Wastewater treatment in a urea plant

WASTEWATER TREATMENT

Wastewater treatment in a urea plant

A detailed rain and wastewater concept is an important part of a urea plant to meet current stringent environmental standards. Wastewater can originate from the process reaction or from outside the process equipment. In order to optimise the wastewater system of a urea plant, both the amount and type of contamination need to be known. With this knowledge, non-contaminated rainwater as well as process drains can be kept separate as much as possible to minimise the amount of wastewater to be treated, saving costs and energy. Wastewater treatment concepts from thyssenkrupp Industrial Solutions and Toyo Engineering Corporation are described.

THYSSENKRUPP INDUSTRIAL SOLUTIONS

State-of-the-art wastewater collection, handling and avoidance management

As a urea contractor for greenfield and revamping projects, thyssenkrupp Industrial Solutions has the knowhow and experience to provides a wide range of local and climate-specific solutions for state-of-the-art wastewater collection, handling and avoidance management for urea plants and the downstream offsites and utility systems.

In a urea plant the reaction of ammonia and CO2 via carbamate ultimately produces urea and water:

This water, the so-called process condensate or ammonia water, is separated from urea by evaporation. As it contains reasonable amounts of ammonia and also urea, it is purified before it is discharged from the urea plant and transferred to offsites and utility units for further use and treatment. Urea plants built by thyssenkrupp Industrial Solutions, using the Stamicarbon urea synthesis process, discharge the process condensate with less than 1 ppm of ammonia and 1 ppm of urea to the wastewater treatment section. Due to its suitable quality for reuse, the urea process condensate is not considered as waste but as a recycle stream to the water treatment facility, where the process condensate is subject to further treatment. Filtration and ion exchange allow the condensate to be treated to produce demineralised water quality suitable for high pressure steam production.

Fig. 1: Basic wastewater collection concept

Wastewater from outside the process equipment originates from rain or open process drains (e.g., pump seals). As this wastewater can possibly contain ammonia and urea as well as other contaminants such as oil, it must also be accurately collected, treated, and discharged from the urea plant. At best contamination can be avoided and rainwater purely discharged into the clean stormwater system, which is a sewer system, usually connected to the public sewer grid.

Table 1: Basic wastewater collection concept

Basic concept for wastewater and stormwater collection

There can be several trench systems, piping and pits in the urea plant to collect rainwater. Fig. 1 and Table 1 explain a typical arrangement that can be found in plants built by thyssenkrupp Industrial Solutions. There are three main areas in a urea plant: synthesis building, granulation building and tank area.

This basic concept can be extended and adapted to the layout or climatic conditions as required. There are six wastewater pits located in the urea plant area. The three pits U-2, U-3 and U-6 collect wastewater which is then returned back to the urea process for direct reuse, U-2 from synthesis, U-3 from the diked urea storage area in which the acid storage vessel and the UF tank are located, and U-6 from the granulation section. There are three other pits that collect wastewater from drains, granulation washing and stormwater: Pit U-5 is located in the diked urea storage area in which the urea solution and the ammonia water tank are located. If the water in U-5 is not contaminated (pure rainwater without spillages) it is disposed of in the stormwater system. If contamination has occured (e.g., spillages or leaking tanks), the water is routed to the central wastewater pit U-1 in the synthesis area. Pit U-4 collects wastewater from the truck loading area for an acidic storage vessel and a urea formaldehyde tank. The solution collected in pit U-4 is sent to pit U-1. Pit U-1 in the synthesis area is equipped with an oil separator and also collects stormwater from the granulation and synthesis sections from an open trench system. After oil separation the wastewater is sent to the offsites and utilities plants for further treatment.

Recovery of waste streams

As urea is a crystalline product, some dedicated pump seals have to be flushed with tempered water (API Plan 32). For pumps handling urea or carbamate solution at a higher concentration a combination of different seal flush setups is used. Just before start-up or shutdown a seal flush with clean hot condensate (API Plan 32) is applied to ensure that there is no crystallisation inside the pump seal, which would damage the seal and lead to leakages of urea or carbamate solution into the wastewater system. During normal operation, when it can be assured that the process media delivers enough heat by itself to prevent any crystallisation, the seal flush can be switched to process media from the discharge side of the pump (API Plan 01 or 11). During operation of the pump a steam quench on the atmospheric side of the pump seal (API Plan 62) ensures that no possible leakage leads to a build-up of crystalline urea, which might damage the seals. As steam condenses at the outer seal, this condensate needs to be collected continuously. For urea melt pumps with a hydrodynamic seal can be used, which tolerates even a small solid content inside the process media. In order to avoid wastewater, a seal system is usually constructed in such a way that the seal water is flushed into the process. If the seal system fails, process medium may be leaked from the pump seal into the environment. The medium released is collected and discharged into pipes. The water that escapes from the pump seals in the synthesis area is collected in pit U-2 (see to Fig. 1) and returned to the urea synthesis section. The water from the pump seals in the granulation area is collected in pit U-6 (see Fig. 1) and returned to the recycle vessel in the granulation section. From there, it is recycled to the process.

