CALCULATIONS ESTIMATE PROCESS STREAM DEPOSITIONS

Yiing Mei Wu
Mobil Research & Development Corp.
Paulsboro, N.J.

A calculation method has been developed to estimate the conditions and extent of ammonium chloride and ammonium hydrosulfide depositions in refinery process streams containing ammonia, hydrogen chloride, and hydrogen sulfide impurities.

Corrosion caused by ammonium chloride (NH4Cl) and ammonium hydrosulfide (NH4HS) has long been a problem in the refining industry. Refining units that can be affected by underdeposit corrosion, or by plugging as a result of salt deposits, include the crude overhead s),stem, hydrocracker, catalytic reformer pretreater, and hydrodesulfurization units.

These units usually process streams containing sulfur and nitrogen compounds, a portion of which will be converted to, respectively, hydrogen sulfide and ammonia. Another impurity hydrogen chloride can be produced by hydrolysis of calcium and magnesium chlorides or by hydrogenation of organic chlorides.

Because salt deposition is a function of feedstock impurity, process temperature, and pressure, it is beneficial to be able to evaluate deposition propensity deductively for each susceptible stream. The evaluation should predict:

Where, or at what temperature, the salt starts to deposit
The kind of salt that deposits
The approximate amount of the depositions.

If salt deposition is indicated or predicted, several preventive measures can be considered to minimize any deposit related damage. These measures include:

Inspecting affected equipment more frequently (i.e.,
equipment downstream of the salt depositions)
Changing to a cleaner, less susceptible feed
Installing a water washing operation to remove the
deposits.

For the third option, the total amount of deposition and the location of the first deposits are important process parameters. Enough wash water should be injected upstream of the first deposits to dissolve all the accumulated deposits.

EQUATIONS

Ammonium chloride and ammonium hydrosulfide deposits are formed in the vapor phase by the following reactions:

NH4CL(S) = NH3(G) + HCL(G) (1)

NH4HS(S) = NH3(G) + H2S(G) (2)

Depositions start when the vapor pressures of the reacting gases exceed certain values. Numerous researchers have measured or calculated those threshold pressures in an attempt to predict the deposition tendency.1 7

The most reliable of these data will be presented and used to estimate the conditions and extent of those deposits. Note that this approach is purely thermodynamic. The important kinetic aspects, such as flow patterns and residence time, are beyond the scope of this work.

Most of the data used are based on ideal conditions; that is, no interaction between other species is taken into consideration. This can be justified because the reactions occur in the gas phase; thus the interactive force between gas molecules should be small. One should not, however, exclude the possibility of such interactions.

THERMODYNAMICS

The equilibrium constants for Reactions 1 and 2 can be written as:

K1 = PNH3 X PHCl

K2 = PNH3 X PH2S

where: PNH3, PHCl, and PH2S are the partial pressures of, respectively, NH3, HCl, and H2S in the vapor phase (see Nomenclature).

Figs. 1 and 2 show the variation of K1 and K2 with temperature. If the product of the vapor pressures exceeds the corresponding equilibrium constant at the same temperature (for example, Point A in Fig. 1) NH4Cl Will precipitate until the vapor pressure product decreases to its equilibrium value (Point B in Fig. 1).

In estimating these depositions, it is the product of the vapor pressures that matters. Deposition will occur even when the stream has a small amount of HCl, as long as the NH3 partial pressure is high enough, and vice versa.

Another important observation from Figs. 1 and 2 is that the deposition tendency of NH4Cl is much higher than that of NH4HS. For example, when the temperature is 120 F., a stream with an NH3/HCl pressure product of 10 10 psia2 will precipitate NH4Cl, while a stream with the same pressure product of NH3 and H2S Will not precipitate NH4HS.

DEPOSITION TENDENCY

The first step in determining whether deposition will occur is to do an isothermal flash calculation at the temperature in question. The vapor pressure product of NH3 and HCl and, if appropriate, NH3 and H2S is then compared to the corresponding equilibrium value.

If the vapor pressure product so calculated lies below the equilibrium curve (in other words, in the region where the vapors are the stable phases), this procedure is repeated with a new, lower temperature. Because the equilibrium Kp values for both salts decrease as temperature decreases, lowering the stream temperature will introduce the onset of the salt deposition if the impurity concentration is high enough.

