The use of active alumina as a catalyst in refining processes (e.g., reformation, hydro-treatment, and hydrocracking) and as an adsorbent, in particular, a natural gas desiccant, is widely known. The high activity of aluminium oxide in interaction with polar adsorbates (first, with water vapours) provides deep drying of associated petroleum gas to a dew point of −60 °С and below. Water resistance is an important positive property of aluminium oxide and often determines the selection of aluminium oxide as an adsorbent for drying and the treatment of media containing condensed moisture. The possibility of multiple temperature regeneration by means of coke burn-off ensures the long-lasting work of the adsorbent as a desiccant of olefin-containing flows [1
Finding new, more effective aluminium-oxide-based adsorbents [2
] and the use of an appropriate complex loading of desiccants into adsorbers, which allows for receiving the associated petroleum gas (APG) with the required parameters, remains relevant. It is known, for example, that loading an aluminium oxide protective layer into an adsorber extends the service life of zeolites [3
]. The use of a more effective desiccant as a protective layer will allow for not only the protection of a bottom layer from condensed moisture but also an enhancement of the effectiveness and operational life of the adsorber.
There are studies on highly effective alumina desiccant adsorbents based on low-temperature forms of aluminium oxide (η-, γ-, and χ-) received by means of the incineration of alkaline hydration products of thermally activated alumina containing bayerite phases of 50% and more [2
]. A characteristic of bayerite-containing hydroxide is the formation of low-temperature phases of aluminium oxide (first, η-modification) at a calcination temperature >300 °С. This provides an opportunity to obtain samples with a developed specific surface area, the required combination of surface sites, a large quantity of micropores, and therefore a larger static capacity than that of the desiccants based on γ-Al2
that are obtained on the basis of pseudoboehmite using the reprecipitation method [5
]. The study [6
] demonstrated that the dynamic capacity of pseudoboehmite-based desiccants synthesized with the centrifugal–thermal activation of gibbsite with its subsequent hydration under mild conditions is significantly lower than the dynamic capacity of desiccants based on η-Al2
. The chemical modification of surfaces with acids and bases has become a more widespread method for increasing the adsorption capacity of active aluminium oxide. A good example is the adsorption capacity of acids or bases, which allows us to modify the concentration of acid–base sites and the reactivity of alumina surface hydroxyls [7
When sulphuric acid was introduced at the preparation stage of moulding a sorbent mass, the samples received from pseudoboehmite-containing aluminium hydroxide were comparable in dynamic capacity rates to bayerite-based adsorbents, and even exceeded them in static capacity rate [6
]. This is related to change in phase composition, textural characteristics, and acid–base surface properties. Moreover, a greater modifying effect was observed in desiccants based on γ-Al2
that had a greater quantity of Bronsted acid sites (BAS) and potent Lewis acid sites (LAS) after the introduction of sulphate ions, and the average diameter of pores was reduced [5
]. The dependence of the static capacity value at low relative humidity on the concentration of electron-withdrawing sites has been established. Impregnation with alkali metal hydroxides may also result in the modification and increase of the concentration of surface base sites. Interaction between structural-phase and surface characteristics of adsorbents and their sorption capacity in relation to water is relevant for study.
The purpose of this work is to study the acid–base, texture, and water-absorbing properties of active adsorbents for pseudoboehmite-based samples and for samples modified with alkaline ions.
2. Materials and Methods
Samples of bayerite- and pseudoboehmite-based alumina desiccants synthesized with centrifugal–thermal activation gibbsite (CTA GS) followed by its subsequent hydration under mild conditions [6
], and also samples of pseudoboehmite-based aluminium oxide modified with sodium and potassium ions, were taken as study objects. The modification was carried out introducing sodium hydroxide and potassium hydroxide solutions at the preparation stage of moulded plastic pastes from pseudoboehmite received by means of the mild hydration of CTA GS product. Adsorbent granules had a diameter of 3.75 ± 0.15 mm at a length of 5 ± 1 mm after the completion of extrusion and the thermal treatment.
A series of studies to determine adsorbent dynamic capacity values on the basis of water vapours under realistic industrial conditions has been carried out for the synthesized samples using a pilot two-reactor adsorption plant (PAP) [8
]. Nine (9) adsorption–regeneration cycles were performed for each sample. At set pressure and space velocity values, gas with 100% relative humidity by water had been supplied for drying at an operating temperature of adsorption (22–27 °С): (1) Р = 30 atm; V(N2
) = 16 m3
О) = 11–15 mL/h; (2) Р = 30 atm; V(N2
) = 10 m3
/h; and V(Н2
О) = 7 mL/h. When the dew point temperature (DPT) dropped to −40 °
С, the adsorption process was completed and the plant switched automatically to the adsorbent regeneration mode (Р = 25 atm, V(N2
) = 2 m3
/h, t 360 min). The final desiccants are characterized by high stability in multiple adsorption–desorption cycles and the possibility of reaching a minimum dew point temperature of −80 °
С in the course of drying. Samples modified with alkaline metals are characterized by a greater dynamic capacity compared to the unmodified ones [8
A series of studies of the physical and chemical properties of the samples obtained was conducted before and after the nine cycles of vapour adsorption–desorption were conducted.
Sodium and potassium contents in samples were determined with inductively coupled plasma mass spectrometry (ICP-MS) using the Agilent 7500cx (Agilent, Santa Clara, CA, USA).
