RU2116107C1 - Method of preventing foam formation in alkaline absorption solutions - Google Patents

Abstract

FIELD: chemical engineering. SUBSTANCE: invention relates to removing acid components (CO₂, H₂S and others) by alkaline solutions involving foam suppressers added to those solutions. Foam suppressers are mainly used in the form of composite reagents. As a result, foam suppression process in hot alkaline absorption solutions is rendered less expensive, while simultaneously foam-suppression ability and long-term stability of compositionally simple foam suppressor are increased. In particular, polyethylsiloxane liquid is used with silicon content 26-28 wt % and containing 0.1 to 0.25 wt % ethoxyl groups. Liquid is added to alkaline absorption solution with pH 9-011 in amount 0.33-0.66 g/cu.dm at solution temperature 20 C or 0.06-0.1 f/cu. dm at solution temperature 120 C. EFFECT: reduced process expenses. 3 cl, 1 dwg, 3 tbl

Description

The invention relates to chemical technology associated with foaming in alkaline absorption solutions of gas purification processes from acidic components (CO ₂ , H ₂ S, etc.), for example, in the production of ammonia, when carbon dioxide (CO ₂ ) is purified from converted gas from hot ( up to 120 ^o C and above) carbonate solution (pH 9-11) and can be used in chemical, petrochemical, gas and other related industries.

 The cause of foaming may be the presence of resinous products of thermal degradation of organic substances (ethanolamines, etc.), which are part of the absorption solutions and impurities entering the purification system together with the gas (carbon black, scale, corrosion products, solid particles of catalyst decomposition and lining of technological equipment, compressor oil, etc.).

 Sources of increasing the content of mechanical impurities are both the raw materials included in the solution (potash, vanadium pentoxide), and condensate or process water used to feed the gas purification system. Untimely removal of mechanical impurities from the solution leads to its foaming and mechanical entrainment with bubbles of purified gas, which reduces the productivity of the equipment and the activity of the absorption solution, and also leads to the loss of expensive reagents. Moreover, it can lead to an emergency with an uncontrolled increase in the pressure drop in the absorbers caused by mass foaming of the solution.

 Foaming can be eliminated by the complete extraction of foaming substances (which is practically impossible) or by the use of antifoam additives in the form of compositions or a single reagent.

A known method of extinguishing foam using a mixture of monoalkyl ethers of polyethylene glycol based on primary fatty alcohols (C ₁₀ -C ₁₈ ) or monoalkyl phenol ethers of polyethylene glycol based on polymeric distillate (C ₁₀ -C ₁₈ alkyl) with a degree of hydroxyethylation 3-12 (surfactant) space-hindered phenol and block copolymer of propylene and ethylene oxides based on glycerol mol. m. 1000 - 8000 [1].

 The disadvantage of this method is the complexity of the composition of the antifoam, a narrow scope, low efficiency in an alkaline environment at high temperatures and the formation of resinous products of thermal degradation of organic substances that are part of the antifoam, polluting the absorption solution and contributing to its foaming.

Known antifoam composition, including polymethylsiloxane liquid (35-50 wt.%) And surfactants - polyethylene glycol derivatives of fatty alcohols fraction C ₁₀ -C ₁₈ or amides of synthetic fatty acids fraction C ₁₀ -C ₂₀ with a degree of hydroxyethylation 5-20 and polyethylene glycol derivatives of carboxylic or hydroxycarboxylic acids of the C ₁₆ -C ₁₈ fraction with a degree of hydroxyethylation of 5–9 [2].

 The disadvantage of this composition is the complexity of the composition, the use of expensive components of organic origin, low efficiency due to the lack of stability in hot alkaline environments.

 Known antifoam composition containing, wt.%: Polymethylsiloxane - 48, polyvinyl alcohol - 2, aerosil - 2, water - the rest [3].

This composition is stable only at temperatures up to 45 ^o C and is used in aqueous media. Its use for extinguishing foam in an alkaline environment and at high temperatures (up to 120 ^o C and above) is impractical.

 A known method of extinguishing foam by treating a solution with polymethylsiloxane (PMS-200A) in the form of an aqueous emulsion (0.01-0.3%) or a 1% solution of stearic acid in kerosene [4].

