The present invention relates to a process for the complete removal of heat stable salts (HSS) from an aqueous alkanolamine solution containing HSS by treating the solution with an ion exchange resin.
BACKGROUND AND PRIOR ART
In gas processing complexes and refineries, alkanolamine solution is used to adsorb H2S and CO2. During adsorption, the alkanolamine forms salts of formate, acetate, oxalate, thiocyanate etc. in presence of oxygen. Other salts like salts of sulphate, sulphite usually come from the water used in the preparation of alkanolamine solution. The presence of heat stable salts in amine circulation loop encounters high corrosion leading to stress corrosion cracking, high maintenance cost, frequent filter replacement, foaming, plugging in amine absorption column, and decrease in amine absorption efficiency, heat exchanger and reboiler tube fouling and overall unit malfunctioning. Reduction of heat stable salts in alkanolamine solution ensures a stable and uniform operation of amine treating plants. Different processes are used in removal of heat stable salts from alkanolamine solutions. Each process has its merits and demerits. Distillation process is used where heat stable salts are concentrated by leaving the amine in vapor phase. This process is not energy efficient and vacuum system is adopted to reduce the temperature to avoid thermal degradation of the alkanolamine. Another process is electrodialysis which uses ion selective membrane for separation of heat stable salts by maintaining potential difference across the membranes. The efficiency of the process depends on membranes used but the cost of the electrodialysis system is relatively high.
The EP0956141B1 discloses a process for purifying an amine solution containing a heat stable amine salt by neutralizing the amine solution containing heat stable amine salt by contacting with an aqueous solution of potassium hydroxide. This process is non-regenerative and has high operating cost.
The U.S patent 3984314 discloses a method for purification of industrial effluents containing cyanide ions, and cyanide precursors like acetone-cyanohydrin. by utilizing a complexing compound followed by treatment with an anion exchange resin and optionally cation

exchange resin to remove the cyanide complexes. However, the process is not applicable to heat stable salts.
U.S. patent 5912387 discloses a process for neutralization of amine or alkanolamine heat stable salts formed when an amine solution is used in gas conditioning. It does not use ion exchange resin for removing the heat stable salt from the alkanol amine solution. This process is also non-regenerative and has high operating cost.
U.S. patent 5292407 discloses a process for converting heat stable amine salts to heat regenerable amine salts which is achieved by passing the amine solution having heat stable salt through an electrodialysis zone. This method does not remove the heat stable salts from the amine solution rather converts them to regenerable amine salts which need to be further treated to obtain the amine solution free of HSS salts. This method is cumbersome and not cost effective as it requires addition steps to achieve the amine solution.
U.S. patent 5277822 discloses ion exchange process which uses type I resin. It uses moving bed and regeneration done by 1-2 M NaOH solution. It uses hot water for washing of resin. It does not disclose the type of anions in heat stable salts to be removed and also did not provide information of extent of HSS removal.
The prior art methods disclosed above suffer from major drawbacks such as: a) non-regenerative menthod and b) high operating costs. Therefore, there exist a need for enhancing the ion exchange process for HSS removal. Ion exchange processes have also been found to be cost effective. Application of the processes depends on type of resin used, feed conditions and extent of heat stable salts removal desired by the user industries. Resins to be used need to be highly selective for removal of HSS and must be regenerative at minimum treatment. The present invention effectively addresses these drawbacks of the reported methods.
OBJECTIVES OF THE INVENTION
The primary object of the present invention is to develop a process for complete removal of
Heat stable salts (HSS) from the aqueous alkanolamine solution comprising the steps:
(a) treating an aqueous alkanolamine solution containing Heat Stable Salts (HSS) with ion
exchange resin, which removes anionic ions by exchanging basic anion of the ion exchange
resin;

