SIMULATED MOVING BED CHROMATOGRAPHY

Extract

FeedRaffinate

Eluent

Direction of the flow and column switching

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AN OVERVIEW.The Simulated Moving Bed (SMB) technology has been applied to both small and large molecules. Compared to single column batch chromatography, SMB separations achieve higher productivity and purity while at the same time reducing solvent consumption and labor not to mention deriving a reproducible and reliable process.The challenge created by the increase in biological treatment in medicine (monoclonal antibody or vaccines) has made the manufacturers look for new directions other than batch mode chromatography. Very small differences in the composition of these large biomolecules as well as very low levels of impurity can have serious heath detriment for the patient with an already compromised immune system. Thus the need for high quality processing methods that include non leaching and non contaminating process media.The downstream purification processes is also under pressure to keep up with the higher titer achieved during the upstream production.Although the interest in bio-separation processes has been rising it has yet to reach the widespread use it has in other area of continuous and automated separations. Other applications have had considerable use such as separation of proteins, sugars, ionic molecules, optical isomers and even some monoclonal antibodies.

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AN OVERVIEW.

A number of API (Active Pharmaceutical Ingredients) are now manufactured using SMB technology. It is also the case for the separation and purification of optically active chiral compounds.

There are well established methods for reliable operations and scale-ups that have resulted from analytical experiments.

In the last few years, rapid developments have been made in the areas of design, improved process schemes, optimization and robust control.

The purpose of this overview is to provide the fundamentals of the SMB science and technology and some practical issues concerning the operation of SMB units.

The “triangle theory”, a design tool that is used both in the academia and industry for the design of SMB processes is also mentioned very briefly.

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AN OVERVIEW.Traditional batch chromatography had an important role in the purification however the large amounts of solvent, as well as buffers makes the process prohibitive as the volume of load increases.

Furthermore when it consists of large scale production the cost of labor is no longer competitive with an automated continuous process such as SMB.

In light of such given a number of equipment manufactures have indeed designed purification systems based on SMB chromatography.

A series of high-pressure and preferably high performance columns are used in series making a circulating loop.

There are two sets of ports between each column:

Inlet ports for “feed” and “solvent” streams and outlet ports for “extract” and “raffinate” streams.

The feed and solvent are supplied continuously while at the same time extract and raffinate are

also drawn continuously from the ports.

The inlet and outlet ports are switched in a coordinated rhythm and with known intervals in the same direction as the flow of the liquid.

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BASIC OPERATION.

Extract

FeedRaffinate

Eluent

Direction of the flow and column switching

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BASIC OPERATION.

Extract

Feed

Raffinate Eluent

Direction of the flow and column switching

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BASIC OPERATION.

Extract

Feed Raffinate

Eluent

Direction of the flow and column switching

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BASIC OPERATION.Extract Feed

RaffinateEluent

Direction of the flow and column switching

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BASIC OPERATION.

Extract

FeedRaffinate

Eluent

Direction of the flow and column switching

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BASIC OPERATION.

Extract

Feed

Raffinate Eluent

Direction of the flow and column switching

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BASIC OPERATION.

Extract

Feed Raffinate

Eluent

Direction of the flow and column switching

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BASIC OPERATION.Extract Feed

RaffinateEluent

Direction of the flow and column switching

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SIMULATED MOVING BED CHROMATOGRAPHY HAS ALSO BEEN PICTURED AS A FAST MOVING, COMPONENT (RABBIT) AND A SLOW MOVING COMPONENT, (TURTLE) ON A CONVEYOR BELT.CONSIDERING THE BELT SPEED AS THE SPEED OF THE RABBIT AS THE SPEED OF THE TURTLE AS BOTH IN THE OPPOSITE DIRECTION,

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THE TURTLE WOULD THEN BE CARRIED BY THE BELT AT A SPEED OF WHEREAS THE RABBIT WOULD OUTRUN THE BELT AT A SPEED OF

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WHEN CONSIDERING THE INTERCONNECTED COLUMNS OF A SIMULATED MOVING BED AND SIMPLIFYING IT TO A SINGLE COLUMN WE WOULD GET A SYSTEM TO WHICH “FEED” AND “SOLVENT” ARE INJECTED CONTINUOUSLY AND FROM WHICH “EXTRACT” AND “RAFFINATE” ARE WITHDRAWN CONTINUOUSLY AS WELL.

Feed

SOLVENT

Simulated Movement of the Bed resulting from column switching RaffinateExtract

The Simulated Movement of the Bed through switching of the columns, creates the effect that the “Feed”, “Raffinate”, “Eluent”, and “Extract” are moving in the same direction as the flow of the fluid while the movement of the bed is in the opposite direction with a very controlled speed.

This apparent speed of the bed is given by the length of the column divided by the switching intervals.

The goal is to add length to “column/s” and therefore increase the resolution of the separation while at the same time evolving it to a continuous mode.

It is clear that the final “resolution” is an increment of the sum total of the “resolution” of each column in the system.

The process is well established for a binary mixture and an isocratic solvent.

Attempts are made to move beyond and there should be no surprise to see it happen.

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The first step is an analytical one in order to choose the right media with the right resolution as well as the appropriate solvent/buffer.

The analytical performances to look for are:

• High resolution,

• High selectivity,

• Low retention time.

During that process one should keep in mind that the intent of the separation is to isolate and purify a product and avoid any added contamination. It is therefore important to check for:

• Low column bleeding,

• High column stability

• Low column back pressure

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The preferred column would have high resolution, low retention time, no bleeding , high stability and therefore long life.

t1<t2< 15 min using a 4.6 mm ID column at a linear flow rate of

360 cm/hr and a length of 250 mm.

Absorbance

t1 t2 Retention time

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The column loadability is another parameter to be considered.

That is the maximum load the column can support before reaching its saturation point under kinetic conditions.

Any two products in this example can be considered to assess the media for its suitability for the operation of a binary mixture.

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• The higher the load the faster the overall operation.

• The higher the capacity of the columns the fewer needed for a system.

• The more stable the media the more usage in the process.

• The less bleeding from the stationary phase the higher the purity of the end product.

• The lower the back pressure of the media the faster the run.

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The “Triangle Theory” is a complex tool used to determine the starting conditions of the SMB.

Nowadays the manufacturers of SMB systems as well as independent software company have addressed the issue. Far less trial and error steps are needed to reach the optimal conditions for the separation.

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b

HBHA

Impure ExtractImpure Raffinate

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m3

Pure ExtractImpure Raffinate

Impure ExtractPure Raffinate

Feed in

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Pure Extract

Pure Raffi

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Solve

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Feed in

Raffi

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The isolation of biotechnological products however has special criteria to be met in order to have widespread use with Simulated Moving Bed technology.

As early as 1997, Gottschlich and Kasche have reported the biospecific purification of monoclonal antibodies by SMB.

Although they report a ≥ 90 % yield and a removal of more than 99 % of the contaminating proteins and most importantly a single step process to reach those results, it leaves room to achieve that golden number of 100 % uncompromised should the end product be used as a therapeutic in humans with poor heath.

Thus far the majority of the stationary phases used were unstable reversed phase silica based compound with minimal capacity compared to stable hard gel and porous polymeric.

Affinity media has also been scarcely used and so have all other chromatographic regimen that are to pave the way for the SMB in becoming a universal process in the isolation and purification of biopharmaceuticals including vaccines.

https://www.orachrom.com

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