 The disadvantage of this method is the narrow specification and low efficiency in an alkaline environment, which limits the possibility of widespread use of the method. Moreover, laboratory experiments established that PMS-200A in an alkaline environment behaves as an initiator of foaming.

The closest to the invention in terms of technical essence and the achieved effect is an antifoam based on polymethylsiloxane, used in amine processes for gas purification from acidic components (CO ₂ , H ₂ S, CS ₂ , COS, etc.) in the presence of anti-corrosion additives in absorption columns [5 ].

Known antifoam [5] contains polymethylsiloxane (PMS) as a polyorganosiloxane, additionally contains CaA brand zeolite, hydrochloric acid, sulfanol, monoethanolamine and water in the following ratio, wt.%
Polymethylsiloxane - 27.1-37.0
Zeaol CaA - 4.28-3.80
Hydrochloric acid - 8.57-7.80
Sulfanol - 0.59-0.40
Monoethanolamine - 3.71-3.0
Water - The rest.

 However, this antifoam is difficult to prepare, uses expensive reagents (sulfanol) in its composition, does not exclude injuries and corrosion of technological equipment when preparing a composition containing hydrochloric acid, and its use as an antifoam. When using CaA zeolite, abrasive wear of rotating parts occurs with possible emergency situations during equipment operation.

 The purpose of the invention is to reduce the cost of the process of extinguishing foam in hot alkaline absorption solutions, while increasing the antifoam ability and stability of the action of a simple antifoam for a long time.

The goal is achieved by developing a reliable and effective method of defoaming, using as a defoamer in an alkaline absorption solution polyethylene siloxane brand PES-5 in an amount of 0.33-0.66 g / dm ³ at a temperature of 20 ^o C and 0.06-0.1 g / DM ³ at a temperature of 120 ^o C and above. The pH of the alkaline absorption solution is 9-11.

 PES-5 brand liquid contains, wt.%: Silicon - 26-28, ethoxyl groups - 0.1-0.25 [6] and is supplied to the alkaline absorption solution at any stage of its use without preliminary preparation (mixing, dilution, heating, etc.).

 The drawing shows a diagram of an installation for foaming, where 1 is a foam apparatus, 2 is a rheometer, 3 is a tee, 4 is taps, 5 is a rubber tube.

The effectiveness of defoaming was tested in a laboratory setting (drawing) by determining the foaming coefficient of the test alkaline solution before and after introducing the defoamer into the foamed solution at a nitrogen feed rate of 1 dm ³ / min into the foam unit 1 filled with the solution. A series of laboratory experiments using an alkaline solution containing, wt.%: K ₂ CO ₃ - 28.3, V ₂ O ₅ - 0.5, diethanolamine - 3.6, solids in solution - 0.03 g / dm ³ the rest is water. pH 10.1-10.3 and foaming coefficient 0.40 - 0.45.

Defoamers tested: PES-5 polyethylsiloxane liquid; PMS-200A polymethylsiloxane liquid
The test results of PES-5 and PMS-200A are presented in table. 12.

The foaming coefficient of the test solution of 0.16-0.18 (table. 1) at a temperature of 20 ^o C is achieved by introducing PES-5 in the amount of 0.33-0.66 g / DM ³ .

The introduction of PES-5 into the solution of less than 0.33 g / dm ³ did not provide complete quenching of the foam, and more than 0.66 g / dm ³ did not give an additional effect.

Under similar conditions, PMS-200A (control experiment) operates in the foaming mode (table. 2). From the table. 2 shows that the dosage of PMS-200A in the test solution in an amount of 0.20 - 0.99 g / dm ³ leads to a stable increase in the foaming coefficient from 0.40 - 0.43 to 2.40 - 2.70.

The proposed method for preventing foaming in alkaline absorption solutions was tested under industrial conditions by ANKhK in the production of ammonia at the stage of purification of converted gas from carbon dioxide (CO ₂ ) with an alkaline solution similar in composition to that tested in laboratory conditions.

Antifoam PES-5 was dosed in an alkaline hot solution (t - 120 ^° C; pH 9-11) at a rate of 20 dm ³ / h (the total amount of reagent introduced was 40 dm ³ , which corresponds to 0.06 g / dm ³ ).

