Enrique Aguilar-Rodriguez, Ciro Ortiz-Estrada, Martin Aguilera-Lopez
Instituto Mexicano del Petroleo
Mexico City
The 25,000 b/d fluid catalytic cracking unit (FCCU) at Petroleos Mexicanos' idle Azcapotzalco refinery near Mexico City has been relocated to Pemex's 235,000 b/d Cadereyta refinery.
The results of a thermal-integration analysis are being used to revamp the unit and optimize its vapor-recovery scheme.
FCCU REVAMPS
New features of modern FCC design, catalyst, and equipment have led to important improvements that have attained higher conversion and better gasoline yield and quality.
These improvements have increased profitability substantially. And because of these changes, old units present a good opportunity for revamps.
When an old side-by-side model is replaced by a riser-type reactor with high-efficiency regenerator and improved feed-nozzle design, major changes can be realized in the reactor/regenerator section.
This type of revamp changes product yields and operating conditions in the reactor/regenerator. A revision of the fractionation and vapor-recovery section (FVRS) is required to fit the equipment and topology to the new heat-exchange conditions and requirements.
For the case of the Azcapotzalco FCCU, the old unit was designed in the 1950s, so modifications to the reactor/regenerator section incorporate many important changes, including a new riser, feed nozzles, cyclones, air distributor, and other internals. Unit capacity (25,000 b/sd) is not being increased because of feedstock limitations in the new location, but the unit could have been redesigned for this purpose.
The original FVRS scheme is shown in Fig. 1. The major heat demands are for the feedstock, the fractionator/stripper reboiler (E-3), and the debutanizer reboiler (E-4). Heat is supplied by hot streams made of heavy cycle oil (HCO) and slurry from the main fractionator.
For the new scheme, the analysis was based on the following restrictions:
- Two cases concerning gas oil feed conditions must be met. In the hot-feed case feed is introduced from a processing unit outside battery limits (OSBL) at 188 C. For the cold-feed case, feed is introduced from OSBL from storage tanks at 70 C.
- No new fire heaters are to be installed.
- Existing equipment must be reused whenever possible.
ORIGINAL SCHEME
Table 1 shows, for both the original design and the new conditions, the flow rates, temperatures, and heat requirements and availability of the process streams involved in the heat integration.
As shown, heat requirements for the revamp conditions increase substantially, mainly because of higher gasoline and C3/C4 yields, and because of higher heat demand for the feed. This is true because, for the most-critical new conditions (cold-feed case), the feed will be supplied from storage tanks at 70 C., while in the original design, the feedstock temperature was fixed at 117 C.
The difference between the heat requirements and the availability is just 1.2 MM kcal/hr at normal operating conditions. At maximum heat load in the fractionator/stripper reboiler (E3), however, this difference is only 0.2 MM kcal/hr. The heat balance and thermal conditions in the original FVRS design are shown in Fig. 2.
One option to ensure the required gasoline specifications is to reduce the C3/C4 recovery by lowering the cold debutanized-gasoline recycle to the fractionator/ stripper (T-2). This reduces its heat requirements, as well as those for the debutanizer (T-3).
This reduction has an adverse impact on the absorption efficiency of the gasoline stream, thus resulting in C3/C4 losses in the dry gas stream at the unit battery limits. This occurred before the unit was shutdown, as shown in Table 2.
For the new conditions, hot-recycle streams from the main fractionator (T-1), with a higher heating potential, are available (Table 1). This is true because the reactor effluent is at a higher temperature (521 C.) than the design case (482 C.), necessitating greater heat release from this stream.
Although greater heat release occurs in the revamped unit, more heat is needed for feedstock and reboilers, so the stiffness in the heat supply/demand ratio remains. Total heat demand is 27.9 MM kcal/hr (including feed preheating and reboiler duties), while heat availability from hot streams from the main fractionator is 28.5 MM kcal/hr.
For the new unit conditions, simulation and thermohydraulic analyses of the equipment, under design specifications, were carried out. Results are presented in Table 3. The table shows that there are thermal and hydraulic limitations to meeting the new operating conditions.
