Heat stable salts

Work continues on improving the efficiency of this process, such as for freeing the alkan olamine from heat-stable salts that can form (125). Formulations have been developed which inhibit degradation of mono- and diethanolamine in processing (126). Models (127), computer programs (128), and kinetics and enthalpies (129—136) have been developed to help determine equiUbria of the acid gas—alkanolamine—water system. Additional references relate to the use of tertiary alkan olamines, such as triethanolamine, for gas conditioning (137—139). 

Other components in the feed gas may react with and degrade the amine solution. Many of these latter reactions can be reversed by appHcation of heat, as in a reclaimer. Some reaction products cannot be reclaimed, however. Thus to keep the concentration of these materials at an acceptable level, the solution must be purged and fresh amine added periodically. The principal sources of degradation products are the reactions with carbon dioxide, carbonyl sulfide, and carbon disulfide. In refineries, sour gas streams from vacuum distillation or from fluidized catalytic cracking (FCC) units can contain oxygen or sulfur dioxide which form heat-stable salts with the amine solution (see Fluidization Petroleum). 

A recent article explains why the common practice of caustic addition is bad for amine systems. Heat stable salts do build up in amine systems causing reduction of the amine solution s effective capacity, corrosion, aggravation of foaming, and amine loss. 

When heat stable salt buildup becomes a problem a variety of options may manage it. These include partial or total solution replacement, heat stable salt removal, or adding caustic to neutralize the heat stable salts. Many operators choose caustic addition because it is perceived to be a more economical way to stop corrosion and subsequent foaming and loss problems. 

Tests to simulate real-world amine plant operations have shown that caustic addition doesn t substantially improve solution corrosivity and in some cases corrosion rates increase. Maintenance of low heat stable salt anion levels is a better way to go. Concentrations as low as 250 ppm are encouraged and 5,000 to 8,000 ppm seem to be tolerable. Caustie doesn t reduce the heat stable salt content of amine solution. 

These reactions are reversible by changing the system temperature. ME A also reacts with carbonyl sulfide (COS) and carbon disulfide (CSi) to form heat-stable salts that cannot be regenerated. At temperatures above 245°F a side reaction with CO2 exists that produces oxazolidone-2. a heat-stable salt, and consumes MEA from the process. 

The normal regeneration temperature in the still will not regenerate heat-stable salts or oxazolidone-2. Therefore, a reclaimer is usually included to remove these contaminants. A side stream of from 1 to 3% of the MEA circulation is drawn from the bottom of the stripping column, This stream is then heated to boil the water and MEA overhead while the heat-stable salts and oxazolidone-2 are retained in the reclaimer. The reclaimer is periodically shut in and the collected contaminants are cleaned out and removed from the system. However, any MEA bonded to them is also lost. 

When MEA is used in the presence of COS and CS2, they react to form heat-stable salts. Therefore, MEA systems usually include a reclaimer, The reclaimer is a kettle-type reboiler operating on a small side stream of lean solution. The temperature in the reclaimer is maintained such that the water and MEA boil to the overhead and are piped back to the stripper. The heat-stable salts remain in the reclaimer until the reclaimer is full. Then the reclaimer is shut-in and dumped to a waste disposal. Thus, the impurities are removed but the MEA bonded to the salts is also lost. 

The absorbent in the CANSOLV SO2 Scrubbing System accumulates nonregenera-ble salts [also called Heat Stable Salts (HSS)] and dust that are removed from the gas over time. These contaminants must be removed from the absorbent continuously to avoid excessive build-up. An APU incorporates both an ion exchange unit (IX) for the removal of HSS and a filtration unit for the removal of dust. 

Finally, compounds can react with the amine to form heat-stable salts that are not dissociated at the temperatures in the regenerator. The concentration of heat-stable salts can build up to such an extent as to cause foaming problems. 

Removal of metal ions and heat stable salts from industrial lean amine solvent using polymeric hydrogels from gas sweetening unit... 

Keywords Gas sweetening, adsorption, heat stable salts, hydrogel, isotherm... 

Removal ofmetal ions and heat stable salts from industrial lean... 

The composition of these solvents is typically subject to change during operation due to a variety of causes, for example solvent degradation (oxidative and/or thermal), solvent evaporation (vaponr pressnre and/or aerosols), solvent inactivation due to heat stable salt formation, and imbalance of the water management. Some of these canses are inherent to the process and solvent that are used, while others depend on the quality of the gas stream that is treated. In addition, these canses can also enhance other unwanted effects such corrosion and foaming. 

Table 10.41 Common sources of anions of heat-stable salts.

Figure 10.376 Separation of heat-stable salts in a "semilean" W-methyIdiethanolamine solution on a methacrylate-based anion exchanger. Separator column lonPac AS9-SC eluent 1.8mmol/L Na2C03-I-1.7 mmol/L NaHCOa flow rate 2 mL/min detection suppressed conductivity injection volume 25 pU peaks (A) 1.9mg/L fluoride (1), 3.2mg/L...

Fig. 9-227. Separation of heat-stable salts in a N-methyIdiethanolamine solution. - Separator column lonPac ASIO eluant 0.1 mol/L NaOH flow rate 1 mL/min detection suppressed conductivity injection volume 25 pL solute concentrations 48 mg/L acetate (1), 54 mg/L formate (2), carbonate (J), 5.9 mg/L chloride (4), 12 mg/L sulfate (5), and J.5 mg/L oxalate (6).

To a lesser extent, the above warning applies to all amine systems. The presence of carboxylic acids produced from heat-stable salts, overloaded amine, and cyanides accelerates the process of hydrogen-assisted stress-corrosion cracking. 

Acetic acids are known to promote hydrogen ion penetration of vessel walls. Acetic acid is formed from the thermal decomposition of heat stable salts formed due to oxidation of the MEA. These heat stable salt precursors were supposed to be removed in an MEA reclaimer. But Unocal had not used the reclaimer unit for many months. 

MEA reclaimer Used to remove heat-stable salts from amine. 

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