Aspen Plus Reverse Osmosis
Sourn Rose
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Abstract: Nowadays, there is increasing interest in advanced simulation methods for desalination. The two most common desalination methods are multi-stage flash distillation (MSF) and reverse osmosis (RO). Numerous research works have been published on these separations, however their simulation appears to be difficult due to their complexity, therefore continuous improvement is required. The RO, in particular, is difficult to model, because the liquids to be separated also depend specifically on the membrane material. The aim of this study is to model steady-state desalination opportunities of saline process wastewater in flowsheet environment. Commercial flowsheet simulator programs were investigated: ChemCAD for thermal desalination and WAVE program for membrane separation. The calculation of the developed MSF model was verified based on industrial data. It can be stated that both simulators are capable of reducing saline content from 4.5 V/V% to 0.05 V/V%. The simulation results are in accordance with the expectations: MSF has higher yield, but reverse osmosis is simpler process with lower energy demand. The main additional value of the research lies in the comparison of desalination modelling in widely commercially available computer programs. Furthermore, complex functions are established between the optimized operating parameters of multi-stage flash distillation allowing to review trends in flash steps for complete desalination plants. Keywords: modelling; multi-stage flash distillation; reverse osmosis; desalination
Reverse osmosis (RO) technology is used in the water purification process to separate dissolved solids and other large molecules from water. Typical applications of RO technology are seawater desalination, microelectronics production, laboratory testing, biotechnology, and other process that require highly purified water. In osmosis process, water with a lower concentration of solids naturally flows through a membrane to an area of higher concentration through naturally occurring osmotic pressure, equalizing the concentration of the solute on either side of the membrane. The RO technology applies pressure to a stream of water to overcome the natural osmotic pressure. The feed water is forced through a semi-permeable membrane, emerging as purified water and leaving behind a concentrated solution of dissolved solids. Therefore, RO system permeates have very low hardness and alkalinity, and therefore, it has quite high corrosion potential. The nanofiltrations (NF) are known by their higher water permeability than the RO membranes and significantly lower TDS rejection: from 95% down to non-significant. However, the NF membranes have sufficiently high rejection of selected constituents, such as hardness, metals and organic matters. Characterization RO NF reported that new insights can be reached regarding the best choice of membrane, based on the minimization of electrical conductivity and the ability to reject specific ions in different operating conditions.
The design of the full-scale reverse osmosis/nanofiltration modules was performed using WAVE software from DOW Chemical. Different design patterns have been developed to reduce the TDS and hardness. One and two stage brackish RO modules and NF module are used to develop for the softened water treatment from above salinity of the water. RO brackish water membranes are a non-selective process that decreases the TDS, while the NF membranes are a selective process that decreases only bivalent ions. Both structures are same as shown in Fig. 1. And the characteristic parameters of the RO brackish module (BW30 4040, XLE 4040) and NF module (NF90 4040, NF270 4040) are presented in Table 1. For this work, both processes have 6 pressure vessels arranged in parallel for the stage 1 case. And 4 PV arranged in parallel and 2 PV in the stage 2 case. The flow requirements for the system were flow permeate output of 100m3/day. The design was constrained by limits on maximum permeate recovery factor of at most 75% as recommended by a membrane manufacturer to preserve a membrane lifetime. The salinity of input data is the same as the LIAS groundwater (3846 ppm of TDS and 340 ppm of total hardness). Variation of temperature and feed pressure were carried out to fix the optimal operating.
Water treatment can be simple if good quality source water with low total suspended solids (TSS), low divalent ions, and no bacteria is available. In this case, only a simple filtration device is needed in the field. However, if the water quality does not meet the ASP slug specifications, then special equipment will be necessary to process the brine before usage (Morrow and Buckley 2011). In this case, brackish raw groundwater from LIAS aquifer is used for this simulation to mimic the real water injection at TFT field. An analysis of a water sample from the LIAS aquifer groundwater revealed a high amount of calcium, magnesium and sulfate, as detailed in Table 2.
The effect of temperature and feed pressure on recovery ratio was simulated to determine the adequate membrane configuration for the EOR process. The temperature was varied from 25 to 45 to imitate the real field temperature at TFT oilfield during seasons, while the feed pressure was varied from 3.1 to 22 bar to include the pressure difference between each process (RO and NF). Figures 2 and 3 show the variation of feed pressure at 75% of recovery ratio (as recommended by manufacturer) for stage 1 and stage 2, respectively. The highest feed pressure required to achieve 75% RR is obtained by BW 30 4040 membrane, while the lowest feed pressure is obtained by NF270 4040 membrane. XLE membrane required more feed pressure than NF90 membrane. The feed pressure required to achieve 75% RR decreases with increasing temperature. This is due ions diffusion transport that increases with temperature causing the rising on water flux. Thus, increasing temperature causes flexibility of membrane chains and allowing more ions to migrate throw membrane pores (Al-Obaidi et al. 2018). Previous simulation results show that the optimum to feed pressure for each membrane BW30, XLE, NF90 and NF270 is 15 bar, 10 bar, 9 bar and 5 bar, respectively. Figures 4 and 5 exhibit the effect of temperature on recovery ratio at optimum feed pressure. As seen, the NF270 reached maximum RR at 40 C. It is noticed that all membranes reached around 75% of RR at 30 C.
This is due to the highest required feed pressure. These results could serve as engineering tools to adjust operating conditions in front of temperature changing at field during seasons to maintain the required recovery ratio and desired permeate composition (TDS, total hardness, sulfate concentration) without having recourse to heat exchanger in the aim to minimize energy consumption.
Two membrane configurations were investigated during this work for the chosen membrane module NF90 4040. In order to identify the most efficient configuration regarding RR, TDS, total hardness and sulfate removal percentage, Figs. 14, 15, 16 and 17 respectively, show that it follows the same pattern. Insignificant difference was observed between the configurations. It is explained by the low flow rate of the process. However, simulation results show imbalance in the input TDS between stages. Input TDS was up to 10 998 ppm for the first membrane module at 2nd stage, which might cause a fouling problem and require a systematic cleaning and antiscalant injection. Therefore, for a small range of flow rate, it is preferable to opt for one stage configuration with 3 modules PV to avoid fooling problems.
Two membrane processes were investigated.RO membrane BW30 provided the higher removal percentage of TDS, TH and sulfate, although, it was the most energy consuming. TH removal percentage was not enough to be used for ASP using NF270. XLE membrane provided acceptable removal percentage to be used for ASP. However, NF90 provided a better removal percentage than XLE with less energy consuming. As a result, RO membrane BW30 is oversized for EOR process. On the other hand, NF90 seems to be the most adequate membrane module. At this scale of study (100 m3/d), insignificant difference between one and two stages configuration. To avoid scaling problems, one stage configuration seems to be suitable choice. This work may be used as an operational tool as regard of temperature changing at field. Moreover, it could provide a solution for decision making concerning process option for on shore oilfield.
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