Introduction
The absorption of hydrogen chloride

Hydrogen chloride gas is encountered in many well established processes. The gas is usually a bi-product of a chlorination process, such as the manufacture of epichlorohydrin, which is used in the production of glycerin and epoxy resins, or carbon tetrachloride.  Chlorination of organic compounds frequently yields hydrogen chloride gas as a bi-product.

Hydrogen chloride is readily absorbed by water.  The problem is not hydrogen chloride's affinity for water, but how best to provide the gas access to the water.  There are two basic widely accepted and used methods:

- Failing film absorption

- Adiabatic absorption

Almost all absorption systems are centered around these two basic types. 

The two absorber types have significant differences in the way they  function, each having its own merits.
The choice of which absorber type to use is normally determined by the user's process feed conditions and product requirements. 

The failing film absorber consists of a series of narrow tubes enclosed in a vertical outer shell.  The hydrogen chloride gas flows co-currently with water down the inside of the tubes with a coolant passing on the outer side of the tubes.  The efficiency of a falling film absorber is closely related to the extraction and heat from the absorption zone, and a vertical shell and tube arrangement provides an ideal environment for this to occur.

The adiabatic absorber consists of a packed tower, through which, the hydrogen chloride gas passes counter currently to water flowing down the tower.  The efficiency of packed tower absorbers is closely related to the nature of the column packing.

top

The nature of column packings effects the operating conditions of any column.  Technical detail Is on packed columns for hydrogen chloride absorption using both glass Raschig rings and Intalox saddles are not available.

top

Gas Scrubber With and Without Neutralisation

The gas scrubber represents the simplest form of HCI absorber which can be operated continuously.  It finds particular application where the concentration of the hydrogen chloride is low and further processing to form hydrochloric acid has no economic advantage. 

In this type of unit the liquid to gas ratio in the column is usually very high.  Any heat of solution generated in the process will therefore be taken up by the liquid phase, thus avoiding the need for coolers.

In many instances there is a requirement to Combine the absorption of hydrogen chloride with a neutralization process.

To achieve this a vessel together with a pump is situated at the base of the column, to hold the neutralizing agent. The agent is circulated through the column, absorbing a sufficiently large quantity Of gas until a neutral solution is formed.  This can be monitored by continuous pH measurement.  The heat generated is removed from the system by a cooler placed in the recirculation loop.

The maximum gas loadings in absorbers of this type, which are almost always operated discontinuously, reaches about 80% of the maximum possible gas loading for a column operated adiabatically, This is based upon a liquid flow of about 10 M3/M2 h.

top

If the hydrogen chloride gas produced can be economically processed into hydrochloric acid and the gas loading does not fall below a certain minimum value, which is 50% of the designed maximum, then adiabatic absorption is the most effective means of production.  Adiabatic absorbers produce, under normal operating conditions, hydrochloric acid of 30 - 31% strength.  This compares with a theoretical maximum of 35% under similar operating conditions.

These absorbers as their name implies, operate without any heat input into the reaction zone.  In the process hydrogen chloride gas is absorbed by fresh water flowing down the column. The heat generated by this reaction vaporises approximately 30% of the water which rises to the head of the column with any other non-soluble components of the gas stream.

Pure HCI feeds

Pure hydrogen chloride streams containing only air, water vapour and/or non-condesibles, are condensed within the column and are returned together with the make up water to the packed section. Atypical arrangement is shown in figure

Contaminated HCI feeds

If the hydrogen chloride is contaminated with condensibles (otherthan water), i.e. the by-product of a chlorination process, an arrangement as shown in figure 3 must be used.  In this case the steam generated together with any impurities rises to the head of the column and is condensed in an external heat exchanger.  In some cases it is then possible to separate aqueous and organic phases and return the water to the column.

Whilst approximately 80% of the heat of solution is removed by the condensers at the head of the column, the remainder is removed at the base before the product is run off.  The heat exchanger supplied with the unit guarantees an acid outlet temperature of approximately 30'- 40 'C.

The maximum possible gas loading and therefore the quantity of acid produced is the same for both types of absorber, and depends basically upon the type of packing material in the column.

top

Operation 

Hydrogen chloride feed gas enters at the top of the falling film absorber and flows co-currently with weak acid coming from the tails tower.  Heat of solution released as a result of the absorption process is removed by the cooling water flowing in the shell side of the falling film unit.  Thus absorption is effected at lower temperatures enabling a higher concentration of acid to be produced.

Product acid leaves at the bottom of the falling film absorber at a suitable temperature for storage and is ready for pumping.  

The remaining unabsorbed hydrogen chloride gas and any inert gases present leave at the bottom of the falling film unit and enter at the bottom of the tails tower.  Process feed water entering at the top of the tails tower absorbes the hydrogen chloride to form dilute hydrochloric acid.  The dilute acid flows by gravity to the top of the falling film absorber to serve as the liquid feed to the tubes.

Applications

1.       Types AFT 3/1, AFT 6/5, AFT 9/10, AFT 12/20

Production of. hydrochloric acid up to strengths of 33% w/w and

Production of hydrobromic acid up to strengths of 63% w/w by absorption from pure gas feeds.

Approximately 5% of the gas is unabsorbed in the falling film absorber and a tails tower is necessary to absorb this remaining quantity of pure gas.

2.       Types AFT 3/1.25, AFT 6/16, AFT 9/12, AFT 12/24 

Production of hydrochloric acid up to strengths of 38% w/w and

Production of hydrobromic acid up to strengths of 63% w/w by absorption from pure gas feeds. 

Approximately I 5 to 20% of the pure gas is unabsorbed and the tail tower has to be made correspondingly larger to absorb this increased throughput of pure gas.

top

Where special problems have to be met, the modular construction of Garg Process Systems plant enables absorbers to be designed which are different from those types previously mentioned. 

For example, if a hydrogen chloride absorber is used after a batch chlorination process, where a gas feed rate which decreases with time is to be processed to form hydrochloric acid.  It is advisable to use the system as shown in figures .

In this unit a specific quantity of water contained in a holding vessel is pumped through the column unit the desired acid concentration is reached.  

Using a recirculatory system, fully wetted packing and tubes can be maintained whilst the hydrogen chloride gas loading varies from 0 - 100% on the absorber. 

If the aim of the process is to obtain concentrations greater than 30 - 31 % which is the maximum which can be obtained in an adiabatic packed tower  absorber then the falling film absorber should be used, and hydrochloric acid strengths up to 38% can be achieved.

top

top