Development Of Gas Carrying Evaporation And Study Of The Fouling On Its Heated Surface

To avoid or mitigate scale formation or thermal deterioration in the process of evaporation in pharmaceutical industry, food industry and chemical industry, Gas Carrying Evaporation (GCE) has been developed by Research Institute of Chemical Engineering, Dalian University of Technology. The results of some basic studies of GCE have been used in the processes of concentration and reboiling of solutions. However, these studies paid attention to higher liquid velocity (ui =0.3 ~ 1.6m s"1), which only fit batch operation. In order to shorten the residence time of thermally sensitive solution in evaporator, a single pass gas carrying evaporator is recently developed, which can operate continuously and has higher evaporation rate, and thus can be used in evaporation operation of mass production. The evaporator needs to be operated at lower liquid velocity for the sake of obtaining higher product concentration. On the other hand, it is necessary to introduce a large amount of air (using air as carrying gas and polymerization retardant) for the evaporation of easily polymerizing aqueous acrylamide solution, especially concentrate aqueous acrylamide solution produced through biologic method. Hence, it is urgent to further develop Gas Carrying Evaporation. GCE at lower carrying gas velocity must be further investigated, and a new GCE method for concentration of aqueous acrylamide solution at higher carrying gas velocity needs also to be developed.Fouling is a common problem in industry. It affects on heat transfer, and causes the extreme waste of energy. Fouling is usually very serious in the process of concentration, and even affects on evaporating operation. In GCE, scale formation can be mitigated, due to nucleate boiling at heating wall being suppressed. Therefore, the investigation of scale formation mechanism, especially the influence of carrying gas injection on fouling will provide theoretical basis for broadening the application of GCE.The main achievements of the dissertation are as follows:(1) The heat transfer and operating characteristics of Gas Carrying Evaporation at low liquid velocity ( =0.032 ~ 0.321m s1) is studied (using air as carrying gas). The results show that the heating wall temperature is lowered, which results in raising heat transfer coefficient as compared to that for convective boiling. When the superficial carrying gas velocity at the entrance of heating tube and the operation temperature are fixed, the heat transfer coefficient of GCE increases with increasing in superficial carrying gas velocity at lower superficial carrying gas velocity, On the contrary, it decreases with increasing in superficial carrying gas velocity. Superficial carrying gas velocity that the heat transfer coefficient of GCE is just reduced with increasing in superficial carrying gas velocity is larger with increasing in superficial liquid velocity. Comparing with convective boiling, flow patterns of whole heating tube in GCE move down remarkably and lift velocity of liquid is increased. Moreover, the wall superheat temperature of the whole heating tube is lowered in the process of bubbles sliding. The mechanism of enhanced heat transfer in GCE is analyzed by using simplified model.(2) A new GCE method for concentrating aqueous acrylamide solution (using air as carrying gas) is developed. Sieve tray tower is used to concentrate aqueous acrylamide solution, and the heating tubes are arranged on sieve trays in sieve tray tower. Thus, equilibrium control is changed into heat transfer control in the process of concentration of aqueous acrylamide solution. The new method uses higher carrying gas velocity, can better avoid to polymerization of aqueous acrylamide solution, and operation range and operation turndown are improved, as compared to the past GCE method of concentration of aqueousacrylamide solution. Multiplayer sieve trays can be used in sieve tray tower of GCE. Evaporate rate of the tower has been increased remarkably. Concentration ratio of sieve tray tower, which is arranged heating tubes, can increase...