Full-cycle slow-lift limited optimization analysis of multi-effect distillation heat transfer temperature difference in seawater desalination system

doi:10.11949/0438-1157.20221212

Abstract

Abstract:

Multi-effect evaporative seawater desalination technology is the mainstream seawater desalination method at this stage. In the actual production process, the process designer will redundantly design the evaporator heat exchange area to deal with the scaling and safety problems of the system. On the another hand, the secondary steam valve in the conventional method does not play an appropriate role in the distillation process, which leading to lower anti-jamming capability of the system. Based on the temperature characteristics of the seawater desalination multi-effect system, it was found that the full-cycle operation of the secondary steam valve can not only improve the utilization rate of the heat exchange area and the operating efficiency of the system, but also avoid the occurrence of the problem of internal consumption. Therefore, a slow-lift limited full-cycle optimization method of the evaporative heat transfer temperature difference is proposed. This method increases the operating efficiency of the margin slow-release optimization model, controls the secondary steam valve and considers the full-cycle operating characteristics. Therefore, this new method realizes the effective monitoring of the heat exchange area of the system. At the end of this essay, the full-cycle slow-lift limited optimization of heat transfer temperature difference is verified in the eight-effect seawater desalination device. The results show that this method takes into account the long-term and short-term goals of system operation, enhances the system's adjustment ability during full-cycle operation, reduces the coupling phenomenon between various effect evaporators, decreases internal margin consumption and external driving steam consumption, and realizes slow release of margin of the slow time-varying system.

Key words: process systems engineering, seawater desalination, slow time-varying, accumulation of fouling, margin slow-released optimization, full-cycle optimization

摘要：

多效蒸发海水淡化技术作为现阶段主流的海水淡化方法，在实际生产过程中工艺设计者都会对蒸发器换热面积进行冗余设计以应对系统的结垢和安全问题。在追求蒸发器冗余换热面积的利用率时，通常会伴随裕量内耗现象。与此同时，常规方法在研究的过程中并未发挥二次蒸汽阀的操作性能，系统的抗干扰能力较差。经过深入探究多效海水淡化系统的温度特性后，发现二次蒸汽阀的全周期操作既能够提高换热面积的利用率和系统的运行效益，又可以避免裕量内耗问题的发生。由此提出蒸发传热温差的全周期渐变优化方法，该方法不仅将运行效益加入系统的裕量缓释优化模型，而且对二次蒸汽阀的控制作用和全周期操作特性加以考虑，形成对系统换热面积的有效监控。最后，以8效海水淡化装置为例，对蒸发传热温差全周期渐变优化方法进行验证。结果表明，该方法能够兼顾系统运行的长短期目标，同时能够增强系统在全周期运行过程中的调控能力，降低各效蒸发器之间的耦合关系，减少裕量内耗的发生和外来驱动蒸汽的消耗，实现慢时变系统的裕量缓释。

关键词: 过程系统工程, 海水淡化, 慢时变, 污垢累积, 裕量缓释优化, 全周期优化

CLC Number:

TQ 028.8

Dehong WANG, Lin SUN, Xionglin LUO. Full-cycle slow-lift limited optimization analysis of multi-effect distillation heat transfer temperature difference in seawater desalination system[J]. CIESC Journal, 2022, 73(12): 5469-5482.

王德宏, 孙琳, 罗雄麟. 海水淡化系统多效蒸发传热温差全周期渐变优化分析[J]. 化工学报, 2022, 73(12): 5469-5482.

Figures/Tables 16

Fig.1 Process flow diagrams of the MED-TVC system with multi-effect

Table 1 Design value of heat exchange area of eight-effect seawater desalination plant

效序数	换热面积设计值 $A s p (i)$ /m²
1	3763.76
2	3636.11
3	3478.54
4	3281.99
5	3047.60
6	2775.37
7	2465.31
8	2116.27

Table 1 Design value of heat exchange area of eight-effect seawater desalination plant

效序数	换热面积设计值 $A s p (i)$ /m²
1	3763.76
2	3636.11
3	3478.54
4	3281.99
5	3047.60
6	2775.37
7	2465.31
8	2116.27

Table 2 Specifications of the MED-TVC system with eight-effect

设计参数		设计可调参数
进料方式	平行-交叉	每效进料流量F_f⁽ⁱ⁾/(kg/s)	31.59
海水温度/℃	30	加热温差ΔT_h⁽ⁱ⁾/℃	3
海水含盐量/(g/kg)	30	TVC引射流量F_ent/(kg/s)	4.43
冷凝器出口温度/℃	35	外来驱动蒸汽流量F_mot/(kg/s)	7.35
驱动蒸汽压力P_mot/kPa	500	淡水产量F_rated/(kg/s)	75.81
驱动蒸汽温度T_mot/℃	151.8	造水比GOR	10.31
运行周期/月	24	预热温升ΔT_p⁽ⁱ⁾/℃	4

Fig.2 Feeding situation and existing problems of conventional allowance slow-release optimization method

