October 2020

Special Focus: Plant Safety and Environment

Optimize flare gas recovery system design to reduce emissions

Emissions from flares worsen air quality and produce waste gas. A flare gas recovery system is designed to facilitate CO2 reduction.

Lu, S.-G, CTCI, Corp.

Emissions from flares worsen air quality and produce waste gas. A flare gas recovery system (FGRS) is designed to facilitate CO2 reduction.

In a recent project, an engineering and construction company optimized an FGRS design by utilizing a high-performance ejector as a compressor, a three-phase separation tank, a self-purification loop of sour water, a software simulationa of the amine scrubber and a dynamic simulation of flare gas discharge.

Flaring regulation background

The flame of a continuously combusting flare stack is an eye-catching but unwelcome signature of a refinery. The flare stack is also one of the main sources of refinery odors and subsequent complaints from nearby residents. On February 1, 2011, Taiwan’s Environmental Protection Agency amended its volatile organic compounds air pollution control and emission standards. Article 4 of the amendment ordered, “Exhaust gases emitted under normal operation in public and private premises shall not be treated with a flare stack.”

The amendment became effective on July 1, 2014, directing refineries to follow regulations and make suitable improvements within a specified time frame. If improvements are not made within this time frame, in severe cases, the refinery can be shut down. In Taiwan’s refineries, the exhaust gas generated under normal operating conditions is discharged to the ground flare for treatment, and the excess exhaust gas generated during abnormal operating conditions is discharged to the elevated flare for treatment. The elevated flare does not meet the new regulations and will require engineering improvements.

Before the exhaust gas enters the flare stack, exhaust gas recovery and compression equipment must be put into place to recover the flare gas, which will be desulfurized and then introduced into the fuel gas system for use in the process. After this project is completed, it must meet statutory regulations that prohibit the exhaust gas from being treated using the flare stack. The law stipulates that the concentration of hydrogen sulfide (H2S) in the recovered gas shall not exceed 80 ppmv to meet the requirements of stack sulfur dioxide (SO2) emission control.

To meet the amended air pollution standards, flare gas discharge must be minimized. A refinery sought to implement an EPC project for an FGRS, as shown in FIG. 1, to minimize air pollution from the refinery. The engineering project team proposed an optimization to the design, which received positive feedback and was subsequently selected as the best design.

FIG. 1. Location of the FGRS in the refinery.

Process scheme

The FGRS is located in Zones C and F of the refinery. Two sets of flare gas recovery systems were designed. The No. 2 and No. 3 flare stacks share one set, and the No. 4 and No. 5 flares share the other. The throughput design of each system is 9,600 Nm3/hr.

Setting up the FGSR required changes to the existing flare gas system. The flare gas flow sequence consists of the flare gas recovery system, a ground flare and an elevated flare, as shown in FIG. 2. Considering that the flare stack is the last pollution-controlling equipment in the refinery, the water-sealing height of the flare gas system seal drum needed to be modified.

FIG. 2. Process flow diagram for the FGRS in the refinery.

The engineering techniques adopted and the optimization achieved with the project are discussed in the following sections.

Ejector as compressor

Advanced and highly durable flare gas recovery and compression equipment (high-performance ejector) was used for the FGRS. The concept is to use high-pressure water to drive the flare gas into the recovery system and process it.

In the past, most flare gas recovery systems used liquid ring compressors. These compressors have disadvantages, such as complicated maintenance procedures and high maintenance costs, large equipment and relatively long delivery times. For these reasons, a high-performance ejector was selected as the flare gas recovery compression equipment instead of the conventional method using complicated, large compressors (FIG. 3).

FIG. 3. High-performance ejector for flare gas compression.

Rapid injection of high-pressure water is used in this equipment to drive the flare gas through the ejector, after which it is mixed with water and then enters the FGRS. The high-performance ejector flare gas compression system is the most advanced flare gas recovery and compression equipment on the market. It has been used for more than 10 yr without maintenance. The equipment also has the advantages of high stability, high durability, simple construction and lower maintenance costs. It can also handle aerosol-containing gases without affecting performance, shortens the time from start to full load and has no lubricant contamination problems.

Unlike other compressors, high-performance ejectors have the following characteristics:

  • Not sensitive to changes in the suction gas load
  • Not sensitive to changes in the molecular weight of the incoming gas
  • Works with a closed suction gas valve, with a load ratio of 0%–100%
  • Controls the upstream side pressure with a suction pressure control valve
  • Easy to start and stop
  • Only moving parts consist of the shaft and the impeller of the centrifugal pump
  • Only parts that require maintenance are the bearings and seals of the centrifugal pump
  • Zero-maintenance solution
  • As an absorption tower, any particle in the feed gas can be cleaned by a motive fluid
  • Ability to handle feed gases containing solids, particles or mists/liquids without compromising performance
  • At low molecular weight, there is no loss of incoming gas capacity, unlike liquid ring compressors
  • At high molecular weights, there is no cavitation problem, unlike liquid ring compressors that can cause heavy loads and vibrations on the shaft and coupler
  • Faster startup time to full load, unlike liquid ring compressors
  • Faster gas supply than all other compressors
  • No lubrication problems, unlike screw compressors
  • No moving parts inside the compression chamber
  • No metal-to-metal contact.