(b) regenerating the resin obtained in step (a) with aqueous caustic solution;
(c) washing the resin obtained in step (b) with demineralized (DM) water; thereby obtaining aqueous alkanolamine solution free of Heat stable salts.
SUMMARY OF THE INVENTION
The present invention provides a process for complete removal of heat stable salts (HSS) from an alkanolamine solution containing theses salts. The disclosed process is cost-efficient and high-efficiency process for eco-friendly generation of aqueous alkanolamine solution completely free of heat stable salts (HSS). It uses high HSS loading resin to remove heat stable salts comprising salts of oxalate, acetate, formate, sulphate and thiocyanate. It is regenerated by 4-10 wt% caustic solution. Complete removal of HSS by the resin according to the process of the present invention provides amine completely free from heat stable salts after each application.
BREIF DESCRIPTION OF FIGURES
Fig. 1 shows schematic flow diagram illustrating the amine regenerating unit conventionally used in oil refining industries.
Fig. 2 is schematic flow diagram illustrating the present invention of amine purifier.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the present invention relates to a process for the complete removal of heat stable salts by contacting aqueous alkanolamine solution containing heat stable salts with an ion exchange resin in an effecient manner which is free from the problems encountered in the methods described in the prior arts.
This exemplified by fig. 1 which shows schematic flow diagram illustrating the amine regenerating unit conventionally used in oil refining industries. Rich amine from amine treater (1) unit is fed to the amine regenerator (2). Lean amine from the regenerator is cooled in lean rich exchanger (3) and followed by lean amine cooler (4). A part of lean amine is fed to amine purifier (5) for the removal of heat stable salts. After treatment the purified amine is fed to amine storage tank.

Similarly, fig. 2 shows schematic flow diagram illustrating the present invention of amine purifier. Referring to figure 2, the adsorption column (1) consist of SS mesh at the bottom of the column, over which ceramic supporting balls (2) is placed. Another mesh is placed over the ceramic balls, over which ion exchange resin (3) is placed. Another mesh is placed over the top of the resin bed. Three separate storage vessels (4, 5, and 6) are used for storing HSS contaminated amine solution, caustic solution and DM water respectively. All these storage vessels are connected in series with one feed pump (7). Isolation valves are provided to facilitate injection of one material at a time to adsorption column (1). Two reservoirs (8, 9) are used to collect treated HSS-free amine solution, and spent solutions (caustic and DM water) independently. Valves are provided in each outlet lines to control flow of the fluid to adsorption column. The flow through the adsorption column is measured by using measuring cylinder and stop watch. Regeneration and washing of resin is done by demineralized water. It is important to wash the resin bed properly by demineralized water after deactivation of the resin bed in 1st water wash. The optimum quantity of water is (6-10 times of the resin used) to be used to wash the resin bed. If water quantity is less than the optimum value, amine is lost during regeneration of the bed. If the water quantity is more than the optimum value, the excess water dilutes the amine solution used in amine treater unit given in figure 1.
After the regeneration, the resin bed is washed with demineralized water in 2nd water wash. The optimum quantity of water (15-20 times of the quantity of the resin used) is used to wash the resin bed before amine purification. If the water quantity is less than the optimum value, the caustic solution contaminates the amine solution and the performance of amine solution in amine treating unit is affected. If the water quantity is more than the optimum value it affects the effluent treatment where caustic solution is feed.
Resins tested in the technology development have unique properties. The properties are given in Table no 1.
Table 1: Physical properties of resins to be used in the process developed

Property Units Resin for Design
Functional Group Dimethylethanol ammonium groups
Total Exchange Capacity(min) meq/ml 0.9-1.3
Moisture Holding Capacity % 38-64
Density Kg/m3 685-720
Effective Size mm 0.4

The present invention relates to a process for complete removal of Heat stable salts (HSS) from the aqueous alkanolamine solution comprising the steps
(a) treating an aqueous alkanolamine solution containing Heat Stable Salts (HSS) with ion exchange resin, which removes anionic ions by exchanging basic anion of the ion exchange resin;
(b) regenerating the resin obtained in step (a) with aqueous caustic solution;
(c) washing the resin obtained in step (b) with demineralized (DM) water; thereby obtaining aqueous alkanolamine solution free of Heat stable salts.
One embodiment of the present invention, wherein alkanolamine is selected from tertiary amine that is methyl diethanol amine (MDEA).
In another embodiment of the present invention, wherein the concentration of alkanolamine in the aqueous solution is in the range of 10 - 50 wt%.
Yet another embodiment of the present invention, wherein the anionic ions are selected from the group organic anions selected from oxalate, acetate and formate and/or from the group of inorganic anions selected from thiocyanate, sulphite, and sulphate.
Yet another embodiment of the present invention, wherein the ion exchange resin is selected from the anionic ion exchange resin.
Yet another embodiment of the present invention, wherein the concentration of aqueous caustic solution is in the range of 4-10 wt%.
Yet another embodiment of the present invention, wherein DM water is free of ions selected from chloride (C1-), Sulfate (S04-2), Sulfide (S203-2).
Yet another embodiment of the present invention, wherein the amount of DM water utilized in washing is 15-20 times of the resin amount.
Yet another embodiment of the present invention, wherein the resin completely removes the anions of claim 5.
Yet another embodiment of the present invention, wherein the amount of oxalate removed by unit weight of resin is 0.05-0.1, and the amount of thiocyanate removed by unit weight of resin is 0.15-0.3.

