Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By contact with solid sorbent
Reexamination Certificate
1995-07-06
2004-09-28
Caldarola, Glenn (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By contact with solid sorbent
C585S824000
Reexamination Certificate
active
06797854
ABSTRACT:
Processes for removing water from industrial gases or from a mixture of gaseous or liquid hydrocarbons by passing the latter through adsorbents are known.
Molecular sieves are particularly employed for such processes. However, molecular sieves deteriorate rapidly because of the presence of so-called insoluble, that is to say liquid, water in the gases or liquids to be dried. In fact, this results in a loss of water removal efficiency and accelerated aging of the sieves.
To avoid this, it is known to protect molecular sieves with a layer of alumina, silica gel or active carbon, the purpose of which is to adsorb the liquid water. Combinations of alumina layers and of molecular sieves make it possible to extend the lifetime of the sieve and to ensure good drying of the gases or liquids.
However, there is still a perceptible need for an increase in the lifetime of these molecular sieves and of their adsorption effectiveness.
The aim of the invention is to propose a process for the removal of water from industrial gases or from a mixture of gaseous or liquid hydrocarbons with the aid of an adsorbent composed of the combination of layers of molecular sieves and of alumina, in order to obtain a satisfactory water removal efficiency while considerably reducing the aging of the adsorbent.
The objective of the invention is to propose a process for drying a gaseous or liquid mixture by passing the said mixture into an adsorber, characterized in that the water adsorption equilibrium zone of the adsorber comprises an upstream alumina zone and a downstream molecular sieve zone.
Other characteristics of the invention will appear on reading the description and the nonlimiting examples.
The invention relates first of all to a process for drying a gaseous or liquid mixture by passing the said mixture into an adsorber, characterized in that the water adsorption equilibrium zone of the adsorber comprises an upstream alumina zone and a downstream molecular sieve zone.
More precisely, this process deals with the removal from gaseous or liquid mixtures. The composition of this mixture may be very varied. Industrial gases or mixtures of gaseous or liquid hydrocarbons may be especially mentioned. Gases to be dried may be, for example, natural gases of methane, ethane type, etc., or associated gases obtained during the liquid/gas separation of petroleum fractions, gases originating from steam cracking or from fluid catalytic cracking (FCC), industrial gases such as pure gases: N
2
, O
2
Ar, etc., coke plant gases or gases originating from processes such as catalytic reforming, hydrocracking etc. The liquids to be dried may be natural-gas condensates of CO
2
type, liquid air or LPGs (liquid petroleum gases) such as, for example, propane or butane.
These mixtures may be unsaturated, saturated or supersaturated with water.
The process uses the principle of separation of water from gases or liquids containing it by adsorption thereof on layers of aluminas and of is molecular sieves. The process consists in introducing the gases or liquids to be dried into an adsorber where they will encounter in succession a layer of alumina and a layer of molecular sieve. The introduction of the gases or liquids can take place equally well via the top as via the bottom of the adsorber, as long as the gases or liquids pass firstly into the alumina layer and only subsequently into the molecular sieve layer.
In the use of the process the alumina adsorbs water in gaseous form, but also essentially water in liquid form, whereas the molecular sieve now adsorbs only the gaseous water.
In the process according to the invention it is essential that the alumina and the molecular sieve should be introduced in precise conditions. It is necessary, firstly, that the alumina should not be situated in the water adsorption mass transfer zone of the adsorber.
The concept of mass transfer zone, employed here, corresponds to the conventional concept of mass transfer zone in the technical field of adsorptions on beds, that is to say that it is defined as the portion of the adsorber in which the concentration of the adsorbate, in this case water, on the adsorbent varies from zero concentration to the maximum adsorption concentration at the equilibrium. This mass transfer zone is generally of constant length and moves during the adsorption cycle, right from the entry of the adsorber up to its exit (in the direction of introduction of the gases or liquids).
Very precisely, the mass transfer zone is defined in this invention as the mass transfer zone taken at the instant when the water breaks through at the exit of the adsorber, that is to say at the instant when the adsorption capacity of the adsorbent is saturated and the cycle ended.
Secondly, it is necessary that the alumina should be placed in the so-called upstream zone of the water absorption equilibrium zone of the adsorber and the molecular sieve in the so-called downstream zone of the said equilibrium zone of the adsorber.
The upstream zone, comprising alumina, corresponds to the part of the equilibrium zone which is the first one in contact with the gases or liquids to be dried, and the downstream zone, comprising the molecular sieve, corresponds to the part of the equilibrium zone which is the last one in contact with the gaseous or liquid mixture to be dried.
The equilibrium zone itself also corresponds to the conventional concept of an equilibrium zone in the technical field of adsorptions on beds. It is therefore defined as the portion of the adsorber in which the concentration of the adsorbate, in this case water, on the same single adsorbent is constant. The length of this equilibrium zone increases during the adsorption cycle.
In the context of the invention the equilibrium zone is also defined as the equilibrium zone taken at the instant when the water breaks through at the exit of the adsorber, that is to say at the instant when the adsorption capacity of the adsorbent is saturated and the cycle finished, and therefore at the same instant when the mass transfer zone is defined.
In practice, the mass transfer zone is defined from adsorption isotherms of the adsorbents employed, at a pressure and temperature which are fixed, and from the internal and external diffusion coefficients of the adsorbates in the adsorbents; the equilibrium zone is determined from the adsorption isotherms of the adsorbents employed at fixed temperature and pressure.
In the equilibrium zone of the adsorber the ratio of the volume of alumina to that of the alumina and of the molecular sieve (Q) is generally not more than 0.95.
In this case and throughout the description the volumes of alumina and of molecular sieve correspond to the volumes determined at the time of charging of the adsorber.
In the equilibrium zone of the adsorber the ratio of the volume of alumina to that of the alumina and of the molecular sieve (Q) is preferably between 0.05 and 0.95; still more preferably this ratio is between 0.05 and 0.8 and more preferably between 0.2 and 0.8, as a function of the embodiments.
In the case where the gaseous or liquid mixture is supersaturated or saturated with water and according to a first embodiment, the ratio of the volume of alumina to that of alumina and of the molecular sieve in the water adsorption equilibrium zone (Q) is preferably between 0.5 and 0.8. Such a ratio of volumes will allow the water to be removed efficiently. It is considered that a gaseous mixture is supersaturated with water when it contains, at a given temperature and pressure, water in gaseous phase and in liquid phase; and that a liquid mixture is supersaturated with water when it contains miscible water and water which is not miscible with the constituents of the liquid.
According to a second embodiment, which corresponds to the case where the gaseous or liquid mixture is undersaturated or saturated with water, the ratio of the volume of alumina to that of alumina and of molecular sieve in the water adsorption equilibrium zone (Q) is preferably between 0.05 and 0.30. A gaseous mixture is undersaturated with water at a
Caldarola Glenn
Nguyen Tam
Rhone-Poulenc Chimie
LandOfFree
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