The feed/main-fractionator bottoms exchanger (E-1) is thermally limiting because it can transfer only 10.6 MM kcal/hr-a lower value than the 12.4 MM kcal/hr needed by the process. Additional equipment would be necessary to comply with process conditions.
The debutanizer reboiler (E-4), allows the heat transfer as required, but presents a rather high pressure drop (35.5 psia) for HCO and is therefore hydraulically limited. A modification in the geometry of the exchanger (number of passes and nozzle diameter) is thus required to meet process conditions.
The fractionator/absorber reboiler (E-3) also exhibits a relatively high pressure drop (12.4 psia vs. 8 psia for the design case). This is not a serious problem because the limiting HCO stream is the one entering the debutanizer reboiler.
These results show that, to comply with the thermal conditions under the new process features, modifications to the existing equipment are necessary.
ALTERNATIVE SCHEMES
A number of processing schemes can be proposed to meet the heat requirements imposed on the FCC under the new reactor/regenerator design. For this purpose, hot streams from the main fractionator can be used, including not only HCO and slurry, but also light cycle oil (LCO) and decanted oil.
Table 3 shows flowrates, temperature levels, and heat loads to be supplied or released from each stream. As shown, product streams would contribute only slightly to the beat requirements. On the other hand, hot recycle streams from the main fractionator have a high thermal potential to be used as an energy source to exchange heat with cold process streams.
Three alternatives were proposed for the new integration scheme.
ALTERNATIVE 1
Fig. 3 shows an integration scheme which includes all the available hot process streams.
Incorporation of LCO and decanted oil in the feed preheating network results in an increase in the feed temperature from 70 to 90 C. The addition of new heat exchangers is necessary, however, as is a design for LCO and decanted-oil coolers flexible enough to handle the hot-feed case. Existing heat exchangers (E-3 A/B) do not meet the new conditions.
HCO recycle is used for feed preheating so the heat duty of the debutanizer reboiler will be supplied by the slurry recycle instead of by HCO. The slurry recycle meets the heat demand and does not exceed the allowed pressure drop.
ALTERNATIVE 2
This scheme, involving integration including only HCO and slurry hot streams from the main fractionator, uses those streams with the greatest heat potential. Fig. 4 shows this proposed scheme.
ALTERNATIVE 3
Integration under the original process scheme with modifications and additional equipment maintains the original topology. This option, however, includes the addition of heat exchangers in the feed-heating train (E-1)-including all the slurry from the main fractionator and modifications to the mechanical design of the debutanizer reboiler (E-4) to lower the pressure drop of the HCO stream.
Fig. 5 shows this proposed scheme.
ANALYSIS
Table 4 shows the most relevant data for each alternative, concerning heat balances and heat loads for each service.
ALTERNATIVE 1
Alternative 1 requires the addition of three new exchangers for feed preheating, with LCO, decanted oil, and HCO recycle. Together with the original exchangers (E-1 A/B), these exchangers will supply the heat for increasing the feed temperature from the 70 C. required at the unit battery limits (new design conditions) to the 218.3 C. required at the reactor inlet.
On the other hand, the heat load to the debutanizer reboiler (E-4) will be supplied by the slurry recycle, instead of by the HCO stream. This will preserve the original heat exchanger and maintain an adequate pressure drop (4 psi), which is even lower than that obtained in the original design (34.5 psi). The use of the HCO stream is limited by the high pressure drop that would result in this case, as was shown in Fig. 2.
The total heat added by the inclusion of the LCO and decanted oil streams in the feed-preheating train accounts for about 12% of the total requirements (1.5 MM kcal/hr), which is equivalent to a temperature increase of 20 C. (from 70 C. to 90 C.). For the hot-feed case, the design of the coolers for the heavy fractions must be flexible enough that the required battery-limit conditions are met.
