Background
In the field of municipal water supply and municipal sewage treatment, the outdoor water supply design standard (GB 50013) prescribes that the effective contact time is not less than 30min when chlorine disinfection is adopted. In general engineering design, when calculating the contact time of the contact disinfection tank, the hydraulic retention time of the contact disinfection tank is often adopted to replace the hydraulic retention time. The reactor model of the chlorine contact tank is a plug flow reactor. The hydraulic retention time is a calculation result when the plug flow reactor is in an ideal state, but the hydraulic retention time is in an nonideal state in the actual application process, so the hydraulic retention time cannot represent the effective contact time and is larger than the effective contact time. The effective contact time can directly influence the calculation of the effective pool volume, when the hydraulic retention time is used for replacing the calculation, if the selected value is 30min, the effective contact time of the chlorinating agent can be actually less than 30min, and the disinfection effect can be influenced; and the excessive hydraulic retention time wastes the tank capacity and raises the engineering cost. Therefore, the main factor affecting non-ideal flow, namely fluid axial dispersion, needs to be considered in the calculation process, reasonable values are judged through theoretical calculation, and proper pool capacity is selected, so that the effective contact time of the chlorine contact pool can be ensured, and the pool capacity can be saved to the greatest extent. In addition to contact time, existing calculations tend to ignore plug flow efficiency for a plug flow reactor, which in practice tends to correlate with pool type. The invention provides two evaluation systems, namely, plug flow efficiency evaluation and effective contact time evaluation, which can solve the problems of pool judgment and pool capacity selection and help to select proper values in design.
Two concepts are first clarified: 1. hydraulic retention time refers to the ratio of effective tank capacity to design scale; 2. the effective contact time refers to the residence time of the chlorine agent in reaction with the wastewater to achieve its effect.
The prior art and the calculation instruction book only require that the effective contact time of the chlorinating agent is 30 minutes, and the details of the calculation are not instructed. Engineering technicians often use hydraulic retention time to replace effective contact time in design calculations, or calculate effective pool capacity in terms of the amount of retention time reserved for 10% -20%.
The reactor type adopted by the chlorine contact disinfection tank is a plug flow reactor. The plug flow reactor is divided into innumerable small units axially from an inlet to an outlet. In each small unit of the ideal plug flow reactor, the disinfectant reacts with water uniformly; in our practical engineering, one factor that is the greatest in the non-ideal state of the reactor is the axial dispersion of the fluid, which can lead to a difference in the reaction between all the small units and the adjacent small units before and after the small units, and part of the disinfectant can flow out of the reactor before the small units due to the axial dispersion. Thus, if the hydraulic residence time is used instead of the effective contact time, a portion of the disinfectant will flow out of the reactor during the hydraulic residence time first, resulting in an unexpected disinfection effect. Thus, to achieve a greater degree of disinfection, while ensuring that the cell volume is minimized, an accurate calculation is required to calculate the effective contact time in place of the hydraulic dwell time.
Disclosure of Invention
The patent relates to a method for calculating the disinfection time of a chlorine contact disinfection pond considering axial dispersion. The whole calculation process involves two core evaluation systems, namely, first, the plug flow efficiency is evaluated, and the evaluation index is dispersity d; secondly, evaluating the effective contact time, wherein the evaluation index is that the contact time of 90% of chlorine disinfectant under the average flow is more than or equal to 30min and less than or equal to 32min.
The whole calculation process is divided into five steps, which are respectively as follows.
S1 is to determine the initial channel width, the total channel length, and the effective water depth. The key parameters are primarily obtained by calculating the hydraulic retention time, so that the calculation deviation is not too large, the channel width is the width of a single channel, W is used for representing the width of the single channel, the total channel length is the total length of the flow channel, L is used for representing the total length of the flow channel, and the effective water depth is H. The water-taking residence time of the initial data is about 30-40 min, the width-depth ratio is about 0.6-1.0, and the effective water depth is determined by calculation and takes a value in the range of 2-5 m according to different scales.
S2 is to calculate the dispersion at the average flow. d → infinity, meaning that the reactor is thoroughly mixed; d=0, which is an ideal plug flow. d is expressed as:
wherein d represents dispersity and dimensionless. In the axial dispersion model, d characterizes the extent of axial dispersion. d=0 is an ideal plug flow, and d= infinity is a complete mixing. D is the axial dispersion coefficient v, the forward flow velocity of the fluid, and L is the total length of the flow passage.
The chlorine contact pool model is in a turbulent flow state, and the calculation method of D is expressed as follows:
NR and v is the forward flow rate of the fluid.
In turbulent flow regime, NR The calculation method of (1) is expressed as follows:
where v is the fluid forward flow rate, R is the hydraulic radius, and μ is the kinetic viscosity.
The calculation method of d can be obtained by sorting and deducing the calculation method, and is expressed as follows:
whereas μ and ρ are constants, the whole formula variable has 5 values Q, W, H, L and d, respectively, and the degree of freedom=4.
Substituting Q and W, H, L for calculation, judging whether the dispersity d is smaller than 0.015, if not, finely adjusting the input value W, H, L in S1 to enable the dispersity d to meet the dispersity condition; if so, S3 computation is entered. This step completed the first step of dispersion evaluation of plug flow efficiency evaluation.
