New line for slurry-oil filter solves plugging problems

Hyung Soon Kim, Tae Gyung Park, Hyun Joo Yoo, Suk Jin Jung
LG-Caltex Oil Corp.
Yosu, South Korea

At LG-Caltex Oil Corp.'s Yosu, South Korea, petrochemical complex, a piping reconfiguration of the fluid-catalytic cracking unit (FCCU) slurry-filter system solved low-pressure and low-temperature problems upstream of the system.

Generally, handling slurry streams is troublesome, and few successful operations of slurry filters exist.

Slurry-oil filters remove sediment, water, and catalyst fines from the slurry-oil product, which is sold as marine-bunker fuel oil. If the slurry oil contains catalyst fines, the fines can accumulate on an engine's piston cylinders and cause inefficiencies. The fines can also plug the throttle valve of fuel-injection systems and damage the engine.

The LG-Caltex Yosu complex began operations in 1967. Currently, there are about 1,300 employees in the refinery, and it is the second largest refinery in the world (OGJ, Dec. 22, 1997, p. 33).

The refinery has 650,000 b/d crude-distillation capacity with four crude-distillation units and a 70,000 b/d resid fluid-catalytic cracking unit. The complex also produces 165,000 metric tons/year (mty) of polypropylene and 1 million mty of aromatics.

FCCU

The resid FCCU started up on Dec. 7, 1995. Stone & Webster Engineering Corp. engineered the basic design of the unit. The unit is designed for 50,000 b/d of unhydrotreated Arab Light crude or 63,000 b/d of unhydrotreated 50/50 Oman/Murban ATB (atmospheric tower bottoms) crude.

Typical feed to the unit is a mix of unhydrotreated Mideast, African, and other ATBs. Feed quality typically has a Conradson carbon residue (CCR) of 5-7 wt %, a nickel content of 3-17 ppm(wt), and a vanadium content of 5-32 ppm(wt). Total metals content on the equilibrium catalyst (Ecat) is usually 10,000-12,000 ppm(wt), and the Ni/V ratio is normally about 50/50.

The unit's operating philosophy is maximum gasoline output. It operates against multiple constraints depending on the time of the year and the feed qualities.

Slurry system

Fig. 1 [59,529 bytes] shows the original configuration of the LG-Caltex slurry system. The main purpose of the slurry stream is to remove heat from the bottom of the fractionator. About 90% of the slurry flow returns to the quench zone of the main fractionator, and the rest goes to a fuel pool to be sold as marine bunker fuel.

The slurry-oil filters remove catalyst fines from slurry-oil product before it goes to tankage.

The slurry-oil filter system was provided by Pall Corp. Table 1 [43,907 bytes] lists the design conditions of slurry oil filters. The system filters particles that are 18+ µ in size. There are three filter vessels, and each filter vessel has 372 filter cartridges.

There are two operational modes. Mode 1 occurs when one vessel is in service, one vessel is being backwashed (occurs every 3-4 hr), and one vessel is in standby. The maximum allowable service time of each vessel is 8 hr.

Mode 2 occurs when two vessels are in service and one vessel is being backwashed. LG-Caltex operates its filter system in Mode 2.

Fig. 2 [50,471 bytes]shows a sketch of the slurry-oil filter system and associated process valves. There are two methods of backwashing the filter. Method A purges the slurry oil with nitrogen, and Method B purges the slurry oil with heavy cycle oil (HCO).

LG-Caltex uses HCO for backwashing. Table 2 [30,328 bytes]gives the sequence and time for Method B backwashing. Minimizing the amount of backwashed material to the riser is better for FCC operations because the material contributes to coke make.

Operational history

During the commissioning and initial operation of the filter system, there were mechanical and installation problems that prevented its acceptable operation. Acceptable operation was achieved by working with the vendor and making some mechanical modifications.

Low inlet pressure to the filters, however, was still a problem. To prevent fouling and plugging of E-210, a minimum flow was required. The heat exchangers upstream of the filter were fouled. Fortunately, the slurry-oil product quality met the specifications for the sediment and water content (<0.5 vol %) without the use of the filter system. thus, lg-caltex decided that it did not need the slurry-oil filter, and it bypassed the filter system. this decision was made before there were catalyst-fine specifications for the slurry oil.

After the system was bypassed, the quality of bunker-fuel oil suffered. LG-Caltex discovered that the plugging at the end point of the customer's burner tip was primarily an Al/Si component, the main component of FCC catalyst.

LG-Caltex concluded the problem was from the slurry oil. The company took advantage of a shutdown period for the resid FCCU to troubleshoot the slurry-oil system in November 1996.

Troubleshooting

During the resid FCCU shutdown, LG-Caltex isolated one filter and steamed it out. Using an air-bubble test (submergence of the filter element in water and blowing air through the filter), 40% of the filter elements (372 elements) were found plugged.

