Continuous Chromatography
Continuous or moving bed adsorption chromatography was first introduced by Freund, Benedek and Szepesy in 1956 who used it to extract acetylene from a hydrocarbon gas mixture. The concept was extended by Scott in 1958 to gas liquid chromatography separations and used the technique to extract, in a continuous manner, pure benzene from crude coal gas. Essentially the system consists of a long tube or column down which the support coated with stationary phase falls; at the bottom of the column, the support is removed at a pre-determined rate by a turn table and then returned to the top of the column by an air lift system. In the separation by Scott the stationary phase (that absorbed on the support) was polyethylene glycol which retained aromatics strongly but aliphatic hydrocarbons only relatively weakly. The gas/vapor mixture is fed into the center of the column and the gas passes up the column and out at the top. Those solutes moving faster than the bed (e.g. hydrocarbons, olefins etc.) are eluted at the top of the column whereas those moving slower than the bed (e.g. benzene, toluene, xylenes etc.) are brought down by the moving bed below the feed point. A section of the bed below the feed point is then heated to increase the elution rate of the solutes and any solute now moving faster than the bed (e.g. benzene) can be eluted through a side tube by a second flow of gas while those solutes still moving at a slower rate than the bed (e.g. toluene and xylenes etc.) continue to move down the column in the stationary phase. The higher fractions can be removed in the same way by a section of the column heated to an even higher temperature. In this way, the hydrocarbons in the coal gas are extracted and separated into aliphatic hydrocarbons, pure benzene and toluene, xylenes and higher molecular weight materials. Using an appropriate valve system and a static arrangement of short columns, the moving bed can be closely simulated and used for liquid chromatographic separations.
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Alternative-Techniques Moving-Bed-System
The Moving Bed Continuous Chromatography System
The idea of a
continuous moving bed extraction process as a means of large scale separation
is almost as old as gas chromatography itself. The first description of a
continuous gas-solid extraction process
that was reported in the chromatography literature was in 1956 by Freund et
al. (10). However, much of the experimental work had been reported
considerably earlier but in the 'not readily available' Hungarian technical
literature. Shortly afterwards, the use
of a moving bed for the continuous preparative scale product isolation using gas-liquid chromatography was reported by Scott
(11,12)
The principle
of the moving bed separating system is
basically simple, but can be quite difficult to carry out in practice.
Preparative Alternative-Techniques Moving-Bed-System
Author: RPW Scott
Book:Gas Chromatography
Section:YES Preparative-Gas-Chromatography Moving-Bed-System
Moving Bed Continuous Chromatography System
The concept of
the moving bed extraction process was originally introduced for hydrocarbon gas
adsorption by Freund et al. (13) and was first applied to gas liquid
chromatography by Scott (14). A diagram of the moving bed system suitable for
GC was proposed by Scott and is shown in figure 39.
The
feasibility of this process was established for a gas chromatographic system,
subsequently, its viability was also confirmed for liquid chromatography which
will be discussed in Book 19. The moving bed system takes a continuous
sample feed and operates in the following way. The stationary phase, coated on
a suitable support, is allowed to fall down a column against and upward stream
of carrier gas. In the original device of Scott, the packing (dinonyl phthalate
coated on brick dust) was contained in a hopper at the top of the column and
was taken off from the bottom the column by a rotating disc feed table and
returned to
YES Preparative-Gas-Chromatography Moving-Bed-System
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Alternative-Techniques Moving-Bed-System Benzene-from-Coal-Gas
The Continuous Moving Bed Process for the Isolation of Pure Benzene from Coal Gas
The moving bed
technique was first applied to gas-liquid
chromatography by Scott (11,12). A diagram of the moving bed system suitable
for gas-liquid chromatography (GLC) was proposed by Scott also in 1956, and the
basic system is depicted in figure 25. The original GC system will be used here
to explain the principal of the separating process.
Figure 25.
A Continuous Chromatography Separation System
Preparative Alternative-Techniques Moving-Bed-System Benzene-from-Coal-Gas
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Introduction
Although
chromatography was discovered late in the 1890s its development was almost
negligible until the 1940s and this was largely due to the lack of an inline
sensitive detector. The first, effective inline liquid chromatography (LC)
detectors were the refractive index detector reported by Tiselius and Claesson
(1) in 1942 and the conductivity detector described by Martin and Randall (2)
in 1951. These two devices should have evoked a growth in LC development, but,
in the early fifties, gas chromatography (GC) was invented which completely
eclipsed the development of LC. It was not until the early 1960s that the
renaissance of LC took place, initially based on the use of the refractive
index of Tiselius and Claesson. Although a significant number of GC detectors
were developed over two or three years, the development of LC detectors was
much slower, largely due to the fact that low concentrations of solute in a
liquid do not change the properties of a liquid nearly as much as they
HPLC-Detectors Introduction
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Alternative-Techniques Moving-Bed-System Acetylene-GSC-Purification
Considering
the very limited chromatography knowledge that was available in 1956, together
with the limitations of the equipment that was accessible, the process and
plant design was a very impressive achievement at that time.
Figure 24.
Apparatus for the Extraction of Pure Acetylene from a Gaseous Hydrocarbon
Mixture By Continuous Adsorption Chromatography
Preparative Alternative-Techniques Moving-Bed-System Acetylene-GSC-Purification
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Basic-HPLC Sample-Valve
the next thrust. The inlet from the solvent supply and the
outlet to the column are fitted with non-return valves in the usual manner. Due
to the large volume of the pumping cavity, any gradient profile would be seriously
distorted so this type of pump is not often used for analytical purposes but is
often used in preparative chromatography.
The Sample Valve
In general, LC
sample valves must be able to sustain pressures up to 10,000 p.s.i., although
they are most likely to operate on a continuous basis, at pressures of 3,000
p.s.i. or less. The higher the operating pressure the tighter the valve seating
surfaces must be forced together to eliminate any leak. It follows that any
abrasive material, however fine, that passes into the valve can cause the valve
seating to become scored each time it is rotated which will ultimately lead to
leaks. This will cause the sample size to vary between samples and eventually
affect the accuracy of the analysis. It follows that any solid
HPLC Basic-HPLC Sample-Valve