The calculation method of the Peclet number in S3 is expressed as follows:
where Pe is the Peclet number, which represents the mass transfer ratio caused by translation and dispersion. The number of Pe is the reciprocal of the dispersity d; according to the principle of the chemical reaction engineering axial dispersion model, in order to simulate a plug flow reactor with axial dispersion, the number n of the series of complete mixing reactors is approximately equal to the number of Pe divided by 2, so that the result of dividing the number of Pe by 2 is rounded downwards in the calculation process to obtain n, and the series of n complete mixing reactors is used for simulating a chlorine contact reaction tank under the practical application condition.
S4, a corrected Residence Time Distribution (RTD) curve calculation table of n complete mixing reactors connected in series is compiled. The RTD curve is composed of two curves, E (θ) is a residence time distribution function, F (θ) is a cumulative residence time distribution function, and F (θ) is obtained by integrating E (θ). The calculation formula of F (θ) and E (θ) is as follows:
θ is the corrected residence time, which represents the meaning: the ratio of the effective contact time to the hydraulic residence time of the plug flow reactor with axial dispersion; n is the number of the complete mixing reactors. The values of θ are calculated from 0 at intervals of Δθ=0.01, and the values of E (θ) and F (θ) are calculated, respectively, to create an RTD calculation table. The RTD curve is a continuous curve, and the simulated value can be obtained through computer programming, but in the conventional project design calculation, the RTD curve can be simplified into a table form, and the calculation and the reading are convenient. Examples of RTD patterns are shown in the accompanying drawings, and examples of calculation are shown in the attached tables.
S5, the section of the outdoor water supply design standard (GB 50013-2018) describes disinfection, and the contact time needs to be the residence time which can be achieved by 90% of disinfectant. It is therefore desirable to tabulate values of F (θ) greater than 0.1 and closest to 0.1, assuming that it is F (m) and the value of θ corresponding to F (m) is m, that F (m) represents the meaning that a flow of about F (m) has carried chlorine agent away from the chlorine contact tank before the hydraulic residence time of m has elapsed. Thus, taking F (θ) to a value greater than 0.1 and closest to 0.1 means that 90% of the chlorine agent remains in the pool before this point in time m, i.e., the effective contact time is met. The value of m is the corrected residence time, which is the product of the hydraulic residence time, i.e., the effective contact time for 90% of the chlorinating agent. After the effective contact time of 90% of the chlorinating agent is obtained, judging whether the effective contact time is within 30-32 min, if so, ending the calculation; and (3) adjusting the input value in the step S1 until the requirement is met. This step completes the effective contact time evaluation.
Fig. 3 shows a scheme of the present invention for displaying data by computer programming in a specific implementation process. The claimed display scheme is intended to include, but is not limited to, such a display form.
Detailed Description
The following is a specific demonstration in one case, assuming a treatment scale of 20000m3 And/d, setting 1 pool body, and totally setting 1 flow passage. According to the step S1, the hydraulic retention time is roughly calculated for 30min, and the effective tank capacity is about 417m3 Assuming that the channel width W is 2.5m and the effective water depth H is 2.5m, the total water flow length L is assumed to be 68m, and the hydraulic retention time after calculation is 30.6min. Then, according to step S2 of the present invention, the dispersity d=0.0114 is calculated, and the condition d < 0.015 is satisfied. According to the invention S3, the Pe number is calculated to be 87.7, and the number n of the complete mixing reactors connected in series is calculated to be 43. According to the invention, an E (theta) and F (theta) calculation table is compiled according to the S4. According to the invention, S5, F (theta) =10.64% is taken from the calculation table, the value of theta is 0.81, and after hydraulic retention time correction, the actual effective contact time is only 24.79min, so that the requirement is not met, and the S1 adjustment is returned. When S1 is adjusted, W and H can be fixed, only L is adjusted, and when L is adjusted to 84m in this case, the effective contact time is 31.37min. Meets the requirement that the effective contact time is in the range of 30 min-32 min, and the calculation is finished.
1. The calculation method provides effective contact time calculation logic of detailed steps and an implementation method.
2. The invention provides two evaluation systems, namely, plug flow efficiency evaluation and effective contact time evaluation.
3. The first optimization of the invention is that the evaluation of plug flow efficiency is considered, the reaction efficiency reduction caused by the axial dispersion of the chlorine contact reaction tank is fully considered, the dispersion degree judgment index is given, and d is required to be smaller than 0.015. Or designing a deviation correcting mechanism, continuing the step S4 when d is less than 0.015, otherwise, repeating the step S1; .
4. The other optimization of the invention is that the effective disinfection time of 90% of the chlorinating agent is regulated on the reaction efficiency, the effective disinfection time of 90% of the chlorinating agent is strictly controlled within 30-32 min, and the pool capacity is saved under the condition of meeting the disinfection effect. Or designing a correction mechanism, outputting a result when 90% is reached, otherwise reporting an error, and repeating the step S1.
5. All calculation processes only need to compile calculation logic, and three numerical values in the step S1 are continuously adjusted, so that an optimal result can be obtained through a verification program.
And 6, continuously taking the delta theta to be smaller to 0.005 or even smaller to 0.0001 in the step S4, and obtaining more accurate calculation results.
RTD curve calculation example