Crude Oil Tank Bottom Recovery by Industrial Centrifuges

Crude Oil Tank Bottom Centrifuge

Table of Contents

Synopsis


Industrial centrifuges offer the most efficient way to recover oil from crude oil tank bottoms. In this article, we highlight the various way in which centrifuges bring efficiency to this process.

Specifically, we will discuss Dolphin Centrifuge’s Oil Recovery Systems (ORS), its features, and its benefits.

We have intentionally formatted this article in the form of a technical article followed by a case study so readers can relate to it better.


Dolphin Centrifuge’s Oil Recovery System (ORS)


Dolphin Centrifuge specializes in the design, manufacture, and implementation of Oil Recovery Systems. These systems are perfect for efficient oil extraction from the crude oil tank bottoms.

Industrial centrifuges are the main components of our modular systems. Our ORS feature Alfa Laval decanter and disc stack centrifuges.

 

Advantages of Dolphin Centrifuge’s ORS

 


Significant Reduction in Processing Time


Gravity settling uses the force of gravity to settle out the water and sludge from the oil. An industrial centrifuge exerts up to 10,000 times the force of gravity. This high centrifugal force causes instant separation of the phases.

In other words, a centrifuge can separate the water and sludge from crude oil much faster than gravity settling. This reduces processing time to a fraction as compared to gravity settling.


Reduced Tank Bottom Sludge Volume


The high centrifugal force reduces the sludge volume through compacting the sludge. It also extracts greater amounts of oil from the sludge. Thus, a centrifuge can reduce the tank bottom sludge volume to a fraction compared to gravity settling.


Reduced Transportation & Disposal Costs


Reduced sludge volume and drier sludge results in reduced transportation and disposal costs.


Emulsified Oil Recovery


Crude oil tank bottoms contain emulsified oil. This emulsion does not separate under gravity settling. The high centrifugal force in a disc stack centrifuge can separate the oil and water in the emulsion.

Emulsion separation is often augmented with chemicals that work in tandem with centrifuges for maximum oil extraction.

Separation of the emulsified phase increases the oil yield while reducing the waste stream. Centrifuges also reduce the amount of chemicals required.

 

Tank Cleaning Cost Savings


The centrifuges in our ORS replace settling in tanks. So, the sludge that was settling in the tanks is now separated from the oil by the centrifuges during the processing. No accumulation of sludge in tanks leads to tremendous costs related to tank cleaning.

Oil Recovery System (ORS) – 3-Step Process Overview

 

Our ORS processes crude oil tank bottoms in three distinct steps. A brief explanation of each step follows.

Step 1: Pre-treatment of incoming raw crude waste oil (grading) from day-tank.


(i) Pre-heating and agitation of day-tank stock through Dolphin’s own Expanded Metal Heat Loop (EMHL) system.
(ii) Introduction of Chemical / Polymer via the in-line static mixer.
(iii) Screening of large debris, rocks, organic material. Our hermetically isolated back-flushing strainer system.

 

Step 2: De-sludging of Heavy Solids (down to 200µ)


(i) Re-heating of strained crude oil.
(ii) Processing through Horizontal Decanter Centrifuge.

 

Step 3: Polishing Crude Oil for final recovery (down to 1µ; < 1% BS&W)


(i) Re-heating of de-sludged crude oil.
(ii) Processing through Dolphin’s specially modified Hi-Speed Disc Centrifuge. Special modifications allow for the separation of large volumes of salt-water.

Crude Oil Tank Bottom Recovery System Configurations (Fixed and Mobile)

 

Our ORS (Oil Recovery System) is available in 2 configurations. Each of these is briefly described below.

Skid Mounted Crude Oil Recovery System


In this configuration, each of the 3 process steps (described above) is accomplished through a separate skid-mounted package. In other words, the ‘pre-treatment system’ on one skid. The de-sludging unit with a decanter centrifuge is on a separate skid. Finally, the polishing skid is on a third, individual skid.

The design intent of this configuration is for indoor installation. A preexisting structure or building is an ideal location for skid mounted ORS.

 

Containerized Crude Oil Recovery System


The containerized ORS is perfect for remote field locations without the need for a preexisting structure. A prefabricated shipping container houses all 3 steps of the process listed above. It also includes servicing tools (builtin crane), internal lighting, service area, surge tanks, etc.

