Recovery of NGL

Natural Gas Liquids (NGL) can be valuable by-products when treating natural gas, and in some cases, recovery of liquids is the main source of revenue, particularly with “stranded” gas. Mackenzie Hydrocarbons has used the recovery method of low temperature treating (see Low Temperature Separators), which can be achieved by expanding the gas (Joule-Thomson effect), but this is not as efficient as using a refrigeration system. However, the refrigeration system is more expensive, for both capital cost and running costs. Mackenzie Hydrocarbons will make the assessment which method is the optimum on the basis of availability of the individual products in the feed stream, product prices versus capital cost and running costs.

In days gone by, adsorption (Silica Gel) was used, and this could be still of interest, particularly if the liquids are mostly C5 and above, but it is not always suitable for smaller plants, as capital costs can be relatively high, and operation can be difficult with the high temperatures required for the regeneration step. Mackenzie Hydrocarbons is prepared to consider the silica Gel adsorption process if it looks suitable. For heavy hydrocarbons recovery, the adsorption process is usually worked on a “short cycle”, which helps this technology be more competitive price-wise. The heater could be either salt bath with a fired U-tube heating the salt solution, or direct electric element heaters. These two options are more economic when looking at direct-fired heaters since these plants are usually mid-ranged with respect to capacity.

With recovery of natural gas liquids, care must be taken to avoid hydrate formation, hydrates being a mix of water ice with some C3, mainly C4s and possibly some C5s. The hydrates deposit quickly if the conditions are suitable, and can easily block the flow if allowed to propagate. An anti-freeze agent (often glycol) can be injected to depress the formation of hydrates. In some cases it is possible to recover the anti-freeze agent downstream, but more often than not, the anti-freeze agent is swept away in the gas stream.

A key issue in recovery of the liquids is separating the LPG (C3s and C4s) from the heavier hydrocarbons. Once, the heavy cut of natural gas liquid was often referred to as “natural gasoline”, but we would not recommend that for automobile use in modern day vehicles.