“Managing Reboiler Steam-side Operation

1 Managing Reboiler Steam-side Operation Improves Equipment Reliability and Steam Quality by Loraine A. Huchler, P. E. MarTech Systems, Inc., Lawrenceville, New Jersey USA. presented at: First International Conference on Practical Industrial Water Treatment Technology for the New Millenium Marriot Houston Westside Houston, Texas January 27 29, 1999. Organized by Gulf Publishing and HYDROCARBON PROCESSING. Published in HYDROCARBON PROCESSING, June 1999. by Gulf Publishing Abstract Reboilers are perceived as an integral part of the production process but not part of the steam system since they are located far from the main boilers or powerhouse. Conse- quently, the Steam-side Operation of reboilers receives minimal attention until a failure occurs. A proactive approach of mechanical optimization and chemical treatment and monitoring is recommended to minimize deposition and corrosion and maximize system reliability and energy efficiency.

2 Mechanical optimization requires an assessment of the Reboiler design and Operation to ensure proper venting, correct arrangement of steam traps on Reboiler supply lines, compatibility of materials of construction, and impact of current system Operation ( dry, partially or totally flooded on the steam side). Select- ing the correct chemical treatment and monitoring program is a balance between eco- nomics, feed considerations, process compatibility and plant preferences. Maintaining the efficiency of the Steam-side Operation of reboilers is critical to improving the process equipment reliability and maintainability. Introduction cal Operation , monitoring and chemical treatment. Centrally located powerhouse boilers produce steam for refining processes Refinery Reboiler Review and power generation. Operators add chemicals for water treatment and moni- Large quantity distillate products such tor condensate quality at this central lo- as gasoline, jet fuel and diesel fuel dic- cation.

3 Water and steam not used in the tate basic refinery Waste powerhouse is routed to the refinery for streams from each of these products are waste heat boilers, reboilers or process refined to a variety of finished materials requirements. in various process units. Process units that commonly use steam as an indirect Reboilers are located far from the pow- heat source in reboilers include: erhouse and, consequently, the steam- side Operation of these units receives Gas plants minimal attention until a failure occurs. light ends unit One study showed the most common vapor recovery unit high risk failure events in a fluid catalytic Alkylation units (HF and Sulfuric). cracking unit (FCCU) were heat ex- Sulfur recovery units changer shell or tube leaks or Aromatics recovery units Reboilers, like heat exchangers, are BTX.

4 Equally vulnerable to tube and shell fail- benzene/cumene ures. This paper will discuss Reboiler UDEX. design, location and Operation within a Catalytic Reformer typical refinery and methods to maxi- mize reliability through proper mechani- A summary of the reboilers typically found in each process unit is shown in Table 1. 1. Rooney, J., Turner, J., Arendt, J., Preliminary 2. Gary, J., Handwerk, G., Petroleum Refining, hazards analysis conducted on FCCU complex, . Dekker, New York, 1984. Oil & Gas Journal, August 8, 1988, pages 60-66. 2. Table 1 Refinery Reboiler Summary Process Reboiler Name/Column Purpose Alkylation (Sulfu- Depropanizer Remove and collect propane from alkylate stream ric) Deisobutanizer Produce low vapor pressure alkylate gasoline Alkylation (HF) Acid Rerun Recycle acid stream to remove contaminants 3.

5 Depropanizer Remove and collect propane from alkylate stream Acid Stripper Remove acid from propane stream Deisobutanizer Produce low vapor pressure alkylate gasoline Debutanizer Remove and collect butane from deisobutanizer stream Catalytic Re- Stabilizer Remove butane and lighter hydrocarbons from reformate former stream Aromatic Recov- Stripper Remove and collect aromatics from solvent hydrocarbon ery stream Pre-Extractor Remove and collect lighter hydrocarbons from reformate charge Water Still Remove water from solvent Benzene Remove and collect benzene from aromatics stream Toluene Remove and collect Toluene from aromatics stream Xylene Remove and collect xylene from aromatics stream Gas Plant Stabilizer Remove butanes and lighter hydrocarbons from gasoline product Stripper Remove very light hydrocarbons such as methane and ethane De-ethanizer / Absorber Produce low vapor pressure alkylate gasoline (Stripper).

