Problem No. 79 What happens in the low-pressure carbamate condenser?

The low-pressure carbamate condenser (LPCC) in a Stamicarbon urea plant is far more than a simple condenser for ammonia and carbon dioxide vapours. It also plays a critical role in concentrating the carbamate recycle stream before it returns to the urea reactor.

Minimising water in this recycle stream is essential. Higher water content in the urea reactor reduces conversion because water shifts the overall reaction equilibrium towards the reactants. As a result, excess water in the carbamate recycle lowers reactor conversion and can significantly reduce overall urea-process efficiency.

However, the water content cannot simply be reduced indefinitely. The lower limit is determined by the crystallisation point of the carbamate solution. If the solution contains too little water, solids may form during pumping and recycle to the reactor. If it contains too much water, the plant may not achieve its maximum efficiency.

Achieving the right balance is complex. Ammonia, carbon dioxide and water can form different products depending on their relative concentrations, each with a different crystallisation temperature. Yet operators often have only laboratory analyses showing the weight percentages of ammonia, carbon dioxide and water in the carbamate recycle stream. Consequently, crystallisation behaviour may be estimated primarily from water content, which can be imprecise.

Operators must also avoid entering a “water recycle operation mode”, in which water recirculation progressively increases throughout the plant. If this continues, plant load may eventually need to be reduced substantially to restore stable operation.

In a submerged condenser, not all vapours should condense. A residual vapour phase is needed to maintain a high heat-transfer coefficient, while inert gases must also be accommodated. The following round table discussion highlights the practical challenges associated with operating an LPCC.

Treyzzztyler, a student at the University of Malaya in Malaysia, opens the discussion: Can anyone explain the operating conditions of the LP carbamate condenser? I have found values of 65°C and 3 atm. Under these conditions, ammonia and carbon dioxide would both be gases, so how can condensation occur? If condensation takes place, do all gases condense to form ammonium carbamate?

Mohamad Kamal of Abu Qir Fertilizers Company in Egypt replies: The condensation temperature depends on the N/C ratio, water content and pressure. Not all gases are condensed.

Prem Baboo, retired from Dangote Fertilizers in Nigeria, shares his operating experience: Condensation is favoured by higher pressure and lower temperature. In the low-pressure section, typical operating conditions are 39–41°C and 3.1–3.5 kg/cm² for the design N/C ratio. According to the condensation chart, maximum condensation occurs under these conditions. In the Saipem process, ammonium carbonate rather than ammonium carbamate forms.

Zadpanchal of GNFC Ltd in India adds: The vapour mixture also contains water vapour. At approximately 3–3.5 bar, water condenses first. The condensed water absorbs ammonia, forming an ammoniacal solution, which then absorbs carbon dioxide. Ultimately, condensation occurs as an ammonium carbonate solution.

Mark Brouwer of UreaKnowHow.com in the Netherlands joins the discussion: In a Stamicarbon plant, the LPCC also concentrates the carbamate recycle as much as possible to minimise water recycle to the synthesis section, thereby achieving a minimum H/C ratio.

In a Saipem plant, this function is performed in the medium-pressure carbamate condenser (MPCC).

The minimum water content depends on the N/C ratio and the margin between the condensation and crystallisation temperatures – typically around 8–10°C by design.

In a Stamicarbon LPCC, subcooling of approximately 10°C also typically occurs. Different carbamate forms may be produced at different CO2, NH3 and H2O compositions. Refer to https://vmgurea.appspot.com/ which provides crystallisation-temperature estimates based on measurement data from the 1930s.

Rajan Kher of Koch Fertilizer LLC in Wever, United States, raises an operational question: What could cause the LPCC liquid-carbamate outlet temperature to decrease by 20°F (11.1°C)? What does this indicate, and how can the temperature be restored?

Mark Brouwer suggests several areas for investigation: Is the temperature reduction continuous or temporary? Has the cooling-water duty changed? Review both the temperature difference and flow of tempered cooling water. Has LPCC pressure changed?

Rajan Kher provides further operating details: The reduction is continuous. The LPCC level-vessel temperature is stable at 175°F (79.4°C), with cooling-water temperatures of 126–128°F (52–53°C). However, I can no longer achieve 170°F (76.7°C), even with cooling water at 131°F (55°C). Operations manually set the LPCC cooling-water temperature.

Finally, Majid Mohammadian of Fertiglobe in Abu Dhabi summarises the likely causes: Assuming no change in the cooling-water temperature difference, a low carbamate temperature is generally caused by a low N/C ratio, high water content, or a combination of both.