RU2213085C2 - Process of drying and cleaning hydrocarbon gases to remove mercaptans and hydrogen sulfide - Google Patents

Abstract

FIELD: oil and gas production. SUBSTANCE: process is carried out by bringing natural or associated gas consecutively into contact with adsorbent (silica gel) and synthetic zeolite followed by regeneration saturated zeolite and adsorbent with counter current cleaned hydrocarbon gas at elevated temperature (180-220 C). Contact of gas with adsorbent and zeolite is performed at their ratio (1-10):1, respectively. EFFECT: simplified procedure, reduced power consumption, and prolonged lifetime of zeolite. 2 cl, 5 tbl, 3 ex

Description

 The invention relates to the processing of natural or associated petroleum gas, namely, to the drying and purification of gas from sulfur compounds by adsorption, and can be used in the gas, oil, oil refining and petrochemical industries.

A known method of adsorptive purification of hydrocarbon gases from sulfur compounds by contacting with NaX zeolite followed by regeneration of the zeolite by blowing dried and purified hydrocarbon gas at a temperature of 300-350 ^o C. The regeneration at a temperature of 300-350 ^o C due to the peculiarity of water desorption from pores of zeolite [ Keltsev N.V. The basics of adsorption technology. M .: Chemistry, 1985, p.396].

The disadvantage of this method is the rapid decrease in the dynamic adsorption capacity of zeolite for sulfur compounds due to the simultaneous adsorption of water vapor, C ₃₊ mercaptans, heavy hydrocarbons, tarry substances (in associated petroleum gas), as well as a reduction in the service life of the zeolite due to the partial decomposition of mercaptans and tarry substances during their desorption with the formation of coke deposits and hydrogen sulfide in the process of high-temperature regeneration of zeolite.

The closest to the claimed technical essence and the achieved result is a method of drying and purifying hydrocarbon gases from hydrogen sulfide, mercaptans and other polluting components (carbon disulfide, asphalt-resinous substances) by sequential contacting with activated carbon, clinoptilolite and synthetic A- and X-form zeolites followed by regeneration of saturated zeolites, clinoptilolite and activated carbon countercurrent purified hydrocarbon gas at elevated temperature (initial rate Atur gas 230-250 ^o C) [RF patent 2047589, IPC 7 C 07 C 07/12, publ. 11/10/95, bull. 31]. When gas is contacted with activated carbon, carbon disulfide, tarry substances, oils and surfactants are cleaned. Then the gas is sent for drying by clinoptilolite, and then for purification on synthetic zeolites: low molecular weight sulfur compounds, hydrogen sulfide, methyl and ethyl mercaptans, are adsorbed on CaA zeolite. On the NaX zeolite, the hydrocarbon gas is refined from traces of carbon disulfide and higher molecular weight sulfur compounds - heavy mercaptans: sulfides, disulfides, etc. Preliminary purification of hydrocarbon gas on activated carbon from carbon disulfide and asphalt-resinous substances, drying of the gas to be cleaned on clinoptilolite not only increase the dynamic adsorption capacity of the zeolite by target components, and therefore the duration of the adsorption cycle, but also exclude the accumulation of difficultly desorbed polycyclic compounds on zeolites, which are formed due to the thermocatalytic transformations of carbon disulfide and asphalt-resinous substances during the regeneration of zeolites. This allowed to increase the service life of zeolites from 0.5 to 2-3 years.

The disadvantages of this method are its complexity and increased energy costs due to the multi-stage process and the high temperature of regeneration. In addition, the service life of zeolites is insufficient. The adsorption capacity of zeolites decreases with each adsorption-regeneration cycle due to the formation of coke substances during the decomposition of mercaptans adsorbed by zeolites during high-temperature regeneration (decomposition of mercaptans occurs at temperatures above 220 ^o C). This leads to a deterioration in the economic performance of the process.

 The objective of the proposed technical solution is to increase the efficiency of the method due to its simplification and improvement of economic indicators: reducing energy consumption, increasing the service life of the zeolite.

The problem is solved according to the claimed method of drying and purification of hydrocarbon gases from mercaptans and hydrogen sulfide by successive contact with an adsorbent and synthetic zeolite, followed by regeneration of saturated zeolite and adsorbent countercurrent with purified hydrocarbon gas at elevated temperature due to the fact that silica gel is used as adsorbent, and regeneration zeolite and silica gel are carried out at a temperature of purified hydrocarbon gas 180-220 ^o C. Another difference of the claimed joint venture the person drying and purification of hydrocarbon gases is the contacting of the purified hydrocarbon gas with silica gel and zeolite in their mass ratio of 1-10: 1, respectively.