Yet another embodiment of the present invention, wherein the process of adsorption and regeneration are either in continuous mode and (or) in batch mode.
Yet another embodiment of the present invention, wherein the operating temperature is ambient.
Yet another embodiment of the present invention, wherein aqueous alkanolamine solution comprising of heat stable salts (HSS) passes through the ion exchange bed in the adsorption column and it continues till the resin reaches its maximum adsorption capacity.
Yet another embodiment of the present invention, wherein the regeneration of resin bed in the adsorption column is performed by washing with aqueous caustic solution followed by DM water.
Yet another embodiment of the present invention, wherein the amount of DM water required for regeneration of the washing resin bed in the adsorption column is 6-10 times of amount of resin in the adsorption column.
The advantages of the disclosed invention are thus attained in an economical, practical and facile manner. While preferred embodiments and example have been shown and described, it is to be understood that various further modifications and additional configurations will be apparent to those skilled in the art. It is intended that the specific embodiments herein disclosed are illustrative of the preferred and best modes for practicing the invention, and should not be interpreted as limitations on the scope of the invention.
Example 1:
The ion exchange resin was loaded in adsorption column (1) above the supporting balls. The resin was activated by passing 1 (N) caustic solution till the pH of the outlet caustic solution remained same as pH of inlet the caustic solution. The bed was then washed with DM water till the pH of DM water leaving the bed is same as DM water.
Methyl Diethanolamine (MDEA) was added with DM water to make up 40 wt% solution. Specified amount of sodium oxalate was added to the solution. The amine solution containing sodium oxalate was passed through the resin bed at flow rate of 0.2 liters per minute. The amine solution leaving the adsorber was analysed by aqueous solution of CaCb. The operation was continued till white precipitate appeared. The loading of the oxalate ions with respect to resin was then calculated.

The resin was regenerated by passing through caustic solution (IN) and operation was continued till pH of caustic remained unchanged. The bed was further washed with DM water till pH of DM water remains unchanged.
The bed was regenerated and was used further for ion exchange of oxalate anions in the amine solution. Table 2 shows the result of different operating parameters and loading of heat stable salts. Table 3 and 4 shows the DM water requirement for the 1st washing and 2nd washing of the bed. pH of fresh DM water used was 7.20 and end point was taken based on steady state value of pH.
Result:
• Amount of the resin in the bed = 1 kg
• 4 kg of MDEA (analytical grade) is added in 6 kg of DM water to make 40 wt% solution.
• 0.1 kg of sodium oxalate was added into it.
Table 2: Operating parameters and loading of heat stable salts

Type of operation Feed Specification Feed Flow, Kg/min Temperature, °C Pressure, Kg/cm2 Time of
operation,
min
Adsorption 0.99 wt% oxalate of40wt%MDEA 0.2 36 1 30
1st washing DM water 0.3 36 1 20
Regeneration Caustic solution 0.25 36 1 35
2nd washing DM water 0.3 36 1 60
Table 3: DM water requirement in 1st water wash

Type of operation Feed Specification Feed Flow, Kg/min Time of
operation,
min pH of the
outlet DM
water
1st washing DM water 0.3 start 9.35

0.3 4 8.60

0.3 8 8.20

0.3 12 7.85

0.3 16 7.48

03 20 7.48

Table 4: DM water requirement in 2nd water wash

Type of operation Feed Specification Feed Flow, Kg/min Time of
operation,
min pH of the
outlet DM
water
2nd washing DM water 0.3 start 10.85

