CONSTRUCTION OF THE FIRST PROTOTYPE OF THE PUMP

Once the idea was developed, to build a skimmer equipped with a Archimedes screw pump but able to separate part of the recovered water, we start manufacturing of the prototype.

Although our final aim is to manufacture a robust pump capable of working with any type of fluid, and being stainless steel the material that responds to such requirements, we decided to make a first prototype using the technique of selective laser sintering of nylon powder.

In this way we built the first pump with a very fragile material, so that if it passed the test we would have guarantees of adjustment and operation to proceed with the very superior investment represented by the metallic prototype.

The technology used responded very well to expectations and not only. We got a prototype that geometrically validated our design but it was also a functional prototype with which we could verify the hydraulic behavour of the pump.

LET'S START!

Analyzing performance and design of the available pumps we are inclined to use screw pumps in our aim to reduce water content in the skimmer.

Its geometry and the way in which power transmission takes place makes us think that they are the most suitable to work together with a centrifugal separator.

Now is the time to take action: We will create a prototype that will allow us to verify the results of our theoretical study.

The challenge in front of us is huge. We have the need to design and manufacture three devices in one: a weir skimmer, an Archimedes screw pump and a centrifugal separator.

Since we have a long experience in skimmers manufacture we will leave it for the last moment, so we will concentrate at first on the design and manufacture of the pump as it represents the core of this new equipment for Oil Spill Response. We will call it:

OWSKIMMER

RECOVERED OIL PUMPING

In order to find a solution to the problem of the huge amount of water recovered together with the spilled hydrocarbons we need to know how these are pumped from the skimmers, because their knowledge will help us to find the best way to integrate the industrial separation methods presented in the previous post.

Once recovered the spilled oil using the different types of existing skimmer we need to pump it to the temporary storage tanks.

Sometimes the skimmer's workplace and the location of the operator advise the use of a suction pump or on the contrary a delivery pump.

When access to the spill zone is complicated it may be more interesting to use lightweight skimmers by working with a self-priming pump located in the proximity but on the ground or on the deck of the response vessel. This configuration has an important limitation, the pump maximum suction head. A self-priming pump will never be able to suck from a height greater than 9.8 meters, a height that is reduced as the viscosity of the product to be pumped increases.

To avoid this handicap skimmers can be equipped with transfer pumps. In this way the limitation of height becomes a minor problem. A logistical problem arises; however, to move and raise these skimmers it is necessary to use cranes.

Among the first type of pumps, the most used are lobes pumps, peristaltic pumps and membrane pumps. All are self-priming positive displacement pumps capable of working with high viscosities fluids even in the presence of solids. The differences between them focus on the necessary maintenance, diameter of solids, ability to work dry and price. In any case, they are bulky and heavy pumps, characteristics that hinder, but not prevent, their use as a transfer pump in skimmers.

Usually skimmers equipped with pumps use the following types:

• Lobe pumps: their weight does not make them the best solution
• Open impeller axial centrifugal pumps: emulsify the water-hydrocarbon mixture and work especially poorly with viscous hydrocarbons.
• Sealing disc Archimedes Screw Pumps: the most widely used and best-performing pumps to be uses with water-hydrocarbon mixtures and viscous hydrocarbons.
  

INDUSTRIAL SEPARATION TECHNIQUES

In this post we will present different methods of oil/water separation and will analyze feasibility of using them to improve the efficiency of skimmers and pumping units.

Gravity Differential Separation: The oldest and more extended separation method. It’s usually the first step in the treatment of oily water. Ruled by the Stokes' law is based on the different density of two immiscible fluids:

•         The more different densities are, the better separation.
•         To reach higher efficiency a long residence times and large oil drops are required.

Normally, separation efficiency ranges between 20% and 60%. However, to accelerate the process can make use of coalescence.

