One of the main purposes of the IIoT, and of analytics tools like Plantweb Insight Applications, is gathering and using data to improve plant performance and profitability. This idea can and should extend to cooling system performance.
Refinery managers need to closely monitor cooling systems to avoid costly problems, and these systems are often surprisingly simple and inexpensive to monitor and control. This is the main point of my article in the November 2019 issue of Hydrocarbon Processing, Improved Cooling System Performance Begins with Data. Why is cooling system performance so important? Because it takes lots of energy and water to refine oil.
Refineries consume large amounts of energy and water to refine crude oil into products. Up to 10% of crude oil’s energy content is consumed during processing, and it takes 1.5 bbl of water to process one barrel of crude oil. Refining processes also generate large quantities of excess thermal energy that needs to be expelled into the environment using a once-through or recirculating cooling system.
That’s a lot of water and accompanying costs. Consequently, the mechanisms which handle these processes merit serious consideration.
Unless a raw water source is abundant and readily available, recirculating cooling water as much as possible is critical not only to reduce the cost of water treatment, but also to conserve the water supply. Unlike once-through systems, recirculating systems reuse the cooling water and employ evaporative cooling towers to transfer heat from the process to the atmosphere. Evaporative cooling towers have high construction, operational and maintenance costs, while consuming large quantities of water, often as much as 90% of the total water consumption in a refinery.
Yes, that’s what it said: 90% of total water consumption. The article has extensive discussion of ways to monitor the mechanical elements of cooling towers, which are well worth following up. But for right now, let’s look at the water itself and examine ways to get more out of each gallon by figuring out its condition so it can be used more effectively. Water quality in this context is calculated in cycles of concentration (COC), which compares the dissolved minerals in the recirculating water compared to fresh makeup water. A high COC value suggests water flowing through the system has picked up lots of unwanted stuff.
Increasing COC introduces several problems that can impact cooling system performance, such as corrosion, scale deposition, fouling from airborne contaminants, microbiological growth and degradation of a cooling tower’s structural integrity. The severity of these problems depends on multiple parameters such as chemical composition of the makeup water, cooling tower location, cooling system materials of construction and operating conditions. In addition, these problems are interrelated and addressing one may exacerbate the other. For example, lowering pH of the cooling water by adding acid can help control scale deposition, but may intensify corrosion and make controlling certain types of microbiological growth more difficult.
It's easy to see that the condition of recirculating water can rapidly degrade, which takes a toll on equipment in a variety of ways. There are methods to treat water in the system, but these can be costly, and trying to solve one problem may make another worse. Fortunately, these factors can be measured in real time with the right instrumentation to determine exactly what needs to happen.
Effective control of COC and chemical treatment to maintain water quality requires continuous online measurement of water quality. At a minimum, plants should continuously monitor their cooling water pH and conductivity, using temperature-compensated pH sensors to monitor the alkalinity of the cooling water, along with conductivity sensors to monitor the concentration of dissolved minerals to maintain an optimal COC. General-purpose pH sensors and contacting conductivity sensors are suitable for most cooling water systems; however, for systems with a high degree of fouling, pH sensors resistant to fouling and toroidal conductivity sensors are recommended.
An excellent place to start when looking at better instrumentation is Emerson’s Rosemount 3900 General Purpose pH/ORP Sensor. This combination sensor's double-junction reference improves its resistance to harsh environments and helps prolong sensor life. For conductivity, the Rosemount 400 Contacting Conductivity Sensor can accurately measure electrolytic conductivity in a broad range of applications from high purity water to clean cooling water. For water with an especially high COC value, there are other sensors designed to handle tougher applications.
You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Liquid Analytical Group and other specialty areas for suggestions and answers.
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