Air Stripping for Treatment of Produced Water

Summary

In a laboratory study, air stripping shows a promising potential for treatment of produced water to meet new government regulations on total organic carbon (TOC). Reservoir hydrocarbons dissolved in water, such as volatile paraffins and aromatics, can be removed by air stripping through interphase mass transfer. However, air stripping cannot remove many chemicals added to crude oil by the operator.

Introduction

In the production of oil and gas, a large amount of produced water is generated that must be treated and disposed of to meet government requirements. It is interesting to observe the recent use of TOC in specifying the discharge quality, in addition to the conventional term "oil and grease." A recent survey shows that oil skimming, gas flotation, or both are used to treat produced water. The TOC in treated produced water varies greatly, depending on the treatments used. The TOC in produced water in some operations is reported to range from 500 to 2,000 ppm. The Environmental Protection Agency (EPA) verification study for 30 platforms in the Gulf of Mexico shows that benzene, toluene, ethylbenzene, and naphthalene are in the ranges of 0.002 to 12.15, 0.06 to 19.8, 0.006 to 6.01, and 0.19 to 1.45 ppm, respectively. These organic compounds make up part of the TOC in produced water.

The TOC includes oil and grease, as well as dissolved organic compounds. Therefore, the use of TOC in environmental regulations is a significant extension because the facility already in operation can easily remove oil and grease but not dissolved organic compounds. Recently, in a discharge permit for an operation in wetland (swampy marsh areas of southern Louisiana), the daily allowable maximum of TOC in the produced water discharge was reduced to 50 mg/L (50 ppm), which is far less than the average TOC of that particular facility and pushing close to the limit of current treatment technology.

To remove dissolved organic compounds is a new game for the oil and gas industry. Activated carbon adsorption was tried and not found cost-effective. Because the volume of produced water is large and the ratio of adsorbate to carbon is low, usually in the range of 1 to 2 %, the carbon adsorption process will require a large amount of activated carbon at about 60cts/45 kg [60cts/100 lbm] and frequent carbon regeneration. Therefore, air stripping, an alternative technology, is studied.

Organic Compound Characteristics

Organic Compound Sources. Two sources of organic compounds in produced water are (1) those extracted from crude oil and (2) those added by operators to help production or to protect equipment, such as antifreeze, biocides, and corrosion inhibitors.

Crude oil contains paraffins, naphthenes, and aromatics, some of which have moderate solubility in water, as shown in Table 1. Crude oil and water have good contact in the reservoir and the production tubing during their upward flow. As a result, some of these compounds that have moderate solubility will dissolve in water. For example, when water is saturated with benzene, 1,750 ppm of benzene in water is observed. In the case of hexane, it is 140 ppm. These organic compounds that can dissolve in water are low-molecular-weight, nonelectrolytic, and volatile compounds. These characteristics give the basis for consideration of air stripping.