Nowadays, the granulation section is usually equipped with both dust and acidic scrubbers to remove dust as well as ammonia from the granulation air. The two acids most often used to capture the ammonia are sulphuric acid and nitric acid. Sulphuric acid and the ammonium sulphate formed from the reaction with ammonia can be recovered together with the urea, so that it finally ends up in the product together with urea by application of the so-called Ammonia Convert Technology (ACT). Consequently, such streams can be collected in the recycle vessel U-6. Nitric acid and ammonium nitrate cannot be mixed with the urea, so these streams are collected in a dedicated pit (not shown in Fig. 1). From there, unreacted nitric acid can be reused in the acidic scrubbing stage of the vent gas scrubber, while ammonium nitrate needs to be discharged e.g., to a UAN plant.

Avoiding rainwater contamination

Rainwater falls onto all surfaces of a process plant. If rainwater comes into contact with process media or oil, e.g., from the lubricating greases of pumps, it is contaminated and must be drained off separately and reprocessed. The best measure is always to avoid contamination. Mitigation is always the preferred measure compared to treatment. Especially in tropical regions, avoiding contamination is key. This is achieved by covering pumps and tanks. In a urea plant, there are several tanks in the tank farm. This area is usually free of process fluids and oil. Nevertheless, in case of leakage or spillages, contamination of the rainwater may occur. The rainwater is collected in a pit in the tank cup and pumped away. Since the tank cup and the tank itself are very large and have a correspondingly large area on which the rain falls, the amount of water can also be quite large. Especially in tropical regions, this can lead to problems, e.g., when designing the required capacities for wastewater pumps and oil separators. If needed, therefore, thyssenkrupp Industrial Solutions builds rain gutters on the tank so that the tops of the tanks act as collecting roofs as shown in Fig. 2. The rain falling on the tank roof is not contaminated and can be directly drained to the clean stormwater system. For light and medium rainfall, the remaining water in the tank farm can evaporate. In case of high rainfall, it is collected in pit U-5 and can be discharged by pump (see Fig. 2). Provided any contamination is within the permissible emission range, it can then be discharged to the clean stormwater system. In case contamination exceeds the permitted levels, the stormwater is pumped directly to the offsites and utilities for further treatment.

Fig. 2: Tropicalisation of a tank farm

Fig. 3: Separation of oily wastewater from non-oily wastewater

The steel structure of the synthesis section can be roofed over so that rainwater cannot contaminate the area. Additional cladding can prevent rain from entering from the side, e.g., during strong winds. Whether roof and cladding are necessary is a trade-off between investment costs and the resulting wastewater load. Since thyssenkrupp Industrial Solutions absorbs the risk of contamination from machines, cladding is not necessary in thyssenkrupp Industrial Solutions plants. The granulation building is usually enclosed due to process needs, so no further measures are necessary there. Granulation also includes a so-called wet section including the scrubbers for dust removal from the granulation air. Nowadays, this scrubbing system usually includes an acidic stage for reducing ammonia emissions. Due to the possible contamination with media from the wet section, rainwater that falls on the surface of the wet section is collected in trench systems and discharged from the plant via wastewater collection pit U-1.

To reduce the oil load in the wastewater, machines can be enclosed or covered so that no oil can be washed away by rain. In case machines cannot be roofed, dedicated catch basins can collect any water that may accumulate and direct it to the oil separator in the pit U-1 shown in Figs 1 and 3 by means of a closed pipe system. Base frames with drip pans can be considered for machines where applicable to minimise cross-contamination with water as much as possible and to keep contamination local, i.e., to keep dedicated areas curbed and separated.

In addition to the aforementioned measures of using tank roofs to drain clean rainwater and covering the roofs of steel structures or directing the wastewater depending on the waste load to stormwater or offsites and utilities, thyssenkrupp Industrial Solutions also uses other measures to keep the overall amount of wastewater low. thyssenkrupp Industrial Solutions has already built many plants in different locations, with different climates. Most recently, tropicalisation was used in a plant located in Southeast Asia contributing to a reduced energy requirement due to reduced treatment requirements.

An effective measure is to cover the open ends of funnels with a removable cover so that rainwater cannot penetrate. Rainwater from areas potentially contaminated with oil or grease is channelled separately through an oil separator as shown in Fig. 3.

Wastewater from places without potential oil contamination is routed downstream of the oil separator. In this way, the oil separator remains efficient even with high rain loads and can be dimensioned appropriately.

Table 2 shows the design requirements for water collected in pits and representative requirements for a location in North Africa. Flow rates will vary depending on the precipitation of the geographic location as well as the amount of paved area. Pits U-2, U-3 and U-6 are not indicated because the solution is recycled back to the process for direct reuse, thus mitigating wastewater disposal.

TOYO ENGINEERING CORPORATION

TOYO zero effluent approach to wastewater treatment

Toyo Engineering Corporation (TOYO) applies a zero effluent approach to wastewater treatment in urea plants to meet current stringent environmental standards.

Different types of wastewater are generated in the urea plant, some of which need to be treated before discharge outside the urea plant.

