with the chemical formula C16H34 used as the standard for assessing the ignition properties of the diesel) and increasing the characteristic density, which cannot be completely reversed by the hydrotreatment.

The secondary aromatic forming reactions, undesirable for obtaining a better quality middle distillate, are favored by high severity conditions including: high reaction temperature and high contact temperature between the batch and the catalyst in the riser.

For this reason, the task of simultaneously obtaining high olefin and better quality LCO yields in a single reaction zone is difficult.

All these modifications in the operational variables lead to a reduced conversion, with the consequent increase in the production of DO and a worsening of the quality of the cracked naphtha.

The problem is that when this solution is applied, some undesirable side effects occurs, such as increased production of DO and lower quality cracked naphtha, what prevents the adition of that naphtha into the gasoline pool.

Generally, another associated undesirable factor caused by the reduction of the reaction severity is reduced production of LPG and, consequently, of light olefins in the LPG.

A negative consequence resulting from the FCC operation for maximization of middle distillates by the reduction of the reaction temperature, is the excessive deposition of coke observed in the reaction zone and in the rectification zone.

This problem reduces the operating reliability of the unit and often requires its complete shutdown for removal and cleaning of the coke.

The primary cause of the excessive deposition of coke is the incomplete vaporization of the feedstock, due to the low resultant temperature in the mixing zone between the feedstock and the catalyst, as a result of the lower reaction temperatures used for maximizing the middle distillates.

The excessive temperature rise of the regenerator is detrimental, because it increases the deactivation rate of the catalyst inventory of the unit, in addition to bringing the regenerator vessel close to its metallurgical design limits.

Furthermore, it has the undesirable effect of significantly increasing the potential gum of the cracked naphtha, resulting in greater thermal cracking to which the feedstock is subjected upon mixing with the hotter catalyst resulting from a hotter regeneration zone, despite the lower TRX.

The fast rising of the regenerator temperature during the reduced reaction temperature operation is the main cause of the increased entrainment of hydrocarbons to the regenerator.

The deleterious effect of the FCC operation with a reduced reaction temperature on the regenerator is worse when processing heavy and residual feedstocks, which tend to be more difficult to vaporize and convert.

Aside from increasing the complexity of the equipment, its use has the drawback of increasing the air demand to the regeneration zone, due to the increased total coke yield, resulting from its effect on the overall energy balance of the converter.

Like the catalyst cooler, this alternative has the drawback of increasing the total coke yield of the unit, consequently requiring the use of greater combustion air-flow rate.

The above techniques do not eliminate the problem of hydrocarbon entrainment to the regenerator.

At the same time, however, the bottom cracking reactions are extended, which also entails an undesirable increased production of DO, similar to that occurring when the reaction temperature is reduced.

This fact—the existence of a single converter—prevents the ideal optimization of the catalytic system, which limits the results that could be achieved through the configuration proposed in the patent application PI 0504321-2.

The fact of having a single riser prevents the optimization of the ideal catalytic cracking conditions of these streams, also limiting the results that could be achieved.

In addition, the converter operating reliability is reduced due to problems related to coke deposition on the equipment in the reaction and rectification section associated with lower riser temperature.

In practice, the application of this process can be restricted to the processing of lighter feedstocks.

U.S. Pat. No. 7,632,977, which seeks simultaneously to increase the production of diesel and LPG in a single converter equipped with two risers (a low severity one for maximizing LCO and a high severity one for re-cracking naphtha), has the same disadvantage of using a single catalytic system for the feedstock and naphtha cracking.

Furthermore, the DO recycling is sent to the low severity riser at a point above the feedstock injection, complicating the cracking of this stream, which requires greater severity for the converter.

Therefore, do not expect a sharp reduction in the production of DO, which continues to be a problem for the refiner desiring to increase production of LCO diesel without the offsetting increase in the generation of DO.

In addition, coke deposition problems in the low severity reaction section are compounded by the recycling of decanted oil, the vaporization of which is hindered by selected injection point.

However, the disadvantage is due to the production of a greater amount of non-specified naphtha and DO in the first riser.

Moreover, it is extremely difficult to obtain a quality LCO and naphtha specified as gasoline from cracking the feedstock in a single riser.

As a result, the quality of the naphtha coming from this riser will not be good;the naphtha and LCO streams of the first riser are not separated in the fractionator associated with this riser, but instead mixed with the same cuts obtained in the second DO riser.