Design of three-phase separation tank

The three-phase separator tank functions as a separator of water, oil and gas, with an internal diverter to disperse the gas-liquid and minimize the impact on the gas-liquid separation tank. The calming baffle is set to reduce level fluctuations. The double baffle is used for liquid and gas three-phase separation, as shown in FIG. 4.

FIG. 4. Process sketch for high-performance three-phase separation tank.

A gas demister is installed at the outlet to reduce the droplets brought out, and another tank is built with oil-water two-phase liquid-level control functions. Oil-water separation uses an oil-water-gas three-phase gas separation tank and an oil-water separation tank to accomplish a two-step separation procedure; it separates oil and water effectively and reduces waste oil and wastewater at the outlet.

Self-purification sour water loop

A self-purification loop for sour water is incorporated to improve the circulating water quality, thereby reducing wastewater discharge, as shown in FIG. 5.

FIG. 5. Process sketch for circulation of water self-purification loop.

The benefits of establishing a sour water self-purification loop include reductions in emissions, supplemented water consumption, H2S concentration and corrosion, as well as extending the equipment life. It also increases fuel gas recovery and reduces the impact of emulsification caused by the recovery system.

To maintain the quality of process water in a three-phase separator for the FGRS, the typical design includes discharging part of the process water and adding the equivalent amount of supplemented water, which maintains the overall quality of the process water. However, using sour water self-purification circulation brings many advantages, such as a reduction in the amount of sour water blowdown/supplemented water, less corrosion, an extension of equipment service life, an increase in fuel gas recovery and a reduction in the impact of emulsion phenomenon.

In the flare gas recovery system, the refinery’s requirement of recovered fuel gas pressure is a minimum of 4.5 kg/cm2g, and the separator’s operating pressure must be at least 6 kg/cm2g in the front process. These operating conditions would increase the water solubility of the hydrocarbon and reduce the separation efficiency.

To maintain the quality of the process water, the typical design is to blow down part of the process water (approximately 1%–2%) and add the equivalent makeup water. Sour water self-purification circulation (FIG. 6) would route a portion of the sour water into a low-pressure flash drum. The water solubility of hydrocarbon decreases immediately due to a reduction in pressure, and a portion of the sour water flashes into vapor. The flash gas is then sent to the flare header, and the residual liquid is sent to the separator by a circulation pump, creating a sour water circulation loop.

FIG. 6. Sour water self-purification circulation flow scheme.

Sour water self-purification circulation has the following advantages:

  1. Decreases the amount of blowdown/makeup water required. TABLE 1 shows that sour water self-purification circulation can remove approximately 80%–90% of hydrocarbons, which significantly enhances oil-water separation efficiency, improves the quality of process water and reduces the amount of blowdown/makeup water.
  2. Decreases H2S concentration, which reduces corrosiveness and extends equipment service life
  3. Increases the amount of recovered fuel gas
  4. Reduces the impact of emulsion.

Although the cost savings from water reduction (18 m3/hr) and fuel gas recovery are not significant, the benefits of decreasing wastewater are great.

Optimization of amine liquid dosage design

Proprietary software is used to simulate and control the temperature of the exhaust gas into the amine washing tower, so that the volume concentration of H2S in the exhaust gas is less than 80 ppm. Additionally, about 20% of the amount of amine liquid is saved, as shown in FIG. 7.

FIG. 7. Process sketch of amine scrubber.

Amine liquid absorption tower design. The refinery requires the installation of two sets of FGRSs. According to this requirement, the processing capacity of each set of recovery equipment is 9,600 Nm3/hr. The treated exhaust gas must have an H2S volume concentration below 80 ppm.

Each FGRS comprises two sets of amine liquid absorption towers, and the H2S in the exhaust gas is removed by a 27 wt% solution of diisopropanolamine (DIPA).

Thermodynamic mode and simulation tools. The softwarea used for simulating the amine liquid absorption tower has a rigorous electrolyte mode and an accurate liquid phase-activity coefficient mode, which is suitable for amine liquid absorption systems. Other commercial process simulation software programs, which use the amine thermodynamic model, consider the electrolytic properties of the amine absorption system; however, the basis is regression, not electrolyte calculation.

The amine liquid-absorption system contains a chemical reaction of H2S and CO2 with amine. The reaction rate of H2S with amines is very fast and can be considered as equilibrium, while CO2 is not. If CO2 is present in the system, it will significantly affect the absorption of H2S.

The software has a reaction kinetics model for CO2 to simulate selective absorption of H2S by amines in the presence of CO2 and H2S. It calculates the lean and rich approaches for the amine absorption tower, which engineers use to determine the efficiency and space of the absorption tower. These approaches are the two most important indicators for the absorption tower.

