Droplet catcher

Since practically all separators are equipped with droplet catchers of different construction, let us consider them in more detail. 

The separator can be represented as consisting of two sections connected in tandem the settling section and the end droplet catcher section. 

The separation process in such a separator can be described as follows. The gas-liquid mixture enters the separator with a certain distribution of drop radii no(R), at the entrance (Fig. 19.1). In the settling section, characterized by a minimum drop radius Rm, all drops with radii R> R are separated. In a settling section of horizontal gravitational type, besides these drops, a part of the drops with sizes in the interval 0 < R< Rm (shown by the horizontal hatched area) is also separated from the gas. The gas-liquid mixture that remains after the removal of the large drops, proceeds to the end section, which is equipped with droplet catcher orifices with their own characteristic minimum radius, < Rm- As a result, drops having radii greater than this value are additionally removed from the distribution (vertical hatching) and at the exit of the separator only a small part of the distribution with 0 < R< R i remains. As will be shown later, the minimum drop radius does not necessarily exist for all types of orifices therefore, the introduction of R i is conditional. 

Fig. 19.3. A comparison with a similar dependence (see Fig 18.4) shows that a jalousie droplet catcher section increases the separator CE. It should be noted that when the flow velocity exceeds the critical value, this can lead to an ablation of the liquid film formed at jalousie walls, which would lower the separation efficiency.

The body of a string droplet catcher consists of a set of separating packages (sections), each of which is composed of rows of frames with stretched threads made from wire, nylon, etc. The threads are oriented parallel to gravity and perpendicular to gas flow. Frames and packages are placed in series. 

As before, we understand the coefficient of efficiency (CE) to be the ratio of the amount (volume) of the liquid phase caught by a droplet catcher, to the amount of the liquid phase at the entrance... 

The value of tj depends on gas velocity, volume concentration and dispersiveness of the liquid phase, geometrical size of the droplet catcher section, pressure, temperature etc. 

So, to determine the CE of a string droplet catcher section, it is sufficient to know the CE of a single frame K... 

If we have a gas-liquid mixture with the volume concentration of liquid Wo = 5 10 " m /m at the entrance of the string droplet catcher, then at the exit, we have Wi = 4 10 m /m. The dependence of t on gas velocity U is shown in Fig. 19.10. For the chosen values of parameters, the critical velocity, i.e. the velocity at which S = S r, is equal to Uc = 1.56 m/s. It means that when U < UcP, the efficiency of the droplet catcher section is practically equal to zero. Keep in mind that if drop have different radii, then the critical Stokes number will be determined by the average radius of drops. 

From (19.31), it follows that an increase in the flow velocity reduces the minimum radius of droplets that can be captured, and thereby increases the capture efficiency (CE) of the string droplet catcher. On the other hand, an increase in the flow velocity can destabilize the liquid film formed on the string surface, stripping drops away from the surface, and thus, reducing the CE. 

The number of strings rows chosen according to this inequality, will ensure the operation of string droplet catcher without ablation of droplets. 

Determination of the critical gas flow rate and the CE of a separator equipped with a string droplet catcher orifice requires a preliminary determination of gas-liquid flow parameters in the entire system, including the supply pipeline, the settling section of the separator, and the droplet catcher orifice - in the same way as it was done earlier for a separator with a centrifugal orifice. The outcome is the following pair expressions for the CE s of a vertical and a horizontal separator... 

Mesh droplet catcher orifices are made from hose knitted grid and placed in vertical and horizontal separators in the device cross-section. 

Each layer of the grid will be modeled by two frames with stretched strings located perpendicularly to each other. We can then use the results from the previous section. If all drops in the oncoming flow running into the mesh bundle have the same size, then the CE of the mesh droplet catcher is... 

Consider now the absorption extraction of heavy hydrocarbons under the conditions of counter-current flow in a column absorber presented schematically in Fig. 20.7. Gas of a given composition yo = (yoi, yo2, , yon), where yoi is the molar fraction of i-th component, with the flow rate Qgo enters the bottom part of the column. At the same time, an absorbent with composition xo = (xoi 5 02, , on) and flow rate qo enters the top part of the column. The number of contact stages is equal to N. Each stage is equipped with a perforated plate operating in the ablation regime. This means that the liquid is not collected on the plate, but exists in a dispersed state in the inter-plate space. Each contact stage contains a separation device, for example, a mesh droplet catcher, in which the exhausted absorbent is separated from the gas and directed toward the next plate. 

We assume, for simplicity, that the droplet catcher completely separates the liquid from the gas. 

---