Fired reboilers

Condenser and eboiler AT. The losses for AT are typically far greater than those for reflux beyond the minimum. The economic optimum for temperature differential is usually under 15°C, in contrast to the values of over 50°C often used in the past. This is probably the biggest opportunity for improvement in the practice of distillation. A specific example is the replacement of direct-fired reboilers with steam (qv) heat. 

For a fired reboiler, a pump-around system is used with an FRC to maintain constant flow. There will be a low flow alarm plus fuel shutoff. There will also be a high flow alarm plus fuel shutoff, since a tube rupture would reflect itself in a high flow. 

Problem A fired reboiler outlet temperature would not rise above 350°F no matter how hard the heater w as fired. 

For direct-fired reboilers, estimate area on basis of heat flux ... 

Most often, forced circulation is used with fired reboilers. If flow is lost to such a reboiler, furnace tube damage is likely to result. Hopefully, this is less likely to occur with a forced-circulation reboiler. Also, the higher pressure drop of a furnace may force the designer to use a pump. Sometimes, we also see a forced-circulation reboiler system, if the reboiler heat is to be recovered from a number of dispersed heat sources that are far away from the tower, and hence a lot of pressure drop has to be overcome. 

Feed Benzene (100%) 653 Fuel for heating purposes (hot oil, steam, fired reboiler) 0.60 Gcal... 

If a column is operating in isolation and using a plant utility for heat input (steam at an appropriate pressure level or fuel if a fired reboiler is... 

Fired furnaces are frequently used in chemical plants to provide energy at high temperatures. If a column, a reactor, or some other unit requires energy at a temperature level above that attainable by steam at reasonable pressure levels, a fired reboiler or heater is used. Steam in a chemical plant is seldom available for process use at pressures above 300 psia. The saturation temperature of 300 psia steam is 417°F. so if the column base temperature is above about 350°F, steam cannot be used. 

A typical triethylene glycol (TEG) dehydration unit is made up of two main components. The absorber, often known in the industry as the glycol "contactor" and the regenerator, is normally based on direct fired reboiling. The feed gas enters the bottom of the contactor and travels upward. The glycol enters the top of the tower and travels down. Thus dry gas leaves the top of the contactor and rich glycol (containing more water) leaves the bottom. 

Over the years some heuristics have been developed for estimating liquid holdups in most systems. Holdup times (based on total flow in and out of the surge volume) of about 5 to 10 minutes work well. If a distillation column has a fired reboiler, the base holdup should be made larger. If a downstream unit is particularly sensitive to rate or composition changes, then holdup volumes in upstream equipment should be increased. Such matters should be considered when the vessels are designed. 

Fired heaters. When the process temperature and the heat duty are high, a fired heater is occasionally used as a forced-circulation reboiler. Fired heater technology is outside the scope of this distillation book, and the reader is referred to review articles (30, 319) as well as some troubleshooting guidelines (237, 239) on this subject. A few of the process-side-related considerations pertaining to fired reboilers are... 

Compared to a forced-circulation reboiler, a fired heater often operates at a higher pressure drop, higher velocities, and a larger fractional vaporization. It is common to have fired reboilers operate at 30 to 50 percent vaporization. 

Fired reboilers are principally used in refineries. Usually, these are fired with a mixtime of gases vented from various units, supplemented by natural gas. A problem with direct-fired reboilers is variation of heat input due to changes in fuel gas composition. For this reason, it may be unsatisfactory to control the fuel flow rate to the furnace, and heat input control may be necessary. 

A fired reboiler control S3rstem using the Wobbe index is shown in Fig. 17.3. The density meter should be installed directly in the fuel gas hne, downstream of the knockout drum and any points where fiiel gas streams are added, and where it would be unaffected by vibrations (370). 

Figure 17.3 Fired reboiler heat input control.

Hydrocarbons may be removed in oil scrubbers. However, the natural vapour pressure of these solvents sometimes requires the use of higher boiling oil fractions in the pre-absorber, which can then no longer be boiled out indirecdy with steam in a conventional heat regenerator, but can be heated to the boiling point only by means of live steam or in fired reboilers. 

Condense the regenerator overhead vapor in a partial condenser and combust the remaining vapor. The imcondensed vapors are typically routed to an incinerator or, if a direct-fired reboiler is used, routed to the reboiler fuel gas. The liquid hydrocarbons are collected and disposed of by blending into a crude oil or condensate stream. The condensed water is typically routed to produced water disposal. 

Vcan be any measure of reboiler duty. It might be the steam flow to a condensing reboiler. If heat integrated it might be the flow of heating fluid multiplied by its temperature change. For a fired reboiler it might be the fuel flow. 

Feed from the Toluene tower is preheated (1) by the MX tower distillate product and then enters the MX tower (2) at approximately the middle tray. The MX tower overhead is totally condensed using heated water firom the gas fired reboiler (3). Steam is produced in the condenser (4) and dehv-ered to a utility header. Flow of water into the shell side of the condenser is manipulated by a level controller. The condensed MX tower overhead material is collected in an accumulator (5). A level controller manipulates the reflux to the tower to maintain the accumulator level. The overhead product flow is set externally, either by an advanced controller or manually. At the bottom of the MX tower the flow is divided into two streams. One of the streams is circulated through the gas fired reboiler, where it is partially vaporized and then returned to the MX tower. The vaporization rate is set... 

Vaporization rate in the gas fired reboiler a feedforward variable manipulated by a separate controller for the reboiler. 

Two other types of reboilers are sometimes encountered (1) reboilers with hot oil as a heating medium, and (2) direct-fired reboilers. Both are more common in the petroleum industry than in chemical plants. 

For higher temperatures than available with steam, use is made of other ccHidensing media such as Dowtherm or p-cymene. Few even higher temperatures, direct-fired reboilers may be used. 

In the refinery, there are usually three sources of heat-steam, high temperature oil which must be cooled before being stored and direct fired heaters. Steam is the first choice for bottoms product temperatures up to approximately 350 degrees F. Above this level, either hot oil or a direct fired reboiler will be required. The following guidelines should be observed in selecting operating utilities. 

Heat for the distillation is provided by a direct fired reboiler. Hot lean glycol leaves the reboilcr and flows to a surge tank (which often contains cooling coils) and is pumped through the glycol/glycol heat exchangers and back to the contactor. 

Towers and vessels should be located at least 30 meters from fired heaters or fired reboilers. 

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