As part of a turnkey project, VWS Westgarth engineers the chemical storage, dosing and injection system for the chemicals required to operate the SRP. More importantly VWS Westgarth's experience has shown that the choice and compatibility of chemicals and consumables will have a direct impact on the performance, availability and working lifetime of the SRP and its membranes.

VWS Westgarth has the experience and capability to implement a chemical & consumables management service. Under such an arrangement VWS Westgarth would be responsible for the supply of sufficient chemicals and consumables to maintain the plant at maximum availability and at a competitive price.

VWS Westgarth has the ability to engineer a cost effective chemical storage, dosing and injection system for the sulphate removal process. The factors that will influence the design of the systems are:

Storage Capacity

Typically the chemical storage tank size will be designed around the clients' logistics.

Pump Capacity

In today's industry, process demands are changing with an ever changing environment and therefore VWS Westgarth requires a versatile approach in their design. VWS Westgarth has the capability to provide an array of injection pumps to meet the clients' requirements such as turn down capacities. VWS Westgarth can advise the most economic solution whether it to be three single headed pumps or one triple headed pump.

Materials of Construction

VWS Westgarth can provide suitable materials for chemical storage, dosing and injection systems which will not only stand the aggressive environment of the offshore industry but also be compatible with the harsh chemicals required for the process.

Chemical dosing is an important factor in optimising the operation of the sulphate reduction plant. The diagram below shows the necessary chemicals and where they are injected.

Biological organisms, such as bacteria, are abundant in seawater and this causes fouling in process equipment. An economic solution to biological fouling is chlorination. The most economic approach is the use of electrolysis to generate sodium hypochlorite from seawater since the main components, electricity and chlorides are readily available.

Electrolysis is a method of separating bonded elements and compounds by passing an electric through them. The predominant ions in seawater are sodium (Na⁺¹) and chloride (Cl^-1) which are reacted according to the following equations:

Anode: 2 Cl^-1 - 2 e^-1 ------------> Cl₂

Cathode: 2 Na⁺¹ + 2 H₂0 +2e^-1 ------------> 2 NaOH + H₂

Combine: 2 NaCl + 2 H₂0 ------------> 2 NaOCl + 2 H₂

Chlorine is provided in the form of sodium hypochlorite

Fouling by colloids and silt are a major cause of loss in performance of membrane separation systems. Media filters are an economic solution to removing the particles. Coagulant is dosed to improve the efficiency of the filtration system and aid removal of organic components.

Due to the characteristics of polyamide SRP membranes, it is necessary to remove free chlorine in the feed water before passing through the membranes. Failure to do so results in hydrolysis of the membranes which irreversibly damages the membranes and radically reduces their performance. Free chlorine removal is achieved by the addition of Sodium Bisulphite. See chemical equation below.

Due to the permeation of the water, the salt in the seawater becomes more concentrated. This will result in the salts exceeding their solubility limits and precipitating onto the membrane. Antiscalant dosing is therefore necessary to minimise precipitation of the sparingly soluble salts.

Antiscalant is paramount for optimising the sulphate reduction process because without antiscalant dosing, scale crystals will form rapidly and this will cause a reduction in permeate production and also increase salt passage.

Due to the polyamide membranes' intolerance to chlorine and other oxidants, the chlorine is removed from the process and therefore there is the potential for biological growth to occur downstream from where the chlorine is removed. Biological growth results in increased cleaning and maintenance cost, poor product water quality and therefore element deterioration. Biofouling is controlled by the injection of non-oxidizing biocides into the SRP feed stream.

Primary oxygen removal is achieved by a vacuum Deaerator. Further oxygen removal is achieved by the addition of an oxygen scavenger downstream of the deaeration vessel. De-oxygenation of the water is required to reduce corrosion and aerobic bacterial growth. See chemical equation below.

2NaHSO₃ + O₂ = 2NaHSO₄

Antifoam is dosed upstream of the deaerator to minimise the effect of foaming in the deaerator.

As well as supplying chemical storage, dosing and injection systems for our own package, VWS Westgarth provides chemical storage, dosing and injection systems for the client's overall process. The advantage of this is one single point of contact, removing multiple project interfaces.

Examples of chemicals VWS Westgarth has provided to clients:

• Oil Defoamer
• Polyelectrolyte
• Client Antiscalant
• Client Biocide
• Wax Inhibitor
• Oil Demulsifer
• Gas Corrosion Inhibitor
• Gas Hydrate Inhibitor
• Nitrogen Generation System