New Efficient Configurations for Sour Wastewater Treatment-

Background: Due to the technological development, the environmental legislations on pollutant concentrations in aqueous effluents tend to tighten and increase. As a result, sour water must be handled and processed properly to provide a high quality of stripped water with insignificant traces of NH3 and H2S in it. This must be achieved within the minimum operating costs. This research investigates the stripping configurations of sour water effluents from various industries. The research also offers an insight on different scenarios and configurations to accomplish set targets satisfying the environmental law criteria. Methods: This research introduces a range of heat integration schemes for better energy savings, further vapor recompression VRC technique is opted for its ability to maximize energy savings. This research analyses the effect of operating and design variables on the stripped water quality such as feed temperature, feed location, reflux split, and steam flow rate. The option of adding new equipment is also addressed to maximize heat integration and enhance the efficiency of the process. Thus, several schemes and process configurations are explored to treat industrial sour water waste streams seeking better efficiency. These configurations differ from one another in heat integration layout and whether VRC is utilized or not. Energy efficiency and economics of the proposed configurations are considered as decisive factors in this research study. The case study adopted are based on published data taken from some iron and steel factories in South Korea named POSCO (Pohang Iron and Steel Corporation). Results: Results of the treated wastewater streams guarantee that the effluent sour water obeys standard environmental regulations, i.e., NH3 contents range from 30 to 80 ppm and H2S concentration falls below 0.1 ppm. The obtained results of the seven different scenarios are compared to the original case study. It is found that scenario 7 is the most economical solution saving 51.54 % in total annual cost when compared to the original case study, while satisfying the treated water environmental regulations with a concentration of 3.19 ppm NH3 and 0.05 ppm H2S. Scenario 7 creates its own steam unlike the original case study where steam utility is needed extensively. However, scenario 7 consumes 15 % more electricity than the original case study but still shows 56.34 % less utility cost in overall. Conclusion: The optimum process configuration can be employed for other sour water purification systems such as those from petroleum refiners. An ongoing research work is focusing on the use of internal heat integration for more energy savings and economics improvement.

Exergy analysis and performance study for sour water stripper units, amine regenerator units and a Sulphur recovery unit of a refining plant

A refining column in the middle east that started its official production in 2020 provides its sour wastewater from all refinery plants to two sour water units (SWS1 and SWS2) to strip H2S and NH3. Sour gas from the refinery uses a lean amine solution for gas sweetening to absorb H2S in different absorbers. Rich amine with H2S is then stripped in two amine regeneration units (ARU1 and ARU2). The overhead of SWS and ARU units provide the acid gas feed to the sulphur recovery unit (SRU) to produce sulphur and prevent any acidic emissions against environmental regulations. First, the SWS1 unit is simulated using Aspen HYSYS V.11. A complete exergy study is conducted in the unit. Exergy destruction, exergy efficiency and percentage share in the destruction are calculated for all equipment. The highest exergy destruction rate was in the stripper with 5028.58 kW and a percentage share of 81.94% of the total destruction. A comparison was conducted between the exergy results of this study with two other exergy studies performed in the same refinery plant. The columns in the three studies showed the highest destruction rates exceeding 78% of the total destruction of each unit. The air coolers showed the second-highest destruction rates in their units with a percentage share exceeding 7% of the total destruction. The pumps showed the lowest destruction rates with values of less than 1% of the total destruction of each unit. Then, an individual simulation is conducted for stripper1 of SWS1, stripper2 for SWS2, regenerator1 of ARU1 and regenerator2 of ARU2. The individual simulations are combined in one simulation named combined simulation to compute the composition of acid gas from SWS and ARU units feeding SRU. Then, the SRU unit is simulated via a special package in HYSYS V.11 named SULSIM. The computed composition from SWS and ARU is exported to excel where it is linked with SRU simulation to calculate sulphur production. For the first time in any article in the world, all data feeding SWS, ARU, and SRU units are connected to a live system named Process Historian Database (PHD) to gather live data from the plant and perform plant optimization.

Evaluation of Process Control Schemes for Sour Water Strippers in Petroleum Refining

For the sour water strippers in petroleum refinery plants, three prediction models were developed first, including the estimators of sour water feed concentrations using convenient online measurements, the minimum reboiler duty and the corresponding internal temperature at a specific location (Tstage,29). Feedforward control schemes were developed based on these prediction models. Four categories of control schemes, including feedforward, feedback, feedback with external reset, and feedforward-feedback, were proposed and evaluated by the rigorous dynamic simulation model of the sour water stripper for their dynamic responses to the sour water feed stream disturbances. The comparison of control performance, in terms of the settling time, integrated absolute error (IAE) of the NH3 concentration of the stripped sour water and IAE of the specific reboiler duty, reveals that FFT (feedforward control of Tstage,29) and FBA-DT3 (feedback control with 3 min concentration measurement delay) are the best control schemes. The second-best control scheme is FBAT (cascade feedback control of concentration with temperature).

Loss Prevention Mitigation in Sour Water Stripper (SWS) Cooler Using Process and Computational Fluid Dynamics (CFD) Modelling

Sour water stripping (SWS) units are similar to other process units that can bring many operational challenges to meet target stripped water specifications. Failure of any equipment in the unit leads to unplanned shutdown and hence increases the downtime of the unit. In this study we investigated the outlet nozzle and elbow downstream the finfan cooler as they occasionally leak due to apparently metal thinning. Process simulation using appropriate electrolyte thermodynamic package to predict vapour liquid equilibrium and streams flow rates was used. Subsequently, Computational fluid dynamics (CFD) simulation was use to predict the erosion patterns. To mitigate and prevent unit upset, many option have been recommended to change the operating mode of the finfan cooler so as to circumvent alloy change. The CFD simulation results matched the erosion pattern that caused the loss of wall thickness. Different cases were investigated addressing elbow size, flow regime and elbow angle. The results, however, have indicated that the erosion is unavoidable irrespective of the fluctuation in the throughputs to the unit.