Once the stream temperature is so low that the calculated point is just above or on the curve, that temperature is defined as the deposition starting temperature. Ammonium chloride, ammonium hydrosulfide, or both, will deposit out of the vapor phase, thereupon bringing the pressure product back to the equilibrium value.

EXTENT OF DEPOSITION

To calculate the amount of deposition formed, a stepwise approach is used.

Theoretically, when the stream cools gradually from the deposition starting temperature, the system will undergo a continuous deposition process with infinitesimal changes in concentrations and temperature each time, so that the equilibrium conditions are always satisfied.

This continuous deposition can be illustrated by the thick curve in Fig. 3. In this process, the stream drops out whatever amount of NH4Cl is necessary to follow the equilibrium curve once the temperature is below the deposition starting temperature.

In reality, temperature changes are not infinitesimal. Supersaturation is a common phenomenon. A stepwise decrease in temperature in the calculation therefore is employed.

As shown in the steps in Fig. 3, NH4Cl is not formed until the system overshoots by 20 F. from the deposition starting temperature. After depositing out a certain amount of NH4Cl, the system is back in equilibrium. Then the next overshooting begins.

This procedure is repeated until the temperature reaches the end point (usually the water dew point). The total amount of deposition is the sum of the salt formed in each step.

ALGORITHM DESCRIPTION

The algorithm used to accomplish the salt deposition calculations is shown in Fig. 4.

Using the necessary stream data (composition, temperature, and pressure), the isothermal flash temperature and pressure are determined using any process simulation software (OGJ, Jan. 14, 1991, p. 55). The partial pressures of NH3, HCl, and H2S are then calculated using Equations 1 3. The equilibrium constants K1 and K2 are calculated using Equations 4 and 5.

Once these values have been determined, one of the following four cases is possible: No deposition, only NH4Cl deposition, only NH4HS deposition, or both NH4Cl and NH4HS deposition. Except for ?he first case, the amount of deposition will be calculated using Equations6 8.

The stream composition of NH3, HCl, and H2S will be adjusted accordingly to account for the loss of concentration attributable to solid deposition.

The deposition starting temperature will be recorded. The temperature is then reduced by a predetermined, arbitrary increment and the calculation is repeated at the new temperature. This process stops when the temperature reaches the minimum (usually water dew point or boot temperature).

The amount of deposition (delta m or delta n) can be reported as a function of temperature or as a sum over the temperature range from starting deposition temperature to Tmin.

SAMPLE PROBLEM

A process stream at 361 F. and 430 psia is cooled after passing through the tube side of a bank of heat exchangers. The outlet temperature and pressure are, respectively, 225 F. and 420 psia. The stream composition is shown in Table 1.

Fig. 5 shows the changes in NH3/HCl pressure product of the sample system as the temperature decreases. The data used to produce Fig. 5 are the results of isothermal flash calculations from 361 F. to the water dew point temperature, which is about 160 F.

The NH3H2S pressure product was too small for NH4HS formation, as is shown in Fig. 6, so only NH4Cl deposition was considered.

The pressure product first crosses the deposition curve at 300 F. As the temperature continues to decrease, the vapor phase becomes supersaturated with NH3 and HCl. Ammonium chloride therefore deposits out between 300 F. and 160 F.

The deposition rate at each temperature is shown in Fig. 7. Note that Fig. 7 is not a cumulative plot.

The amount of deposits was initially high, but as the temperature decreased gradually (from 300 F. to 160 F. in 10 increments), less and less NH4Cl deposited out. The total amount of deposits in this sample problem (delta n, as calculated by Equation 6) is about 9.1981 x 10 3 lb-moles/hr.

REFERENCES

Thermodynamic Research Center Thermodynamic Tables, Vol. 5, Texas A&M University System, College Station, Tex.
Kirk, R.E., et al., Kirk Othmer Consise Encyclopedia of Chemical Technology, 3rd ed., John Wiley & Sons, New York, 1985.
Isambert, F., Comptes rendus, Vol. 92, 1981, pp. 919 77.
Lin, Y.R., and Crynes, B.L., OGJ, Nov. 28, 1983, p. 111.
Ehmke, E.F., Materials Performance, Vol. 14, No. 7, 1975, pp. 20 28.
U.S. Bureau of Mines, Bulletin 406, 1937, p. 56.
Wayaku, M., Aromatikkusu, Vol. 34, Sept. 10, 1982, pp. 219 31.