Zeolites NaA-type

Polymeric (PERVAP 2510, Sulzer Chemtech GmbH) and Ceramic membranes (NaA type zeolite, Mitsui Co.)... 

Kita et al. (2003) reported on a tubular-type PV and vapor permeation module with zeolite membranes for fuel EtOH production. They used two types of zeolite membranes (i) NaA-type zeolite membrane, which was grown on the surface of a porous cylindrical mullite support and (ii) T-type zeolite membrane, which was also grown hydrothermally on the mullite support. Both membranes were studied for the flux and the separation factor of PV and vapor permeation for water-alcohol mixtures at 50°C and 75°C. The membranes were selective for permeating water preferentially with the high permeation flux. The separation factor of the T-type zeolite membrane was slightly smaller than the NaA zeolite membrane. They also claimed that this can provide more energy-efficient concentration of the EtOH to fuel grade EtOH. 

Carbon dioxide adsorption causes changes in the sizes of all the zeolites studied similar to the variation observed for faujasite-type zeolites after water adsorption (Figure 6). For all zeolites, an increase in the adsorption of carbon dioxide leads to contraction this reached a minimum in the adsorption range 3-5.5 mM/gram. The final length of the pellets is below the initial value up to a relative pressure of p/ps 0.7 for zeolites CaA, CaY, and NaY while for NaA and NaX the contraction passes to an expansion, reaching 0.11% of the initial length at p/ps = 0.66 for NaX and 0.32 for NaA. 

The absence of an initial expansion in faujasite-type zeolite (Figure 5) is probably associated with the peculiarities of their structure and with the number and location of the cations. The heats of adsorption of water on zeolite NaX, in contrast with that of NaA, rapidly decrease in the... 

The adsorbents have been prepared fi-om the halloysite (H) - mineral fi-om kaolinite group with an admixture of carbonaceous materials refinery waste deposits (RSI), sediment communal sewage (CSew) and cellulose (Ce), and the fiaction of these mixtures were within 30 - 70 wt.%. The mbcture of raw material was thermally (carbonaceous materials carbonization, 973 K) and hydrothermally (crystallization of the amorphous metahalloysite in alkaline solution to zeolitic structure of NaA type, 373 K) pretreated in order to cilitate their specific structure [1,2]. 

As Fig. 25 shows, the intracrystalline self-diffusion coefficient of methane in ZSM-5 is between coefficients in zeolites NaCaA and NaX (5,71,114,115,). This order can be interpreted in terms of the minimum apertures of the zeolite channels, which are approximately 0.45, 0.55, and 0.75 nm for 5A, ZSM-5, and X-type zeolites. Due to the hydrophobic nature of ZSM-5, the mobility of water in ZSM-5 considerably exceeds the mobility in zeolites NaA and NaX. A change in the Si02/Al203 ratio of ZSM-5 does not alter the self-diffusion coefficient of methane. On the contrary, for water in ZSM-5 an increase in the self-diffusion coefficients with decreasing A1 concentrations in the framework is indicated. 

Synthetic zeolites of various types differ in the number of cations in their voids which are accessible for direct interaction with the molecules adsorbed. Table I lists, for typical examples of zeolites, the numbers of accessible cations Na per zeolite void and their number Z in mmole/ gram for dehydrated zeolites. When passing from zeolite NaA to zeolite L, the number of accessible cations Z—i.e., the number of adsorption centers in the void—decreases almost by a factor of 10. Therefore, in the case of zeolite L, the relative role of interactions among cations and molecules adsorbed, conventionally called electrostatic, will be approximately one order lower than for zeolite NaA. In adsorption on this zeolite of substances with slightly pronounced nonuniformity of distribution of electron density in molecules— for instance, saturated hydrocarbons— one may expect that electrostatic interactions will not play the decisive role. As a result, we obtain the limiting case of adsorption on zeolites like zeolite L and erionite with a weak electrostatic interaction. 

A membrane reactor using a H-ZSM5 membrane was used by Bernal et al. [3.42] to carry out the esterification reaction of acetic acid with ethanol. An equimolar etha-nol/acetic acid liquid mixture was fed in the membrane interior, while He gas was used as an inert sweep on the shell-side. In this particular application the membrane, itself, provides the catalysis for the reaction. NaA and T-type zeolite membranes have been utilized for esterification reactions in a PVMR and in a vapor permeation membrane reactor (VPMR) by Tanaka et al [3.43, 3.44]. Both membranes are hydrophilic and show good separation characteristics towards a number of alcohols. The NaA membrane was used to study the oleic/acid esterification in a vapor permeation membrane reactor (VPMR) at 383... 

