Designing a crude unit heat exchanger network
Preheat train design for heavy Canadian crudes can be very challenging, requiring an approach not normally required with other crudes
Tony Barletta and Steve White, Process Consulting Services
Krishnan Chunangad, Lummus Technology Heat Transfer
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A well-designed crude and vacuum unit (CDU/VDU) heat exchanger network is essential to meet product yield, product quality, unit reliability and crude processing flexibility objectives when processing heavy crudes. Preheat trains conceived with the wrong flow scheme or those with multiple parallel paths that are complex to operate rarely have the flexibility needed to handle a range of crude blends or even the variability of many heavy Canadian crudes. Standard shell and tube exchangers designed with low velocity are prone to rapid and heavy fouling.
It is becoming ever more important to temper crude train design that has been developed from composite curves, optimal energy targets and pinch points with crude unit experience and know-how. Practical concerns include operability, reliability, exchanger type and minimal fouling design. Real-world experience using flexible preheat networks, good exchanger design practices and proven exchanger technology is proving to be more important than theory. This article covers practical considerations when designing CDU/VDU preheat networks for heavy crude processing.
Heavy crude challenges
The desalter is an integral part of the crude unit, and unit reliability is directly related to desalter performance. Desalting is becoming increasingly important as crudes get heavier and contain more contaminants that increase the difficulty to desalt. Poorly performing desalters with a high desalted crude salt content are dramatically increasing unit corrosion and wreaking havoc on unit reliability.
With heavy crude processing —particularly with some heavy Canadian crudes — it is becoming more important to have the flexibility to operate the desalter at an optimum temperature. The optimum desalter temperature is no longer a single, fixed design target; it is an operating variable that must be adjusted to maintain peak desalter performance. In heavy Canadian crude processing, the optimum temperature can change by 15-25°C, depending on the crude or crude blend, to avoid massive asphaltene precipitation. The ability to change the desalter temperature by 15-25°C must be part of the preheat train’s design objectives. This requirement must be identified, since a preheat train designed with normal methods will not provide the massive amount of swing heat that will need to be shifted to/from the cold crude train.
Many heavy Canadian crudes have a high asphaltene and solids content and considerably higher fouling potential compared with other crudes. Blending these crudes with paraffinic diluents or other paraffinic crudes can precipitate asphaltenes in the preheat train or desalter. Special exchanger design considerations are required to reduce fouling.
Refiners have noticed a variable composition with some heavy Canadian crudes. Some of these crudes, such as Western Canadian Select (WCS), are blends of other heavy crudes. As blend ratios change, so does the composition. Some heavy Canadian crudes are distillate laden, while others contain more gas oil. It is becoming apparent that refiners with preheat trains flexible enough to handle variable product yields will benefit most from processing these crudes.
Current network design practice
CDU/VDU preheat train designs are relying more and more on theoretical constructs such as pinch analysis without sufficiently considering realities such as fouling tendency and system operating flexibility required for heavy crude processing. Advances in computer speed and easy-to-use targeting programs have made pinch analysis a prerequisite for network design. While pinch technology can be a very useful tool, a preheat network cannot be designed for a single theoretical optimum point, nor can it ignore the practical realities of running today’s ever more challenging crudes.
The preheat train is part of an integrated system and needs to have the flexibility to process varying crude blends while meeting seasonal or economic product yield targets. These days, few refiners have the luxury of running one crude or even a consistent crude blend. Increasingly, refiners are processing larger quantities of opportunity or heavy crudes to remain profitable.
To make matters worse, outmoded design approaches that rely on exchanger experts and vendors to design around an allowable pressure drop, without sufficient understanding of the integrated system, continue to be used. Today, most projects use system engineers to develop P&IDs, with hydraulic calculations for each circuit setting hydraulic allowances for exchangers, control valves, strainers and other equipment. In many cases, system engineers do not do the process modelling and therefore do not have a thorough understanding of the variability of all potential operating scenarios that are required of the exchanger network.
This approach then uses in-house specialists or vendors to design the exchangers, with a major focus on allowable pressure drop. While this approach may be efficient from an execution standpoint, it is a prescription for poor performance. Reliable, low fouling exchangers should be the goal of preheat train design, with pressure drop simply a factor that the hydraulic system needs to handle.
Crude unit exchanger networks must operate reliably for four to six years. Proprietary exchanger technologies with helical baffle designs, such as the HelixChanger heat exchanger, have proven essential in reducing fouling when designed at high velocity. Yet, many recent designs have not taken advantage of the benefits of this technology because of the perceived added exchanger cost. It is not surprising, then, that many crude heater inlet temperatures degrade by 25-40°C within the first few months of operation.
A more effective approach
Exchanger networks must have the flexibility to meet critical objectives such as desalter temperature in addition to satisfying column heat balances that may be variable as a result of changes in crude composition. Process engineers must first identify the need for and degree of flexibility required for specific crudes or crude blends and make that flexibility requirement part of the preheat train design. There must be more interaction and communication between process and systems engineers to assure flexibility is incorporated into designs.
Secondly, strict adherence to “allowable pressure drop” as the main design criterion must be tempered so that low-velocity, high-fouling designs can be replaced with high-velocity, low-fouling designs. Larger acceptance of the benefits of reduced fouling that a properly designed exchanger can bring is needed.
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