Filtration can manage costly contaminants in refineries

Thomas H. Wines, Tore H. Lindstrom
Pall Corp.
Port Washington, N.Y.

Filtration systems can assist process operations by managing costly contaminants.

These systems have benefited various applications in the refinery and enhanced the ever-present need to supply clean products.

Table 1 [124,335 bytes] lists terms and definitions related to filter operations.

Filtration generally applies to the removal of solids from liquid or gas streams. Coalescing defines the removal of liquids, with or without some solid contaminants, from gases, in addition to the separation of aqueous/hydrocarbon liquids.

Filter design

In refineries, filters can capture mineral matter, such as scale and rust, which often foul equipment and catalyst beds. Additionally, they can remove catalyst and coke from and within various refinery-process units.

Liquid-gas coalescers can "dry" streams down to 3 ppb liquid with essentially no solids remaining. Therefore, they can effectively protect compressors and furnace burners, reduce foaming, and improve process-solvent recovery.

Liquid-liquid coalescers with high-efficiency, polymeric media can process hydrocarbon streams that hold as much as 10% aqueous phase. This can be done at very low interfacial tension (>0.5 dyne/cm), down to 15 ppm(vol) of free aqueous contaminants or lower.

Coalescers can similarly process aqueous streams, such as wastewater, down to 5 ppm (vol) of free hydrocarbon contaminant.

Disposable filters

Disposable filters offer the advantage of a low initial capital investment. They are typically one-tenth the cost of fully automated backwash-filtration systems. This benefit is, however, offset by the cost of replacement filters and the labor associated with filter change outs.

Disposable filtration may be advantageous over automated regenerable filter systems for process streams that contain less than 10 ppm solids. Typical service lives for disposable filters vary from weeks to months, depending on the nature and contaminant burden of the service. Filter materials include cellulose, cotton, glass fiber, polypropylene, and nylon.

Three types of disposable filters have evolved: pleated, depth, and bag.¹

Coalescers

Coalescers are constructed with materials similar to disposable filters, but are designed to separate liquid from gas or liquid-liquid emulsions. In practice, coalescer systems, with proper prefiltration, typically achieve service lives from 6 months to 2 years.

There are two types of coalescers:

Liquid-gas coalescers. Liquid-gas coalescers are the latest development in the history of liquid-gas separation units. Their performance is superior to knock-out drums, vane separators, mesh pads, and combinations of filter separators and vane or mesh packs. Older coalescers rely on inertial separation mechanisms and work well for larger aerosol droplets (5m) but lose efficiency at reduced flow rates. High-efficiency, vertical liquid-gas coalescers have been used extensively in gas processing in the last decade. They can remove fine droplets (0.3m to levels as low as 3 ppb) and are able to operate efficiently at low flow rates.

A recent innovation in liquid-gas coalescer design is to use a surface treatment² to prevent the wetting of the coalescer media with the aerosol liquids. This increases the allowable flux, decreasing fouling tendency and decreasing pressure drop losses.

Liquid-liquid coalescers. Liquid-liquid coalescer systems are used extensively to dry jet fuel.³

Traditional coalescers use glass fiber media, which works well for emulsions that have interfacial tensions greater than 20 dyne/cm. New coalescer media, constructed with novel formulated polymers and fluoropolymers⁴ are effective for emulsions having interfacial tensions as low as 0.5 dyne/cm.

An improved design of the vertical liquid-liquid coalescer has been to stack the coalescers on top of the separators. Older designs located the separators in a different section of the housing. This new design improves the flow distribution, and thereby increases the separator utilization.

Continuous operation

Three regenerable element systems have been commercially applied: backwash for solid-liquid feed, gas-assist backwash for solid-liquid feed, and blowback for solid-gas feed.

All operate on the same principle. When an element has reached designated maximum solids loading, it is isolated from the continuous operation and fluid is applied in reverse flow to wash the collected solids from the element. Solids loading is mainly monitored by pressure-drop increase.

The regenerable system is designed with parallel numbers of elements, and the elements are cleaned in sequence by automated control.

The backwash system, which uses liquid to remove filtered solids, needs only one material. The gas-assist system requires both liquid and gas, which would seem to increase capital and operating costs. The gas-assist method, however, substantially reduces the amount of backwash liquid and allows for higher feed throughput.

The blowback system for solid-gas separation uses media with very small pores to attain 99.99%+ removal of contaminants, surpassing efficiencies expected from bag houses, electrostatic precipitators, and many cyclones. Since gas is the only regenerating agent, catalyst fines can be removed.

