PTQ Q2 2025 Issue

100 80 120

Separate vapour species

Before wash rate increase

After wash rate increase

T system

RNH

HCl

Salt ∆ T

20 40 60

T formation

-100 -20 -40 -60 -80 0

RNH Cl + –

SALT

Salt ∆T = System temperature – Salt temperature

Dewpoint s alt ∆T Afte r wash s alt ∆T

Partial pressure product of amine and HCl

Date

Figure 2 Amine-HCL salt formation phase diagram

a couple of seconds and only fractions of a second when injecting water near an exchanger inlet. Keeping salts washed requires continuous 100% water-wetted surfaces within the condensing exchangers, which is also highly unlikely given the complex exchanger geometries and large heat transferring surface areas. Instead, the purpose of a water wash is to move enough salt-reacting contaminants from the vapour phase into the water phase so that salts can no longer form until liquid water is thermodynamically stable. Practically, in terms of the CRM, this means that a negative salt delta T value at the water dew point reference is made positive. Thus, designing and operating a water wash becomes an exercise in maxim- ising mass-transfer efficiency.3 Understanding the complicated factors governing the rate of transferring salt forming contaminants from the vapour phase into the water phase can be simplified using an analogy of moving rush hour traffic. The concentration of contaminants is like the number of vehicles making trips. The residence time from the wash injection to the exchanger is analogous to the desired travel time. The total interphase surface area is like the number of traffic lanes. And the amount of convective transfer generated is like the available speed limit. Just like more vehicles, tighter schedules, fewer traffic lanes, and slower speeds are a recipe for late arrival, more contaminants, less residence time, less interphase surface area, and less convection will make conditions more chal- lenging for the wash to succeed at mitigating salt corrosion.4 The program’s spray model has been developed to evaluate the impact of a water wash injection on the salt formation risk for the specific set of conditions and equipment config - uration in a crude overhead system. The model calculates the wash injection’s upgrading of the salt ΔT value compared to the water dew point tem - perature. A wash would be successful at mitigating salt risk if the ΔT value is improved to a positive number. Figure 3 shows the model results on a unit overhead operating under the needed flow rate for the nozzle installed. Most of the after-wash salt ΔT remained at negative values during the time of lower flow rate. Periodic high- Figure 3 Topguard spray model results showing impact of wash nozzle flow on salt formation

of amine and HCl in the vapour phase. The phase diagram in Figure 2 is an example of a typical amine. In the phase diagram, if the system is operating to the left or above the phase boundary line (as shown by the sample point for T system ), the amine and HCl will remain as separate vapour species (salts will not form). If the system operates to the right or below the phase boundary line, the amine and HCl will combine to form salts. A ΔT Factor (system temperature – salt formation tem - perature) is used to quantify the salt formation risk and is interpreted as follows: • ΔT Factor >0 – Salt reaction is not thermodynamically spontaneous. • ΔT Factor <0 – Salt reaction is thermodynamically spontaneous. A buffer control margin of 10°F (5°C) is recommended to account for measurement errors in the input data supplied to the CRM program. A larger buffer of 25°F (14°C) is used in the tower top to account for non-equilibrium thermal zones caused by the reflux return injection. The CRM program is now an internal web application providing enhanced data security and quality control. The web application can also be automated when paired with online stream analysis and secure data transfer technologies. Absorbing threats: Water wash Water wash injection has been used in crude overhead systems for more than 50 years, yet the success of these injections at mitigating salt formation has been varied. In fact, a survey by SC16 Oil and Gas – Downstream of the Association for Materials Protection & Performance (AMPP) showed that only 53% of responses giving an overhead tube bundle life, while noting a continuously injected water wash via a spray nozzle, listed a bundle life of more than two years. The success rate reveals a gap in understanding what makes a water wash successful, making it more difficult to realise a return on capital invested in overhead water wash systems. The conventional understanding of the purpose of an overhead water wash was to ‘force water dew point’ and to keep salts ‘washed’ from the condensing exchangers. However, forcing water dew point is an equilibrium con- dition that is highly unlikely in the very short residence time of an overhead line, which is typically no more than

PTQ Q2 2025