This is a major disadvantage of this alternative because, in addition to requiring new equipment, a flexible cooler design and modifications to pipelines in reboiler E-4 are needed. Additionally, HCO must be cooled down to 170 C., with an increase in the cooling-water consumption to provide 2.2 MM kcal/hr. This load is even higher than the one required for cooling only the LCO and decanted oil streams (1.75 MM kcal/hr).
ALTERNATIVE 2
For Alternative 2, the integration scheme is similar to that for Alternative 1. This scheme, however, includes only the HCO and slurry recycle streams from the main fractionator. And it requires chancing the hot stream that provides the heat duty to the debutanizer reboiler (E-4) in order to obtain an adequate pressure drop.
As shown, only one new heat exchanger must be added in the feed-preheating train, and the pipelines for the hot stream to the debutanizer reboiler (E-4) also must be modified.
For this scheme, the heat load for the HCO recycle cooler (E-5) decreases from 2.2 MM kcal/hr to 0.7 MM kcal/hr. This reduced heat load decreases cooling-water demand because most of the heating potential from all the main-fractionator hot recycle streams is used.
For this alternative, the slurry operating temperature is higher than the design temperature of the exchanger in which it is to be used (350 C. vs. 330 C.). Modifications to some pipelines and tubes, therefore, are needed to meet the new thermal conditions.
In summary the advantages of Alternative 2 are:
- The heat requirements of reboiler E-4 are met with a hotter stream, enabling the expected quality and recovery of the debutanized gasoline, C3, and C4 fractions to be met.
- For feed preheating, there is enough flexibility to handle both the cold feed (70 C.) and the hot-feed cases (188 C.).
Because the temperature of the hot streams is higher, the remaining heat from those streams is higher. This heat can be used for steam production in exchanger E-2.
ALTERNATIVE 3
The integration scheme for Alternative 3 maintains the original topology. One additional exchanger is needed to meet the feed-preheating requirements, and only the slurry recycle is used because the existing exchanger (E-1) does not meet the energy demand for the new design conditions. Additionally the geometry of Reboiler E-4 must be modified to lower the pressure drop for the HCO stream that, under the actual design, increases to 34.5 psi.
If the number of passes and nozzle diameters are changed, the pressure drop may be decreased to 17.1 psi, which is acceptable for the hydraulics of the HCO loop. Under this option, few new exchangers are required and the modifications to pipelines and equipment location (plot plan) are not needed. Table 5 shows the pros and cons of each of the proposed alternatives.
RESULTS
From the analyses of the proposed schemes, Alternative 3 was chosen as the most attractive because it requires the addition of only one new heat exchanger in the feed-preheating train for the slurry recycle and some modifications to the geometry of Reboiler E-4.
On the other hand, Alternative 2 is also attractive. Despite the limited heat availability of the hot streams, compared to the requirements of cold streams in this option, the heat required for Debutanizer Reboiler E-4 is assessed so that its adequate performance guarantees the required recovery of gasoline, C3, and C4 fractions.
Additionally, this option exhibits the flexibility required for handling both the cold and hot-feed cases because, for the former, steam generation occurs.
Under either option, the heat availability of the hot streams is very limited in relation to the energy requirements of the unit. Flexibility, therefore, is low, which may lead to product losses (mainly C3S and C4S).
To avoid this, other modifications must also be included, such as altered column internals, additional cooling services, and other changes that permit the addition of extra energy to the unit. These modifications must lead to a more satisfactory design.
BIBLIOGRAPHY
Cerda J., and Galli M.R., "Synthesis of Heat Exchangers Networks," Computers Chem. Eng., No. 2, 1990, pp. 213-27.
Kotjabasakis, E., and Linhhoff, B., "Sensitivity Tables for the Design of Flexible Processes (1)," Chem. Eng. Res. Des., Vol. 64, May 1986, pp. 197-211.
Del Rosal, R., Briones, V., Mancilla, R., Chavez, L., and Vega, C., "Sintesis de Redes de Intercambio de Calor eficientes bajo multiples condiciones de operacion," Advances en Ingenieria Quimica, Mexico, 1991, pp. 253-57.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
Issue date: 11/07/94
