Study On Optimization Of Pretreatment Techenology Of Alcohol Wastewater In Gas Field

In an alcohol-containing sewage pretreatment system in a natural gas treatment plant in Changqing Oilfield,the mixture of sewage and reaction agent is not uniform,the dosage and flocculation is insufficient and the effect of pretreatment is not satisfactory.Meanwhile,after the pretreated raw material water enters the methanol recovery system,fouling reaction occurs after the high concentration of Ca²⁺is heated,resulting in the blockage of the pipeline,heat exchanger and the tray of the tower,which seriously affects the stable and safe operation of the methanol recovery system.In view of the problems existing in the pretreatment process of alcohol-containing sewage,the process optimization of the pretreatment system is studied to ensure that the alcohol-containing sewage is pretreated to meet the requirements of the production and operation of the methanol recovery unit.The main tasks are as follows:（1）XRD and SEM analysis are carried out to decide fouling components on the inlet of the distillation column and the scale of the heat exchanger.According to the problem of high calcium ion content and poor iron ion removal in the existing sewage pretreatment system,an experimental study of calcium removal and iron removal is carried out.Four kinds of calcium removal agents are selected to conduct single and composite calcium removal,optimize iron removal agent and carry out the experimental study on removing iron and calcium.In the separate calcium removal experiment,the temperature has the greatest effect on the removal of calcium ions from alcohol-containing sewage.When the temperature is high,the overall calcium removal rate is greater than that of the lower temperature.Among the four calcium removal agents,calcium carbonate has the best calcium removal effect,and its maximum calcium removal rate is 95.41%.In the compound calcium removal experiment,the combined effect of sodium carbonate--sodium oxalate--sodium hypophosphite is better than that of the other three groups,in which the optimal removal rate of calcium is 93.57%,but the effect of calcium removal is not improved compared with the single calcium removal experiment.Considering the cost of production and the simplicity of operation,calcium carbonate is selected as calcium removal agent to achieve a better calcium removal effect.The iron removal efficiency of H₂O₂ is better than that of sodium hypochlorite in the optimization experiment of removing iron.When the dosage of hydrogen peroxide is 120mg/L,the iron removal efficiency is the best,and the iron content is 0.23mg/L at this time,which achieves the standard of water reinjection.In the experiment of calcium and iron removal,the order of adding medicine is studied.The best order is Na₂CO₃→NaOH→H₂O₂→PAC→PAM.In the static time experiment,the static time is 5h and the treatment effect of alcohol-containing sewage is the best.The iron content is 0.25mg/L,the removal rate of calcium is 93.3%,the transmittance is 91.6%,and the oil removal rate is 90.5%.（2）In view of the insufficient flocculation effect in the pretreatment system,single component and compound flocculation experiments are carried out.In the single component flocculation experiment,four kinds of flocculants（PAC,PAM,CMC,CTS）are selected and the flocculation effect under different dosage is investigated.The flocculation effect is the best when the content of PAC is added to 60mg/L,and the transmittance and oil removal rate are90.5%,87.5%respectively.In the experiment of compound flocculant dosage,the flocculation effect of PAC-PAM and PAC-CTS is better than that of PFS-PAM and PFS-CTS,and the flocculation effect of PAC-PAM and PAC-CTS is better when pH value is 7 to 8 in the experiment of pH value on the flocculation effect.In the experiment of mixing time on flocculation effect,when mixing time is 5min,the flocculating effect of PAC-PAM and PAC-CTS is the best,the transmittance and deoiling rate of PAC-PAM are 97.1%,98.5%respectively,and the transmittance and deoiling rate of PAC-CTS are 97.7%,97.3%.The deoiling rate of PAC-PAM is superior to PAC-CTS,but the transmittance of PAC-PAM is lower than PAC-CTS.In the preparation of PAC-CMC compound flocculant,the dosage ratio of carboxymethyl chitosan and poly aluminum chloride is 1:7.The reaction temperature is70℃,the reaction time is 2.5h,the transmittance of the alcohol-containing sewage can reach to 98.7%,and the better flocculation effect has been obtained,meanwhile,the production and operation requirements of the methanol recovery unit are met.