Thermal and gravitational (TG) tests of aluminium oxides were performed in oxidizing medium using the NETZSCH STA 409 synchronous thermal analysis machine. During analysis, the heating rate and exposure time at the selected temperature were varied. Textural characteristics of the CTA GS product and adsorbents were determined by isotherms of nitrogen adsorption at 77 К using the Asap 2400 sorptometer (Micromeritics, Norcross, GA, USA).
Specific surface area was measured using the Brunauer-Emmett-Teller (BET) method, and the micropore volume using the t-method. Mesopore volume was determined by analyzing the integral pore volume distribution curve depending on radius (along adsorption branch); average pore diameter (in nm) was determined with the equation dave
/A, where А is the granule surface area [10
Determination of the crush strength of samples was carried out using a catalyst strength tester, Lintel PK-21.
The study of acid–base properties of the surfaces of the samples was performed using the pH measurement method in conformity with the technique [11
]. The measurement of the pH of the suspension from its formation to the achievement of electrochemical adsorption equilibrium was registered every 5–10 s according to the readings of the IPL-101 ion meter and a рН-meter (рН 673 M) using glass and standard silver chloride electrodes. The values of рН at 5, 10, and 15 s of the sample coming into contact with water and the рН of isoionic state of matter (рНiip), which characterizes the equilibrium state, were selected as the parameters characterizing the acid–base state of the surface.
The dynamic method was used in this study to observe the adsorption of water vapours. The adsorption value was determined by the weight method using a McBenna-Bakr spring scale with a sensitivity of 2.9 × 10−3
g/mm. Prior to the performance of the adsorption measurements, each sample was trained (regenerated) at 220–240 °С in a nitrogen flow (extra pure grade nitrogen of impurity content not more than 10 ppm) supplied for 1 h at 5 L/h. Regeneration at temperatures up to 250 °С is considered to be acceptable [12
], since it leads to the minimum reduction in the time of useful use of the adsorbent. Wet nitrogen (moisture content >80%) was supplied to the sample to perform adsorption of water vapours. The elongation of the spiral was fixed with a V-630 cathetometer (Instrument-making plant, Kharkiv region, Izyum, Ukraine). All experiments for the study of water vapour kinetics were performed at 26 °С and at atmospheric pressures on the aluminium oxide samples with a fraction of 0.5–1.0 mm. To exclude an impact of the advance speed of a substance on an outer granule surface, a series of experiments on water vapour adsorption at different gradually increasing flow rates up to 36/h had been conducted beforehand. It was found that if the carrier gas velocity was equal to or exceeded 27 L/h, the kinetics of water vapour adsorption did not depend on the nitrogen delivery rate. It was found the optimal adsorbent weighed amount (0.02–0.03 g) allowed us to place adsorbent granules with a fraction of 0.5–1.0 mm in a layer in a foil cup. After the completion of adsorption and the establishment of adsorption balance, dry nitrogen was supplied to the sample at 10 L/h, and the changes in the weight of the sample were recorded at preset time intervals after the water desorption.
A comprehensive study of desiccants based on aluminium oxide modified with alkaline metals, including their physicochemical, mechanical, and physical properties and water vapour adsorption kinetics, has been conducted. The samples were studied before and after nine water adsorption–desorption cycles under maximally realistic conditions to the industrial parameters during the drying and transportation of the associated petroleum gas. It was found that the samples of bayerite- and pseudoboehmite-based alumina desiccants have potent base sites on their surface. The dynamic capacity of adsorbents with respect to water vapours increases if modified with alkali ions against the background of an enlargement of the average diameter and pore volume. This is associated with the growth of the concentration and strength of surface base sites due to the effect of alkaline cations [18
As soon as these samples were used in the nine adsorption–regeneration cycles at the pilot adsorption plant under the pressure of 30 atm, the phase composition of samples А-2, А-3-Na, and А-4-K underwent minor changes: an additional pseudoboehmite phase, a pore size enlargement, a decrease in fine pore quantity, and a reduction in the specific surface area. The specific surface area of the samples А-2 and А-3-Na is reduced more intensively and an increase in the surface acidity of the samples was observed after nine adsorption–regeneration cycles. The value of pHiit for the samples А-1 and А-4-K before and after their use as desiccants coincides, i.e., the total surface basicity did not change. It can be noted that quite a high stability of these samples was observed during the adsorption–regeneration cycles at the pilot plant.
The testing of the sample with the TGA and DTA methods demonstrated that the peak displacements of physically adsorbed water to a lower temperature region (81–86 °С) were observed for the samples used in multicyclic drying with an increase in the volume of water removed at temperatures up to 240 °С. A greater amount of water released at these temperatures was observed in the modified samples А-3-Na-9C and А-4-K-9C having a larger volume and pore diameter, which confirms the larger water capacity of these samples.
The adsorption kinetics of all the samples in the region of average coverage are expressed by the Roginsky–Zeldovich equation, which characterizes adsorption with a proportionally non-uniform surface. The desorption is carried out from the uniform surface. The water vapour adsorption on active aluminium oxide is the combined result of three processes: chemosorption, physical adsorption, and capillary condensation. As follows from the data, the first layer is formed on active surface sites of aluminium oxide due to dissociation chemosorption of water molecules. The second and the next layers are adsorbed physically by means of van der Waals strength. The capillary condensation at a relative vapour pressure below one P/P0 and at a temperature above the dew point of the liquid is typical for this adsorbent. The factors of the adsorption and desorption equations calculated are connected with the binding energy of the surface sites’ water adsorption. There were higher adsorption factor values set in the Roginsky–Zeldovich equation for aluminium oxides modified with alkali metal cations.