If desired, catalyst can also be recovered from the continuous catalytic reformer regenerator to protect the recycle-hydrogen compressor, or from fluid-catalytic cracker (FCC) regenerator flue gas to meet environmental regulations and protect downstream equipment.

Refinery filter applications

A typical refinery flow plan, shown in Fig. 1 [196,326 bytes], illustrates potential applications for filtration. Filters can clean feeds, process and product streams, and blended products.

Filtration applications and potential benefits for some of the key process steps are discussed in this article. The filter numbers correspond to the numbered filters on the refinery flow plan (Fig. 1).

Crude unit

Essentially all refineries use the C ₁-C ₂ cut from the vapor-recovery units (VRU), light ends, or saturated/unsaturated-gas plants as fuel for furnaces, boilers, or burners to meet overall energy demand. This refinery-fuel gas contains variable amounts of C ₃+ depending on VRU design and operation, as well as the refinery-operations mode.

Liquid-gas coalescers will effectively remove condensables and particulates to improve atmospheric and vacuum-tower furnace efficiency and reduce maintenance costs (Filter 8). These coalescers are equally effective for furnaces in other refinery processes, such as catalytic reforming, steam reforming, hydrotreating, and hydrocracking.

Hydroprocessing

Nearly all hydroprocessing (hydrotreating, hydrorefining, and hydrocracking) units use fixed beds of catalyst. One of the important operating parameters that affect catalyst activity and run length is pressure drop ( deltaP) through the bed. The lower the deltaP, the better the distribution of feed and hydrogen across the reactor area and throughout the catalyst-void volume.

Solids from process equipment scaling, corrosion, and tankage fill catalyst bed voids, raising DP and reducing activity. This, in turn, can dictate higher furnace-outlet temperatures or lower space velocities, thus lowering conversion and increasing coking probabilities. The net result will be higher operating costs and shorter run lengths.

Solid-liquid continuous backwash or gas-assist backwash filters (Fig. 2) [51,818 bytes] remove feed solids. Solids removal ensures less frequent catalyst regeneration or change-out, maximizes reaction conversion, and lowers upgrading costs associated with upgrading products (Filter 4).

Many hydroprocessing applications have transitioned from wedgewire elements to a backwash system that uses substantially more-efficient media,⁵ as discussed later in this article.

Liquid-liquid coalescers (Fig. 3) [55,205 bytes] may be used to protect catalyst for those feeds that might be prone to water contamination from tankage or from purchased feedstocks. This is especially true if the water is contaminated with salts (Filter 12).

Hydroprocessing units, along with catalytic reforming and isomerization units, incorporate a hydrogen-recycle loop to control the reaction and minimize coke formation on the catalyst.

The recycle-gas composition can carry liquid droplets and aerosols to the recycle compressor. This carryover can foul compressor internals, reduce efficiency, and cause unscheduled downtime. Liquid-gas coalescers are employed to remove the condensables upstream of the compressor (Filter 9).

FCC

FCC-regenerator flue gas is normally scrubbed of catalyst fines using cyclones, bag house filters, electrostatic precipitators, or wet and dry gas scrubbers. These systems, to varying degrees, can lose efficiency from changes in gas-flow rate and solids loading. As environmental standards for particulate emissions continue to become more stringent, these systems may prove unacceptable or may require an expensive revamp. Continuous filter systems that use ceramic or porous metal media and blowback-element regeneration have been proven in flue-gas conditions. They can completely remove particulate as small as 1m (Filter 10). This not only helps in meeting regulations, but also improves performance and reduces maintenance of downstream units such as turboexpanders and CO boilers.

Another potential source of catalyst loss is from fresh and spent catalyst storage vents during filling and discharge. Either in situ regenerable or disposable filter systems can effectively recover all of these solids.

Cat slurry oil, decant oil, or heavy cycle oil (HCO) often contains the catalyst exiting the riser-reactor section of the FCC. While the regenerator-side catalyst is collected and disposed of, the reactor-side catalyst is collected and recycled to the process. More efficient catalyst/oil separation results in less catalyst loss and minimal particulate contamination of the heavy product.

Although slurry settlers have been the primary means of separation, the high-efficiency backwash filter is becoming more popular. The backwash filter accommodates the increased constraints on desired or allowable solids content in heavy fuels or specialty carbon black applications (Filter 6).

Catalytic reforming/aromatics recovery

Fixed bed, semiregenerative, and cyclic reformers (CRU) have similar needs for catalyst protection.

