July 2017

Special Focus: Refinery of the Future

Design and troubleshoot the orifice chamber in an FCCU

The fluid catalytic cracking (FCC) process remains one of the most important secondary conversion processes due to its inherent capability to upgrade residual streams and its flexibility to maximize the desired product, depending on market dynamics.

The fluid catalytic cracking (FCC) process remains one of the most important secondary conversion processes due to its inherent capability to upgrade residual streams and its flexibility to maximize the desired product, depending on market dynamics. Data on global capacities indicated that FCC capacity (approximately 840 MMtpy) in 2010 exceeded the combined capacity of hydrocrackers and delayed cokers. In general, FCC units (FCCUs) are significant contributors to refinery margins and usually operate under constraints that push their operational boundaries.

FCCUs are highly versatile with respect to handling a variety of feedstocks, adjusting the operating severity and catalyst composition without requiring shutdown of the unit to meet the changing market demands. The units are often revamped significantly beyond their original design capacities and regularly debottlenecked to push higher throughputs.

Continual advances in zeolite-based catalyst have significantly contributed to enhancing FCCU performance. Depending on unit configuration, operating conditions and feedstock quality, FCCUs are operated with bottlenecks. While debottlenecking results in improved operation, it also generates/hits a new bottleneck. Regular debottlenecking essentially leads to an extreme operating environment for the unit hardware, especially the internals.

Erosion in cyclones, air distributors, standpipes, double-disc slide valves (DDSVs) and orifice chambers (OCs) in the reactor-regenerator section is quite common due to continuous contact with circulating catalyst. Erosion challenges are aggravated when the units are operated beyond 120% of original design capacity. Restoring damaged cyclones, air distributors and slide-valve discs is generally carried out during maintenance and inspection shutdowns. However, the restoration of partially eroded nozzles in an OC is difficult within the limited shutdown period.

FCC process

An FCCU encompasses two main sections, the reactor-regenerator (R-R) section and the separation section, which comprises the main fractionator and gas concentration plant. In the R-R section, the hydrocarbon feed (with steam) is contacted with up-flowing hot solid catalyst in the riser, where it cracks into smaller and more volatile compounds. During the reactions, catalyst is deactivated due to the deposition of coke on the active sites. The catalyst is rejuvenated by burning the coke using air in the regenerator. The coke burning process is highly exothermic, which meets the heat demand for feed vaporization, endothermic cracking reactions and the increase in sensible heat for all incoming streams, such as air, steam, purge gas and losses.

The flue gas from the regenerator is passed to a cooler via the DDSV and OC—the DDSV controls the pressure of the regenerator, while the OC kills the pressure before venting through the flue gas stack. In some units that process resid feed, regeneration is carried out in two stages. Regenerators are typically operated at a temperature of 650°C–760°C (1,202°F–1,400°F) and a pressure of 1.8 kg/cm2(g)–3.5 kg/cm2(g), depending on the configuration.

The reactor effluent is fed to the main fractionator for separation of the liquid products, depending on the boiling points as side draws. The top stream from the main fractionator is passed through the gas concentration plant for further separation based on molecular weight. Debottlenecking in an R-R section is more complex compared to a separation section due to the involvement of simultaneous cracking and combustion reactions in the presence of circulating catalyst.

Orifice chamber

The OC contains perforated plates designed to progressively reduce the flue gas pressure while achieving stable pressure downstream of the DDSV. Without an OC, the flue gas velocity through the DDSV would be very high, generating considerable noise and causing faster erosion. Pressure drop across the OC and DDSV varies in proportion to the square of the velocity. Typical orifice velocities across the DDSV and OC are approximately 250 m/sec and 150 m/sec, respectively. Theoretically, it is possible to operate the DDSV and OC at much higher velocities, but increased noise and erosion can cause mechanical damage.