Distinctive features of the claimed invention are:
- use of silica gel as an adsorbent (for drying and preliminary purification of hydrocarbon gas);
- the implementation of the regeneration of zeolite and silica gel at a temperature of the regeneration gas - purified hydrocarbon gas 180-220 ^o C;
- the implementation of the contacting of gas with silica gel and zeolite with a mass ratio of 1-10: 1, respectively.

Silica gels are spherical glanules with a diameter of 3-6 mm of the chemical composition Al ₂ O ₃ - 3.0%, SiO ₂ - 97%; Designed for drying and topping hydrocarbon gas (absorption of water and heavy hydrocarbons).

 The authors experimentally established a new property of silica gel - adsorb mercaptans.

 When conducting a patent information search in the field of natural gas purification and in related fields, no technical solutions were found in which silica gels are used to absorb mercaptans.

 The manifestation of a new property by a well-known feature in the claimed technical solution allows us to conclude that it meets the criterion of "inventive step".

 The technical result from the use of silica gel as an adsorbent is to provide the possibility of combining the drying of hydrocarbon gases and their purification from mercaptans and heavy hydrocarbons, as well as other pollutants (for example, asphalt-resinous substances when refining petroleum gas) in one step, which reduces the amount stages of the process, simplify the method and reduce energy costs. The absorption of water vapor, mercaptans and heavy hydrocarbons by silica gel and, as a result, the complete elimination of their contact with the zeolite allows not only to increase its dynamic adsorption capacity for hydrogen sulfide, but also to reduce the temperature of the regeneration gas. This leads to a reduction in energy consumption, and also eliminates the accumulation of difficultly desorbed polycyclic compounds in zeolite, which are formed as a result of thermocatalytic transformations of mercaptans during high-temperature regeneration, which prevents the rapid deactivation of zeolite and allows to increase its service life.

 The method of drying and purification of hydrocarbon gases from mercaptans and hydrogen sulfide is illustrated by schemes. Figure 1 shows the drying and purification of sulfur compounds on silica gel and zeolite, placed in separate adsorbers, in Fig. 2 - on a combined layer of silica gel and zeolite, placed in one adsorber.

 The method is as follows.

Hydrocarbon gas to be dried and cleaned of sulfur compounds (hydrogen sulfide and mercaptans) is fed sequentially (Fig. 1) to adsorber 1 and adsorber 2 from top to bottom (similar pair of adsorbers 3 and 4). Adsorbers 1 and 3 are filled with silica gel 5, and adsorbers 2 and 4 are filled with zeolite 6. On silica gel 5, competing adsorption of water vapor, mercaptans and C ₅₊ hydrocarbons occurs, on zeolite 6 adsorption of hydrogen sulfide and residual amounts of C ₅₊ hydrocarbons (slip amount) . In this case, the adsorbers 3 and 4 operate in a regeneration / cooling cycle.

 The process can be carried out on a combined adsorbent layer (Fig. 2) in one adsorber 1 (similar adsorbers 2, 3, 4), while the ratio of the layers of silica gel 5 and zeolite 6 is determined by calculation taking into account the specific composition of the dried and purified hydrocarbon gas. Depending on the dew point in the water, the content of mercaptans and heavy hydrocarbons, on the one hand, and hydrogen sulfide, on the other hand, the necessary volume and the optimal ratio of silica gel and zeolite are determined to achieve the required adsorption cycle duration. This condition is also true for the placement of silica gel and zeolite in separate adsorbers. The optimum is the mass ratio of silica gel and zeolite in the range of 1-10: 1, respectively. Three adsorbers operate simultaneously in the adsorption cycle and one in the regeneration / cooling cycle.

 Dried and purified gas enters the line 7 of commercial gas.