Separator Types:

•         API separators
•         Circular separators
•         Plate separators (parallel and corrugated)
•         Curved-Plate Finger Separators

Rotational Separation: It was conceived to accelerate the gravity differential separation processes, since centrifugal forces are much greater than gravity. A difference of 5% between densities is enough to separate fluids, but the larger is such difference the faster the separation and the lower the energy consumption. The separation efficiency ranges between 77% and 91%. Using coalescence these values may increase.

Types:

• Centrifuges: Oily water is moved along a circular path by the rotational motion of the device.
• Cyclones: The liquid is forced into circular motion due to tangential injection of the oil/water misture.
• Vortex: Separation of oil/water mixtures is accomplished by imparting relatively large rotational motion to the mixture, in a cylindrical vessel.

Filtering is one of the oldest methods used. It is very effective for removing suspended matter especially hydrocarbons.

•       Multimedia: consists of passing the oil/water mixture through a set of layers of different solid materials of different size and density. Its biggest advantage is the lower pressure drop for high flow rates to be treated without reducing the quality of the effluent.
•     Absorption adsorption filters: The activated carbon and graphene, during the recent years, have demonstrated a high adsorption capacity. Saturation of cartridges and regeneration of materials are the main problems that researchers are facing today to overcome the barrier of feasibility. Efficiency between 95 and 100%.

Membrane: In recent decades they have developed water-repellent membranes and hydrophilic materials. To be effective, The membranes must be chosen according to the characteristics of the substance to be separated and find difficulties to separate viscous hydrocarbons. It is still a very expensive technology and can be justified as the last filtration step to remove the last traces of hydrocarbon from water.

Electrodialysis: Separation takes place through the application of an electric current. It has found greater application in the field of desalination plants.

Reverse Osmosis: Applying a pressure to the substance to treat oil and water are separated on the membrane surface since the former have a molecular dimension greater than water, that can pass through the membrane.

Ultrafiltration: Similar to the reverse osmosis works at much lower pressures.

Coalescence: Used to group dispersed hydrocarbons in aqueous solution. Generally it helps speed the aforementioned separation processes (gravitational, rotational, filtration). In some cases you can reach efficiency values ​​of 90-98%

Other technologies:

•         Electromagnetic separation
•         Thermal separation
•         Ultrasonic separation
•         Chromatographic separation

To achieve an almost perfect separation we will need the succession of different methods used in cascade with the right combination to meet the needs of each case.

Reverse osmosis, ultrafiltration and adsorption by active carbon or graphene are the most potential methods but entail high operating costs, mainly energetic, and auxiliary equipment. Meanwhile gravity separation needs major work volumes and long residence times is used as a pretreatment. While filters, because of their ease of saturation and high costs in some cases are suitable for a final stage of separation in which well reach 100% efficiency.

This analysis of separation technologies follows that the most appropriate to improve skimmers performance are coalescence and rotational separation.

A NEW ROUTE OPENING

The goal has been focused: We need to increase the efficiency of skimmers and improve the energy balance of decontamination operations.

We could analyze step by step response operations up to final treatment trying to adopt better techniques and integrating low consumption motorizations, we will, but there is a job we can do before and will contribute more than anything else to the improvement we seek: We need to separate the maximum amount of water possible inside the skimmer and before pumping. 

We will analyze the different separation techniques that are being used in industry and will search for ways to integrate them into recovery equipment.

SOLUTIONS TO THE PROBLEM OF MECHANICAL RECOVERY EFFICIENCY

As mentioned in previous post, the low efficiency of mechanical recovery devices causes high additional costs of decontamination and purification.

Among the factors that impact on mechanical recovery efficiency we have cited the weather conditions and the type of skimmer. Improving weather is not something that is in our hands but improve existing skimmers performance and create new recovery systems.

Skimmers

Several manufacturers of mechanical skimmers and recovery systems are making significant efforts in innovation to improve its products and propose new ones that improve the performance of the above. Unfortunately only a few manufacturers have chosen the path of innovation versus a majority stake by traditional products almost unchanged since its release.