0.3 5 10.55

0.3 10 10.04

0.3 15 9.48

0.3 20 9.01

03 25 8.65

0.3 30 8.54

0.3 35 8.32

0.3 40 8.01

0.3 45 7.81

0.3 50 7.54

0.3 55 7.45

0.3 60 7.45
Estimation of loading:
Concentration of Oxalate in the solution (A) = 0.1/10.1*100 = 0.99 wt%
Total oxalate adsorbed at equilibrium (B) = 30* 0.99/100*0.2 =0.0594 kg
Amount of Oxalate adsorbed per unit mass of the resin at equilibrium i.e., loading of the oxalate in the resin = 0.0594/1.0 =0.0594 kg of oxalate/ kg of resin
Regeneration:
Concentration of the caustic (1N) used = 4 wt%
Amount of caustic used = 35* 0.25 * 0.04 = 0.35 kg
Amount of caustic used per kg oxalate removed = 0.35/0.0594= 5.89 kg of caustic/kg of oxalate
Washing:
Amount of water used in 1st wash per kg of resin loaded = 6 kg
9

Amount of water used in 1st wash per kg of oxalate removed = 6/0.0594 =101 kg of water/kg of oxalate
Amount of water used in 2nd wash per kg of resin loaded = 18 kg
Amount of water used in 2nd wash per kg of oxalate removed = 18/0.0594 = 303 kg of water/kg of oxalate
Total amount of water used per kg of resin = 6+18 =24 kg
Example 2:
Methyl Diethanolamine (MDEA) was added with DM water to make up 40 wt% solution. Specified amount of sodium thiocyanate was added to the solution. The amine solution containing sodium thiocyanate was passed through the resin bed at flow rate of 0.25 liters per minute. The amine solution leaving the adsorption column was analysed by aqueous solution of FeCl3. The operation was continued till red blood colour appeared. The loading of the thiocyanate ions with respect to resin was calculated at the end of the run.
The resin was regenerated by passing through caustic solution (1N) and operation was continued till pH of caustic remained unchanged. The bed was further washed with DM water till pH of DM water remained unchanged.
The bed was regenerated and was used for ion exchange of thiocyanate anions further. Table 5 shows the result of different operating parameters and loading of heat stable salts. Table 6 and 7 shows the DM water requirement for the 1st washing and 2nd washing of the bed.
Result:
• Amount of the resin in the bed = 1 kg
• 4 kg of MDEA (analytical grade) is added in 6 kg of DM water to make 40 wt% solution.
• 0.2 kg of sodium thiocyanate was added into it.
Table 5: Operating parameters and loading of heat stable salts.

Type of operation

Feed Specification

Feed
Flow,
Kg/min Temper
ature,
OC Pressure,
Kg/cm2 Time of
operation,
min

10

Adsorption 1.96 wt%
thiocyanate of 40
wt% MDEA 0.25 38 1 32
1st washing DM water 0.32 38 1 20
Regeneration Caustic solution 0.31 38 1 40
2nd washing DM water 0.3 38 1 65
Table 6: DM water requirement in 1st water wash

Type of operation Feed Specification Feed Flow, Kg/min Time of
operation,
min pH of the
outlet DM
water
1st washing DM water 0.3 0 9.82

0.3 4 8.54

0.3 8 8.10

0.3 12 7.62

0.3 16 7.37

03 20 7.37
Table 7: DM water requirement in 2nd water wash

Type of operation Feed Specification Feed Flow, Kg/min Time of
operation,
min pH of the
outlet DM
water
2nd washing DM water 0.3 0 10.98