Part of the dried, purified from mercaptans, heavy hydrocarbons and hydrogen sulfide hydrocarbon gas (marketable) is used as a gas for cooling and regenerating adsorbents. Regeneration and cooling of zeolite and silica gel according to both schemes is carried out by a single stream of purified hydrocarbon gas in countercurrent flow (from bottom to top) in the zeolite-silica gel sequence. Part of the commercial gas is first used as a cooling gas of the zeolite and silica gel that have passed the regeneration stage, then it is heated in a recuperative heat exchanger 8 and then in the furnace 9 to a temperature of 180-220 ^° C and sent to the adsorber, which is currently being regenerated. Hydrogen sulfide and the residual amount of heavy hydrocarbons are desorbed from the zeolite, but they do not decompose at this temperature. The relatively low regeneration temperature is due to the absence of water in the zeolite, which is more easily adsorbed and more firmly retained by zeolites (for desorption of water from zeolites, a temperature increase of up to 300 ^° C is required, this temperature is 80-100 ^° C higher than that required for desorption of mercaptans). During the regeneration of silica gel, desorption of mercaptans, heavy hydrocarbons and water occurs, while the formation of coke deposits also does not occur. Water from silica gels is desorbed in the temperature range 100-180 ^o C. Saturated regeneration gas is sent to the heat exchanger 8 for heat recovery and then cooled in the refrigerator 10 and for separating moisture into the separator 11, after which the cooled regeneration gas stream is sent for purification.

 Example 1

The adsorption properties of silica gels during drying and purification of natural gas from mercaptans and C ₃₊ hydrocarbons were studied in a pilot adsorption unit. The installation includes a unit for preparing the initial gas mixture and compression, an adsorption unit, consisting of two adsorbers made of stainless steel with an internal diameter of 50 mm and a height of 3000 mm, equipped with external heating, and a gas sampling and analysis unit. The studies were carried out under conditions as close as possible to the operation parameters of industrial adsorption plants for drying and purifying natural gas from impurity components: pressure in the adsorber - 5.0 MPa, adsorption temperature - 30-35 ^o С, linear gas velocity in the adsorber - 0.07 m / s, average gas flow rate - 23 nm ³ / h, contact time "gas-silica gel" - 43 s.

 The gas flow rate was controlled by a rotameter and measured by a gas meter at the outlet of the adsorber. The moisture content of the gas supplied to the adsorber was determined by the dew point using a laboratory instrument equipped with a polished metal mirror, which was cooled in liquid nitrogen. The moisture content of the dried gas at the outlet of the adsorber was measured using a Bikle-1 moisture meter.

 As a model hydrocarbon, n-heptane was used, introduced into the natural gas as an impurity, and isopropyl mercaptan as a mercaptan.

 The following silica gels were used as silica gel: KS-Trokenperlen "N", KS-Trokenperlen "WS", KS-Trokenperlen "N" (Engelhard, Germany), as well as domestic silica gels KSMG, KSMK. Characteristics of silica gels are given in table. 1.

The dynamic adsorption capacity of silica gels in water and n-heptane was determined. The dynamic adsorption capacity was estimated by the weight gain of the sorbent during the adsorption until the breakthrough of sorbate vapor behind the silica gel layer. Before the experiments, silica gels were activated in an argon flow at a temperature of 180 ^o C. The test results of silica gels are given in table. 2.

 As can be seen from the table. 2, the dynamic capacity of KS-Trokenperlen silica gels both in water vapor and in heptane is much higher than domestic silica gels. Therefore, further studies were carried out on silica gels KS-Trokenperlen. The dynamic adsorption capacity of silica gels by n-heptane and isopropyl mercaptan was determined similarly, the results are shown in table. 3.

Tests in a pilot plant showed that silica gels in the presence of water and n-heptane adsorb mercaptans well, especially with increasing their partial pressure. Moreover, with competing adsorption on silica gel, the activity of the components increases in the series:
hydrocarbons C ₅₊ <mercaptans << water.

 Example 2

The patterns of silica gel regeneration were investigated. During desorption, the regeneration gas was supplied countercurrently. The heating rate is 1 deg / min. Studies have shown that the temperature range of the predominant removal of mercaptans from silica gel was 80-120 ^o C, desorption of water - 100-180 ^o C, desorption of n-heptane - 90-180 ^o C. Complete desorption of all adsorbed components was observed at 200 ^o C.

In order to determine the presence of coke deposits on silica gel (due to the possible decomposition of adsorbed mercaptans), after 20 adsorption-desorption cycles, it was heated to a temperature of 300-500 ^o C. Exothermic effects in this temperature range, which are usually characteristic for the burning of coke deposits, were not detected. This indicates the progress of the process of regeneration of silica gels without decomposition of mercaptans and heavy hydrocarbons and the formation of coke deposits.

