Control corrosion in washwater systemsause equipment failures

Intermittent or continuous water iujection?Watercools to the salt-deposition temperature. Unforthe resulting aqueous solution can be very corunits are operating at rates above may be a good choice. If continuous washwater is not used, the NH4C1 andlor NH4HS may build up in the reactor pressure drop and lower heat transfer. These dry salts are not considered as very corrosive. When an intermittent'.

hydroprocessing reactors by converting feed. Hydrochloric acid can enter the unit

Where should the water be injected? Wakr should be injected into the reactor effluent upstream of where the effluent is cooled down to the deposition temperature.

, ,

If chlorides are not a problem, the normal injection

Fig. 2. Typical reactor effluent.configuration with hot separator.

Fig.. 3. Common washwater recyc

point is just before the effluent air cooler (Points A and C in Figs. 1,2). Typically, the effluent is cooled to 275OF to 400F'. This temperature may be even higher if a hot separator is used. The injection point should always be downstream of the hot separator. Some engineers like to design an intermittent injection ahead of the last exchanger before the air cooler (Points B and D in Figs. 1,2). However, this method can impact the water rate required ifthe system is designed for a higher injection temperature. If parallel exchangers are used before the air cooler (Point B in Fig. I), exchangers czin be cleaned intermittently without increasing the design washwater rate. Process conditions. After the washwater injection, the process stream is in three phases: vapor, liquid hydrocarbon and an aqueous phase. Normally, the vapor phase will be 70% to 90% of the total stream volume. With a high-pressure hot separator configuration, only a small amount of hydrocarbon liquid is present. If there is no hot separator, then the stream (typically 60%-85%) will be mostly hydrocarbon liquid by weight. Hydrogen sulfide by itself is not very soluble in water; NH3is much more soluble. When large quantities of N & l and H2S are present in the vapor phase and water is introduced, t h e NH3 dissolved in the water causes a n electrolytic reaction and an approximately equivalent number of moles of H2S a s moles of NH3 will be absorbed into the aqueous phase. Be careful when using simulation software to calculate sour water composition, because not all simulation programs have correlations that correctly model this relationship. ' \ have excess aqueous water, the hydrocarbon liquid I, and vapor must be saturated with water. Normally, only a small quantity of water will be dissolved in the hydrocarbon liquid. 9% have free water, calculate how much water is required to saturate the vapor phase. This calculation can be done by simulation. Alternatively, the Appendix shows how to estimate saturating the vapor phase by water injection.Mditional design guidelines. Construction materials

since it has the highest likelihood of maldi water, plugging or local high-velocity proble Washwater source. The two sources use water are steam condensate and stripped sour w refiners prefer steam condensate. Stripped so often recommended because i t contains no diss gen. However, most refiners elect to use s t sate due to its availability and reliability. Ca taken to ensure that the washwater is oxyg than 15 ppm to 50 ppm). Most licensors spec washwater is also free of cyanides.

engineers defined the NH4HS concentration by measuring the total concentration of NHs the water and summing them. Others have rep lines based solely on the NH3 content. Many studies indicate that carbon steel c related to the NH4HS concentration. Most lic industry experts have threshold guidelines for,

cient washwater is injected and some wat the aqueous phase, then the resulting aqueo

for the effluent system after the water injection point is a n economic decision. The cost of alloy systems have a trade-off in reduced inspection costs and reduced risk. Materials are often selected based on the perceived severity of expected NH4HS corrosion and some of the industry guidelines are dependent on the material Downstream of the washwater injection, modern designs use either carbon steel, Alloy 825 (UNSN08825), or Alloy 2205 (UNS S31803) materials for exchangers.-

Superficial fluid velocity. I t is well acc the superficial, fluid velocity affects the rate 0 solution corrosion. The normal velocity for materials is 20 ft/s maximum. For alloy mat is normally used, but this is considered a les guideline. The actual threshold corrosion vel alloys has not been determined. In air cooler is often used as the minimum velocity to ens tribution. Normally, minimum velocity is no since 20 ft/s is a very low velocity for these rn streams. Superficial velocity for a multiphase be easily calculated by taking the total stream flow and dividing by the cross-sectional area. : Piping design. The goal of the washwater,-

88

KYDROCARBON PROCESSING / JUNE 1997

&rve showing simulated NH4HSconcentration with tem-

air cooler i s to get an even distribufluid and the washwater through each air e. To accomplish this goal, these design guidelines distribution piping-commonly called

Fig. 5. Nomograph relating NH4CLdepositiontemperatures with varying CI-concentrations.

water i

of the two guidelines is controllingfor a given application, since it determines which process variables are most critical and what design alternatives are permitted. If the NH4HS concentration is controlling, the only way k ~ a r a t flow orifice for each water iniection ~ o i n t . to decrease the amount of required water is to reduce the e NH3 content in the reactor effluent. This is not normally done, since it would require significant modification to &high salt concentration could build up. ~ h i s j s the unit or reactor conditions. Operating engineers must &y important in units using an intermittent water be aware of the feed nitrogen levels. Changing from design &ce the salt concentration during washing will to higher-nitrogen feeds should not be done without conBy be very high. sidering potential corrosion implications. b of thumb. The washwater rate is ~ r o ~ o r t i o n a l If the second specification is controlling, refiners can optimize the process condition at the injection point and the washwater rate. For example, depending on the calculated NH4HS deposition temperature, it may be possible to decrease temperature a t the injection point, thereby &ed solely by a "rule of thumb" basis. decreasing the washwater requirement. Another option may be to install a washwater recycle pump, taking some K," factor. In the late 60s and early 70s. R. separator water and pumping it back to the injection point. This requires installing a high-pressure, low-head pump (Fig. 3). The concentration of the separator water will remain unchanged, but significantlymore water is available at the injection point to saturate the vapor and ensure gon charts).-The "Piehl KPnfactor is: an aqueous phase is present. This scheme decreases the ! I amount of make-up water, and reduces the quantity of = (mol% NH3 in reactor effluent) X (mol % produced sour water. Both reductions are benefits. : H2S in reactor effluent) (1) Process conditions at water injection point. There are two theories about the NJ&HS solution conditions at 1's proposed guidelines, Kp less than ervice, above 0.15 is severe service and the injection point and in the downstream heat transfer equipment: severe service. At the injection poin