PES-5 was commissioned at the following process parameters for absorbers (two absorbers in operation) [7]: converted gas consumption of 109,000 nm ³ / h for each absorber (normal - not more than 110,000 nm ³ / h; resistance to absorbers (in average) 0.40 - 0.45 kg / cm ² (the norm is not more than 0.4 - 0.5 kg / cm ² ); the foaming coefficient of the absorption alkaline solution is 0.12-0.12 (the indicator is not standardized [7 ]).

During a long test (more than five months), an additional input of antifoam brand PES-5 at the stage of purification was not performed. With a stable technological mode during the test, the following positive results were obtained:
- reduction in resistance to absorbers to 0.20 - 0.29 kg / cm ² (approximately 2 times from the original 0.40 - 0.45) and its stabilization during the entire test period;
- decrease in the coefficient of foaming of the alkaline solution to 0.05-0.07 (approximately 2 times from the original 0.12 - 0.13);
- the residual CO ₂ content in the purified gas averaged 0.06 vol. % (norm no more than 1 vol.% [7]) at gas loads in the range of 90,000-109,000 nm ³ / h for each absorber;
- the iron content in the working solution averaged 39.2 mg / dm ³ (the norm is not more than 100 mg / dm ³ [7]), which indicates the absence of corrosion activity of the test reagent.

More short-term tests using PES-5 in an alkaline solution showed (Table 3) that a reagent concentration of less than 0.06 g / dm ³ reduces the defoaming effect (foaming coefficient obtained 0.08-0.09), and the concentration is greater than 0, 1 g / dm ³ only leads to an unreasonable cost overrun of the antifoam, which is not economically feasible (foaming coefficient, ceteris paribus, remains at the level of 0.06-0.07).

Thus, the use of PES-5 in alkaline absorption solutions increases the antifoam ability and the duration of the antifoam due to inertness, high chemical resistance in an alkaline environment in a wide temperature range - 20 - 150 ^o C.

 PES-5 is used without preliminary preparation (mixing, dilution, heating, etc.), it can be introduced at any stage of purification (absorption, desorption).

 A reliable and effective way to prevent foaming, stabilizing the gas purification process, eliminates the use of expensive organic substances prone to thermal degradation; the use of substances that cause corrosion of equipment or abrasive wear of moving parts; reduces the formation of impurities polluting the absorption solution, and its tendency to foaming and entrainment with gas; increases the activity of the absorption solution and the performance of the equipment.

The main and main features of PES-5 grade polyethylsiloxane fluid are a low temperature dependence of viscosity; low pour point (below -60 ^o C): increased thermal stability; stability under long-term storage and in working condition at temperatures up to 150 ^o C and above.

 Such a reagent is non-toxic and does not have corrosive activity.

 The chemical inertness and oxidation resistance of PES-5 provides it with a rather long service life as a defoamer in alkaline absorption costs in small quantities.

Sources of information
1. A.S. N 1570743, cl. B 01 D 19/04, 1987.

 2. A.S. N 1122336, class B 01 D 19/04, 1980.

 3. Tikhomirov V.K. Foam. Theory and practice of their preparation and resolution. - M.: Chemistry, 1983, p. 200-210.

 4. Tikhomirov V.K. Foam. - M .: Chemistry, 1975, p.216-217.

 5. A.S. N 611640, class B 01 D 19/04, 1978 (prototype).

 6. GOST 13004-77. Polyethylsiloxane fluids, M., 1994.

 7. Technological regulations for the production of ammonia, JSC "ANHK", 1989.

Claims (3)

1. A method of preventing foaming in alkaline absorption solutions of gas purification processes from acidic components (CO ₂ , H ₂ S, etc.), comprising introducing a chemical reagent into the absorbent, characterized in that polyethylsiloxane liquid (PES-5) is used as a chemical reagent a silicon content of 26-28 wt.% and ethoxyl groups of 0.1-0.25 wt.%. 2. The method according to p. 1, characterized in that the PES-5 is administered in an amount of 0.33-0.66 g / dm ³ at a temperature of an alkaline solution of 20 ^o C or 0.06-0.1 g / dm ³ at a temperature alkaline solution 120 ^o C.  3. The method according to p. 1, characterized in that the PES-5 is administered at a pH of the absorption alkaline solution of 9-11.

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