Natural gas as obtained from the well contains water vapor. Before the gas can be passed to the pipeline, it must be dried to prevent problems such as hydrate or ice formation or corrosion.
Glycol dehydration, which is simple and inexpensive, is currently the most widely used method of dehydrating natural gas. The estimated number of glycol dehydration systems operating in the United States alone is at least 55,000.
In a typical dehydrator, wet gas is scrubbed with dry glycol to yield a product gas of lowered water dew point. The glycol absorbs not only water, but also aromatic compounds, such as BTEX, and other hydrocarbon vapors.
The water-rich or “spent” glycol is passed to a regeneration system, which typically includes a flash tank, where methane and other light gases are flashed off, a reboiler and a regeneration still column. The water-laden glycol is heated to drive off absorbed water, and dry glycol is recovered for reuse in the absorber. Unfortunately, this heating also vaporizes hydrocarbons that have been sorbed into the glycol, and these are expelled, along with the water vapor, in the hot overhead vent stream from the still column.
Although the major component is steam, this overhead vent stream may contain as much as 20 mol % or more of organic compounds, including aromatic and non-aromatic organic vapors. Of these organic compounds, a significant proportion maybe the aromatic compounds benzene, toluene, ethylbenzene and xylene, together commonly known as BTEX compounds. These organic emissions are now classified as Hazardous Air Pollutants (HAPs), and are subject to emissions regulations both in the United States and internationally.
There is, therefore, a need for a simple, reliable and cost-effective method to reduce or eliminate the release of these organic components.
So-called “enhanced” regeneration systems have been used to increase water removal from the spent glycol, and some of these systems can reduce HAP emissions. For example, U.S. Pat. No. 5,141,536, to Texaco, describes an apparatus with a condenser unit in the reboiler overhead vent to condense hydrocarbon components and some water, thus venting a hydrocarbon-depleted water vapor stream. U.S. Pat. No. 5,234,552 describes a condensation system to recover liquid hydrocarbons and collect remaining hydrocarbon vapors for use as fuel for the glycol reboiler. U.S. Pat. No. 5,788,745, to Phillips Petroleum, describes the use of both a condenser and a phase separator to recover liquid hydrocarbons.
Another technique for glycol regeneration is stripping against a gas or steam. A typical configuration for a stripper is a packed or multi-tray tower, with the stripping gas normally flowing upward countercurrent to the descending liquid glycol. Depending on the stripping agent used, water, hydrocarbons, or both are absorbed from the glycol into the stripping gas, thus regenerating the glycol for reuse in dehydrating the natural gas.
U.S. Pat. No. 5,643,421, to OPC Engineering, describes a glycol regeneration process comprising flash evaporation to recover hydrocarbon components from the spent glycol, followed by heating and multiple stripping steps using a vaporized solvent as the stripping agent to dehydrate the glycol. U.S. Pat. No. 5,766,423, also to OPC Engineering, uses basically the same process, but includes condensation of a portion of the once-stripped glycol stream for additional removal of hydrocarbons.
U.S. Pat. No. 5,490,873, to Bryan Research and Engineering, combines condensation of the reboiler overhead with further treatment of the lean glycol stream by stripping against a sidestream of the dehydrated natural gas. U.S. Pat. No. 5,453,114, to Ebeling, uses a stripping step to remove hydrocarbons upstream of the glycol reboiler. U.S. Pat. No. 5,209,762, to Gas Research Institute, uses an integrated combination of condensation and steam stripping to produce dischargeable water, organic liquid and an organic vent gas that may be useful as fuel.
U.S. Pat. No. 5,725,636, also to Gas Research Institute, adds potassium salts to the glycol to increase water absorption and decrease hydrocarbon absorption in the natural gas dehydration step. Since hydrocarbons are less readily absorbed, the glycol has greater water-absorbing capacity. The regeneration process also produces a leaner glycol stream and the water vapor vented to the atmosphere has a lower hydrocarbon content.
These and other processes can achieve low water concentrations in the glycol, which in turn provides good dehydration of the raw gas, resulting in low dew points in the treated gas. They can also reduce, but do not eliminate, BTEX and other HAP air emissions through the additional treatment step. However, most of these processes produce an additional gaseous or aqueous waste stream that requires on-site or off-site attention such as incineration, disposal, or further treatment.
Despite these efforts, a cost-effective regeneration technology that truly minimizes or eliminates HAP emissions has not been developed. There yet remains a need for such a process.
Membranes have for many years been known to be usable in pervaporation mode for dehydrating organic liquids. For example, many patents to GFT and others, such as U.S. Pat. Nos. 3,720,717, 4,405,409 and 4,755,299, describe such separations using polyvinyl alcohol (PVA) membranes.
A large number of patents to Texaco also disclose pervaporation processes using membranes of various types for dehydrating organic liquids, such as organic acids. In particular, U.S. Pat. No. 4,802,988 describes PVA membranes and processes for treating ethylene glycol/water solutions to reduce the ethylene glycol content in the permeate to low levels, and U.S. Pat. No. 5,182,022 describes a similar process using ion-exchange type sulfonated polyethylene membranes to produce a low-glycol permeate.
U.S. Pat. No. 5,552,023, to Allied Signal, describes a process to treat spent deicing fluid (ethylene glycol mixture) using a combination of membrane distillation through a porous polytetrafluoroethylene membrane and reverse osmosis. U.S. Pat. No. 5,554,286, to Mitsui Engineering, describes an A-zeolite-type membrane for pervaporation applications, and includes sample data for dehydration of alcohols.
U.S. Pat. No. 5,350,519, to Membrane Technology and Research, describes a process including a condenser to condense the glycol reboiler overhead vent stream and recover liquid hydrocarbon components, followed by a pervaporation step of the waste aqueous stream to recover additional hydrocarbon components and vent a hydrocarbon-depleted water stream.