Reboiler Basics
Reboiler (Reboiler) is a
Distillation Tower bottom or side line of the heat exchanger, used to vaporize a portion of the liquid-phase product back to the tower for gas-phase reflux, so that the tower of the vapor-liquid contact between the two phases of mass transfer can be carried out, while providing the heat required for the distillation process, also known as reboiler. Most of the reboiler for the
Shell And Tube Heat Exchanger. According to the actual production of different needs, the boiling process can occur in the shell process, can also occur in the tube process.
Engineering the basic requirements of the reboiler is stable operation, easy to regulate, simple structure, easy processing and manufacturing, easy installation and maintenance, long service life, safe and reliable operation, but also should consider its footprint and the height of the installation space to be appropriate. In general, while meeting the above requirements is difficult, so the design should be a comprehensive analysis, comprehensive consideration, to find the main, decisive role in the requirements, choose a more reasonable form of reboiler.
First, the type of reboiler
According to the form of circulation, the reboiler is divided into natural circulation type thermosiphon and forced circulation type thermosiphon.
Natural circulation type thermal siphon reboiler in the domestic distillation tower (device) process is most commonly used. Natural circulation thermosiphon reboiler relies on the liquid static pressure head in the kettle and reboiler density difference between the two-phase flow to generate the driving force to form a thermosiphon movement. Utilizing the density difference between the gas-liquid mixture in the reboiler and the liquid at the bottom of the tower as the driving force increases the flow velocity of the fluid in the tube, reduces fouling and improves the heat transfer coefficient.
Forced circulation thermosiphon reboilers use a pump to force the fluid out of the tower and into the reboiler for heating and circulation. Reboiler heated with steam, can be set on the steam pipe regulating valve to control the steam flow, or in the condensate outlet pipe to install regulating valves to change the level of condensation in the reboiler and regulate the heat.
According to the installation form, the reboiler is divided into vertical and horizontal.
Vertical heat siphon type, forced circulation type.
Horizontal type has heat siphon type, forced circulation type, kettle type reboiler, built-in type reboiler.
Vertical heat siphon reboiler
Vertical thermosiphon reboiler is the use of single-phase kettle liquid at the bottom of the tower and the heat exchanger heat transfer tube in the vapor-liquid mixture of the density difference between the formation of the circulation driving force, constituting the process logistics at the bottom of the distillation tower and the reboiler between the flow of the cycle. Vertical and horizontal thermosiphon reboilers do not have their own gas-liquid separation space and buffer zones, which are provided by the kettle.
Vertical thermosiphon reboiler heating medium goes to the shell side, the process stream goes to the tube course, vaporized in the tube, high heat transfer coefficient, low investment, in order to obtain good circulation, may require a relatively high tower skirt height, suitable for vacuum and low pressure system.
Features:
✦ Circulation driving force: kettle liquid and heat exchanger heat transfer tube gas-liquid mixture density difference;
✦✦ Compact construction, small footprint, and high heat transfer coefficient;
✦✦ The kettle liquid has a short residence time in the heating section, is not easily scaled, and the tubing is easily cleaned and adjusted;
✦✦The tower kettle provides gas-liquid separation space and buffer zone;
✦✦ The equipment is installed directly next to the tower due to the simple piping system, so the equipment cost is low.
Disadvantages:
✦✦ Because the shell process can not be mechanically cleaned, is not suitable for high viscosity, or dirty heat transfer media; reboiler requirements of the static pressure head is relatively large, the need for the tower installation height is high; and a single device has a certain limitation of the heat transfer area.
Horizontal thermosiphon reboiler
Heating medium flows in the tube, the tube course can be a single process, can also be a multi-process. The feed is introduced into the reboiler from the bottom of the tower down tube, the liquid in the shell process boiling vaporization occurs, the formation of less dense vapor-liquid mixture, due to the difference in the density of the feed tube and the discharge tube of the liquid, resulting in a difference in static pressure, the natural circulation of the fluid to become the driving force.
Characteristics:
✦Circulation driving force: the density difference between the kettle liquid and the gas-liquid mixture in the heat transfer tube of the heat exchanger.
✦✦Large footprint, low skirt height, medium heat transfer coefficient, easy maintenance and cleaning.
✦The tower kettle provides gas-liquid separation space and buffer zones.
✦✦Higher recirculation rate, resulting in higher flow rates and lower outlet dryness, which prevents accumulation of high-boiling-point components and reduces the rate of fouling.
✦ Easy cleaning and large heat transfer area.
Disadvantages:
✦Large footprint, and in order to obtain good fluid distribution, usually use more than one receiver, which results in a complex piping system, increasing the price of the equipment. ✦Longer outlet piping, larger pressure drop, not suitable for low-pressure and vacuum operation conditions as well as occasions with more serious fouling.
