Vapor-phase inhibitor concentration

In order to assure control of the reaction, the vapor-phase inhibitor concentration must be closely controlled in the ppm range. Although several compounds have been claimed to be useful, it is likely that commercial processes use only ethylene dichloride or some of the simpler chlorinated aromatics (102). In general, the choice between inhibitors is not based on their differences in performance, but rather on the designers preference for dealing with the type of control problems each inhibitor system imposes (102). 

An inhibitor is a chemical substance that, when added in small concentration to an environment, effectively decreases the corrosion rate. There are several classes of inhibitors, conveniently designated as follows (1) passivators, (2) organic inhibitors, including slushing compounds and pickling inhibitors, and (3) vapor-phase inhibitors. 

The per pass ethylene conversion in the primary reactors is maintained at 20—30% in order to ensure catalyst selectivities of 70—80%. Vapor-phase oxidation inhibitors such as ethylene dichloride or vinyl chloride or other halogenated compounds are added to the inlet of the reactors in ppm concentrations to retard carbon dioxide formation (107,120,121). The process stream exiting the reactor may contain 1—3 mol % ethylene oxide. This hot effluent gas is then cooled ia a shell-and-tube heat exchanger to around 35—40°C by usiag the cold recycle reactor feed stream gas from the primary absorber. The cooled cmde product gas is then compressed ia a centrifugal blower before entering the primary absorber. 

Existing methods of technological calculations of the inhibition process [65] are based on the assumption that there exists a thermodynamic balance between liquid (inhibitor) and gas (natural gas) phases. Application of this method allows to determine equilibrium values of concentration of water vapor and inhibitor in a gas at given values of pressure, temperature, inhibitor s mass concentration in the solution, composition of gas, and specific flow rate of inhibitor required for given temperature decrease of hydrate formation ... 

Metals exposed to humid atmosphere corrode by an electrochemical mechanism due to the formation of a thin electrolyte layer on the metal surface (Chapter 3.1, this volume). This type of corrosion can be controlled by Vapor-phase Corrosion Inhibitors (VCIs), that is, volatile inhibiting substances that allow vapor-phase transport to the corroding surface (examples are amines, benzoates, imidazoles, or triazoles [3]). The vapor pressure should be sufficiently high to ensure a protective surface concentration of the inhibitor, but low enough to prevent premature depletion of... 

There is a very wide range of inhibitors available to stabilize MDC, but it is best to avoid aluminium as a material of construction when MDC may be processed or stored. Because of MDC s volatility there is always a danger that it will distil away from a less volatile inhibitor and lose its protection. Low-boihng inhibitors such as te/t-butylamine, propylene oxide and amylene (2-methylbut-2-ene) tend to stay with MDC when it is vaporized whereas dioxane, ethanol, THF, N-methyhnorpholine and cyclohexane tend to remain in the liquid phase. The concentrations required for effective inhibition are low (50 ppm to 0.2%) and, if MDC is used as a reaction medium, it is almost always possible to find one that does not become involved in the reaction itself. 

The phase equilibria data for hydrates with inhibitors are presented below. As in previous results, data plots are provided for those systems which have been considered by more than one investigation, as a first order means of data evaluation. Individual investigators usually include plots of their data in the original reference. Unless otherwise indicated the mass concentration of the inhibitor in the aqueous phase is included in the column marked wt%, hydrocarbon/CC>2/H2S/N2 concentrations are mol% in vapor unless otherwise indicated. 

Consider a spherical liquid drop in a boundless gas medium. The liquid phase represents a solution consisting of inhibitor-water solution of methanol. Denote molar fractions of water and methanol in a drop through Xw and Xm- The gas phase represents multi-component mixture including natural gas with molar concentration of components yi, as well as water and methanol vapor y and ym, respectively. At the initial moment of time t = 0 drop and gas temperatures Tjx) and Tco, molar concentration of liquid x o, Xmo and gas yjo, y o, fmo phases, and drop radius Ro are specified. 

The system of equations (21.19)-(21.21) is solved numerically. As a result, the changes in time of water and methanol concentrations in a drop, temperature and radius of drops are determined. Thermo-physical properties of gas and liquid phases involved in equations can be determined by methods given in [9]. Calculations were carried out for various pressures, initial temperatures of the drop and gas, and initial concentrations of methanol in the inhibitor solution. The composition of gas used in calculations depends on p, T and should be determined in advance from the equations of vapor-liquid equilibrium. Thus, for p = 8 MPa and T = 313 °K, the following composition is obtained (molar fractions) N2 = 0.81 CO2 = 0.22 CH4 = 96.97 CzHg = 1.74 CjHg = 0.16 i - C4 = 0.07 n - C4 = 0.03. 

---