Zeolite membranes have attracted a lot of interest for their uniform pore size at molecular scale, which allows the separation of liquid and gaseous mixtures in a continuous way. Because of their thermal and chemical stability, they can also be used in processes at high temperatures and in the presence of organic solvents where polymeric membranes fail. In addition, zeolite materials exhibit intrinsic catalytic properties which clearly suggests the use of zeolite membranes as catalytic membrane reactors (CMRs). In the last two decades, enormous progress on zeolite membrane synthesis has been made, but only 20 structures are used for membrane preparation even if 170 zeolitic structures are available today (Baerlocher et al, 2007). The high cost and poor reproducibility in the synthesis step hinder the application of the zeolite membranes at industrial level (Caro et al, 2005 Mcleary et al., 2006). Until now, only NaA and T-type zeolite membranes... 

Morigami, Y., Kondo, M., Abe, J., Kita, H., and Okamoto, K. (2001) The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane separ. Purif. Tech., 25, 251-260. 

Another type of mixed-matrix membranes for alcohol/water pervaporation applications was developed utilizing hydrophiUc poly(vinyl alcohol) (PVA) and ZSM-5. The ZSM-5/PVA mixed-matrix membranes demonstrated increased selectivity and flux, compared to pure PVA, for the water/isopropyl alcohol separation [97]. This type of mixed-matrix membranes, however, may have membrane swelling issue due to the hydrophilic nature of the PVA polymer. Mixed-matrix membranes comprising modifled poly(vinyl chloride) and NaA zeolite have shown both enhanced flux and selectivity for the ethanol/water separation at high NaA loadings [98]. 

M. Kondo, M. Komori, H. Kita and K. Okamoto, Tubular-type Pervaporation Module with Zeolite NaA Membrane, J. Membr. Sci. 133, 133 (1997). 

The samples used were a standard Na-A zeolite and five nitrogeneous types of zeolite A, or N-A.(18) The N-A zeolites are siliceous analogues of zeolite A, synthesized with tetramethyl-ammonium cation. The Si Al ratio varied from 0.94 (NaA) to 3.54 for the most siliceous N-A sample.t The ratios were determined by wet chemical analysis, and the structure type and absence of impurity phases were confirmed by X-ray powder diffraction techniques. Adsorption measurements (oxygen, -183°C) showed a zeolite A content of greater than 90%. 

For NaA zeolite the composition was found to be nearly unity across the crystal. For Na-ZSM-5, three types of chemical profiles were found. Small particles of Si Al=10 showed either enrichment of Si near the surface or a nearly homogeneous composition. 

In the case of zeolite A, three different cation sites have been identified most of the cations occupy comer sites in the central cavity (Type I sites), but some of the total of twelve univalent ions (e.g. Na+ or K+) ions per cage must occupy sites within the eight-ring windows and therefore partially obstruct the channels. The effective window size of NaA (i.e. the 4A sieve) is thereby reduced from 0.42 nm to c. 0.38 nm. Since the K+ ion is somewhat larger, the window size becomes even smaller (i.e the 3A sieve). When the Na+ cations are exchanged for Ca2+ or Mg2+, the number of requisite cations is reduced and the effective aperture size and pore volume are both increased (the 5A sieve). 

The gel thus obtained is then hydrothermally treated and can be ion exchanged by conventional methods. The preferred pore diameter of the support is 10 nm and may be obtained alternatively by using a dense support followed by chemical after-treatment to the membrane-support composite. A wide variety of zeolites including NaA-, CaA-, HA-, H-, NaY-, CaY-, REY-, CaX-types all have been produced and they have shown very efficient separation of hydrocarbons. 

In a joint work with A. A. Isirikyan with the participation of G. U. Rakhmat-Kariyev, we carried out direct measurements of differential heats of adsorption of water vapors on crystalline and molded zeolite NaA at 22 °C using a Tian-Calvet-type calorimeter. The calorimetric installation enabled us to measure thermal effects for each point of the adsorption isotherm for a period of 300 hours and more (Figure 1). The squares and circles in the upper part of the graph denote our data for... 

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