✦ medium viscosity of the fluid in the horizontal boiling state than in the vertical in the better, when the fluid viscosity is less than 0.5cP, can be considered to use the vertical thermosiphon; but when the viscosity is greater than 0.5cP, it is considered to use the horizontal thermosiphon reboiler.
Forced circulation reboiler
Forced circulation reboiler is to rely on the pump input mechanical work for fluid circulation. An external boiling bundle is generally appropriate in most systems. Typically, it is ensured that the evaporation rate is less than 1% and the fluid will flash completely after passing through the valve at the outlet pipe. In addition, the circulation rate is controlled by the pump, so the cost of pump power consumption should be taken into account, and the circulation rate can be optimized by the volume of fluid circulated, the initial investment and operating costs.
Applications
✦ Suitable for high viscosity, heat sensitive materials, solid suspensions and high resistance systems with long sensible heat sections and low evaporation ratios.
✦✦ Forced-flow reboilers are best suited for heavily fouled and very high viscosity fluids.
Kettle Reboilers
The kettle reboiler consists of a housing with a gas-liquid separation space and a withdrawable tube bundle with an overflow weir at the end of the bundle to ensure that the bundle is effectively submerged in the liquid. The space outside the overflow weir serves as a buffer for the discharge liquid. The filling factor of the liquid in the reboiler is 80% for non-foaming systems and not more than 65% for foaming systems.
Characteristics:
✦Insensitive to hydrodynamic parameters, high reliability, can be operated under high vacuum, easy maintenance and cleaning.
Disadvantages:
✦Small heat transfer coefficient, large shell volume, large footprint, high cost, long residence time of the kettle liquid in the heating section, easy to scale.
Built-in reboiler
The tower built-in reboiler is characterized by a tube bundle inserted directly into the bottom liquid pool of the distillation column. Other kettle-type reboiler with the same advantages and kettle-type reboiler is the same.
Features:
✦ Built-in type reboiler is the reboiler tube bundle is directly placed in the kettle into the tower, eliminating the need for shells and connecting piping, its structure is simple, the cost is lower than the kettle type reboiler.
Disadvantages:
✦ Due to the tower built-in form, the length of the tube bundle is restricted by the diameter of the distillation tower, so the size is limited. Tower kettle space volume is limited, the heat transfer area can not be too large, the heat transfer effect is not ideal.
Summary:
Reboiler selection, try to use vertical thermal siphon reboiler; control program does not strictly control the liquid level of the tower kettle, the general choice of kettle reboiler; in the absence of sufficient space height, consider the choice of horizontal thermal siphon reboiler or kettle reboiler. High viscosity in the case of forced circulation type reboiler.
Second, the arrangement of the reboiler requirements
⒈ reboiler should be arranged as close as possible to the tower, and even the mouth of the reboiler can be directly docked with the mouth of the tower.
Peck with a double reboiler tower, reboiler to symmetrical arrangement is appropriate.
　　　　The distance between the center of the vapor phase pipe at the bottom of the tower connected with the reboiler and the pipe plate of the reboiler should not be too big, so as not to cause the effect of heat siphonage is not good, which affects the efficiency of the reboiler.
Singed reboiler support position should be determined in this way: reboiler and the tower connected to the upward displacement of the mouth should be the same. For vertical siphon reboiler, the normal liquid level of the tower kettle is usually the same as the reboiler on the tube plate, in order to adapt to the fluctuation of the liquid level, the height of the reboiler tube plate and the tower's gas connected to the lower end of the height of the receiver should be left with an appropriate margin.
Be careful when more than one reboiler and tower arrangement, reboiler location and installation height in addition to process requirements should be ensured, should also meet the requirements of the import and export of the set of pipe layout, and easy to operate and maintenance.
Select the horizontal reboiler with steam or heat carrier heating should be arranged close to the tower, and maintain a certain height difference with the tower, the distance between the two should meet the requirements of the pipeline arrangement, the reboiler should be pumped to the end of the bundle should be access to the maintenance site and access.
Third, the typical equipment layout program
Reboiler rooted in the tower, both full rooted, can also be half-rooted
Whether the reboiler is rooted in the tower, in addition to meeting the process requirements, should also take into account the size and quality of the reboiler itself, as well as the carrying capacity of the distillation tower. Figure (a) in the reboiler volume is relatively small, the mass is not very large, can be completely dependent on the tower. Figure (b) in the reboiler's relatively large size, in the arrangement of space is not very large, it should be used in the way of half-rooted, so that the arrangement is more compact, small footprint, economical.
Reboiler mounted on a separate stand or frame
Whether the reboiler is installed in a separate bracket or frame or on the ground, in addition to meeting the process requirements, but also consider the form of the reboiler. Figure (a) in the reboiler because there is no surrounding framework can be used, so the two reboilers should be jointly arranged to facilitate operation and maintenance; Figure (b) in a horizontal reboiler directly supported on the ground, and another reboiler to use the surrounding framework for the arrangement, so that the arrangement is more economical, the operation is also more convenient.
Source: Reprint
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