Continuous removal of contaminants from amine solutions

The amine-treating unit is of great importance in gas processing and refinery opera-

tions for treating acid gases. It is also attracting increasing attention from the pressing needs of environmental compliance and for meeting strin-gent levels of H2S and CO2 removal.

Amine scrubbing solutions used for gas treatment at refineries, chemical, sulphur and gas process-ing plants remove hydrogen sulphide (H2S) and carbon dioxide (CO2) from gas streams. Yet, one of the problems with amine solutions is the gradual accumulation of byproduct contaminants called heat stable salts (HSS). An increased HSS concentration in the amine loop leads to corrosion-related maintenance problems, frequent filter replacements, amine losses, excessive foaming, capacity reduc-tion and more.

Ion exchange resins have been employed in the removal of HSS, but because of the different affini-ties the various anionic HSS contaminants have towards the resin, successful removal is restricted. However, the continuous online removal of HSS rather than periodic or batch treatment helps to ensure a more stable and uniform conditioning operation (see Figure 1).

A proprietary system from Eco-Tec has proven successful over the years in achieving efficient online HSS removal. To date, 57 of these systems, known as AmiPur, have been supplied in the US, Mexico, Europe, China, India and Saudi Arabia. This article details the tech-nology and its various applications, including CO2 sequestration.

A continuous purification system overcomes amine degradation and the formation of corrosion-promoting heat stable amine salts

Sunil DAnDekAr and Jenny ShAoEco-Tec

Various examples are presented, with two case studies in China and Aruba.

hSS formationLean amine solutions of 20–50 wt% and relatively free of acid gases enter the top of the absorption tower and flow counter-current to the sour gas stream being fed from the bottom of the column. The acid gases in the gas stream are chemi-cally absorbed, resulting in a rich amine solution that exits from the bottom of the tower and the sweet gas from the tower top. Thermal

regeneration of the rich amine is then carried out in a steam stripper, where the absorbed acid gases are liberated and this regenerated (lean) amine is recirculated back to the absorption tower top.

In the absorption process, a common problem is the formation of small quantities of HSS, which are not removed during steam stripping. HSS are formed due to the presence of certain other acidic components in the process gas and liquids, which result in an irreversi-ble reaction with the amine to form HSS. These contaminants include

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Absorber Condenser

Stripper

Flashdrum

Leanfilter

Leancooler

AmiPur®-PLUS

Lean/richexchanger

RefluxdrumFeed

gas

Richamine

Lean amine

Water

NaOH

Acid

Purified lean amine (product)

Treatedgas

Acidgas

Condensate

Waste

Figure 1 Batch treatment such as purging, off-site treatment or on-site mobile services result in continuous changes to the amine circuit characteristics and operating challenges. Continuous treatment results in low, stable HSS concentration with predictable amine characteristics

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amines are restricted from building up and the corrosion rate Is minimised.

Continuous online amine reclaiming is now recognised as the most effective solution for HSS-related problems (see Figure 2). Furthermore, if an upset occurs upstream of the amine unit, the equipment is on-site to address the problem immediately. Such an online system, known as AmiPur, was introduced to this application in 1998 by Canadian-based Eco-Tec.

Some of the features differentiat-ing this ion exchange technology from conventional systems include: • Fine mesh resins • Short and packed resin bed • Counter-current regeneration • Low resin loading • Short cycles • Compact and skid-mounted system• Stainless steel construction• Full automatic operation.

Systems employing this technol-ogy have been used for decades, but for various other industrial applications such as chemical puri-fication, recovery and recycling (in steel finishing operations and elec-tro-plating, for instance).

For the amine purification system, there are basically two steps in the operating cycle: amine contaminant loading and caustic regeneration. This cycle is repeated automatically every 10–15 minutes.

Lean amine solution is pumped through the resin bed. The ion exchange resin removes the HSS, then the purified amine solution is directed to the flash tank or returned to the amine loop:

Loading step: R’OH + RN3+HHCOO- →R’HCOO + RN

3 + H

2O

Dilute caustic soda is used to regenerate the resin column. The unit draws concentrated caustic from the bulk tank, which is auto-matically diluted to the proper concentration before passing through the resin bed. After a few minutes of regeneration, a water wash rinses out the excess caustic from the resin bed and a new cycle starts:

chloride, sulphate, formate, acetate, oxalate, thiocynate and thiosul-phate. The resultant salts have a relatively strong chemical bonding with essentially no dissociation with heat (in the steam stripper). This results in the gradual build-up of HSS in the amine circulation loop and, when the tolerable limits of HSS are exceeded, several opera-tional and maintenance problems are encountered, some of which include: • High corrosion rates, leading to stress corrosion cracking • High maintenance and repair costs and safety concerns• Frequent filter replacements • Foaming and plugging in absorber tower • Decrease in absorption efficiency and productivity • Heat exchanger and reboiler tube fouling • Excessive heat requirements • Overall unit instability.

