Journal of Natural Gas Chemistry 17(2008)249–255 Predicting hydrate forming pressure of pure alkanes in the presence of inhibitors Alireza Bahadori 1∗ , Hari B. Vuthaluru 1 , Saeid Mokhatab 2 , Moses O. Tade 1 1. Department of Chemical Engineering, Curtin University of Technology, GPO Box 1987, Perth, WA 6845, Australia; 2. Process Technology Department, Tehran Raymand Consulting Engineers, Tehran, Iran [ Manuscript received January 21, 2008; revised February 22, 2008 ] Abstract: An inherent problem with natural gas production or transmission is the formation of gas hydrates, which can lead to safety hazards for production/transportation systems, and substantial economic risks. Hydrate inhibition with different inhibitors such as, methanol, ethylene glycol (EG), triethylene glycol (TEG), and sodium chloride solution continues to play a critical role in many operations. An understanding of when the hydrates form in the presence of these hydrate inhibitors, is therefore necessary to overcome hydrate problems. Several thermodynamic models have been proposed for predicting the hydrate formation conditions in aqueous solutions containing methanol/glycols and electrolytes. However, available models have limitations that include the types of liquid, compositions of fluids, and inhibitors used. The aim of this study is to develop a simple-to-use correlation for accurate prediction of hydrate-forming pressures of pure alkanes in the presence of different hydrate inhibitors, where the obtained results illustrate good agreement with the reported experimental data. Key words: gas hydrates; pure alkanes; methanol; ethylene glycol; triethylene glycol; sodium chloride 1. Introduction A gas hydrate is an ice-like crystalline solid called a clathrate, which occurs when water molecules form a cage- like structure around smaller guest molecules. The most com- mon guest molecules are methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide, and hydrogen sulfide [1,2]. It should be noted that normal butane does form a hy- drate, but it is very unstable [3]. It has been assumed that nor- mal paraffin molecules, larger than normal butane, are nonhy- drate formers [4]. Although, many factors influence hydrate formation, the two major conditions that promote hydrate for- mations are (1) the gas being at the appropriate temperature and pressure, and (2) the gas being at or below its water dew point. Other factors that affect hydrate formation include mix- ing, kinetics, type of the physical site, surface for crystal for- mation, agglomeration, and the salinity of the system [5]. Strategies for hydrate mitigation and prevention often modify one or more of the essential elements of hydrate for- mation to destabilize the hydrate and thus remove the prob- lem. Gas hydrate formation can be prevented by several meth- ods. The permanent solution is removal of water prior to pipeline transportation, such as, using an offshore dehydra- tion plant or subsea separation, which is not often the most cost-effective solution. Another way to prevent hydrate plugs is to maintain the pressure and temperature conditions outside the hydrate formation region. The primary practical means of avoiding hydrate formation, is to rely on the idea of pre- venting the hydrate envelop and temperature/pressure produc- tion facilities profile from crossing each other during normal production [6]. This is done by either pushing the hydrate envelope to the left, using thermodynamic inhibitors, either regular, for example, methanol or glycols, or by shrinking the temperature/pressure production facilities profile to the right by insulating and/or heating the flow line [7]. Chemical inhibitors are injected at the wellhead and pre- vent hydrate formation by depressing the hydrate tempera- ture below that of the pipeline operating temperature. This method is expensive if water production is significant. For most oil production systems this cost is prohibitively expen- sive, whereas, it can be the least expensive alternative for gas systems. Hydrate inhibition using chemical inhibitors is still the most widely used method, and the development of alter- native, cost-effective, and environmentally acceptable hydrate inhibitors is a technological challenge for the oil and gas pro- duction industry [7] Traditionally, the most common practical approach to pre- vent hydrate formation in gas production systems has been the addition of massive amounts of methanol or glycols (usu- ally ethylene glycol and triethylene glycol) to the gas/water stream. These chemicals are called “thermodynamic in- hibitors” and have the effect of shifting the hydrate forma- ∗ Corresponding author. E-mail: alireza.bahadori@postgrad.curtin.edu.au