Chemistry of inorganic compounds – Zeolite – Seed used
Reexamination Certificate
2001-06-28
2003-12-02
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
Seed used
C423S305000, C423S306000, C423S716000, C423SDIG002, C423SDIG003
Reexamination Certificate
active
06656447
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for continuously synthesizing a molecular sieve. More specifically, the process involves controlling either the amount of interstage backmixing in a continuous reactor or the number of stages in order to control particle size.
BACKGROUND OF THE INVENTION
Zeolites are crystalline aluminosilicate microporous compositions that are well known in the art. There are over 150 species of both naturally occurring and synthetic zeolites. In general, the crystalline zeolites are formed from corner-sharing AlO
2
and SiO
2
tetrahedra which form a three-dimensional framework and are characterized by having pore openings of uniform dimensions, having a significant ion-exchange capacity and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent crystal structure.
Other crystalline microporous compositions are known which are not zeolitic (non-zeolitic molecular sieves or NZMS), i.e. contain framework elements other than aluminum and silicon, but which exhibit the ion-exchange and/or adsorption characteristics of the zeolites. These include: (1) crystalline aluminophosphate compositions disclosed in U.S. Pat. No. 4,310,440 B1; (2) silicon substituted aluminophosphates as disclosed in U.S. Pat. No. 4,440,871 B1; (3) metal substituted aluminophosphates as disclosed in U.S. Pat. No. 4,853,197 B1; (4) metal sulfide molecular sieves disclosed in U.S. Pat. No. 4,880,761 B1 and (5) metallo zinc-phosphate compositions disclosed in U.S. Pat. No. 5,302,362 B1.
Molecular sieves (both zeolitic and NZMS) are usually hydrothermally synthesized from a reaction mixture in a batch reactor. A continuous process would have the advantage of reduced capital investment, space requirement, operating costs, consistent quality, greater efficiency and waste handling. There are several references which teach continuous processes for the synthesis of zeolites. For example, EP 021675-B1 teaches using a continuous-stream process to prepare zeolites. The process involves preparing a reaction mixture and passing the mixture through a heated reaction zone.
U.S. Pat. No. 3,425,800 B1 discloses a process for the continuous preparation of zeolites comprising forming a zeolitic gel and then supplying the gel to a stratified zone where the zeolite crystals form and settle downward into a lower stratum where they are collected.
U.S. Pat. No. 3,886,094 B1 discloses a continuous process for preparing a hydrocarbon conversion catalyst which involves as one step continuously preparing a zeolite from a reaction mixture containing seed particles. U.S. Pat. No. 5,427,765 B1 discloses preparing zeolite beta by first continuously forming granular amorphous aluminosilicate and then crystallizing the zeolite beta. U.S. Pat. No. 5,100,636 B1 discloses preparing zeolites by sending a reaction mixture through a pipe segment for an initial crystallization and then to an open vessel for a second crystallization.
In U.S. Pat. No. 4,336,234 B1 it is disclosed to use a cascade system in which a zeolite is formed and then milled. U.S. Pat. No. 4,368,174 B1 discloses an apparatus for synthesizing zeolites such as ZSM-5 which among other steps uses an inert gas to pressurize the vessel. A stirring means is also used. U.S. Pat. No. 4,371,510 B1 discloses a continuous process for synthesizing zeolites which contains several stages which have separately zoned mixing areas. U.S. Pat. No. 4,374,093 B1 discloses an upflow zeolite crystallization apparatus containing a stirring means and baffle means. U.S. Pat. No. 4,385,042 B1 discloses a process for the continuous synthesis of zeolites which uses a reactor-separator and which involves the introduction of nuclei particles. U.S. Pat. No. 4,591,491 B1 discloses the same process as in U.S. Pat. No. 4,371,510 B1 but using meta-kaolin. U.S. Pat. No. 4,818,509 B1 discloses a process which includes a series of tanks operated at different temperatures in which the zeolite is synthesized by passing the reaction mixture from one tank to another. In GB 2252305A a process using a cascade system is disclosed. DD152525 discloses continuously introducing a reactant into a vessel. Finally, DE 3011834A discloses first reacting a reaction mixture at 40-50° C. and then transferring it to another vessel and reacting at 85-95° C.
In contrast to this art, applicants have developed a process to not only continuously prepare molecular sieves with high efficiency, but to also control the particle size and the particle size distribution. For example in the case of aluminosilicate zeolites, the process involves continuously adding reactive sources of at least aluminum and silicon into a continuous crystallization reactor having at least two stages. A structure directing agent can be added separately or in combination with the aluminum or silicon containing stream. The mixture thus formed is flowed through the reactor in order to crystallize the zeolite. Finally, an effective amount of interstage backmixing is introduced in order to control the particle size and particle size distribution. The greater the amount of backmixing (deviation from plug flow), the larger the particle size and the wider the particle size distribution. Alternatively, the approach to plug flow can be controlled by the number of stages in the reactor. The greater the number of stages, the closer the approach to 100% plug flow, i.e., batch performance.
SUMMARY OF THE INVENTION
As stated, this invention relates to a process for controlling the particle size of a molecular sieve. Accordingly, one embodiment of the invention is a process for controlling the particle size and particle size distribution of a molecular sieve during its synthesis, the molecular sieve having an empirical formula on an anhydrous basis of:
r
R
2
O:(Si
x
Al
y
P
z
)O
2
(I)
where R is at least one structure directing agent, “r” has a value of 0 to about 1.0, “x”, “y” and “z” are the mole fractions of silicon, aluminum and phosphorus respectively, “x” has a value from 0 to about 1.0, “y” has a value from 0 to about 0.6, “z” has a value from 0 to about 0.545; x+y+z=1; x+y>0; and when x>0 and y=0, then z=0, the process comprising:
a) Continuously introducing separate streams comprising reactive sources of at least each framework element, according to formula (II) into a continuous crystallization reactor having at least two stages, thereby forming a reaction mixture having a formula expressed in terms of molar ratios of:
a
R
2
O:(Si
b
Al
c
P
d
)O
2
:e
H
2
O (II)
where R is at least one structure directing agent, “a” has a value of greater than 0 to about 5; “b” has a value of about 0 to about 1.0; “c” has a value of 0 to about 1.0, “d” has a value of 0 to about 1.0 and “e” has a value from greater than zero to about 500; b+c+d=1; b+c>0; and when b>0 and c=0, then d=0;
b) flowing the reaction mixture through said continuous reactor, at reaction conditions, thereby crystallizing the molecular sieve;
c) controlling the particle size and particle size distribution by introducing an effective amount of interstage backmixing or by adjusting the number of stages, and;
d) recovering the molecular sieve.
This and other objects and embodiments will become more evident after a detailed description of the invention.
DETAILED DESCRIPTION OF THE INVENTION
A process for continuously preparing molecular sieves and controlling the particle size and particle size distribution has been developed. The molecular sieves which can be prepared using this process include any of those which can be prepared by a batch process. This includes any of the synthetic zeolites and especially zeolites A, X and Y. However, the instant process is particularly attractive when the crystallization time (residence time in the reactor) is short or when the material of construction of the reactor is expensive, e.g., special alloys, since a
Marchioretto Sonu
Monson Lyle E.
Tannous Medhat K.
Molinaro Frank S.
Sample David
Tolomei John G.
UOP LLC
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