How do I separate water from diesel?

In the production of diesel, the two immiscible liquids undergo separation in a decanter. Within the decanter, the feed is divided into an organic phase and an aqueous phase. The majority of diesel in the feed is found in the organic phase, while most of the water resides in the aqueous phase.

The separation of two liquid phases, whether they are immiscible or partially miscible, constitutes a common necessity in process industries. For instance, in the liquid-liquid extraction unit operation, a separation stage must follow the liquid contacting step. The separation of small quantities of entrained water from process streams is frequently required as well. The gravity settling tank, known as the decanter, is the simplest form of equipment used for separating liquid phases. Additionally, various proprietary equipment is utilized to encourage coalescence and enhance separation in challenging systems or where emulsions are prone to form. Centrifugal separators also find application in this context.

Decanters are employed for liquid separation when there exists a significant difference in density between the liquids, allowing droplets to settle easily. Essentially, decanters are tanks providing sufficient residence time for droplets of the dispersed phase to rise (or settle) to the interface between the phases and coalesce. In an operational decanter, three distinct zones or bands exist: clear heavy liquid, separating dispersed liquid (the dispersion zone), and clear light liquid.

Typically designed for continuous operation, decanters adhere to the same design principles for batch-operated units. While a variety of vessel shapes are utilized for decanters, a cylindrical vessel is usually suitable and cost-effective for most applications. The figure below illustrates typical design.

Control over the interface position can be achieved, with or without the use of instruments, by employing a syphon take-off for the heavy liquid. The height of the take-off can be determined by establishing a pressure balance. Disregarding friction loss in the pipes, the pressure exerted by the combined height of the heavy and light liquid in the vessel must be balanced by the height of the heavy liquid in the take-off leg:

(Z₁ - Z₃)ρ₁g + Z₃ρ₂g = Z₂ρ₂g

where

ρ₁  = density of the light liquid (here diesel), kg/m³,

ρ₂ = density of the heavy liquid (here water), kg/m³,

Z₁ = height from datum to light liquid (diesel) overflow, m,

Z₂ = height from datum to heavy liquid (water) overflow, m,

Z₃ = height from datum to the interface, m.

Accurate measurement of the liquid interface height is essential when liquid densities are close, one component is present only in small quantities, or the throughput is minimal. In cases where one phase is present in small amounts, it is often recycled to the decanter feed to ensure more stable operation.