The regenerator-recycle elutriation gas contains catalyst that can cause erosion and fouling in the recycle-hydrogen compressor. A high-performance blowback-filter system provides the most efficient protection of the compressor and minimizes maintenance. The filter also prevents loss of catalyst fines (Filter 9).

A higher sustained catalyst activity is achieved by "chloriding" the platinum-promoted catalyst in CRUs. Chloriding can be done using a variety of chemical agents, typically carbon tetrachloride or perchloroethylene. Because of the formation of aggressive acids, water must be excluded from the catalyst beds to prevent deactivation and reduce the potential for acid corrosion.

CRUs are usually fed with hot stock to reduce energy requirements. Cold feed from tankage is typically where there is a potential for excessive water and solids. Liquid-liquid coalescers can be used to assure maximum water removal from the naphtha feed (Filter 12).

Aromatics recovery from reformate to provide BTX as petrochemical feedstock is accomplished by extraction, using an organic solvent such as sulfolane or glycols. The solvent is recirculated within the process after separation from the aromatics by distillation and stripping.

Residual hydrocarbons and particulate in the recirculating solvent can cause foaming and fouling in the extraction column. Backwash-filter systems can be used to minimize operating problems in the extractor (Filter 3).

Amine sweetening/ sulfur recovery

Amines, such as MEA (monoethanolamine), DEA (diethanol amine), or MDEA (methyl DEA), are solvents used to scrub acid gases from refinery C ₁-C ₄ vapor streams.

Liquid hydrocarbons from C₅+ carryover, along with particulate, tend to cause foaming and fouling in the amine-hydrocarbon contactor. This can adversely affect acid-gas removal efficiency, as well as increase amine loss and maintenance costs. After the acid gases are stripped from the rich amine solution, any carryover of amine to the sulfur-recovery unit can diminish activity of the Claus plant catalyst.

If an amine system is employed in the tail gas-treating unit for the removal of residual hydrogen sulfide, similar foaming and fouling can occur. Liquid-gas coalescers (Filter 7) and disposable or backwash-filter systems (Filters 1 and 2) can be employed to improve the operation and efficiency of this section of the refinery. Filter selection is mainly dependent on the source and the anticipated level of particulate.

Product/stream quality

To meet specifications, refiners may need to clean their process streams (prior to fuel product blending) and blended products. This clean-up includes removal of residual caustic from caustic treating units or removal of water and particulate from stream and product tankage.

Liquid-liquid coalescers (Filter 13) or disposable filters (Filters 5 and 11) will assure that product quality requirements are met and will minimize the reprocessing of off-spec material.

Commercial applications

Coalescers and disposable filter systems have been widely and routinely used in the hydrocarbon-processing industries. Their reasonably low cost and ease of operation and maintenance are well recognized.

Backwash-filter systems are also used worldwide. As a result of their "newness" and higher capital cost, however, their value is not as widely recognized.

Hydroprocessing

Continuous, regenerable filters have been successfully used in place of graded reactor beds or guard beds.

Pall Corp. has designed, built, and installed over a dozen backwash units to protect hydroprocessing catalyst beds in Canada, China, Germany, Japan, the Netherlands, Taiwan, and the U.S. Most have been complete new systems, while a few were retrofits wherein improved elements were adapted to existing filter housings.

Key process parameters for hydroprocessing include:

• Capacity: 5,000-74,000 b/d; average 27,000 b/d
• Particulate: 10-550 ppm; average 130 ppm
• Feedstocks: Atmospheric and vacuum gas oils, heavy coker gas oil, lube oil extract, and FCC light cycle oil (LCO).

By reducing the usually anticipated increase in pressure drop, operations have increased run lengths from as little as 3-6 months to as much as 12-24 months. Alternatively, some hydroprocessing units increased space velocity, achieving expanded throughput without revamp or deeper desulfurization. Increasing space velocity is becoming of paramount importance as domestic refineries prepare for Phase II RFG (reformulated gasoline) and increased low-sulfur diesel fuel production, and as the rest of the world evolves into its low-sulfur fuels environment.

A U.S. Gulf Coast refinery has been using a backwash system for 6½ years to filter coke fines from a heavy-coker gas oil feed. This feed is then upgraded in a 20,000 b/d, two-train, multistage hydrotreater. Typical solids content is about 350 ppm, which limits the effectiveness of guard and graded beds.

The backwash system doubled catalyst life from 6 to 12 months. Over 4 years of operation, this represented four fewer catalyst regenerations, or change-outs-an estimated annual savings of $5.5 million, which includes catalyst cost, production loss, and labor. Each shutdown avoidance generates $320,000-480,000 of additional product and saves $6,000-8,000 in labor costs.

