Predicting the Capabilities of Glycol Dehydrators

Abstract

Gas dehydration requirements vary, depending upon processing pressure, temperature and water vapor content. Because there is variation, also, in the capacities and capabilities of glycol dehydrators, it is important to match the equipment to the application. Introduction The glycol absorption process is used extensively in dehydrating high-pressure natural gas to pipeline specifications at the wellhead, on gathering and transmission systems, and from gas storage fields. Since its inception 20 years ago, this process has been developed to a remarkable degree and its capability increased. The performance of a dehydrator is measured by its ability to depress the water vapor dewpoint of the feed gas this depression being the difference between inlet gas temperature and the outlet gas dewpoint temperature, expressed in degrees Fahrenheit. Twenty years ago a 60 degrees F dewpoint depression was considered exceptionally good, but today this dehydration process can be designed and operated to attain 150 degrees F dewpoint depressions or more. Performance of this magnitude is often necessary where Performance of this magnitude is often necessary where gas is compressed to pipeline pressure and air-cooled before dehydration. Not all dehydrators have this capability, but on the other hand not all applications require such high dewpoint depressions. The problem is to assess the requirements of the application and to select a dehydrator with the correct capability. The purpose here is to present and illustrate some criteria for selecting and operating glycol absorption dehydrators. Description of the Process This is a classic example of a simple absorption process that removes and rejects one component process that removes and rejects one component (water) from a gas mixture. Referring to the flow diagram of Fig. 1, we see that water-saturated gas from an effective inlet scrubber enters the lower side of a vertical absorber and flows upward through multiple trays, or packing, where it comes in counterflow contact with a descending stream of a solution of triethylene glycol (TEG) and water, introduced in highly-concentrated form on the top tray. Because of large differences in concentration and the strong affinity of TEG for water, the water transfers from the vapor to the liquid. In this counter-flow operation, dry gas issues from the top of the absorber and diluted TEG flows from the base of the column by way of a filter to power the glycol solution pump. The small amount of free gas required to help power this pump is separated in a scrubber operated under a 20- to 30-psig backpressure and is discharged to the fuel or stripping gas systems. Gas-free TEG flows from this vessel to a heat exchanger, where it picks up heat from the stream of reconcentrated TEG and flows to the still inlet. Here the water is distilled overhead in a trayed or packed column to minimize the TEG vapor losses. Distillation heat is supplied through a reboiler, which is often direct-fired but which may be heated by other meda. Unless this is a vacuum still, this part of the process takes place at atmospheric pressure, and the temperature maintained in the pressure, and the temperature maintained in the reboiler establishes the concentration of the TEG that is recirculated from this point. Reboiler temperature may range from 375 degrees to 425 degrees F. JPT P. 925