Fig. 1: TOYO urea process

TOYO’s wastewater treatment for urea plants can be broken down into treatment and disposal methods for the following types of wastewater:

• process drain water;
• process condensate;
• oily water;
• contaminated surface drain water including rainwater.

Process drain water

Since process drain water contains a large amount of urea, carbamate, and ammonia, it is essential to store the drain water in the pit or tank for reuse it in the process. The capacity of the pit or tank must be carefully determined to prevent liquid overflow from the system e.g., during upset conditions, start-up or shutdown.

Fig. 2: Process condensate treatment with on-line analyser

Fig. 3: Schematic flow of on-line process condensate analyser

Process condensate

Water (process condensate) is the largest by-product of the urea production process. Stoichiometrically, 0.3 tonne of water is produced per tonne of urea, but in industry it can be as high as 0.5 t/t-urea. The treatment of process condensate containing ammonia and urea has been a challenge since the urea production process was industrialised. Water as a by-product is evaporated in the vacuum evaporation section of the urea plant and sent to the process condensate treatment section. Fig. 1 shows the process flow of the typical urea plant. In the process condensate treatment section, deep urea hydrolysis and steam stripping technology reduces the urea and ammonia content to lower than 1 ppm, enabling the treated process condensate to be used for make-up for boiler feed water (BFW). When using the treated process condensate for BFW makeup, the urea content must be strictly controlled because urea cannot be removed in an ion-exchange resin bed. In case of excessive urea in the BFW, it is hydrolysed in a boiler to form carbon dioxide which lowers the pH but can result in excessive corrosion in the boiler. As the conductivity meter, which is typically installed at the line of treated condensate, does not show the level of urea content (urea does not become electrolytes in aqueous solution), urea in treated process condensate must be periodically analysed in a laboratory and it typically takes a few hours to obtain the urea analysis results. To eliminate the risk of corrosion in the boiler due to low pH caused by excessive urea in BFW, the urea of the treated condensate should be analysed continuously in real-time. TOYO and Mitsui Chemicals Inc. (MCI) have developed a proprietary online urea analyser for the treated process condensate, which analyses urea continuously in real-time in the range from 1 ppm to hundreds of ppm. As the TOYO-MCI online urea analyser is simply configured and does not require any chemicals and reagents, its initial and running costs are fairly low. Figs. 2 and 3 show an example of its application and schematics of the analyser. The TOYO-MCI online urea analysers have been running in TOYO’s urea plants for more than a decade. The TOYO-MCI online analyser can be installed for the rigorous control of process condensate quality in all existing urea plants in the world.

Oily water

As major rotating machines have a dedicated lube oil system, during maintenance periods some lube oil may spill onto the paving area and contaminate floor washing water or rainwater. This effluent (oily water) should be segregated from other types of effluent because the removal of oil requires specific technology for the treatment. As such, the area where oily water is generated should be enclosed and isolated from other areas. The collected oily water is sent to a dedicated oil separator, for which API oil-water separator or CPI separator is predominantly used, for the separation of oil. As some oil still exists even after the treatment by such gravity-type oil separators, another type of technology, e.g., a coalescer can be used in conjunction with oil separators, depending on local regulations or end-user’s requirements. It is necessary to identify and carefully discuss with the end-user which rotating machine is to be targeted for the treatment of oily water and decide on a segregation plan during the initial phase of a project so that unexpected oil contamination does not happen.

Fig. 4: TOYO’s effluent treatment plant

Fig. 5: Outline of wastewater treatment methods steam

Contaminated surface drain water including rainwater

Floor washing water and rainwater in the paved area may also be categorised as wastewater in some circumstances. In the urea plant, urea or carbamate may spill onto the paved area due to a variety of reasons, such as local maintenance activities or an unforeseen leakage from piping flanges. If it rains during the period when the process fluid spillage occurs, or when floor washing water is used to clean the spillage, it is possible that the process fluid may contaminate the cleaning water or rainwater. The amount of urea, carbamate, and/or ammonia in these wastewaters is often small; therefore, in many instances, they have just been diluted by another water and discharged to the outside of the urea plant. Even now, depending on local regulations, some urea plants still apply this method in combination with a neutralisation system; however, considering the growing environmental interests in the world and the difficulty of removing urea by the neutralisation technique, a dedicated wastewater treatment facility should be used in the urea plant.

TOYO has developed a remarkable wastewater treatment facility that removes urea and ammonia from such surface drain water from the paved area and has installed and operated it in several plants. Fig. 4 shows the schematic process flow of TOYO’s effluent water treatment system. The urea contained in the surface drain is first decomposed in the hydrolysis tower and then steam stripping technology is applied to separate the ammonia. The wastewater can be sufficiently cleaned to a level where it can be sent out of the system directly. In general, surface drain water contains a lot of foreign material, which often causes mechanical problems, however, TOYO’s robust and careful design provides a trouble-free system.

Fig. 5 summarises TOYO’s wastewater treatment and disposal methods. By employing these wastewater treatments in this way, TOYO urea plants do not discharge any dirty wastewater outside of the system, complying with all environmental standards around the world. In future, TOYO aims to contribute to the global fertilizer business while paying the utmost attention not only to the main process performance but also to the environment.