Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 85

Although the energetics of the fusion process may be considered as satisfactorily assessed, the kinetic aspects of fusion are not yet well enough understood. The efficiency of heat exploitation η_ex of a furnace varies according to a hyperbolic law of the type η_ex = 1/(A + B·p) with the production rate p (t/h). Thus, furnaces are preferentially operated at the highest achievable rates. The limits for p are determined by the rate of heat transfer or the time demand of the fusion process required to achieve an acceptable glass quality. As of now, however, one does not even known which of the above constraints controls the melting rate. As a matter of fact, the answer depends on both furnace and batch design.

Table 5 Calculation scheme for the energetics of a soda‐lime silicate glass (composition in wt %).^a

Oxide	wt %	Compound k	H°k,GL	H k,1300	c P,k,L	m(k)	m(k)· H°k,GL	m(k)· Hk,1300
			MJ/kg	MJ/kg	kJ/kg·K	kg/t	MJ/kg	MJ/kg
SiO2	71.84	hm	4.4313	3.0196	0.9217	0.18	0.8	0.5
Al2O3	1.50	FS	8.7888	7.3999	1.0589	0.22	1.9	1.6
Fe2O3	0.03	MS	14.9599	13.2740	1.4582	74.47	1114.1	988.5
MgO	2.99	NS2	13.4194	11.6862	1.4335	284.99	3824.4	3330.4
CaO	9.47	NC3S6	14.0278	12.6137	1.3301	332.51	4664.4	4194.2
Na2O	13.96	NAS6	14.7131	13.2234	1.2358	65.46	963.2	865.6
K2O	0.21	KAS6	14.0258	12.5775	1.3755	12.41	174.1	156.1
Sum	100.00	S	15.0023	13.6179	1.4347	229.75	3446.9	3128.8
						Sum	H°GLASS	H1300,MELT
						1000.00	14 189.7	12 665.9
							ΔH1300
							1523.8

^a H°_k,GL = standard enthalpy of component k in the glassy state; H_k,1300 = enthalpy of k in the liquid state at 1300 °C; c_P,k,L = isobaric heat capacity of liquid k; m(k) = equilibrium amount of k in the multicomponent phase diagram; H°_GLASS = standard enthalpy of the resulting glass; H_1300,MELT = enthalpy of the melt at 1300 °C; ΔH₁₃₀₀ = heat content of the melt at 1300 °C relative to the glass at 25 °C.

^b hm = FeO·Fe₂O₃, F = Fe₂O₃, M = MgO, C = CaO, N = Na₂O, K = K₂O, S = SiO₂.

A better understanding of redox and acid base reactions in real furnaces is also desired. Although these reactions are well understood at the laboratory scale, the transfer to a real production situation is still set by experience rather than by scientific principles. In view of the large impact of these reactions on glass quality, progress in this area would be highly appreciated.

Finally, the glass industry is engaged in a quest to lower its overall energy consumption to decrease its operating costs and to satisfy increasingly stringent legislation imposed on high‐temperature industrial processes. The design of faster conversion batches is becoming important in this respect. Conventional glass formulae and batch recipes are no longer taken for granted. Efforts are in particular made to design batches that would melt along reaction pathways ensuring higher turnover rates than current randomly mixed batches. Progress may be achieved with selective batching, granulation processes bringing the reaction partners into close contact at the μm scale, preparation of core‐shell type pellets, or selective preheating of specific raw‐material combinations of the batch. In each case, of course, a prerequisite would be that the obtained energy savings are not offset by increased batch costs.