Overcome the challenges of cooling desalter water effluent
Crude oil desalting is a process in which crude oil is contacted with hot water and chemicals to remove impurities such as chloride salts and particulate matter from crude in the preheat train. A specialized piece of equipment called an electrostatic coalescer or desalter is integral to the process, along with the associated heat exchangers that supply crude and water at the appropriate temperatures for efficient desalting. The produced water, salts and sediment flows to the bottom of the desalter where it is cooled and then sent to wastewater treatment. (See figure 1) This article will address the challenges in cooling desalter water effluent; we’ll also share our experience in mitigating these challenges with Alfa Laval’s spiral plate heat exchanger technology.
Figure 1: Simplified Desalter effluent cooling system with Alfa Laval spirals installed
Desalter water composition basis
Desalter “water” is not really water at all; it is a mixture of metal salts, particulate matter and even heavy oils and asphaltines. Desalter effluent water quality also varies widely from crude to crude, and from day to day. Therefore, any heat exchanger designed to cool only water will undoubtedly fail when subjected to common desalter conditions. Providing a heat exchanger to successfully cool desalter water requires, not only an understanding of the design basis, but also the transient conditions like desalter upsets and mudwashing* that significantly change the water quality. Reliably cooling this water is important, since it makes up a significant portion of the total refinery wastewater and high temperatures will destroy downstream biological processes. This is a common motivation for refiners to seek better technology for cooling desalter effluent water.
Figure 2: Spiral plate heat exchanger flow paths
Heat transfer challenges
The predominant challenge to successful heat exchange on the water side of the desalter is fouling and plugging. Fouling can occur on either (or both) side(s) of the heat exchanger. As the feed water is heated, fouling can occur depending on the source of the water. If the water is fresh water or well water, minerals can deposit on the heat exchanger as the water is heated. Other sources of water, like drum boot waters, carry a significant amount of particulate matter and trace oils that affect heat transfer. Having knowledge of these fouling mechanisms and design techniques to limit their impact is an important part of heat exchanger design.
The desalter effluent water is usually the most problematic due to its composition. The water, now contaminated with oil, suspended solids and dissolved solids, is cooled in two stages before discharge to the wastewater treatment plant by way of the API separator. Oil often coats the heat transfer surface area, rendering sophisticated design tools useless to accurately model performance. Solids from the desalter can also plug heat exchanger channels, requiring unplanned downtime for mechanical cleaning. Lately, Alfa Laval has seen heavy oils and asphaltines carry-under with the water, mixing with suspended solids and severely plugging heat exchangers. This phenomenon is most noticeable in refineries that process heavy bitumen such as Canadian crudes and will foul any type of heat exchanger. Deploying a heat exchanger designed to minimizing the impact of these fouling mechanisms is critical to reliable performance.
Over the years, Alfa Laval has gained experience with many heat exchanger types in desalter water service, including various plate and shell and tube types. Most heat exchanger types struggle to address the various fouling mechanisms and deliver disappointing performance. The main reason is most heat exchangers use parallel channels or tubes to distribute flow inside the heat exchanger. During mudwashing or solids upsets, it is impossible to distribute the solids evenly to all tubes, so many channels foul and plug in a short time (Figure 3). This results in a hydraulic restriction, as well as diminished heat transfer, and can lead to unplanned shutdowns. A heat exchanger that eliminates the preferential flow paths of parallel heat exchangers is needed to successfully pass solids; and this is the key to the operation of spiral plate heat exchangers.
Figure 3: S&T heat exchanger operating 1 month in desalter service
The spiral plate is a single-channel heat transfer device that will pass solids through the channel without becoming lodged in the heat exchanger. (Figure 4) This eliminates the hydraulic restriction concern and allows the spiral to operate without unplanned shutdowns. Other deposition fouling mechanisms such as asphaltine deposits will inevitably stick to any heat exchanger surface, spiral plate included. The impact of this fouling mechanism is lowered heat transfer performance and pressure drop increase. Our experience has shown that while the spiral plate exchanger can foul in this specific type of mechanism, the runtime can be 3-5 times longer than other technologies due to the high channel velocities and single channel arrangement of the spiral plate technology. Another advantage is when cleaning the spiral plate is needed, steamout or chemical cleaning is very effective since there are no plugged areas of the heat exchanger, ensuring all cleaning chemicals reach the thin fouling coating on the surface. Mechanical cleaning is also easy to perform by removing one or both covers from the unit and hydroblasting with typical refinery tools.
Figure 4: Spiral plate exchanger cold and hot side after two months operation in the same desalter. No cleaning needed.
If these challenges sound familiar to you, give one of Alfa Laval’s refinery experts a call to discuss potential solutions. A spiral plate exchanger may be exactly what you need.
Chris Wajciechowski is Business Development Manager for Alfa Laval, applying high-efficiency and problem-solving heat exchangers in the Refining industry for over 20 years. His focus is on improving process performance and increasing reliability, thereby improving the profitability of refineries worldwide. Chris holds a Bachelor of Science in Chemical Engineering from Virginia Tech and is a licensed Professional Engineer in the USA.
SpiralPro heat exchangers
This animation illustrates how a spiral heat exchanger functions when operating in fully counter-current mode as a horizontally installed heat exchanger for heating, cooling or recovering energy from process liquids.