Some gas sweetening plants utilize thermal fluid heaters to help remove carbon dioxide

and hydrogen sulfide from natural gas, purifying t he product for market.

By Zane Rhodes, GTS Energy

I mproved shale oil and gas extrac­tion technologies have led to a renewal in the energy industry in North America, and thermal fluid

heaters are used in many processes to help bring these products to market. One of these processes is amine gas treating.

Sometimes called gas sweetening plants, an amine plant is used to remove carbon dioxide (C02) and hydrogen sulfide (H2S) from natural gas so that it can be put into pipelines and used by customers. Collectively, C02 and H2S are known as acid gases, and an amine plant uses a water-based, heat-regenera­ble solvent - or amine - to remove these impurities from the natural gas.

During the process, the natural gas is introduced into the bottom of a tall vertical vessel called an absorber, and the amine is introduced into the top of the vessel. The gas and amine are brought into contact with one another

using trays or packing. The amme absorbs the C02 and H 2S, and the purified natural gas exits the top of the absorber. This is an exothermic reac­tion, and a properly designed system will ensure that the temperature is not

excessive, which can prevent the amine from absorbing the contaminants.

Then, the amine is sent to the regen­eration system. In the regeneration sys­tem, the amine is filtered to remove par­ticulates and any heavy hydrocarbons

This 200 gal/min amine plant has the t hermal flu id heater separated from the rest of t he system for safety.

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Thermal Fluid Heaters •

Simplified Process Flow Diagram

..-- -+ Treated Gas Outlet

Amine Solution Pumps

Contactor Level Valve

Lean Amine Cooler

Set at 18 psig ~

Reflux Accumulator

Booster Pumps

Flash Tank Level

tJ: Accumulator Level Valve

Reflux Pumps

Rich/ l.ean Exchanger

Amine Reboiler/Surge

Tank

Still Overhead

Set at rc 226•F

Hot Oil

Natural gas is introduced into the bottom of the absorber whi le the amine is introduced in the top. The gas and amine are brought into contact with one another using trays or packing. The amine absorbs the C02 and H2 S, and the purified natural gas exits the top of the absorber. Then, the amine is sent to the regeneration system, where it is filtered to remove particulates and any heavy hydrocarbons that have been entrained.

that have been entrained in the amine. The actual regeneration of the amine

occurs in two pieces of equipment: the still and the reboiler. The still is a tall vertical vessel filled with packing or trays. The amine is introduced into the top of the still, where it comes into contact with steam that is produced in the reboiler. In the still, the steam pro­vides the heat necessary to reverse the reaction and dilutes the C02 and H2S released during regeneration. The high temperatures and dilution move the equilibrium point of the amine regen­eration to higher amine purities. The amine flows into the reboiler, which is a shell-and-tube exchanger heated with steam or thermal fluid. In this exchang­er, thermal fluid or steam is used to provide the heat to vaporize a portion of

the water contained in the amine solu­tion. In some smaller plants, the reboiler is a direct-fired heater where a firetube is submerged in the amine to produce the stripping steam necessary for the regeneration of the amine.

The thermal fluid or steam tem­perature must be maintained to pre­vent thermal degradation of the amine. Common practice is to limit the ther­mal fluid temperature to less than 360°F (182°C) with a return tempera­ture of approximately 280°F (138°C) and steam systems to 50 psig saturated steam. The use of steam is limited to systems that are installed in some refin­eries or where a surplus oflow pressure, saturated steam is available. For the balance of the installations, thermal fluid systems are used.

Controlling the Regeneration Temperature In thermal fluid systems, the most prevalent flow scheme is to vary the flow through the reboiler exchang­er - using a control valve and a bypass valve - to maintain a constant flow through the thermal fluid heater and a constant temperature setpoint. Because there is boiling in the reboiler, changing the heat input does not change the temperature of the reboiler by an appreciable amount.

To control the reboiler, some systems are set to control the overhead tem­perature. This is the temperature of the noncondensed steam and acid gases exiting the top of the still.

There are a few problems with this strategy. First, there is a significant

www. process·heating. com NOVEMBER 2013 4 17 t

II

• Thermal Fluid Heaters

time delay between increasing the heat input into the rebo iler and when the overhead temperature cha nges.

This delay can lead to the control valve fully opening a nd closing a nd never settling down to steady-state

operation. However, this can be miti­gated to some degree by reducing the operating range of the thermal fluid control valve to ± 10 percent of the nominal setpoint.

Second, another issue that can arise when using this control scheme is that the ambient temperature can greatly affect the measured overhead tern­perature. This occurs because the tern­perature transmitter often is placed in piping close to the g round for ease of access for maintenance and not at the top of an 80-foot tower. T he gas will cool as the acid gases flow through the pipe to the temperature measurement, and this coo ling will vary based on ambient temperature.

Th is 400 gal/min amine plant has a thermal fluid heater (left), regeneration equipment , including the still (middle), and an absorber (right).

Third, because a cool reflux stream is injected periodically into the top of the still, it can cause a sudden drop in overhead temperature. This can wreak havoc on the control system and desta­bilize it if no t properly accounted for in control scheme design.

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The Role of Operators in Regeneration Control Even though most systems are designed and programmed to control using the overhead temperature, operators often disable this and manually set the flow of hot oil through the reboiler. The advan­tage to this is that the control system does not have to be tuned for various operating scenarios. The disadvantage to this is that operators generally use a conservative setting that ensures the treated gas meets the specifications. This conservative sett ing wastes energy that could have been avoided.

This problem is exacerbated when the inlet gas conditions vary because the operator then has to choose a setting that works for all scenarios even if that heat load is only required a few min­utes a day. This results in much higher energy costs than should be required.

The general specifications fo r a thermal fluid system for an amine plant will include:

To maximize the run time, there should be 100 percent backup on the pumps.

• No yellow metals (copper alloys) should be used in any valves or piping due to the possible presence of H 2S and corrosion issues that can result.

• Class I, Division 2 burner, motors and electronics should be used due to the possible presence of natural gas.

• The heater should be separated from the high pressure gas streams by at least 50 feet.

The requirements of the thermal fluid systems are further complicated if other users are added to the thermal fluid system. For instance, a mol-sieve dehydration system can require up to 600°F (315°C) thermal fluid for the regeneration of the mol sieve. The mol-sieve regeneration is an intermit­tent process, which must be taken into consideration when designing the

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overall system. In installations such as these, the amine plant can be put into a secondary loop to prevent oil a t 600°F (315°C) being introduced into the reboiler.

Likewise, TEG dehydration units require thermal fluid at approxi­mately 460°F (238°C) , and stabilizer and fractionation trains require oil at various temperatures and flow rates. Therefore, it is important to work with a thermal fluid system supplier that has experience with natural gas applications so the various needs of each system are addressed in a robust, efficient and cost-effective manner. ·:!-

Zane Rhodes is president of GTS

Energy, Norman, Okla., a provider of

process heating equipment for indus­

trial applications. For more informa­

tion from GTS Energy, call 405-321-

9800 or visit www.gtsenergy.com.

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