Preferably, the orifice should be circular, as it greatly influences the discharge co-efficient and the pressure drop. Due to a lack of adequate maintenance for restoring the eroded nozzles in the OC, the load—in terms of pressure drop on the DDSV—often increases. This essentially erodes DDSV discs, resulting in a reduction in feed throughput and the eventual replacement of the DDSV discs. Since refiners generally do not have the facilities to fully repair damaged DDSV discs, the debottlenecking of the OC through the incorporation of additional grids is recommended.

Design principles

An OC typically contains four to eight grids with nozzles of 1-in. to 2-in. diameter, depending on unit capacity and the required pressure drop. The pressure drop across each grid can be calculated using Eq. 1 and Eq. 2: 

            (1)

where:

ΔP       Pressure drop through plate, kg/cm2
v          Velocity through orifice, m/sec
ρ          Gas density at upstream condition conditions, kg/m3
Af         Orifice plate free area, m2
Ap        Orifice plate area, m2
gc         9.81 m/sec2
C         Discharge coefficient
Y          Compressibility factor 

            (2)

where:

t           Plate thickness
D         Hole diameter
P          Hole pitch.

Eq. 2 is valid for a t/D ratio between 0.4 and 2.4, and a Reynolds number between 4,000 and 20,000. The volumetric flowrate of flue gas increases from one orifice grid to the other due to pressure reduction. Accordingly, the required number of nozzles are kept open to maintain uniform velocity across the orifice grid. Excessive velocity at the orifice grid is avoided to minimize damage of the OC grids.

The quantity of flue gas passing through the DDSV and OC can be estimated by material and energy balance calculations. Similarly, pressure drop across the DDSV and OC can be worked out based on single-gauge pressure survey data. Erosion in downstream equipment—DDSVs, OCs, CO boilers/flue gas coolers—can be caused by high gas velocities and the presence of catalyst fines. Erosion of orifice nozzles further aggravates catalyst loss from the regenerator and can be caused by inferior fresh catalyst properties, pressure fluctuations and damage of R-R hardware internals, among others. An active open area in the orifice grids increases due to erosion, which reduces the OC pressure drop. This increases the pressure drop requirement across the DDSV.

Troubleshooting through debottlenecking

Debottlenecking is the removal of any obstacles or constraints in any equipment, plant or process to increase efficiency and workability, improve conversion, selectivity and reliability, and troubleshoot specific problems. The most important aspect of debottlenecking any equipment or unit is the capture of performance data through a controlled and comprehensive test run. Three case studies on the debottlenecking of OCs in different Indian Oil Corp. FCCUs are presented here.

Case study 1

The first case study is on the reliability improvement of an FCCU operated in distillate mode with a partial combustion regenerator. The unit was revamped to 150% by expanding the regenerator capacity and associated modifications, including the feed injectors. Furthermore, the main fractionator and gas concentration sections were debottlenecked to enhance feed throughput by an additional 20%.

The unit was experiencing high catalyst loss due to higher superficial velocity in the regenerator due to higher feed throughput. Apart from severe erosion in cyclones due to high catalyst loss, the unit was also facing the challenge of a low DDSV opening. The pressure drop across the DDSV was almost twice the design value. As the problem was aggravated over a period of operation, the DDSV opening gradually reduced to 3%–4%, leading to an unplanned unit shutdown. The DDSV discs and support guider were severely damaged. Due to the erosion of the guider, one of the discs was dislodged downward, increasing the vertical gap between the discs. This essentially provided more flow area, as compared to the indication of the DDSV opening.

The higher pressure drop across the DDSV was mainly due to higher regenerator pressure to accommodate incremental feed throughput. Incidentally, measures such as the inclusion of an additional orifice and the plugging of nozzles in an existing OC were not implemented during the revamp to achieve the required pressure drop. Due to the passing of catalyst fines with flue gas, severe erosion of nozzles in the OC was observed, particularly on the initial grids. An increase in active flow area reduced the pressure drop across the OC, thereby increasing the DDSV pressure drop.