 Example 3

Tested the proposed method of drying and purification of hydrocarbon gases from mercaptans and hydrogen sulfide. The tests were carried out on a model installation containing an adsorption column with a combined adsorbent layer: silica gel 79.3 wt.% (3395 g), zeolite 20.7 wt.% (884 g). KS-Trokenperlen-N silica gel was used as silica gel; NaX or CaA zeolites were used as zeolite. A study of the operation of the combined adsorption layer was carried out on model gas mixtures close in composition to real gas. The model gas mixture contained n-heptane as C ₅₊ hydrocarbons, and isopropyl mercaptan as mercaptans.

 The composition of the model mixture according to the quantitative content of the components was maximally approximated to the real gas to be cleaned (Table 4).

 The model gas mixture was fed into the column with a combined adsorbent layer from the bottom up; at the regeneration stage, dry gas was supplied from top to bottom, i.e., countercurrent.

 A chromatographic analysis was performed of the purified gas leaving the adsorption column. The first to exit the column was hydrogen sulfide, then heptane. This is due to the fact that heptane is displaced from silica gel by water and isopropyl mercaptan and reaches the zeolite, from where hydrogen sulfide begins to displace.

 The results of gas purification on a combined layer of adsorbents (silica gel and zeolite) are given in table. 5.

According to OST 51.40-93 “Combustible, natural gases supplied and transported via gas pipelines,” the content of hydrogen sulfide in natural gas should be no more than 7 mg / nm ³ , and mercaptan sulfur no more than 16 mg / nm ³ . The above results show that the proposed method provides gas purification from mercaptans and hydrogen sulfide in accordance with the requirements of OST 51.40-93.

 By calculation, we obtained the range of the preferred ratio of the layers of silica gel and zeolite for drying and purification of natural gases, which differ in the content of water vapor, mercaptans and heavy hydrocarbons. It was determined that it is advisable to dry and purify natural gas from mercaptans and hydrogen sulfide after amine purification by contacting silica gel and zeolite at a ratio of 1: 1. It is advisable to carry out deep gas drying and purification from mercaptans with a silica gel-zeolite 9 layer ratio: 1. Carrying out adsorption drying and purification from mercaptans and hydrogen sulfide at silica gel and zeolite ratios outside the proposed limits is impractical, because does not ensure the completeness of gas purification from hydrogen sulfide. When the silica gel content is less than 50%, water vapor and mercaptans that are not adsorbed by silica gel settle on the zeolite, which reduces its adsorption capacity for hydrogen sulfide. When the zeolite content is less than 10%, the adsorption of hydrogen sulfide is also not complete.

The process of desorption of hydrogen sulfide and heptane from zeolite was investigated. The temperature range of the preferred removal of hydrogen sulfide is 40-80 ^o C, heptane - 80-185 ^o C.

It was found that complete desorption of all components from the combined layer of silica gel and zeolite occurs already at a temperature of 200 ^o C.

The data obtained, taking into account the data on the regeneration of silica gel (example 2), allowed us to determine the preferred temperature range for the regeneration: 180-220 ^o С. Conducting regeneration at temperatures below 180 ^o C can lead to incomplete desorption of heptane and water, regeneration at temperatures above 220 ^o C to the formation of coke deposits due to the decomposition of mercaptans. The implementation of the regeneration in the claimed temperature range will completely desorb the impurity components of the gas and maintain the adsorption capacity of zeolites. According to estimates, the service life of the zeolite when applying the proposed method will increase to 4 years.

 Thus, the implementation of the method in two stages: drying and purification on silica gel and purification on zeolite (instead of 3 according to the prototype), lower temperature of regeneration of adsorbents can simplify the method, reduce energy consumption for the process of cleaning and regeneration, increase the service life of the zeolite. All this increases the efficiency of the proposed method of drying and purification of hydrocarbon gases from mercaptans and hydrogen sulfide.

Claims (2)

1. The method of drying and purification of hydrocarbon gases from mercaptans and hydrogen sulfide by sequential contacting with an adsorbent and synthetic zeolite, followed by regeneration of saturated zeolite and adsorbent countercurrent with purified hydrocarbon gas at elevated temperatures, characterized in that silica gel is used as adsorbent, and zeolite and silica gel are regenerated carried out at a temperature of purified hydrocarbon gas 180-220 ^o C.  2. The method according to p. 1, characterized in that the contacting of the gas with silica gel and zeolite is carried out at a mass ratio of 1-10: 1, respectively.

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