If the severity in both risers is low, the resultant LCO is of good quality, but reprocessing of the DO loses its effect, and the process usually has the drawback of high production of DO (and low quality naphtha).

On the other hand, if the second riser is more severe, the DO conversion increases, but the quality of LCO of the second riser declines, and this stream is mixed with the LCO obtained in the first riser, with the overall result of a less significant improvement in the quality of the LCO;in the design shown in the patent, the catalyst system of the two risers is the same;coke deposition problems associated with the first riser can limit the reduction of the reaction temperature within it and also limit the quality of the feedstock to be processed.

The lower severity in the area for cracking the feedstock can lead to an unwanted increase in the production of DO; however, the patent indicates the possibility of recycling the DO together with the feedstock.

The disadvantages of this technique are:the use of a single riser for cracking the feedstock and recycling the naphtha and DO decreases the refiner's flexibility, as the operating window of the unit is shorter;a considerable part of the installed riser is used for reprocessing, reducing the refinery's space for converting residues;with a riser, although the injections take place at optimized temperature and C / O ratio points for each stream, the process is restricted to a single catalytic system for meeting such diverse objectives (maximizing diesel and LPG).

The failure to meet this requirement significantly compromises the potential to improve the LCO quality of the process.

First disadvantage: the mixing between the reaction product of the feedstock and the reaction product of the DO results in the LCO of both feedstocks being mixed.

This leads to the following possibilities:

(a) the cracking of the DO is done with high severity, and, in this case, the DO yield is lower (positive effect), while LCO generated is of poorest quality and will be mixed with the LCO produced in the feedstock riser, resulting in reduced quality for this LCO (negative effect); or

(b), the DO is cracked at reduced severity, and, in this case, the quality of the mixed LCO of the two feedstocks is not as adversely affected; however, the overall DO yield from the unit remains high, therefore unsatisfactory.

Although the cracking in the DO riser is done at reduced severity, the LCO obtained from this stream is, generally, of poorer quality than that obtained by the cracking of the feedstock, because the DO has a larger proportion of aromatics in its composition—typically the only possible allocation for the thus obtained LCO would be the fuel oil pool, as a diluent.

This means that whatever the reaction condition in the DO riser, the mixing of the LCO obtained in the DO riser with the LCO obtained in the feedstock riser is not a good practice for the purpose of maximizing the LCO quality.

Second disadvantage: despite the fact that U.S. Pat. No. 7,316,773 puts forth the possibility of using different catalytic systems for the feedstock and for the DO, this characteristic is not possible when both the two risers discharge the two product streams from the cracking into a single gas-solid separation vessel (separator vessel), as described in U.S. Pat. No. 7,316,773, without contamination occurring due to the mixing of the two catalytic systems with conflicting objectives.

This mixing would occur in the separator vessel itself, in the rectification system, in the regenerator, and in the catalyst transfer standpipes, which, over time, would lead to the complete mixing of the two catalytic system inventories, both losing their characteristics—unless the equipment had physical dividers separating them over their entire height and the standpipes were independent.

In practice, this partitioning is not economically attractive, it is technically difficult, and its execution is improbable.

Nevertheless, according to the design of U.S. Pat. No. 7,316,773, the transfer lines of both reaction products must be interconnected, leaving the previously indicated first disadvantage.

Thus, the technique still requires a fluid catalytic cracking process with mixed feedstock aiming the production of middle distillates, which optimizes to the maximum the quality of the LCO obtained in cracking the feedstock, this high-quality LCO stream being isolated from other LCO streams deriving from the reprocessing of cracked fractions that, due to the chemical nature and because they require more stringent reaction, generate a poorer quality LCO.

Without this technique, the entire reaction / rectification section for cracking the feedstock and the re-cracking of the DO would be subject to problems of coke deposition and loss of operating reliability, which could limit the range of possible batches that could be processed in the unit subjected to the technique made known in U.S. Pat. No. 7,316,773.

In these conditions, the second converter will be used to crack part of the feedstock flow of the first, since the increased thermal demand of the first will limit the feedstock processing in it.

However, it will remain difficult to cope with the greater production of decanted oil: if one opts for recycling part of the DO or not to reduce the reaction severity so much, so as not to increase the DO yield, the consequence will be a deterioration in the quality of the LCO and less incorporation of this stream into the Diesel pool, the initial objective of the operation of the converter for maximum middle distillates.

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