Close examination of the deposits with a microscope and with burn tests showed the materials to be hydrocarbon, not catalyst fines.

LG-Caltex sent one filter element to Pall for an analysis of unidentifiable materials. The materials proved to be asphaltenes, tar, and wax. The materials were removed from the filter by washing with HCN (heavy cracked naphtha), and the element was placed back in service.

After installation, the differential pressure across the slurry-oil filter increased to the limit of 3 kg/sq cm-gauge within 30 min. The filter was again bypassed and taken out of service.

Filter inlet temperature

In further investigation, LG-Caltex found that, at temperatures below 260° C., asphaltenes, tar, and wax separate out of slurry oil and form a second, more viscous layer. This layer preferentially plates out on the surface of the filter and "glues" the filter cake to the element.

This glued layer is not easily dissolved by normal HCO backwashing, by HCN washing, or by a temperature increase because the rate of dissolution is extremely low.

To have acceptable filter service, it was necessary to prevent the separation of the asphaltenes, tar, and wax during operations. If the slurry product bypassed the slurry exchangers, the inlet-slurry filter temperature would increase, which would prevent separation of these viscous materials.

Taking the slurry oil to the slurry oil filter to a temperature above 260° C. was difficult because the existing piping of the slurry exchangers was not designed for that temperature. The original design specified a temperature of 240° C. at the slurry-filter inlet, but it assumed no plugging and a low-pressure drop on the slurry exchanger.

Modifications

A new line was installed to route high-temperature slurry oil to the filter to maintain a filter inlet temperature above 260° C. LG-Caltex also installed additional exchangers, E-219A/B, to protect the material of the equipment downstream of the filter. This downstream equipment was designed to be operated below 260° C.

Fig. 3 [65,504 bytes] shows the new line and the additional exchanger with the control scheme revision.

As a result of these modifications, the slurry oil is now operating under 3 kg/sq cm-gauge for 3 hr at a time. Currently, the filter is working well and there have been no sudden differential pressure increases.

Other improvements

To provide sufficient pressure at the inlet of slurry oil filter, LG-Caltex changed the size of the heat exchanger tubes upstream of the slurry filter from 0.75 in. to 1 in. in diameter.

In the near future, LG-Caltex will modify the slurry-control scheme to make it easy to control the main fractionator bottoms level and give steady flow through the slurry system (Fig. 3).

In an associated problem, the backwash-drain receiver pump did not have enough suction pressure. A large pressure drop through the suction pipe as a result of an undersized pipe (2 in.) caused the pump to trip. Enlarging the suction pipe size to 4 in. solved the problem.

Lessons learned

After its experience, LG-Caltex makes the following recommendations for slurry oil filters:

1. Provide a proper inlet temperature to the slurry oil filter. Materials such as asphaltenes, tar, and wax will separate out and form a second, more-viscous layer at temperatures below 260° C. Typically, the viscosity at the slurry oil filter should be about 2-3 cSt at 280° C. Although viscosity is adjusted with the temperature, a minimum temperature of 280° C. is recommended.
2. Provide sufficient inlet pressure to the filter. The design-inlet filter pressure was 5.7 kg/sq cm-gauge. The slurry-oil filter did not work properly because of a low inlet pressure. Tube fouling of the upstream slurry exchangers resulted in a high pressure drop. This drop reduced the inlet pressure to under 4 kg/sq cm-gauge.
   Any exchanger on the slurry stream should use minimum 1-in. diameter tubes to prevent plugging.
3. Use HCO as the soaking medium. Most slurry oils are rich in aromatics. HCO is preferable to light cycle oil (LCO) because HCO contains more aromatics (about 80%). If too heavy a medium is selected, it will be make coke and plug the filter.
   An additional HCN line, installed to use HCN as a soaking medium during manual unplugging or troubleshooting, may be needed. LCO spiking to the slurry stream before the filter, is recommended to reduce fouling of the slurry exchanger tubes and reduce the viscosity of the slurry-oil product.
4. Minimize recycle backwashing material. Any backwashing recycle material becomes coke in the riser and may collect on the vapor line or other cold spots. It will also tend to reduce fresh feed rate.
5. Do not introduce any cold stream at the inlet of the slurry-oil filter. Warm temperatures are necessary to achieve a minimum velocity at equipment downstream of the slurry oil filter, such as fin fan cooler and exchangers.
6. Provide enough pressure difference and time during the backwashing. If backwashing is not done sufficiently, the filter will be plugged in a very short time. The particles which are not removed from filter the vessel will come to the surface again.
7. Provide an alternative for the slurry nozzle. There should be alternative slurry nozzles at the riser in case the nozzle plugs.

The Authors