If replaced, existing wedgewire systems are usually retrofit with new backwash systems.

FCC slurry oil

Backwash filters, in operation since 1990, have enabled refiners to obtain lower solids-content decant oil than slurry settlers. A summary of operating parameters for 12 commercial installations is:

• Oil feed rate: 1,200-12,000 b/d; average 5,300
• Maximum feed catalyst concentration: 1,500-17,000 ppm; average 3,500
• Effluent catalyst concentration: 10-100 ppm
• Backwash liquid: LCO or HCO.

In 1992, Nippon Petroleum Refining Co.'s (NPRC) Negishi Refinery in Yokohama, Japan, installed a backwash filter. The company continuously operates it to remove catalyst from the slurry oil of a resid-catalytic cracking unit.

Before the filter was installed, feed particulate ranged from 3,000 to 17,000 ppm, reaching more than 20,000 ppm during start-up periods. Since the use of the filter system, the decant-oil product has normally contained less than 100 ppm catalyst solids.

In general, minimizing solids content through high-efficiency filtration enables refineries to make a more valuable end product from slurry oil. Consequently, refining operations can substantially increase profits by producing a higher-quality product.

Higher products include carbon-black feed, carbon-fiber feed, and carbon for use in electrode manufacturing, which sell at about $65/bbl. A lower-quality product produced without a filter, such as fuel, will sell for only about $16/bbl.

Compressor protection

High-efficiency, liquid-gas coalescers are being used worldwide in a variety of gas services for protection of reciprocating, centrifugal, and turbocompression units.

Problems are most severe for reciprocating compressors. Valve and cylinder fouling is a common maintenance problem. Although centrifugal and turbocompressors are less prone to fouling, they experience a more gradual degradation as corrosion products and organic foulants build up on compressor internals.

A survey of plants that have installed Pall high efficiency, liquid-gas coalescers found that on average the compressor maintenance costs were reduced by 75%.⁶

Reciprocating compressors located in a sour (15% H₂S) natural gas stream at Chevron Corp.'s Carter Creek gas plant were known to have serious maintenance problems. The problems were a result of chronic fouling of intake valves and cylinders from the liquid and solid aerosols in the inlet gas.

Following the installation of a new compressor system in 1991, including a liquid-gas coalescer, the fouling problems were virtually eliminated. Since the coalescer installation, the compressors have been shut down only for yearly scheduled routine maintenance. The reduction in unscheduled shutdown has resulted in significant savings by decreasing the loss in production, labor, and replacement parts. The average coalescer service life was about 18 months.

In another example, a hydrogen-recycle compressor belonging to an 1,800 psig Canadian hydrocracker experienced maintenance problems as a result of liquid carryover. This carryover led to valve and cylinder fouling. Fouling caused unscheduled shutdowns every 2-3 months.

The fouling material was determined to be polymerized hydrocarbon sludge. Compressor repairs cost $375,000 annually and additional margin losses of $50,000 for each day of shutdown.

After installation of a high-efficiency, liquid-gas coalescer in 1993, unscheduled compressor shutdowns were eliminated. Coalescer elements lasted an average of 8-12 months.

References

1. Swiezbin, J., Uberoi, T., and Janas, J., "Sizing Up Disposable Filters," Chemical Engineering, January 1996.
2. Brown R.L., Wines ,T.H., and Malbrel, C., "Recent Developments in Liquid / Gas Separation Technology," Laurance Reid Gas Conditioning Proceedings, February 1994.
3. Brown, R.L., and Wines, T.H., "Improve Suspended Water Removal from Fuels," Hydrocarbon Processing, December 1993, pp. 95-100
4. Wines, T.H., and Brown, R.L., "Difficult Liquid-Liquid Separations: High Performance, Polymer-Fiber Coalescers Break Up Hard-To-Handle Emulsions and Dispersions," Chemical Engineering, December 1997, pp. 104-9.
5. Lindstrom, T.H., and Cathcart, N., "Optimized Filtration Systems for the Protection of Hydroprocessing Units in Petroleum Refineries," Ascope '97 Conference Proceedings, Vol. l, pp. 281-92.
6. Ruegsegger, N.C., "Using High Efficiency Coalescing Filters to Remove Compressor Suction Gas Contaminants and to Improve Hydrogen Recycle Gas Quality," Chevron Research and Technology Company, 1993-1994 Technology Development Study, June 7, 1994.

The Authors