Introduction

The consequences of the presence of salt in the amine solution are amine contamination, followed by reduced efficiency of the operating unit (Omer et al. 2022). Increasing electrical conductivity of solution accelerates the electrochemical reactions, including corrosion reactions (Verma et al. 2022). The total concentration of heat stable salt in amine should not exceed 0.5 wt.% (Çevirim-Papaioannou et al. 2022). One way to release contaminated amines with heat stable salts is to add strong bases such as calcium hydroxide or potassium carbonate to the solution (Aryal et al. 2022). This does not affect anion content of the heat stable salts, but forms sodium salts instead of amine salts (Yang et al. 2022a). Therefore, it seems necessary to create a de-chlorination unit and a heat stable salt separation unit next to the gas sweetening unit for amine recovery (Yang et al. 2022b; Jeyaseelan et al. 2022). The anions of the salts in gas enter to the amine and form a heat stable salt. For this reason, presence of high chloride ions and heat stable salts has been observed in some refineries in recent years (Yuan et al. 2022). The anionic compounds are separated using resin to solve this problem (Shi et al. 2022). Increasing efficiency of gas sweetening process and quality of amine is always one of the most basic operational plans of refineries (Abuzalat et al. 2022; Zhang et al. 2022; Zhao et al. 2022).

In this study, a strong anionic resin was prepared to remove chloride ions. This research was performed in a laboratory pilot to study parameters such as operating temperature, amine flow rate, pH, chloride ion concentration and concentration of caustic. The purpose of this research is to determine kinetics of the reaction of resin with caustic solution.

Materials and method

A study of following items is necessary in order to conduct this research: (A) Determination of synthetic reaction. (B) Calculation of resin capacity. (C) Calculation of break through time (BTT) of the resin. (D) Determination of consumed water to neutralize alkaline effect of caustic. (E) Determination of functional and optimal pH of chlorine adsorption process. (F) Determination of optimal temperature of amine recycling and resin recovery. (G) Determination of caustic consumption to recovery each liter of resin.

Required chemicals

The following chemicals are used in this study: The Diethanolamine 30%, the nitric acid 65%, hydrochloric acid 37%, silver nitrate tetrazole, sodium hydroxide 99%, phenolphthalein, lead acetate paper, standard solution in acidic medium, standard solution in neutral medium, standard solution in alkaline medium, standard chloride solution 1000 mg/l. All of the solutions are provided by the Merck Company, Germany.

Special properties of M-500 as anionic resin

Table 1 presents main specifications of the resin. In addition, Table 2 shows physical and chemical properties of the resin.

Table 1 Main specifications of anionic resin (M-500)
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Table 2 Physical and chemical characteristics of anionic resin (M-500)
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Results and discussion

Investigation of distilled water at temperatures 40 °C, 50 °C and 60 °C

The resin becomes alkaline due to that the pH of diethanolamine and 4% caustic is in the alkaline range. The alkalinity of resin must be eliminated in order to perform saturation and recovering steps at temperatures 40 °C, 50 °C and 60 °C. Deionized water is used to neutralize the alkalinity of the resin. This study shows that volume of deionized water passing through resin at temperatures of 40 °C, 50 °C and 60 °C is equal to 5500, 5000 and 7000 ml, respectively. Table 3 shows volume of distilled water and caustic at temperatures 40 °C, 50 °C and 60 °C.

Table 3 Laboratory results at temperatures of 40 °C, 50 °C and 60 °C
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Table 3 shows that amount of caustic to recover resin is 8.5 L at a temperature of 50 °C. Also, the amount of distilled water to eliminate alkalinity of resin is equal to 5 L. Table 3 states that a temperature 50 °C is more appropriate than other temperatures. Fig. 1 shows concentration of chloride leaving the resin in terms of volume of amine at temperatures 40 °C, 50 °C and 60 °C.

Fig. 1
figure 1

Concentration of chloride leaving the resin in terms of passing amine

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The steps of recovery and elimination of resin alkalinity in terms of caustic and distilled water at temperatures 40 °C, 50 °C and 60 °C are shown in Fig. 2. Fig. 2 shows that the amount of caustic and water becomes minimum at a temperature of 50 °C.

Fig. 2
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Effect of temperature on the amount of used caustic for resin recovery

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The amount of caustic passing through resin in terms of chloride ion concentration at temperatures 40 °C, 50 °C and 60 °C is shown in Fig. 3.

Fig. 3
figure 3

Caustic passing through the resin in terms of chloride concentration leaving the resin

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The saturation time of resin in terms of chloride ion concentration at temperatures 40 °C, 50 °C and 60 °C is shown in Fig. 4. Fig. 4 shows that the chlorine ion concentration increases at first and then, decreases. The equation in Fig. 4 shows the changes of chloride ion concentration in terms of time. The regression of this equation is equal to 0.9926.

Fig. 4
figure 4

Resin saturation time in terms of chloride concentration changes

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Determination of reaction kinetics

After measuring chloride concentration at temperatures 40 °C, 50 °C and 60 °C, it was found that ion exchange reaction was a quasi-first order. Equation of this reaction is \(- r_{{\text{A}}} = kC_{{\text{A}}}^{{\text{n}}}\), and k is called reaction rate constant. Also, parameter n is presented as degree of reaction. The Eq. 2 was used to calculate reaction order and reaction constant. At the first, a 0.5 wt.% caustic is made, and then, this solution and diethanolamine are heated to a temperature 50 °C. This step is opposite of previous steps, exactly. In other words, first, a 0.5 wt.% caustic solution is passed through the resin to recovery it and then, a diethanolamine solution containing 1400 ppm of chloride ion is passed through resin bed to saturate the resin. The chloride ion measuring testing is performed after 100 ml of caustic and amine passes through the resin. Finally, the Eqs. 1 and 2 are used to calculate values of order and constant of reaction.

$$- r_{{\text{A}}} = kC_{{\text{A}}}^{{\text{n}}}$$
(1)
$$\ln \left( { - r_{{\text{A}}} } \right) = \ln k + {\text{n}}\ln \left( {C_{A} } \right)$$
(2)

According to the chemical equation of reaction, ln is taken from the sides of equation. According to the laboratory results, the diagram for Eq. 2 is drawn. The constant reaction parameters and reaction order are obtained, finally. Table 4 presents laboratory results for determination of reaction constant and reaction order with respect to the chloride ion concentration.

Table 4 Chloride ion concentration
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Figure 5 shows chemical equation of reaction of chloride ion. The equation in Fig. 5 shows the logarithmic of chemical reaction in terms of chloride ion concentration. The regression of this equation is equal to 0.9942.

Fig. 5
figure 5

Investigation of order and constant values of chloride ion exchange reaction rate

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According to Fig. 5, it is shown that reaction order to chloride ion is equal to 0.794, and reaction constant is equal to exp (−1.8753).

Conclusion

The laboratory results show that volume of amine for resin saturation at temperatures 40 °C, 50 °C and 60 °C is equal to 3000 ml. This work shows that resin will be recovered after passing 9500, 8500 and 10,000 ml of 4% caustic solution at temperatures 40 °C, 50 °C and 60 °C, respectively. This research shows that volume of deionized water to eliminate alkalinity at temperatures 40 °C, 50 °C and 60 °C is equal to 5500, 5000 and 7000 ml, respectively. This work shows that a temperature 50 °C is more appropriate than other temperatures. In addition, the amount of caustic to recover resin is 8.5 L at a temperature 50 °C. Also, amount of distilled water to eliminate alkalinity of resin is equal to 5 L. This study shows that the reaction order to chloride ion is equal to 0.794, and reaction constant is equal to exp (−1.8753).