Investigating the Effect of Operating Temperature on the Performance of TGT Absorber Column

International Journal of Energy Science (IJES), Volume 6 Issue 1, 2016 www.ijesci.org doi: 10.14355/ijes.2016.0601.02

Investigating the Effect of Operating Temperature on the Performance of TGT Absorber Column Hamid Reza Mahdipoor*1 Department of Process and Equipment Technology Development, Research Institute of Petroleum Industry, Tehran, Iran *1

*1

mahdipoorhr@ripi.ir

Abstract Hydrogen sulfide (H2S) coming from oil and gas refineries, after purifying in amine sweetening unit, is routed to Sulfur Recovery Unit (SRU), wherein converted to elemental sulfur. In order to compensate the low performance of SRU, the remained acid gas is then fed to Tail Gas Treatment (TGT) unit for more processing. TGT unit increases the overall sulfur recovery and therefore, decreases the environmental pollution. The absorber column is an important equipment in the TGT unit.

The main function of this column is the selective absorption of H2S in the presence of CO2. In this paper, the effect of operating temperature on the performance of TGT absorber column will be investigated. For this purpose, a typical industrial TGT unit is simulated and then, the results are illustrated and analyzed in terms of technical points of view.

Keywords Tail Gas Treatment (TGT), Selective Absorption

Introduction Hydrogen sulfide (H2S) coming from oil and gas refineries, after purifying in amine sweetening unit, is routed to Sulfur Recovery Unit (SRU), wherein converted to elemental sulfur. Claus process is the most common sulfur recovery process. It contains a front head reaction furnace connected to a waste heat boiler (WHB), and two or three catalytic converters followed by sulfur condensers. The overall Claus reaction is as below [1‐6], 2H 2 S  O2  S 2  2H 2 O (1)

At first, one third of H2S entered to reaction furnace is oxidized to SO2. The oxidation reaction is as below, H 2 S  3 O2  SO2  H 2O (2) 2

About 60% of SO2 resulted from reaction (2), react with H2S and converted to elemental sulfur in reaction furnace, as below, 2H 2 S  SO2  3 2 S 2  2 H 2 O (3)

In the next step, the remained H2S and SO2 react regarding to reaction 3 in catalytic converters and converted to elemental sulfur. The overall recovery of Claus process hardly exceeds 96% and the unrecovered sulfur as COS, CS2, H2S and sulfur vapour, burned in the tail gas incinerator which leads to excessive sulfur dioxide sent to the atmosphere and therefore, environmental pollution. In order to increase the total recovery of sulfur and decrease the environmental pollutants, TGT unit is incorporated before incinerator of Claus section. SCOT process is the most common process for treatment of tail gases and has been industrialized in more places. Figure 1 shows the Schematic diagram of a typical SCOT process [6‐13].

As illustrated in Figure 1, tail gas stream coming from Claus unit, after heating in the inline burner, enters the reduction reactor in which all its sulfur components including COS, CS2 and SO2 are converted to H2S. After cooling the gas, this stream enters the amine absorber column. H2S enriched stream (rich amine) is routed to the regenerator column (stripper). Lean amine stream from the bottom of the stripper is recycled to absorber and the

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gas stream with high content of H2S from the top of the stripper is returned to the input of the Claus where it is mixed with Claus acid gas feed. The stream exiting from the top of the absorber (the off gas of SCOT process) which includes a negligible amount of H2S is directed to incinerator where it is burned with fuel gas (see figure 1) [6,11]. Valve tray or packing can be used in TGT absorber as internal. However, structured packing is preferred because of its low pressure drop as well as high performance. In the next section, the absorber column of a typical industrial TGT unit is simulated to investigate the effect of Amine temperature on the performance of TGT absorber column.

