Direct-Fired Reboilers

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). 

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

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. 

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. 

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. 

Reboilers for columns, using relatively small size direct-fired units. 

Various glycol regeneration methods were considered, such as direct-fired glycol reboilers, o direct-fired indirect hot oil system, electricol heating ond waste heat recovery. A direct-fired in-direct hot oil system was finally selectd because it provided a safe, well proven regeneration system which could be accommodated within the platform space limitations. 

The selected system comprises three direct-fired hot oil heaters, surge vessels ond circulation pumps. The hot oil is circulated in a closed loop through the tube bundles in the glycol reboilers. The system allows accurate control of the glycol tempercture ond by designing the fired heaters os a redundant utility system cvailoble to the three process troins the reliability of the system is thereby improved. 

The gas from the glycol/condensate separator can be used for fuel gas. In many small field gas packaged units this gas is routed directly to fire tubes in the reboiler, and provides the heat for reconcentrating the glycol. This se[>.irator is sometimes referred to as a gas/glycol separator or pump gas separator. 

Steam reboiler duty and efficiency can be estimated directly from steam rates and conditions. We can also estimate furnace reboiler duty and efficiency. The amount of fuel fired in the reboiler can be measured from the meter. The heat delivered by fuel combustion can be estimated from both fuel rate and heating value of the fuel. 

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