The containerized version of our ORS is suitable for worldwide installation. It is ideally suited for locations with minimal infrastructure.

 

Key factors affecting ORS efficiency


The ORS’s efficiency is dependent on several fluid properties and operating parameters. The following list describes the factors which have the greatest effect on separation efficiency.

 

Processing Temperature


Centrifugal operational efficiency is very dependent on the viscosity of the process fluid. Heavier oils are best processed at 180F. Lighter oils processed at 145F produce acceptable BS&W levels.

 

Homogeneity of Process Crude Oil


It is essential to keep the solids in suspension in the process oil. Feeding settled oil to a centrifuge can lead to unacceptable solids load on the equipment. This can lead to unplanned downtime and premature failure of equipment.

Besides, the operation of the “self-cleaning” disc centrifuge has a set sludge discharge interval. A non-homogenous product will lead to inconsistent separation performance.

 

Chemical / Polymer / Demulsifier


Emulsion breaking polymers are often needed to aid in the separation of slop oil with a significant emulsion phase.

In such cases, heating and centrifugation may not be adequate to produce the desired results of the produced oil. Chemicals help to split the oil-water emulsion.

 

Unique Features and Upgrades Specific to Dolphin’s ORS

 

Dolphin Centrifuge has incorporated several modifications to our Oil Recovery System. Our field experience led to these modifications. Customer feedback has also helped us make the system more robust for this application.

The following list highlights some of these modifications.

 

Builtin Pump Upgrade


Disc stack centrifuges have internal pumps to pump out the separated liquid phases. The original pump’s design allows a specific oil to water ratio. However, crude oil tank bottoms tend to have significant amounts of water.

We have modified the internal pumps to accommodate this extra volume of water.

 

Salt-water Handling


Crude oil tank bottoms contain salt-water. Key areas of our ORS incorporates stainless steel contact. This upgrade prevents corrosion which ensures the durability of the equipment.

 

Electrical Upgrades


Crude oil processing requires electrical system upgrades to allow the equipment to be used in hazardous areas. Dolphin’s ORS comes with Class 1 Div 1 or Div 2 electrical components.

 

Service Features


Shipping containers have space limitations. So we have incorporated builtin service cranes in our containerized ORS.

Service cranes provide ease of service inside the container. This eliminates the need for external lifting equipment.

Other features include internal lighting, local power sockets, slip-free floor covering, venting manifolds, etc.

Dolphin Centrifuge’s ORS Flow-Diagram

 

The following diagram shows the over-all flow process through our crude oil recovery system.

Crude Oil Centrifuge Process Diagram
Crude Oil Tank Bottom Centrifuge Process Diagram

Tank Bottom Recovery – Case Study


In this case study, we will illustrate the real-world use of our crude oil recovery system. The information & data shared in this application were actually observed on a tank bottom processing site.


Crude oil producer’s background


An oil producer operates several Salt Water Disposal (SWD) sites in the Permian and Anadarko basins. Before the deployment of Dolphin ORS – the main method of oil “recovery” was the age-old method of heat & settle.

Heater-treaters were a big part of the landscape. On this particular location, the producer has several 12 and 15 thousand barrel tanks. These tanks collect and store crude oil waste-streams trucked in from as far as 200 miles in all three states.

On a daily routine – the producer used to processes the oil from each of the tanks through the heater-treaters to heat the oil to about 180 F. Thereafter, settling the oil and skimming the sale oil from the top. The goal was to recover oil with least Basis Sediment and Water (BS&W)

The recovered oil (with over 1% BS&W) sells to tier 2 operators for polishing and recovery of sale oil.

 

Issues with the producers’ current process


The crude oil producer had several bottlenecks that are prevalent across this industry. We have summarized some of these issues in the following list.

 

Time to Process


Gravity settling takes time. So, the conventional ‘heating and settling’ process is time-consuming and thus costly. Also, the produced oil is often not to spec. This is due to the fact that the force of gravity is not enough to pull out all the BS&W from the oil. Producers lose considerable amounts of good oil in the tank bottoms.

 

Tank Bottom Sludge Volume Loss


The above-mentioned limitation generates large amounts of ‘tank bottom sludge’. The producer needs to address these ‘tank bottoms’ which is another added expense.