FIG. 1 SCHEMATIC DIAGRAM OF A TYPICAL SCOT PROCESS [6]

Simulation of a Typical TGT Unit In order to investigate the effect of Amine temperature on the performance of TGT absorber column, a typical industrial TGT unit is simulated using Promax software. Promax is developed by BRE company and is a well‐ known software for simulating sulfur recovery plants, as well as amine sweetening units. More accurate results can be achievable, when this software is calibrated by means of experimental and industrial data. The molar flow of the Claus tail gas which is considered as TGT unit feed is equal with 2300 kmole/h. The mole percent of this stream is shown in table 1. As shown in this table, the main portion of tail gas consists of CO2 while the H2S mole percent is equal with 1%. If high concentration of CO2 was absorbed by TGT absorber, there would be some problems in Claus unit such as increase of the equipment sizes, decrease of the reaction furnace temperature, etc which result in low sulfur recovery. Therefore, the ideal function of TGT absorber is the selective absorption of H2S. In this study, methyl‐diethanolamine (MDEA) which is a selective absorbent, is used in absorber column. The molar flow rate of amine solution is considered to be 14500 kmole/h. The concentration of amine is presented in table 1.

TABLE 1 COMPONENTS MOLE PERCENT OF ACID GAS AND AMINE SOLUTION FEED

component

Amine solution

Acid gas

CO2

0.01

40

N2

0

47

H2S

0.04

1

H2

0

3

H2O

93.95

9

MDEA

6

0

In order to investigating the effect of operating temperature on the performance of TGT absorber, the temperature of amine solution is increased from 30 C up to 60 C. Acid gas temperature suddenly decreased when contacting with amine solution countercurrently. Hence, the temperature of acid gas feed was supposed to be constant which is equal with 50 C, in this study. Figure 2 shows the changes of H2S concentration in the sweet gas from the top of the TGT column vs. changes of the amine temperature. As illustrated in this figure, increasing the amine

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temperature will increase H2S concentration rapidly. Although, H2S concentration is acceptable yet when the amine temperature reaches to about 50 C.

FIG. 2 CHANGES OF H2S CONCENTRATION IN THE SWEET GAS VS. AMINE TEMPERATURE

Figure 3 shows the changes in acid gas rich amine loading (moles of H2S and CO2 in rich Amine per moles of MDEA) versus amine solution temperature. According to figure 3, H2S loading is almost constant when the temperature of amine was changed. On the other hand, CO2 loading increases by raising the temperature of amine solution. Therefore, although raising the amine temperature would not be effective on the H2S content in the sweet gas leaving the top of the absorber column, it increased the absorbed CO2 which is not preferable in the selective absorption process.

In addition to ppm of H2S in the sweet gas, the ratio between H2S and CO2 is an important parameter to evaluate performance of TGT absorber column. The changes in H2S/CO2 ratio with MDEA concentration are depicted in figure 4. As is shown in this figure, H2S/CO2 ratio decreases with the increase of amine solution temperature.

Therefore, using lower temperature is more appropriate in terms of the selective absorption of H2S. Regarding to the achieved results illustrated in figure 2 to figure 4, if the amount of CO2 in acid gas feed of TGT unit was high (e.g. TGT unit of gas refineries with lean acid gas feed), lower temperatures should be considered for operating temperature of TGT absorber column to absorb lower CO2. On the other hand, if the amount of CO2 in acid gas feed of TGT unit was low or negligible (e.g. TGT unit of oil refineries), operating temperature can increase up to 50 C. More temperatures should be prevented to avoid increasing of the H2S concentration in the sweet gas stream in the top of the TGT absorber.

FIG. 3 CHANGES IN ACID GAS LOADING VS. FIG. 4 CHANGES IN H2S/CO2 RATIO VS. AMINE AMINE TEMPERATURE TEMPERATURE

Conclusions TGT unit is considered to enhance overall sulfur recovery as well as respect environmental regulations. The selective absorber column is an important equipment in TGT unit. In this paper, the effect of operating temperature on the performance of absorber column was investigated. For this purpose, a typical industrial TGT unit was simulated. The results revealed that lower temperatures should be considered for operating temperature of TGT

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absorber column to absorb lower CO2. If the amount of CO2 in acid gas feed of TGT unit was low or negligible, operating temperature can increase up to 50 C. REFERENCES

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