0.3 5 10.45

0.3 10 10.10

0.3 15 9.52

0.3 20 9.15

03 25 8.75

0.3 30 8.14

0.3 35 8.02

0.3 40 7.89

0.3 45 7.75

0.3 50 7.40

0.3 55 7.38

0.3 60 7.32

0.3 35 7.32
11

Estimation of loading:
Concentration of sodium thiocyanate in the solution (A) = 0.2/10.2*100 = 1.96 wt%
Total thiocyanate adsorbed at equilibrium (B) = 32 * 1.96/100*0.25 =0.1568 kg
Amount of thiocyanate adsorbed per unit mass of the resin at equilibrium i.e., loading of the thiocyanate in the resin = 0.1568 /1.0 = 0.1568 kg of thiocyanate / kg of resin
Regeneration:
Concentration of the caustic (1N) used = 4 wt%
Amount of caustic used = 40* 0.32 * 0.04 = 0.512 kg
Amount of caustic used per kg thiocyanate removed = 0.512/0.1568= 3.26 kg of caustic/kg of thiocyanate
Washing:
Amount of water used in 1st wash per kg of resin loaded = 6.4 kg
Amount of water used in 1st wash per kg of thiocyanate removed = 6.4/0.1568 =40.81 kg of water/kg of thiocyanate
Amount of water used in 2nd wash per kg of resin loaded = 19.5 kg
Amount of water used in 2nd wash per kg of thiocyanate removed = 19.5/0.1568 = 122.95 kg of water/kg of thiocyanate
Total amount of water used per kg of resin = 6.4+ 19.5 =25.9 kg
Example 3:
Industrially used MDEA solution (40 wt %) containing heat stable salts was passed through the resin bed. The MDEA solution leaving the adsorption column was analysed with RI, conductivity, pH and density. Table 8 shows the different properties of the amine solution before and after treatment with the resin.
12

Table 8: properties of the amine solution before and after treatment with the resin.

s.
No. Property MDEA solvent before treatment MDEA
solvent After treatment Change in property (before- after)
1 Conductivity, mS/cm 6.93 2.75 + 4.18
2 pH 9.62 10.10 -0.48
3 Refractive Index (RI) 1.3643 1.3600 +0.0043
4 Density
(g/cc) 1.0877 1.0834 + 0.0043
ADVANTAGES
1. Complete removal of Heat stable salt to generate free aqueous alkanol amine solution.
2. Regenerative and high efficiency process to generate free aqueous alkanol amine solution
3. The process is applicable for organic and inorganic Heat stable salts removal.

WE CLAIM:

A process for removal of Heat stable salts (HSS) from the aqueous alkanolamine solution comprising the steps
(a) treating an aqueous alkanolamine solution containing Heat Stable Salts (HSS) with ion exchange resin, which removes anionic ions by exchanging basic anion of the ion exchange resin;
(b) regenerating the resin obtained in step (a) with aqueous caustic solution;
(c) washing the resin obtained in step (b) with demineralized (DM) water; thereby obtaining aqueous alkanolamine solution free of Heat stable salts.
The process as claimed in claim 1, wherein alkanolamine is selected from tertiary amine that is methyl diethanol amine (MDEA).
The process as claimed in claim 1, wherein the concentration of alkanolamine in the aqueous solution is in the range of 10 - 50 wt%.
The process as claimed in claim 1, wherein the anionic ions are selected from the group organic anions selected from oxalate, acetate and formate and/or from the group of inorganic anions selected from thiocyanate, sulphite, and sulphate. The process as claimed in claim 1, wherein the ion exchange resin is selected from the anionic ion exchange resin.
The process as claimed in claim 1, wherein the concentration of aqueous caustic solution is in the range of 4-10 wt%.
The process as claimed in claim 1 wherein DM water is free of ions selected from chloride (CI"), Sulfate (S04"2), Sulfide (S2O3"2).
The process as claimed in claim 1 wherein the amount of DM water utilized in washing is 15-20 times of the resin amount.
The process as claimed in claim 1 wherein the resin completely removes the anions of claim 5.
The process as claimed in claim 1 wherein the amount of oxalate removed by unit weight of resin is 0.05-0.1, and the amount of thiocyanate removed by unit weight of resin is 0.15-0.3.
The process as claimed in claim 1 wherein the process of adsorption and regeneration are either in continuous mode and (or) in batch mode. The process as claimed in claim 1 wherein the operating temperature is ambient.

The process as claimed in claim 12, wherein aqueous alkanolamine solution comprising
of heat stable salts (HSS) passes through the ion exchange bed in the adsorption column
and it continues till the resin reaches its maximum adsorption capacity.
The process as claimed in claim 12, wherein the regeneration of resin bed in the
adsorption column is performed by washing with aqueous caustic solution followed by
DM water.
The process as claimed in claim 1 wherein the amount of DM water required for
regeneration of the washing resin bed in the adsorption column is 6-10 times of amount of
resin in the adsorption column.