Water Treatment Cooling Innovations for the Oil & Gas Industry
New technologies to treat water in refineries and petrochemical plants: Giving the right water treatment solution to refineries and petrochemical plants is a big challenge. Nalco has addressed this problem with a holistic cooling water management system.
In the current industrial scenario, refining and petrochemical cooling water systems place most difficult challenges before the water treatment industry. Conditions like high exchanger skin temperatures, variability in heat load, low water velocities due to complex plant and piping layouts, degrading quality of makeup water, risk of hydrocarbon contamination, tightening environmental restrictions and intense pressure on capital and consumable budgets demand that these cooling water systems operate under maximum stress.
In the midst of the above challenges, today’s refineries and petrochemical plants set high expectations from their water treatment solution providers to achieve the following goals:
- Asset integrity and reliability
- Production efficiency
- Process throughput
- Process improvement
- Health, safety and environmental compliance
- Consistent cost optimization
Refineries and petrochemical cooling water system problems Due to their typical operating environment, almost every refinery or petrochemicals cooling water system will face any of the four major problems, viz. potential to form scaling, aggravating conditions for corrosion reactions, transfer of suspended solids leading to fouling and, providing source and substrate for microbiological inoculation and growth. Since, a complex interrelationship exists between all these four water related problems, there is a need to address all of them with equal importance.
Root Causes of Major Water–Related Problems
In all cooling water (CW) systems in the refining and petrochemicals unit, Nalco has consistently adopted Mechanical- Operational-Chemical (MOC) approach to identify and understand all the stress factors, which eventually lead to the four major problems mentioned above. Some common stress factors as observed during Nalco’s MOC studies are depicted on the next page...
Mechanical Stresses for Refinery Cooling and Water Systemsn
- In refineries, the crude unit overhead condensers are the most critical cooling water exchangers in the whole plant. These condensers are located as close as possible to the vacuum towers to minimize the pressure drop. This arrangement makes the condensers vulnerable to low cooling water supply pressure, thus making them prone to fouling.
- Air-finned coolers and condensers often used in the refinery light end fractionation units for both overhead condensers and product coolers. Generally, these are located ahead of cooling water exchangers. If the
- performance of the air-finned coolers drops off, it places more demand on the cooling water condensers resulting in high skin temperature and potential for scale formation. A similar situation can also arise
- in separator condenser/cooler in the hydrotreater unit due to the performance drop in upstream fin-fanned coolers.
- Another threat in refinery fractionator overhead condensers is that the process side carries light hydrocarbon containing mercaptan sulphur compounds similar to H2S. Leaks into the cooling water system
- will form zinc sulphides, consume chlorine and hence lead to corrosion and microbiological excursion.
- In ethylene plants, the maximum heat duty is taken up by a few critical heat exchangers, namely quench water coolers, acetylene converter intercoolers, depropanizer overhead condensers, C3 splitter overhead condensers, propylene refrigerant condenser and cracked gas/refrigeration surface condenser. In many of these heat exchangers, the cooling water outlet is throttled since temperature control in the process side is highly critical. This affects the cooling water flow and velocity leading to increased skin temperature and high potential for scaling.
Operational Stresses – When the Going Gets Tough...
- Considering the variable system stresses in different refinery and petrochemical processes (as described above), control on water chemistry becomes extremely essential. However, in today’s highly competitive scenario, manpower resources are heavily optimized. Thus, conventional monitoring and control methods become inadequate as they are based on a few routine testing of grab samples and are slow in response to unusual upset situations.
- Overfeeding of chlorine/hypochlorite (for microbiological control) and acid (for pH control) due to manual operational error can create highly corrosive environments in the water system.
- Poor phosphate control is another critical stress factor in cooling water system. Maintaining too low phosphate levels will cause corrosion, but too high phosphate level will lead to hard Ca3(PO4)2 or FePO4 deposition and violation of discharge limits.
- In a polyphosphate-based treatment program, the reversion of polyphosphate in a high-temperature situation increases the total inorganic phosphate (TIP) in the circulating water independent of cycles of concentration; thus, leading to potential for scaling.
- Unattended passing blow-down valves or pump glands may cause uncontrolled water leakage from the system, thereby bringing the chemical inhibitor levels down below the safe limit leading to scaling, corrosion or microbiological problem.
Discover how chemical properties can affect cooing assets - especially in petrochemical installations on page 3...
Chemical Stresses – How to Cope With Varying Water quality
- Multiple sourcing of makeup water (bore well, river, RO pretreated, recycled wastewater, etc.) for cooling towers will create a lot of variability in the water quality based on the changes in mix ratio.
- Scarce supply of water often forces the utilities department to operate with reduced blow-down and very high cycles of concentration, which increases the ionic load in the water and also increases the Holding Time Index (HTI). The HTI is a measure of the time needed for any species in the cooling system to fall to half its original concentration. Longer holding times will stress the dispersant polymers and cause reversion of most polyphosphates to orthophosphate.
