DEHYDRATION BY SOLID BEDS
This plant employs the same adsorbent materials used in the short cycle solid bed plant.
The only difference between the two plants is the operating time. As a matter of fact, using the sova-bead as an adsorbent for a short time, we find that it adsorbs mainly hydrocarbons vapors (STRIPPING) on the contrary with the absorption time increasing, the sova-bead mainly segregates the water vapors, which will remove the drops of hydrocarbons that had been previously absorbed (DEHYDRATING).
Scheme and Functioning
In addition to water vapor, this kind of plant can also remove traces of hydrocarbon vapors such as C4, C5 and C6.
The equipment used for a complete gas dehydration plant of solid absorbent is generally composed of:
– 2 adsorbers
– 1 heater
– 1 scrubber
– 1 heat exchanger
– various valves
In order to dehydrate a gas, the solid bed doesn’t have uniform behaviour. The area which is first crossed by the gas adsorb much more humidity than the other bed areas. Therefore, this area is subjected to a quick saturation.
In such conditions, there’s a “saturation front” that starts from the area next to the gas inlet point and follows the same direction of the gas, gradually involving the whole volume of the drying material.
It is for this reasons that when it starts, the dehydrating process shows a high activity and a high adsorbing velocity.
Later, as the dehydrating power decreases, the activity of the desiccant shows a reduction in activity, up to total exhaustion when the saturation front reaches the final area of the adsorbing bed.
After a sudden decrease due to the operations starting, the dew point of the gas coming out of the separator slowly increases. The adsorbing capacity of the bed decreases with time until it is completely saturated and does not
remove any more humidity. Should the operator fail to cut the flow to the absorber before this point, the dew point of the outgoing gas would reach the same value of the ingoing gas; signalling that the bed is saturated.
The operator should stop the flow to the unit before reaching saturation, by knowing the time it takes to saturate and/or by humidity measurements of the gas leaving the bed.
Once the bed is off process, the silica gel has to be regenerated. The regeneration of the drying material is obtained by passing into the adsorber, heated gas with a higher temperature than that of water evaporation.
(200°C). The high temperature causes the evaporation of the adsorbed water, which is carried away, together with the regenerating gas, outside the adsorber and sent to the heat exchanger and then on to the separator,
where it condensates and is separated.
The quantity of gas used in a regeneration cycle is usually about 5 to 15% of the total gas coming from the dehydrating plant and it is obtained from the main cycle, at the outlet point of the free water separator.
To avoid any interruption of the gas flow, the process must be performed by means of double contact equipment. While one of the containers with the dehydrating means operates in the active adsorbing phase, the other is cut
off from the main gas circuit to allow both the regeneration process and the cooling of the exhausted adsorbent.
The purpose of the cooling process is to allow good gas dehydration during the initial phase of the adsorbing cycle.
This alternation in the adsorbing and regeneration cycles to (which the dehydrating material is subjected), is one of the main causes for its efficiency reduction over the time. This phenomenon is called “ageing”.