 

Transportation and Disposal Costs


Large volumes of ‘tank-bottoms’ lead to more transportation and disposal costs.

 

Emulsified Oil Loss


Collected oil often has an emulsified oil phase within. There is good oil trapped in this emulsion. Chemicals help break the emulsion but demulsifying agents are expensive. Chemical treatment requires additional equipment. This also adds to operating costs and maintenance.

 

Crude Oil tank Cleaning Costs


Finally, the producer has to handle the thick sludge left behind in the tank. Cleaning tank sludge is a tough and expensive job. With the conventional method, it is a regular exercise.

Enter Dolphin Centrifuge’s Crude Oil Recovery System (ORS)


Our design process started with site visits and many discussions. The producer then engaged Dolphin Centrifuge to supply a complete ORS for their crude oil recovery process.

The system manufactured by Dolphin is capable of processing up to 2 BPM of tank bottom crude oil. Dolphin designed and manufactured the complete system in consultation with the producer.

The producer’s technicians were instrumental in providing valuable feedback to our design team. Based on their input, we incorporated features specific to this particular site.

The producer’s technical team installed the system with support from Dolphin Centrifuge engineers. After the installation, Dolphin Centrifuge provided on-site startup help. Our startup service includes training for operators on site.

 

ORS Process Details


The Dump Tank (DT in the picture) holds the incoming collected oil. the operator transfers the daily volume of process oil to the Slop Oil Storage Tank (SOST).

Crude Oil Tank Bottom Dump Tank

During regular operation, the operator transfers the slop oil form SOST to the Surge Tank #1 (ST 1 in the picture). At this point, the operator introduces the chemical (if any) into ST#1.

The heat-loop (HL 1) has an in-line agitator for the thorough mixing of the polymer with the oil. This process occurs before straining through the Shale Shaker (SS) at this particular site.

Crude Oil Tank Bottom Shaker Screen

The pre-straining process separates organics (grass, twigs, etc.) and other debris (rocks, bones, etc.). For this particular grade of oil, the temperature for straining was 120 F.

From the shale-shaker, the oil is gravity fed to Storage Tank #2 (ST 2). All 3 storage tanks are continuously agitated utilizing low-speed paddle mixers.

Crude Oil Tank Bottom Decanter Centrifuge

A transfer pump feeds the oil through Heat Loop #2 (HL 2) to the Decanter Centrifuge on the overhead platform. The Decanter platform is designed to allow dump truck access for sludge dumping onto the lined truck bed. 

Decanter operation is optimized through the change of pond-depth & auger-speed. This balances maximum sludge dryness with the least sludge content in the oil for the disc centrifuge. 

For this particular grade of oil, the temperature for de-sludging through the decanter is at 150 F.

The SO going into the Decanter carried 12% (%v/v) of BS. The de-sludged oil contained 0.5%~1% (%v/v) of sludge content.

The de-sludged oil from the Decanter would then flow (by gravity) into Storage Tank #3 (ST 3).

At the final stage, the de-sludged oil from Storage Tank #3 (ST 3) is pumped through Heat Loop #3 (HL 3) to the high-speed disc-stack centrifuge (HSDC). This is the final stage for the removal of fine solids and water from the oil. The temperature for polishing through the HSDC is 180F.

Crude Oil Tank Bottom Disc Stack Centrifuge

The slop oil going into the disc-stack centrifuge had about 1% BS and about 32% Water. The disc-stack centrifuge was fine-tuned to produce 99.6% sale oil (<0.4% BS&W).

The grade of oil processed during the initial startup phase by Dolphin personnel was in the range of 13 ~18 API. This is a challenge to separate through conventional methods.

 

Results


We have categorized the results into fluid-balance (throughput) and ROI (Return-On-Investment)  categories below.

 

Fluid balance calculation


A fluid balance flow diagram below shows the actual production results. This data is from the startup phase on this particular customer site.

Crude Oil Tank Centrifuge Fluid Balance

 

ROI Calculation


Using the process flow-rates above, some preliminary ROI calculations follow.

 

Conclusion


Based on the conservative expectation of about 25% crude oil recovery – it is easy to see the short payback period for our Oil Recovery System. Specifically when used for processing crude oil tank bottoms.

 

Contact Dolphin Centrifuge for more information or if you have any questions. Or call us at (248) 522-2573.