Design considerations

The following items are considered as parameters for the design of the amine liquid absorption tower:

  • Amine liquid circulation flowrate
  • Lean amine inlet temperature
  • Absorption tower retention time
  • Amine liquid circulation flow.

Using the software to simulate amine absorption, the lean and rich approaches can be adopted to determine the closeness of the operating state and the equilibrium state of the system. Conceptually, the lean approach indicates the ratio of equilibrium H2S content to operation H2S content at the absorber overhead, and the rich approach indicates the ratio of operation amine loading to equilibrium amine loading at the bottom of the absorption tower for the rich amine solution.

In an example of the lean approach, FIG. 8 shows the ratio of H2S content of the sour gas when the amine/sour gas feed reaches equilibrium (point B, Ye) to the H2S content of the overhead gas under operation (Point A, Y1).

FIG. 8. The lean approach.

In an example of the rich approach, FIG. 9 shows the ratio of H2S content of the bottom rich amine solution (Point A, X2) to the H2S content of the rich amine/acid gas feed at equilibrium (Point B, Xe).

FIG. 9. The rich approach.

FIGS. 8 and 9 show that the slope of the operating curve is the ratio of the circulating amount of amine liquid to the flowrate of the acid gas. If the ratio of the circulation flowrate of amine liquid to acid gas flowrate (L/V) decreases, then the appeal of the rich approach increases. The value of the rich approach helps quickly determine the appropriate amine circulation flow. As a rule of thumb, the rich approach should be between 70% and 80%.

A rich approach above 80% indicates low volume circulation; for a typical 15–20-tray absorption tower, the amine liquid generally does not absorb a higher proportion of H2S. Under abnormal conditions, such as an increase in the concentration of H2S in the imported acid gas or a sudden increase in the amount of imported acid gas, the concentration of H2S in the outlet gas may be out of specification.

If the rich approach is less than 70%, it suggests excess circulation; at this stage, even if the quantity of amine liquid is increased, the concentration of H2S at the overhead gas outlet is low.

Dynamic simulation of flare gas discharge. A dynamic simulation softwareb calculates the levels of exhaust gas emissions (FIG. 10) and helps determine the circulation pump start/stop settings and the water seal isolation tank’s water sealing height. The goal is to obtain the best control parameters and reach the target of zero emissions.

FIG. 10. Dynamic simulation for exhaust gas emissions from flare main header.

Sour flare gas backflow prevention. Anti-backflow facilities were added to prevent sour flare gas from flowing back into the hydrocarbon flare header facility.

An exhaust gas recovery backflow prevention facility prevents sour flare gas from flowing back into the hydrocarbon flare header and discharging from the ground flare, causing air pollution. This design also allows the exhaust gas recovery system to prioritize the recovery of acid waste gas to clean the air near the refinery, thereby improving the value of investment in this system.

Low consumption for utility system. The temperature difference between the cooling water inlet and outlet is 13°C, and the cooling water consumption is the most economical. As the amount of exhaust gas recovery is ever-changing, the amount of water supplement for the oil-water-gas three-phase separation tank can be proportionally adjusted according to the quantity of exhaust gas recovery needed to save water.

Since the energy consumption of the ejector circulation water pumps is high, energy-saving designs have been incorporated. Four circulation water pumps—two BB3 high-efficiency (80.2%) pumps and two BB2 pumps (efficiency 69.5%)—were added in two trains in Section C of the refinery.

The two BB3 high-efficiency pumps are prioritized for activation during operation, and when the demand is high, the other two BB2 pumps are also activated to improve energy efficiency and reduce operating costs.

Optimization of plant plot arrangement. Due to the limited space in the existing plant, in addition to optimizing process technology, the project optimized plant layout by symmetrical arrangement to reduce the materials required, including pipelines, pipe racks, concrete and others. A three-in-one design is used to integrate two liquid separation tanks and one amine scrubber into one vessel to save space.

Equipment backup system. The two train systems are designed for both Sections C and F. When the first train equipment is under inspection or maintenance, the other train can continue the flare gas recovery operation.

Multistage automatic start/stop. The process system safety control design included multistage control start/stop for the flare gas recovery unit to save power and improve efficiency. Computer simulation software was used to calculate the best setpoint to achieve the highest system efficiency, safety and reliability.

Finally, the dual-feeder power supply system is designed to improve system stability and reliability.

Takeaway

After the completion of the project, the improvement in overall environmental quality in the city is quantifiable. Calculations show that each train in Sections C and F can recover up to 9,600 m3/hr of flare gas. The flare gas recovered in one day is equivalent to the amount of CO2 absorbed by Daan Forest Park in Taiwan in an entire year.

The project was highly praised by the jury members in the Golden Quality Award of the National Public Construction Commission of Taiwan for technology innovation and optimized design. This recycling technology should be promoted to increase environmental protection. HP

NOTES

       a ProMax
       b DySime

The Author

Related Articles

From the Archive

Comments

Comments

{{ error }}
{{ comment.comment.Name }} • {{ comment.timeAgo }}
{{ comment.comment.Text }}