High corrosion rates, typical for a number of amine plants, as well as stress corrosion cracking of stain-less steel, usually attributed to chloride, create serious safety concerns. High corrosion leads to high repair and maintenance costs, potential environmental implica-tions and lost production. Carbon steel corrosion is often attributed to the amine contaminants that cannot be stripped and thereby accumulate in the amine solution. However, preventive measures to reduce corrosion produce good results and improve amine unit reliability at a reasonable cost.

Continuous reclamation to control impuritiesTo prevent HSS from building up beyond critical limits, the most straightforward approach is peri-odic amine purging, which is messy and prohibitively expensive given the current situation of increased competition, cost control and restricted waste discharge. Other periodic amine clean-ups, on- or off-site, are also practised to some

extent by plants that have to employ large equipment for vacuum distillation, conventional ion exchange or electro-dialysis. However, this approach of periodic reclaiming is cumbersome and expensive and does not effectively overcome the operational and corrosion problems caused by the anions.

It became apparent that the ideal solution to overcome HSS-related problems was to have online amine purification (not periodic), which would ensure a stable and uniform gas conditioning operation, while the contaminant levels in the

6

5

4

3

2

1HS

S,

wt%

as

MD

EA

Time0

Batch treatmentContinuous treatment

Figure 2 Continuous online amine reclaiming

Continuous online amine reclaiming is now recognised as the most effective solution for hSS-related problems

Regeneration step: R’HCOO + NaOH → R’OH + HCOONa

Note: R’OH - resin surface, RN3 - tertiary amine,

HCOOH - HSS

Case study 1: Chinese refinery A catalytic cracking unit at a refin-ery in China has a cracking capacity of 3 million t/y. This FCC unit is connected to an amine-scrubbing unit using MDEA, which treats 160 000 t/y of dry gas and 450 000 t/y of LPG.

Major problems with the amine-scrubbing unit were a high concentration of suspended solids, frequent filter clogging, amine losses due to foaming and equip-ment fouling, severe corrosion problems found in the reboiler, higher temperature piping and a lean/rich heat exchanger (see Figures 3, 4 and 5). Corrosion was also observed at all connections. Upon further examination, leaking was found in the gas piping, in both the inlet and outlet piping and the lean/rich heat exchanger. Eventually, the whole unit was shut down for repair. Some 60 tonnes of 24% dirty amine was drained from it.

An online, continuous amine purification unit was selected to keep the system’s HSS level below 1 wt% (as MDEA). There are two operation modes: high HSS and low HSS. The high mode is used to bring the HSS level down to 1 wt% as fast as possible. The low mode keeps the HSS concentration in amine solution below 1 wt%.

The system was installed and started up in less than one month. Following start-up, it had been running for 2300 cycles in high HSS mode and the HSS level was brought down from 3.8 wt% to 1.0 wt%. The system was switched to low HSS mode and the HSS level in solution has been kept below 1 wt% (as MDEA), with the lowest level at 0.27 wt%.

Due to the continuous removal of HSS by the system, the corrosion rate decreased dramatically. The average corrosion rate at the top of the regenerator before installation was 90 mpy (2.3 mm/y). When the HSS level was brought down to 0.5

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Figure 3 Lean/rich heat exchanger piping

Figure 4 Reboiler piping

Figure 5 Erosion inside lean/rich heat exchanger

solution. The skid also uses a six-step process and low-strength caustic to regenerate the resin bed. The operating cycle period can be changed to low, medium or high, depending on the content of the HSS in the lean amine feed.

Before the commissioning of the skid, the amine unit was frequently replenished with fresh MDEA to maintain a low HSS in the amine solution. The contaminated HSS amine was removed from the amine unit and stored in ISO containers; this amine continues to be slowly reintroduced to the unit for HSS reduction by the skid. At the amine unit, the HSS was reduced to below 4 wt%, which resulted in fewer filter change-outs. No MDEA has been purchased since start-up. Without the skid, amine change-out would have to be done every three months at an annual cost of over $250 000 in amine costs alone.

The problems of high amine losses, high levels of H2S in the fuel gas and poor unit reliability are now resolved. The revamps, skid and various parameter changes have resulted in significant savings and improved amine unit reliabil-ity. Based on savings of amine purchases and filter replacement, the HSS removal system had a payback of less than one year.

Co2 removal

CO2 sequestration, or capture and storage applications, offer opportu-nities to reduce greenhouse gas emissions from coal and other fossil fuels used in industrial applica-tions. The most widely used and accepted method of CO2 capture is through the post-combustion use of amine solvents. For over 60 years, amine scrubbing has been instru-mental in oil and gas processing applications. For CO2 sequestration, the underlying difference in the process is the large-scale produc-tion required for the removal of CO2 from flue gas.

Amines used for CO2 capture can present challenges, such as increas-ing solvent efficiency, increasing regeneration efficiency and reduc-ing solvent degradation. Each of these challenges can be addressed by implementing an amine

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wt%, this rate was as low as 2 mpy (0.05 mm/y).

