Reboiler Selection Essentials

Reboilers are vital components in every distillation unit. They exist in almost all petrochemical and refinery plants. Their objective is to vaporize the working fluid and return it back to the process cycle. Reboilers are manufactured in different configurations such as thermosiphon & kettle. Basically, they are tubular exchangers with various shell arrangements. Their heat source is steam or other hot process fluids in order to achieve heat integration. Reboilers are mostly used at the bottom of distillation columns. Due to their critical operating condition, it is important to choose the proper type of reboiler for every unique application. In this article, we will review some basic rules of reboilers and provide guidance for process engineers on proper reboiler selection in oil and gas industry.

Thermosiphon reboilers are the number one choice in terms of reboiler selection. Since they are economic and common in industry, we will mostly focus our attention on this type of reboilers. They are shell and tube reboilers designed in both vertical and horizontal configurations. Boiling rate of these reboilers are not as high as kettle reboilers. Therefore, the maximum vapor quality of the outlet flow is less than 25%. They have the maximum heat transfer rate (especially vertical type) among other reboiler.

They can be utilized in both once-through and circulating arrangements. In a once-through arrangement, the overall down-coming liquid from the lowest tray, first enters the reboiler and then, the unevaporated portion is collected in the tower’s sump. Thus, the bottom tower’s product is obtained after partial vaporization. In this manner, the reboiler acts as a theoretical stage. However, in circulating arrangement, the overall down-coming liquid firstly enters the tower’s sump and then, the bottom product exits the tower before sending into reboiler. As a rule of thumb, if the column’s reflux rate is low, use a once-through arrangement. Otherwise, if the boiling rate (returned vapor to column) is high enough (e.g. more than 40% of bottom product), it is recommended to use a circulating arrangement.

In vertical thermosiphon reboilers, the hot source (usually steam) flows in the shell side and the process fluid passes through the tubes in an upward direction. They are the least expensive type of common industrial reboilers. They usually do not require an upstream pump in order to circulate the flow. Because of the higher velocity in the tubes, the fouling tendency is minimized. Maintenance and cleaning can be difficult due to the installation type and inaccessible parts. However, the tube-side vaporization, makes the cleaning procedures easier. Therefore, in terms of reboiler selection, they are suitable for applications involve with fluids with high fouling characteristics. They can handle process fluids with high pressure easily.

The main disadvantage of this type is their low boilup ratio. In addition, the optimum outlet vapor quality shall not exceed 15%. Because the vaporization occurs in tubular area, the outlet flow can be choked if the outlet vapor quality exceeds this value (15%). Therefore, this type of reboiler is not recommended for towers with large internal diameter or high boiling rate. Because of relatively higher heat transfer rate, they requires large temperature difference between shell and tube side.

Vertical thermosiphon reboiler

In horizontal thermosiphon reboilers, the hot source flows in the tube side and the process fluid passes across the shell in an upward direction. Maintenance and cleaning are relatively easy.

For those applications that require a circulating pump, the tower tends to have a high skirt (tall height) in order to satisfy the required NPSH (Net Positive Suction Head) for the downstream pump by increasing tower’s sump elevation.

Among thermosiphon reboilers, TEMA (Tubular Exchanger Manufacturers Association) type “X” horizontal reboiler provides the most vaporization rate up to 25% of the inlet flow.

Horizontal thermosiphon reboiler

For designing vertical thermosiphon reboilers, the upper tubesheet shall be at least in line with the low liquid level (LLL) of the tower’s sump in order to guarantee that tubes are always filled with liquid. Therefore, If the upper tubesheet’s elevation is more than liquid level of the sump, it may lead to thermal damage of tube-to-tubesheet joints. However, in about 50% of tubes length there is two-phase flow usually of annular regime.

Because of large-sized input/output connections to tower’s shell (for line sizing criteria click here), vertical thermosiphons can be installed without additional foundation supports. Therefore, the installation requires less space and is cost-efficient.

As vaporization does not occur in a constant temperature in a vertical thermosiphon reboiler, there is always a slight temperature difference between inlet and outlet tubes. The reason is the static head produced by the liquid level in the tubes and the pressure drop across the path.

Although slug flow is not of concern in vertical thermosiphon reboilers, the choking problem at the outlet nozzle is probable and has to be avoided. On the contrary, in horizontal thermosiphon reboiler, the slug flow is a critical issue.

Kettle reboilers are relatively large. They consist of a TEMA “K” shell type. Technically, they are once-trough reboilers. Boiling rate of kettle reboilers are very high so they can handle big distillation columns with high vapor load. The outlet vapor quality can reach 80%.

Generally, kettle reboilers require higher capital cost. Maintenance and cleaning are relatively easy. They can easily handle low-pressure applications that operate near vacuum condition. They are capable of operating with the lowest temperature difference between shell and tube sides. Their rear head can be manufactured in both TEMA “U” and “T” type. U-type configuration is more stable and robust. However, if the shell size is important, T-type is preferable due to its compact specification.

Kettle reboiler

In this type of reboilers, the exchanger tubes located in the bottom liquid inside the tower’s sump. They are not separate equipment and can save lots of space. They are not so common in oil and gas industry.

Internal shell reboilers provide long residual time for liquid in heating zone, which may increase scaling problem or material degradation. Since the exact gas-liquid boundary of the surface cannot be obtained, it is challenging to specify liquid level of the sump. Therefore, controlling the level is difficult. In this type of reboilers, cleaning is relatively difficult. They are also subject to unwanted leakage from nozzles flanges.

In this article, we reviewed some engineering tips as a guidance on proper reboiler selection. The term “Reboiler” is mostly used for distillation column’s bottom exchanger. Consequently, other exchangers in the process area, which used for vaporizing liquids, are called vaporizers or evaporators.

In oil and gas industry, process engineers are the ones who make the final decision in choosing the best reboiler according to their applications. Proper reboiler selection is in close connection with ease of operation and low capital cost.

As a brief summary, the following table presents a comparison of reboiler types: