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Problem No. 65 High urea concentration in carbamate recycle

This UreaKnowHow.com round table discusses the process upset condition of a high urea concentration in the ammonium carbamate recycle. It is commonly known that the urea formation reaction from ammonia and carbon dioxide via ammonium carbamate is an equilibrium reaction and that the urea conversion in any urea plant is limited. That is why downstream of the urea synthesis section the urea is separated from the ammonium carbamate in a recirculation section. Ammonium carbamate is dissociated into ammonia and carbon dioxide gases by means of low pressure and the addition of heat. The ammonia and carbon dioxide gases are dissolved in water and recycled back to the urea synthesis section. The urea content should be minimal as urea in the ammonium carbamate recycle leads to lower efficiencies. What are the causes and remedies for a high urea concentration in the ammonium carbamate recycle?

Leandro Blum of Petrobras in Brazil starts this round table discussion: What could cause a higher urea concentration in the low-pressure carbamate condenser level tank and thus in the ammonium carbamate recycle?

Muhammad Farooq of SABIC Agri-Nutrients in Saudi Arabia asks for more information: Can you provide details of the inputs to the carbamate tank at your plant?

Leandro replies: In our recirculation section we have a rectifying column and two low-pressure carbamate condensers where vapours from the column are condensed and go to the carbamate tank. According to our mass balance we should expect a urea content of some 0.2 wt-% in the tank, but we normally see 1-1.5 wt-%. We think that the vapours from the column are carrying over urea solution that enters the top of the column.

RD Patel of Notore Fertilisers in Nigeria shares his experiences:

The following are causes of high urea concentration in the ammonium carbamate recycle:

• carryover to the rectifying column off gases;
• from the urea recovery system;
• recovery from the desorption/hydrolyser section.

Mark Brouwer of UreaKnowHow.com in the Netherlands joins the discussion: A higher urea concentration in the low-pressure carbamate condenser level tank has sometimes been seen due to entrainment from the rectifying column.

The following factors can play a role:

• high load on the rectifier (actual plant capacity related to design);
• low pressure (increases vapour flows);
• fouling in rectifier causing build-up of liquid;
• too low disengagement height in top of rectifier;
• type of sprayer which causes small droplets;
• a combination of the above.

When did the urea concentration increase? Has it ever been low? Check the urea content in the rectifying column outlet vapour line. In case it shows high urea content it means that your bed system is blocked and needs cleaning or in case there are trays then clean the holes.

Malik Muhammad Sohail of SABIC Agri-Nutrients in Saudi Arabia shares his experiences: In my opinion, the following should be checked:

• heat load to rectifier (relates to entrainment);
• mass flow of solution inlet;
• rectifier outlet urea concentration;
• high liquid level at top of rectifier;
• inspection of top part (plates/packing).

Masood Yousaf of Fatima Fertilizer in Pakistan joins the discussion: Other possible causes of urea carryover in the rectifier are:

• high level;
• blockage of the packing bed.

Akbar Ali of SABIC Agri-Nutrients in Saudi Arabia asks for clarification: What can we expect as a negative impact if the urea concentration increases to 4 wt-% or more due to carryover from the rectifier column top?

Mark replies: If in the past the urea percentage in the carbamate tank was normal but now you are faced with this problem it might be due to displacement of the chimney tray (Chinese hat) in the outlet of the recirculation heater to the rectifier separator.

Norozipour of Khorasan Petrochemical in Iran shares his experiences: The following can cause urea carryover from the rectifying column:

• overload of rectifying column;
• high level;
• fouling in the packing and tubes due to corrosion product and some oil in the process and even partial blockage of packing bed (our case);
• malfunctioning of distribution sprayer in the top;
• low pressure.

We were faced with this problem, and we changed the packing and exchanger. As we know, if urea carries over with the gas phase, it means we lose capacity of the plant and will consume more energy to decompose this product to NH3  and CO2 in the hydrolyser section and send it back to the synthesis section.

Amirzadeh of Pardis Petrochemical Co. in Iran contributes to the discussion: We had the same problem in our unit. Please check these items before taking any actions:

• check your synthesis conditions, if everything is ok, this problem is caused by something else;
• check for any plugging/fouling in the rectifying column, such as plugging in the packing bed, overflow line from top part to the condenser;
• In case you have same the temperature in the top of the column and the loop line your inert valve at exit of the column may have a problem such as choking or bad design in sizing.

Naseem of SABIC Agri-Nutrients in Saudi Arabia shares his experience: Carryover happens if the L/G ratio is on the higher side. Liquid accumulates at the top and gases push it out. In the rectifying column it can happen due to:

• choking of orifice plugs at bottom of rectifying column if available;
• choking of the recirculation heater tubes;
• compression of the packing bed due to pressure;
• decrease in superficial area in bed or voids due to corrosion products;
• partial choking of LV which will appear with level sensor.

Observation: urea content is high in the low-pressure carbamate condenser level tank. Analyse the ammonium carbamate solution in the level tank. In case the urea content is higher than design, it is clear that there is solution carryover.

Short term remedy: Increase the operating pressure gradually up to the design pressure to shift the liquid. Increase to 0.5 bar higher than normal and observe the condition. Repeat it until the desired result and 1 barg below PSV set pressure. If the situation remains the same, shut down the plant.

Permanent solution: Shut down the unit and clean the heater.

Long Term: Shut down the plant clean the packing bed as well as the heater tubes. If the orifice plugs are installed at the bottom, replace them with an orifice plate.

Muhammad Bilal Shabeer of Fauji Fertilizer Bin Qasim in Pakistan asks a question: How does choking of the tubes increase the L/G ratio? Are the tubes blocked?

Naseem replies: L/G means liquid to vapour ratio in the column. When there is less decomposition and liquid is continuously coming then the L/G ratio will be affected. When the heater is choked, and decomposition of carbamate decreases, it reduces the gas flow through bed while the liquid is still coming.

Mark adds another option to the list of possible causes: Another cause could be the liquid distributor plate holes. These holes get smaller over time due to iron-oxide precipitation. Once the diameters become too small, the liquid level rises to over the chimneys resulting in urea carryover. They are easily reamed back to full size, or larger.

Mohamed Tarek Sadek of Helwan Fertilizer in Egypt shares his experiences: We have also experienced the carryover of urea solution in the rectifying column to the LPCC in our Stamicarbon urea plant. The reasons were:

• plugging of the Rasching rings in the packing bed in the upper part of the rectifying column;
• plugging in the tubes of heater and that’s why the opening of the steam valve on the heater recirculation became higher than normal in order to reach the right set point for the urea solution out of rectifying column.

The consequences were:

• level in LPCC level tank increases sharply;
• level in urea tank decreases (due to urea solution carryover in the rectifying column). Therefore, take care, you must act before the urea solution pump and urea melt pump start to cavitate. You could also isolate a header to avoid decrease of the main header pressure to the granulator.
• concentration of solution flow from LPPC to HP scrubber via HP carbamate pump changes, which leads to change in the synthesis loop conditions (so take care).

The solutions are to decrease the possibility of the carryover phenomenon in the rectifying column.

From the process side:

• decrease the synthesis load to decrease load on the rectifying column;
• increase the recycle pressure (more than 4 bar) to damp the liquid solution either in the rectifying column or LPCC.

From the mechanical side during shutdown and after complete drainage of the plant:

• mechanical cleaning must be done using a high-pressure condensate (jet pump);
• chemical cleaning to remove any deposits in the recirculation heater tubes or the Rashing rings in the packing bed. Broken rings in the packing bed should be replaced.

Bob Edmondson, Expert of UreaKnowHow.com shares his experience: We suffered from this problem and corrected it by drilling the holes in the liquid distributor to increase their size. After that carryover was no longer a problem.

Hassan Abdul Raheem of Fauji Fertilizer Bin Qasim in Pakistan contributes to the discussion: Urea carryover from the rectifier may be due to the following reasons:

• low stripping efficiency in stripper e.g., improper distribution of reactor effluent on ferrules, loose ferrule gasket looseness, CO 2 flow disturbance etc.;
• high load on rectifier;
• high heat load on recirculation heater;
• mechanical damage of gas risers or partially blocked holes in tray;
• partially blocked recirculation heater tubes;
• high vapour velocity;
• low bottom level in stripper;
• high liquid level in rectifier.

Carryover Indications are:

• rectifier feed inlet temperature matches rectifier vapour temperature;
• visibly check from vapor sample point;
• sudden level increase in LPCC;
• decreasing level of rectifying column;
• 75 % urea solution tank level drops from normal condition;
• rectifier liquid outlet temperature.

Shoaib of Fauji Fertilizer Bin Qasim in Pakistan confirms Hassan’s observations: We have suffered from the carryover problem in the rectifying column a number of times, specially at particular loads.