The Indian Oil refinery was looking for a solution to the problem of the low DDSV opening. Within the limited shutdown time, modification of the 3rd and 6th grids was not possible due to inadequate accessibility. To overcome this problem, some of the nozzles on the 1st, 2nd, 4th and 5th orifice grids, as shown in TABLE 1, were plugged as a temporary solution rather than nozzle restoration. The nozzles were made circular by welding externally machined plates with the required orifice size. FIG. 1 and FIG. 2 show the nozzles before and after repair of the OC, respectively. Installation of an additional three grids was recommended as a long-term solution for healthy operation of the OC and DDSV.

Fig. 1. Damaged grid.
Fig. 1. Damaged grid.
Fig. 2. Repaired grid.
Fig. 2. Repaired grid.

After implementation, the DDSV opening was found to improve from 3%–4% to 25%–30% due to an increased pressure drop across the OC. The pressure across the DDSV was reduced from 1.45 kg/cm2 to 1.2 kg/cm2. Stable opening of the DDSV was also noticed, and the same operation continued for more than 3 yr without additional maintenance.

Case study 2

Following an FCCU maintenance and inspection shutdown in another refinery, the DDSV opening was found to reduce drastically to 7%. The lower DDSV opening resulted in intermittent, high catalyst loss due to pressure fluctuation. Regenerator pressure was reduced to bring the opening of the DDSV to more than 15%, necessitating a reduction in feed throughput of approximately 40%.

After examining the unit operating data, unlike Case 1, the pressure drop across the OC was found to be higher, even at normal feed throughput. The blockage of some nozzles was the suspected cause of the higher pressure drop. During an inspection of the OC, it was found that the nozzles were blocked by refractory debris. The orifice grid before and after cleaning is shown in FIG. 3 and FIG. 4, respectively.

Fig. 3. Orifice grid before cleaning.
Fig. 3. Orifice grid before cleaning.
Fig. 4. Orifice grid after cleaning.
Fig. 4. Orifice grid after cleaning.

During subsequent operation, the DDSV opening was gradually reduced to 3%–7%, along with an increase in pressure drop across the OC that clearly indicated the erosion of DDSV discs. Based on plant operating data, plugging 21 nozzles in the OC was recommended (TABLE 2), and this was implemented during the next opportunity shutdown. These resulted in the restoration of feed throughput, while maintaining the opening of the DDSV around 30% and an increase in OC pressure drop from 1.1 kg/cm2 to 1.5 kg/cm2.

Case study 3

Another case of OC operation debottlenecking was carried out in a resid reactor plus two-stage regenerator (R2R)-type FCCU with two DDSVs and one OC placed in series in the flue gas line of Regenerator 1 (RG 1). The flue gas produced in RG 1 contains a considerable amount of carbon monoxide (CO), which is burned in a CO boiler to generate steam. Regenerator 2 (RG 2) is operated at a dense bed temperature exceeding 710°C (1,310°F) to ensure complete combustion.

Increased feed throughput was not achieved due to the higher catalyst slide valve opening (> 70%). Although the replacement of slide valves by higher capacity can increase feed throughput, it is time-consuming, as standpipes must be replaced. Another option is to increase the RG 2 dense bed temperature so the additional heat demand in the riser can be met at the prevailing catalyst circulation rate.

Typically, 30% of the coke is burned in RG 2. If this amount is increased by 7%, the RG 2 dense bed temperature is expected to increase by 25°C (77°F) at a given riser outlet temperature (ROT); the catalyst circulation rate will also increase considering a 10% increase in feed throughput. To enable this, some amount of RG 1 air must be diverted to RG 2. A lower air rate to RG 1 is expected to further reduce the opening of the RG 1 DDSV. To achieve a healthy DDSV opening with reduced air to RG 1, plugging some of the nozzles in the RG 1 OC was recommended (TABLE 3). When implemented, this created a feed throughput increase of 10%.

Takeaway

As the major secondary conversion unit in a petroleum refinery, the FCCU presents many challenges to the operator while maximizing the margin and achieving longer turnaround periods. The inspection of the DDSV and OC is critical and as important as the inspection of other internals for improving an FCCU’s overall reliability. The debottlenecking of an OC not only improves unit reliability, but also generates cushion to increase feed throughput and, thereby, higher refinery margins. HP

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