6 Debutanizer Stabilize gasoline component Rerun (Naptha Splitter) Recycle to polish lean oil (solvent) of lighter gasoline fraction Depropanizer4 Remove and collect propane Depentanizer Remove and collect pentane Stabilizer Remove butane and lighter hydrocarbons from gasoline stream Sulfur Recovery Sour Water Stripper5 Remove and collect H2S and NH3 from water stream 6. Amine Regeneration Regenerate amine for recycle Miscellaneous Vaporizer Normally found in fuel system to remove butanes Splitter A distillation of two or more hydrocarbon fractions 3. Reboiler has critical water treatment requirements due to low pressure steam usage. 4. Reboiler has critical water treatment requirements due to low pressure steam usage. 5. Low corrosion potential due to internally generated low pressure steam. 6. Low corrosion potential due to internally generated low pressure steam.

7 3. Types of Reboilers not operate at high pressures because they must have adequate liquid/vapor Reboilers are essentially shell and tube separation. heat exchangers with process fluid on one side and steam or a mixture of Internal or Stab-In Reboilers steam and condensate on the other side. Three of the most common re- A variation of a kettle Reboiler is an in- boiler designs are kettle, forced recircu- ternal or stab-in tube bundle that is in- lation and thermosiphon. serted directly into the column (Figure 2). Kettle Reboilers Kettle reboilers, also known as pool boilers, are often used for light hydro- Steam carbons (propane, butane). These re- boilers handle process flow fluctuations and high heat fluxes better than other Tower Reboiler designs, but kettle reboilers have a greater tendency to foul on the process side (Figure 1).

8 Process Flow to tower (hot). Steam Steam/Condensate Figure 2 - Stab-in Reboiler Process Flow Steam/ Con- from tower (cool). densate The operating characteristics of a stab- Figure 1 - Kettle Reboiler in Reboiler are almost identical to a kettle boiler with the exception that the units have lower heat fluxes due to the size A liquid condensate level must be main- restriction to fit the column and have a tained at or near the top tube row by lower process-side fouling tendency. means of a weir or a level controller. The capital cost of a stab-in unit is lower Typically, the tube bundle has baffles to than the capital cost for kettle reboilers, allow liquid movement and reduce the but maintenance is more difficult. potential for vapor blanketing. Forced Recirculation Reboilers In kettle reboilers there has to be ade- quate head-space to permit the separa- Figure 3 shows a forced recirculation tion of the aqueous liquid phase from Reboiler .

9 This unit uses a pump to move the vapor phase. Demisters can be process liquid through the Reboiler . used to minimize the entrainment of liq- uid in the outlet flow. These reboilers do 4. Process Flow These reboilers do not require a pump to tower (hot) for recirculation and are the least likely Tower to foul in service due to relatively high Steam process flow velocities. Like forced re- circulation reboilers, thermosiphon re- boilers have sensible heat transfer fol- lowed by nucleate boiling. Steam/. Condensate Steam-side Operation Process Flow from tower Reboilers are specially designed steam (cool) heat exchangers that transfer heat to the process by circulating some of the Figure 3 - Forced Recirculation Re- process fluid through the Reboiler and boiler back to the column. Reboiler Steam Control Strategies Forced recirculation boilers have two mechanisms of heat transfer: sensible A temperature control system accurately heat transfer followed by nucleate boil- controls the steam flow and hence the ing.

10 Process flow is typically on the heat transfer rate. tube side of a standard exchanger in the The most common temperature control vertical position. At low process system consists of a flow/pressure con- flowrates, these units are prone to foul- trol valve on the steam inlet to the re- ing. boiler tied to the process outlet tempera- ture as shown in Figure 5. Thermosiphon Reboiler Thermosiphon reboilers (Figure 4) oper- ate using natural circulation with process T. flow on the shell side in horizontal units and process flow on the tube or shell Tower Steam side in vertical units. Reboiler Steam/. Condensate Process Process Flow Flow to Flow tower (hot). Stea Figure 5 - Reboiler Temperature Con- trol System Condensate Process Flow Process temperature fluctuations are from tower (cool) controlled by modulation of the steam flow control valve.