Case study 2: Aruba refinery The Aruba refinery’s amine system uses a 45 wt% MDEA solution for removing H2S from various gas streams and an LPG stream. The amine regeneration units each have a design capacity of 1450 barrels per hour (1015 gallons per minute). These units regenerate the amine solution, removing H2S from a solu-tion of water and amine (MDEA), and provide lean amine solution for the H2S absorbers. The high- pressure H2S absorbers are at the gas oil hydrotreater units. The medium-pressure absorbers are at the diesel hydrotreater unit, purge gas unit and LPG absorber. The low- pressure absorbers are two coker unit absorbers and two fuel gas absorbers.

The Aruba refinery observed high amine losses, a high level of H2S in the fuel gas and poor amine regen-eration unit reliability. High corrosion rates resulted in leaking of the lean/rich exchangers and also the lean amine coolers. For example, the leaking lean amine cooler resulted in seawater contami-nation of the amine at one of the units. After exchanger tube repairs, the unit equipment was washed with clean water and replenished with fresh amine. Contamination and foaming caused great losses of the MDEA solution and off-spec treated gas.

In an effort to resolve the many problems associated with the amine units and at the same time accom-modate higher projected sulphur loads (at higher sulphur crude rates) while minimising cost, it was agreed to revamp the existing amine units. The units were origi-nally designed for MEA but were revamped for MDEA. The revamp directive included higher amine circulation rates and a higher MDEA concentration of 45 wt%.

As part of the revamp, the capac-ity of each amine regeneration unit was increased to 1450 b/h (1015 gal/min) from 1000 b/h (700 gal/min) at two of the amine regenera-tion units, and 1250 b/h (875 gal/ min) at another unit. The revamp

included a larger feed flash drum to increase residence time and prevent hydrocarbon carry-over to the amine regenerator.

After the revamp, the HSS build-up rate was 1.3 wt% per month. This increase can be attributed to better amine operation and lower amine losses. The primary HSS are formate and thiocyanate anions, which are degradation byproducts from the delayed coker units. The upper limit recommended for formate is 500 ppmw, but as high as 19 600 ppmw was measured at the amine unit. These HSS are formed with MDEA in the presence of CO or HCN. The amine stream treating the coker gas showed a high amount of HSS between 3.0 and 8.5 wt%.

The iron in the solution had increased from 11 to 40 ppmw since the revamp, an indication of

increased corrosion in the amine system. The dark colour of the amine and frequent filter replace-ments also confirmed the corrosion mechanism due to high HSS.

hSS removal skidAn AmiPur skid for HSS removal was installed and commissioned. The project took 11 months to purchase, design and construct. This skid includes a resin bed, cartridge filters for lean amine and water, a caustic reservoir and a programma-ble logic control panel for automatic control. The skid is designed to remove dissolved HSS from a 21.8 gal/min flow of lean amine. The key to the process is the ion exchange resin used to absorb the HHS dissolved in the circulating amine

Based on savings of amine purchases and filter replacement, the hSS removal system had a payback of less than one year

compliance. Amine degradation causes the formation of HSS, which can lead to corrosion. Amine unit performance begins to deteriorate as the HSS increase and the H2S absorber becomes less stable. Iron in solution increases, amine filter

life starts to fall, and equipment corrosion and fouling can lead to an unscheduled outage.

One of the most important results of the installation of a continuous amine purification system is the stable and reliable operation of the amine plant, which, in turn, has a

purification system into the design. This must be able to continuously remove any amine impurities to avoid the many problems associ-ated with solvent and overall process efficiency.

By consistently maintaining low HSS levels, continuous purification results in less corrosion and reduced operating costs. This effi-cient ion exchange method for continuously removing impurities offers high purity, reclaimed amine.

Unlike gas processing, CO2 seques-tration operations have larger capacity demands, so the amine purification system must be scalable. Another important consideration is the amine purification system’s abil-ity to reduce solvent degradation and loss. Effective systems that incorporate high capacity with effi-cient regeneration address this consideration and provide opportu-nities for lower operating costs.

ConclusionAmine unit operation is an impor-tant part of environmental

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The continuous removal of hSS has immediate and easily quantifiable results, including a reduction in filtration costs

dramatic environmental impact. The continuous removal of HSS has immediate and easily quantifiable results, including a reduction in filtration costs, elimination of peri-odic chemical cleaning of the absorber tower, improved gas treat-ing capacity of the unit, elimination or reduced use of anti-foaming additives, neutralisers and corro-sion inhibitors, and elimination of the costs associated with previously used methods of HSS removal.

Sunil Dandekar is Global Product Manager with Eco-Tec Inc, responsible for the development of continuous amine purification system in the Americas, Asia-Pacific and the Middle East. He has over 25 years’ industrial experience in construction, project management and sales of engineered process products, and holds a bachelor’s degree in mechanical engineering. Email: sdandekar@eco-tec.comJenny Shao is Vice-president, Business Development – Asia, with Eco-Tec Inc. She has helped develop capital equipment for engineering firms in China, Japan, Canada and the US, has held teaching and research positions with universities, and has a masters in chemical and environmental engineering. Email: jshao@eco-tec.com