At present research is focused on increasing skimmer efficiency working on material and geometry of the contact surface with the hydrocarbon; in the case of drum skimmers creating perimetral grooves that increase the capacity and broaden the spectrum of recoverable hydrocarbons. In the case of disk skimmers these grooves are lateral and increase only recovery capacity.

We have chosen these two types of skimmers because they are the ones that offer higher efficiency, but in contrast they are more limited when working with viscous fluids.

With regard to brush skimmers, more suitable for higher viscosity fluids as above, little has been achieved. Manufacturers usually change brush configuration that seems more focused on reducing costs than on increasing efficiency.

Finally, we refer to the weir skimmers, perhaps the most sold worldwide. Technological advances experienced by these skimmers have been throughout its history almost non existent.

Other recovery systems

To improve performance of skimmers we could also act on their working environment. Improve working conditions help to increase performance.

At present there are different systems that with the assistant of booms and barges allow to concentrate hydrocarbons near the skimmer creating a thicker layer enough to minimize the water content in the recovered substance.

Unfortunately, and although good skimmer and recovery systems builders insist weather ends up being decisive and minimizes progress in innovation. Their improvements are important but not enough.

At this point, what is left to do? Why not act on the pumping phase to minimize the presence of water?

ENERGY BALANCE OF MECHANICAL RECOVERY

Oil spill recovery operations have a huge energy cost as well as causing a secondary pollution caused by the use of polluting engines.

Besides the use of antediluvian technologies, the largest contribution to energy consumption growth during the recovery operations is caused by the decrease in the efficiency of skimmers, or what is the same, the increased volume of water in the recovered substance.

For example, a weir skimmer recovers a large amount of water in a range from 50% (layer thickness above 25 mm) and 90% (layer thicknesses between 1 and 8 mm), this means that if we are facing an oil spill of 1,000 m³ we will collect between 1,000 m³ and 9,000 m³ of water. Among other things, we are multiplying by 10 the time invested in recovery.

A selective skimmer (brushes, discs, drum) can work at a maximum efficiency of 95% (5% water in the recovered substance), that will be greatly harmed by the state of sea, to reduce it to 50%. In this case we will recover between 50 m³ and 1,000 m³ of water.

The consequence of skimmers inefficiency is an increase in energy cost of recovery operations broken down as follows:

1. Increase the operating time of  skimmers. In the case of a skimmer working at 50% efficiency we have to operate twice as long compared to an ideal situation (95-100% effectiveness). A selective skimmer working at 50% consumes 66 kWh compared to 35 kWh of a 95% efficient skimmer.
2. Increase in the transfer pump total consumption (between skimmers and temporary storage tanks). In the case of a weir skimmer working with a oil layer thickness exceeding from 25 mm, energy required to pump 1,000 m³ of spilled oil plus other 1,000 m³ of water is 1.120 kWh while in the case of thickness lower than one centimeter energy consumption soars to 5.600 kWh. In an ideal situation, considering an efficiency of 95%, energy consumption would be 590 kWh, between a 50% and a 90% lower than real cases.
3. Increase in the transfer pumps total consumption (between temporary storage tanks and oily water separator). We’ll be in a similar situation to point 2.
4. Energy cost of separation. In the working vessel we will separate most of the water from hydrocarbons by means of an oily water separator. This water should have a hydrocarbons content below 15 ppm. Energy cost of this operation amounts to 1.800 kWh in the first case and 16.200 kWh in the second one. If we had worked with a truly efficient skimmer (95%) we would have spent 100 kWh to 1.000 m³ of spilled oil.

The above calculations are extremely approximate and may suffer significant variations depending on many factors but they offer us a striking confirmation of the importance to improve methods of oil spill recovery, because we are not talking only about an economic cost but a higher cost in terms of operability, responsiveness and autonomy of the media displaced to the spill.