Fig.3 Single-effect adjustable analysis diagram

Table 3 Single effect operation variables

控制手段	控制变量
进料分离器	进料流量F_f⁽ⁱ⁾
二次蒸汽阀	蒸发器蒸发温度T⁽ⁱ⁾、加热温差ΔT_h⁽ⁱ⁾
预热蒸汽分离器	预热温升ΔT_p⁽ⁱ⁾

Fig.4 Single-effect and holistic multi-angle analysis of the effect of heating temperature differences on system performance

Fig.5 Influence of preheating temperature difference on system performance

Fig.6 Taking the second effect as an example to study the effect of the secondary steam valve on the steam temperature

Fig.7 The impact of secondary steam control valves on system performance

Fig.8 Regular diagram of full cycle operation

Table 4 optimization variables and optimization methods

参数	效序数								全周期优化次数
参数	1	2	3	4	5	6	7	8	全周期优化次数
蒸发温度T⁽ⁱ⁾/℃	[65,67]	[61,63]	[58,60]	[54,56]	[51,53]	[47,49]	[43,45]	[40,41]	12
预热温升∆T_p⁽ⁱ⁾/℃	[3,8]								1
夹带蒸汽F_ent/(kg/s)	[2,5]								12
总进料流量∑F_f⁽ⁱ⁾(i=1,…,8)/(kg/s)	[150,255]								12
二次蒸汽阀温度损失 $T l o s s (i)$ /℃	[0,1.2]								12

Table 4 optimization variables and optimization methods

参数	效序数								全周期优化次数
参数	1	2	3	4	5	6	7	8	全周期优化次数
蒸发温度T⁽ⁱ⁾/℃	[65,67]	[61,63]	[58,60]	[54,56]	[51,53]	[47,49]	[43,45]	[40,41]	12
预热温升∆T_p⁽ⁱ⁾/℃	[3,8]								1
夹带蒸汽F_ent/(kg/s)	[2,5]								12
总进料流量∑F_f⁽ⁱ⁾(i=1,…,8)/(kg/s)	[150,255]								12
二次蒸汽阀温度损失 $T l o s s (i)$ /℃	[0,1.2]								12

Table 5 State variables and their initial design values

变量	效序数
变量	1	2	3	4	5	6	7	8
预热蒸汽分离比ξ⁽ⁱ⁾	0.98	0.96	0.94	0.91	0.89	0.85	0.81	—
蒸发压力P⁽ⁱ⁾/kPa	27.34	23.92	20.86	18.15	15.74	13.61	11.73	10.07
有效换热面积 $A e (i)$ /m²	3002	2914.81	2789.21	2632.69	2445.77	2228.64	1981.19	1702.91
二次蒸汽产量F_s⁽ⁱ⁾/(kg/s)	11.33	11.16	10.68	10.09	9.41	8.63	7.75	6.77
污垢热阻R_f⁽ⁱ⁾/(m²∙K/W)	0	0	0	0	0	0	0	0

Table 5 State variables and their initial design values

变量	效序数
变量	1	2	3	4	5	6	7	8
预热蒸汽分离比ξ⁽ⁱ⁾	0.98	0.96	0.94	0.91	0.89	0.85	0.81	—
蒸发压力P⁽ⁱ⁾/kPa	27.34	23.92	20.86	18.15	15.74	13.61	11.73	10.07
有效换热面积 $A e (i)$ /m²	3002	2914.81	2789.21	2632.69	2445.77	2228.64	1981.19	1702.91
二次蒸汽产量F_s⁽ⁱ⁾/(kg/s)	11.33	11.16	10.68	10.09	9.41	8.63	7.75	6.77
污垢热阻R_f⁽ⁱ⁾/(m²∙K/W)	0	0	0	0	0	0	0	0

Fig.9 Effect of operating conditions on system performance

Fig.10 Optimization results of temperature slow-lift limited of full cycle evaporation heat transfer

Table 6 Comparison of operation results of design conditions， conventional optimization and temperature slow-lift limited optimization of full cycle evaporation heat transfer

参数	设计条件	常规裕量缓释优化方法	蒸发传热温差全周期渐变优化方法
外来驱动蒸汽F_mot/(kg/s)	7.35	[7.13,7.35]	[6.8,6.85]
总蒸汽流量/(10⁸ kg)	4.64	4.55	4.30
造水比GOR	10.31	[10.31,10.62]	[11.07,11.15]
二次蒸汽阀总温度损失􀰑 $T l o s s (i)$ /℃	0	0	5.89
总加热温差 􀰑ΔT_h⁽ⁱ⁾(i=1,…,7)/℃	21	21	[19.10,24.82]

Table 6 Comparison of operation results of design conditions， conventional optimization and temperature slow-lift limited optimization of full cycle evaporation heat transfer

参数	设计条件	常规裕量缓释优化方法	蒸发传热温差全周期渐变优化方法
外来驱动蒸汽F_mot/(kg/s)	7.35	[7.13,7.35]	[6.8,6.85]
总蒸汽流量/(10⁸ kg)	4.64	4.55	4.30
造水比GOR	10.31	[10.31,10.62]	[11.07,11.15]
二次蒸汽阀总温度损失􀰑 $T l o s s (i)$ /℃	0	0	5.89
总加热温差 􀰑ΔT_h⁽ⁱ⁾(i=1,…,7)/℃	21	21	[19.10,24.82]

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