Hydroprocessing units such as hydrotreaters and hydrocrackers, and other units such as vacuum distillation towers, visbreakers, fluid catalytic crackers, etc., involve considerable heat consumption and subsequent heat recovery. Heat recovery is mainly achieved with reactor effluents, fractionator pumparounds or overhead condensing vapors, depending on a plants process flowsheet. Invariably, this heat is used to preheat feed or generate steam. Some recoveries take place in a kettle-type heat exchanger.
Kettle exchangers are mainly used for boiling in the shell sidee.g., steam generation. Normally, cold liquid entering the exchanger is close to its boiling point with respect to the given fluid pressure. The hot fluid flowing in the tube side may be two-phase overhead condensing vapor, reactor effluent, single-phase fractionator bottoms or bottoms from the fractionator pumparound.
For medium-pressure (MP) steam generation, boiler feedwater (BFW) from the deaerator is preheated by heat from available intermediate or final product streams. Preheated BFW close to its boiling point enters the kettle where MP steam is generated.
However, in some revamp cases, BFW available at the site is directly allocated in the kettle for MP steam generation. In such cases, the kettle is expected to handle a sensible heat load that is more than 10% to 15% of the total heat duty. This situation leads to an uneconomical design, resulting in ineffective heat transfer and fouling. To understand this phenomenon, it is helpful to have an understanding of boiling basics.
Basics of boiling in the kettle.
Kettles are unbaffled heat exchangers. The tubes are supported by full baffles. The tube bundle is submerged below a pool of liquid, and nucleate boiling phenomena normally occur. In a kettle, liquid generally enters through the bottom of the shell. Heat is transferred in nucleate boiling through the combined effect of liquid-free convection and additional convection produced by the rising stream of bubbles. Liquid-free convection occurs due to the density difference in the liquid pool.
In MP steam generation, BFW entering at its boiling point is further heated by coming into contact with the hot tube surface, and becomes lighter. The lighter water stream rises across the bundle between the tube pitch. With greater heat intake, bubbles are generated on nucleation sites on the tube surface. These bubbles enlarge and then disengage and rise above the liquid. Hence, across the tube bundle, the warm water gradually rises up along with the bubbles. Meanwhile, around the periphery of the tube bundle, dense, cooler water settles below. The rising bubbles and the density difference help circulate the pool. The circulation across the bundle in a kettle is shown in Fig. 1.1
Fig. 1. Liquid circulation in the pool.
If the BFW entering the pool is too far below its boiling point, a sensible heat duty is needed to attain the boiling point. Thus, the sensible heat duty requirement affects the boiling process adversely.
This adverse effect can be visualized by imagining a pot of water heated over an open fire. If a glass top is placed over the opening of the pot, the boiling process can be easily observed. As the inner heating surface becomes hot, tiny bubbles gradually form and stick to the inner surface of the container. As heating progresses, the bubbles grow and detach from the inner surface, and then rise to the surface of the liquid. At this point, even if a small amount of cooler water is added to the pot, the active boiling process is temporarily suppressed. Similarly, the boiling process in the kettle is affected when BFW enters the pool at a lower temperature than its boiling point.
Boiling heat transfer coefficient in the kettle.
In the shell side, the sensible duty required to heat the liquid to its boiling point is enhanced by guiding the liquid through baffles, enabling a velocity increase and a longer contact time. However, these results are not possible in the kettle, since it is unbaffled.
Keeping this in mind, the boiling heat transfer coefficient correlation considers a correction factor that is related to the ratio of sensible duty and total duty. The presence of sensible duty is essentially penalized. Thus, the smaller the sensible duty, the better the overall heat transfer rate of the kettle.
Effective recovery option for sensible duty.
If a considerable amount of sensible duty is unavoidable, then an effective option is to add a tiny baffled exchanger at the kettle entry nozzle, as shown in Fig. 2. At first, BFW is heated in this baffled shell-side exchanger by the leftover duty of the hot fluid exiting the kettle. This tiny baffled exchanger acts as a relatively cold shell for the kettle.
Fig. 2. Schematic of option for handling
With this modification, 25% to 30% of the heat-transfer area can be reduced, compared to a kettle-only design. The circulation of boiling fluid improves, and the stagnation of cooler fluid below the shell bundle is minimized. As a result, fouling is also reduced.
Sometimes, hot fluids may have a higher degree of superheat in the condenser. In such cases, the small exchanger could be placed above the kettle. The BFW will heat up in the shell side of the tiny baffled exchanger due to superheated vapor in the tube side.
After preheating BFW in the baffled exchanger, hot fluid from the tube side enters the kettle to generate steam. Here, the tiny baffled exchanger becomes the hot shell. During design optimization, it can be further assessed whether this small baffled exchanger at the entry of the kettle is placed as a hot shell above the kettle or a cold shell below the kettle.
The accumulation of a large amount of the sensible duty of the cold fluid should be avoided in a kettle exchanger. If accumulation is unavoidable, then the sensible duty is preferably recovered in a baffled heat exchanger. HP
1 Heat Transfer Research Inc., Advanced Thermal Design of Condensers and Vaporizers Course, 1998.
|The author |
||Tandra Das has designed and provided support and analysis for heat exchangers as a thermal engineer at Fluor, KTI and ABB Lummus Heat Transfer. She also developed engineering packages as a process engineer at Aker Kvaerner. She earned an M.Tech degree in process engineering and design from the Indian Institute of Technology in Delhi, and a BS degree in chemical engineering from the National Institute of Technology in Rourkela. |