ADVANCED FILTER SYSTEM PART OF OILY GROUNDWATER CLEANUP

A new high-capacity groundwater filtering system has a key role in a groundwater hydrocarbon-recovery program at Chevron U.S.A.'s El Segundo, Calif., refinery.

The system was engineered and provided by ABB (Asea Brown Boveri) Impell Corp. Fully operational since April 1990, the system produces clean filtered water for reinjection into the ground beneath the refinery. The filter system design flow rate is up to 2,280 gpm, or 75,000 b/d.

Impell's responsibility included engineering, design, equipment, construction, and installation supervision, as well as start-up and field services.

According to project management, valve reliability, tight shutoff, and resistance to fine-particulate abrasion are prerequisites in handling discharge from the dry cake filters (see flow diagram, Fig. i).

Valves implemented with special hard-coated discs and abrasion-resistant seat material were selected for maximum sealing efficiency, project management said.

All process control valves, instrument root valves, and block valves under 3 in.over 200 throughout the system-were supplied by Neles-Jamesbury.

DESIGN

Design of the groundwater filtration facility was the result of a series of on-going environmental and technical feasibility studies conducted by Chevron. This was followed by installation of a 300 gpm pressure precoat filter of Impell's design in December 1988.

This system was used as a prototype to evaluate different filter media, filtration efficiency, and service variables. This led to the development of a conceptual system design, operational format, and specifications.

DESIGN FEATURES

The final design, according to Impell project managers, incorporates several unique and innovative design features:

Large volumes of water are filtered to get an absolute minimum volume dry solid waste.
Filter aid material is injected and automatically adjusted by the degree of solids and hydrocarbons in the stream, thus further minimizing waste volume.
The system is completely automated. No operator attention is required other than water sampling.
Complete system redundancy eliminates shut-down for maintenance.
A charcoal adsorber system processes all gases prior to venting to the atmosphere.
All liquid releases are contained and reprocessed in the filter system.

OIL RECOVERY

The objective of the liquid hydrocarbon recovery project at the El Segundo refinery is the retrieval of hydrocarbons floating on shallow groundwater beneath the refinery.

Among the techniques used are liquid hydrocarbon skimming wells and dual pump wells which pump large volumes of water to lower the water in the well area. As a result, liquid hydrocarbons flow into the well and can be pumped out by an upper hydrocarbon pump.

In addition to these production wells, there are water reinjection wells for the water produced from the dual pump wells to be fed back into the ground.

It is this produced groundwater that the ABB Impell system filters prior to reinjection. Without appropriate filtration, the particulate contamination in this water would promote rapid fouling of the reinjection wells.

CONTINUOUS PROCESS

The groundwater filtration systems consists of a separation tank and pressure precoat filtration system. Groundwater is pumped from the production wells to a 37,000 bbl collection tank. Here most of the trace hydrocarbons in the water separate from the water and are periodically skimmed from the top. The water is pumped from the bottom of the tank through the groundwater filters and then back into the ground through the reinjection wells.

FILTRATION METHOD

The groundwater filters are pressure precoat-type units which use tubular stainless steel retention element to hold the diatomacous earth (DE) filter aid media. The filter aid is introduced into each filter at the beginning of each process cycle by flowing a mixture of filtered groundwater and DE through the filter with a precoat pump.

This precoat step deposits a thin layer of DE over the entire retention element which helps remove the fine particulate from the groundwater.

To maintain high filtration efficiency during the 8-hr cycle, additional DE is added on a continuous basis in very small amounts by a body feed pump. The DE/water mixture for the body feed comes from the same mix tank as the precoat and is introduced into the filter along with the groundwater.

At the end of the 8-hr filter cycle, another filter is precoated and brought on-line. The filter at the end of its cycle is taken off-line and backwashed. This process consists of draining the dirty side of the filter, then forcing clean filtered groundwater through it at a very high rate in the reverse flow direction. The flow dislodges the filter aid and dirt cake from the elements and flushes the elements and vessel clean. Backwash water is contained in a filter accumulator pressurized with nitrogen.

During the backwash, water and solids are collected in a backwash tank equipped with a mixer to maintain a uniform slurry. This is subsequently processed through one of the two dry cake filters. These are similar, but smaller, in construction to the groundwater filters and operate at a very low flow rate.

After the drain portion of the filtration cycle, nitrogen flows through to dry the collected dirt cake. The dried cake is discharged from the filter vessel by reversing the nitrogen flow at a high flow rate. The nitrogen for the discharge is stored in an accumulator located above the dry cake filter vessels.

Dry cake collected in a 20 cu ft dumpster is removed from the collection enclosure, covered, and taken to a holding area for sampling and subsequent disposal.

KEY EQUIPMENT, VALVING

Each of the five groundwater filters is 60 in. in diameter and contains 475 sq ft of filter element area (Fig. 2). Each processes from 400 to 900 gpm or an average of 760 gpm (25,000 b/d). The two dry cake filters are 30 in. in diameter, each with 50 sq ft of element area. Operating at 50 gpm with a process cycle time of 35 min., two cycles process all of the backwash slurry from one groundwater filter.

Among the over 300 valves supplied by Neles-Jamesbury for the system, the most critical were five 20 in. and two 24-in. discharge valves located on the five groundwater filters and two dry-cake filters.

To provide tight shutoff in the highly abrasive and corrosive environment, the high performance butterfly valves were equipped with a special TTC (titanium, tungsten, carbide) coating on the seating surface of the discs and ultrahigh density polyethylene seats.

Further modifications were made in the two 24-in. butterfly valves located on the dry-cake filters where the dry abrasive discharge created special problems in the seat area.

Process control and data organization are handled by an Allen Bradley System 5 programmable logical controller.

A data link between the groundwater treatment system and the refinery's computer network provides cumulative flow information as well as monitoring of other key process parameters.