- Stringent discharge regulations have driven many refineries and petrochemical plants to look for recycling options of wastewater. When used as cooling water makeup, the biological oxygen demand (BOD) and chemical oxygen demand (COD) levels remain on the brink of the threshold limits.
Disadvantages of Conventional Treatment Methods
Can conventional chemical treatment methods detect and respond to these highly dynamic and variable stress factors? Perhaps not. Conventional chemical treatment operates on one-off survey based control parameters. The chemical dosing equipment is designed for metering fixed stroke and frequency. The treatment activities have to be monitored and maintained through ‘grab’ sample tests, residuals or on-line analyzers. At best, residuals are restored to ‘precalculated’ set points. Often, change is made for a single, ‘out of spec’ parameter and not considering all the KPIs or operational conditions holistically. Thus, these systems are not intelligent enough to respond to any unusual system stress. (see Fig 2).
Dynamic Stress Management – How it's Done
By thoroughly understanding the dynamic stress conditions in various process plants (through years of plant data analyses and bench scale and pilot scale simulation studies), Nalco Researchers have developed the 3D TRASAR technology, a holistic cooling water management system based on the three key principles – detect-determine-deliver. This new technology continuously measures key parameters related to system stress, e.g. pH variability, process contamination, changes to makeup water sources and quality, process changes and heat load variation. It detects upset conditions and takes automatic corrective actions. It controls scale, corrosion and microbiological fouling through precise system monitoring and control.
3D TRASAR Technology
An efficiently controlled cooling water treatment always considers the polymeric dispersant as its backbone, as it not only inhibits scaling and fouling, but also stabilizes the corrosion inhibitors for better cathodic and anodic protection. Both the mechanisms of scale inhibition and dispersancy result in consumption of polymer, and therefore, to ensure ongoing cleanliness, fresh supplies of polymer are continuously required.
The 3D TRASAR program manages the residual active polymer using real-time monitoring and control. Until the introduction of 3D TRASAR technology, it was impossible to measure and control active polymer online. This meant that when particles or salts increased within the cooling system due to changes in water quality, or changes in blowdown, the activity of the polymer was reduced often to below the minimum concentration requirement. 3D TRASAR technology has the ability to automatically detect the loss of “active polymer” and ensure that the correct concentration of polymer is dosed to maintain the optimal level for protection. This, therefore, prevents the polymer becoming overwhelmed with activity dropping to low levels and subsequent deposition.
Real Time Stress Management – The Key Technology?
This real-time stress management capability enables 3D TRASAR Technology to optimize the chemical dosing as per the system demand i.e. higher dosage during high-stress condition and lowering the dosing once stress is abated and normal condition is restored.
Furthermore, this new technology is uniquely positioned to help refining and petrochemical plants to improve process and production efficiency, environmental performance and sustainability. It can directly influence the heart of the refinery and petrochemicals production processes by maximizing heat-exchange efficiency, minimizing demand for fresh water and reducing wastewater flow and the chemical additives contained within.
Here are Few Examples:
- Increasing cooling water cycles of concentration reduces water use, but the reduction comes at a cost. Stress on cooling systems increase proportionately when cycles of concentration increase. With higher stress comes the risk of scaling, corrosion and microbial fouling. 3D TRASAR technology helps to manage the increased system stress by real-time detection and replenishment of chemical actives, thus allowing customers to increase the ionic load in the CW system and save precious makeup water without risking exchanger performance. Reduced makeup consumption also helps in the plants’ TCO reduction and meeting their sustainability goals.
- The heat exchanger’s heat transfer coefficient is a key metric used to measure cooling water system performance. Decreasing heat exchanger performance will become a limiting factor during increased production demand. With effective control of water quality and chemical actives, 3D TRASAR can bring significant improvement in the heat transfer coefficient leading to efficient production and process management and huge TCO reduction. It also reduces the need for expensive mechanical cleaning.
- For example, the fouling of the refinery vacuum overhead condensers in the crude unit will increase the flash zone pressure in the vacuum tower leading to increase in low value residual oil production at the expense of high value cat cracker feed. Similarly, in the ethylene plant, fouling of the C3 splitter overhead condenser may cut back the reflux leading to lower-grade propylene.
- Nalco 3D TRASAR technology’s positive role in maintaining heat exchanger cleanliness helps to reduce the energy consumption in the upstream and downstream processes. Thus, it indirectly impacts the emission to air. This stress management technology also helps in running the cooling water system with wastewater treatment recycled makeup; thus, reducing wastewater discharge. Thus, Nalco 3D TRASAR technology proved to be a holistic cooling water system management with direct contribution to refinery and petrochemicals TCO reduction, energy savings and environmental sustainability.