in the process according to the invention , ethanol is dehydrated under the action of the solid catalyst to form ethylene : and methanol is dehydrated under the action of the solid catalyst to form dimethyl ether : it is well known that the dehydration of ethanol is a strongly endothermal reaction . in the case where pure ethanol is fed , a temperature drop in an adiabatic reactor is about 400 ° c . therefore , in fixed bed processes for preparing ethylene by dehydrating ethanol , a shell and tube fixed bed reactor or a multistage fixed bed reactor is generally employed . if a shell and tube fixed bed reactor is employed , for a large scale process for preparing ethylene by dehydrating ethanol , there are problems relating to engineering enlargement and equipment manufacture . although a multistage fixed bed reactor may maintain the used catalyst in a suitable operation temperature range by supplying heat at a position or positions between the stages , the presence of a larger temperature gradient in the catalyst bed results in that the catalyst cannot function best and that the perfect selectivity to ethylene is hardly achieved . furthermore , the two kinds of reactor suffer from a common problem that energy consumption is high . the inventors have noted that the reaction of dehydrating methanol to dimethyl ether is a strongly exothermal reaction and is substantially the same as the reaction of dehydrating ethanol to ethylene in reaction condition , the catalyst used , and the sequent isolation system . thus , the present invention couples the dehydration reaction of methanol and the dehydration reaction of ethanol , and thereby provides a process for the co - production of ethylene and dimethyl ether . because the two reactions are coupled in situ in heat , there does not need further supplying or removing a large amount of heat . hence , the process is energy - effective , resulting in that the process flow is simplified , equipment investment is reduced , and the reactor can be easily enlarged . there is not a specific limitation to the source of ethanol and methanol used as feedstock in the process of the invention . from the view point of matching reaction heat , the weight ratio of methanol to ethanol in the feedstock may be in a range of from 1 : 10 to 10 : 1 , preferably from 1 : 5 to 8 : 1 , more preferably from 1 : 2 to 6 : 1 , and most preferably from 1 : 1 to 5 : 1 . the catalyst useful in the process of the invention may be selected from the group consisting of alumina catalysts and crystalline aluminosilicate catalysts , which are known by those skilled in the art . the alumina catalysts comprise preferably γ - al 2 o 3 . crystalline aluminosilicate catalysts comprise preferably at least one selected from the group consisting of zsm molecular sieves , β - zeolites and mordenite . in a preferred embodiment , the solid catalyst comprises a zsm molecular sieve , especially a zsm - 5 molecular sieve , having a molar ratio of sio 2 to al 2 o 3 of from 20 to 500 , and preferably from 30 to 200 . in addition to the alumina or the crystalline aluminosilicate , the catalyst may further comprise a conventional binder . the process according to the invention may be carried out under the following reaction conditions : a reaction temperature in a range of from 200 to 480 ° c ., a reaction pressure in a range of from 0 to 2 mpa ( gauge ), and a whsv of the feedstock in a range of from 0 . 1 to 10 h − 1 . the reaction conditions may be further optimized according to the selected catalyst . when the solid catalyst is an alumina catalyst , the reaction temperature is preferably in a range of from 300 to 480 ° c ., and more preferably from 350 to 430 ° c . ; the whsv of the feedstock is preferably in a range of from 0 . 5 to 5 h − 1 ; and the reaction pressure is preferably in a range of from 0 . 1 to 1 mpa ( gauge ). when the solid catalyst is a crystalline aluminosilicate catalyst , the reaction temperature is preferably in a range of from 200 to 400 ° c ., and more preferably from 230 to 350 ° c . ; the whsv of the feedstock is preferably in a range of from 0 . 5 to 5 h − 1 ; and the reaction pressure is preferably in a range of from 0 . 01 to 1 . 0 mpa ( gauge ). in an embodiment , at least a part of the obtained dimethyl ether is further converted to olefins , especially light olefins , mainly ethylene and propylene , through an oxygenate - to - olefin process . the oxygenate - to - olefin processes are well known by those skilled in the art . see , for example , cn96115333 . 4 , cn00802040 . x , cn01144188 . 7 , cn 200410024734 . x , and cn92109905 . 3 . the process of the invention allows the reaction to proceed at a lower temperature , for example about 250 ° c ., under a higher space velocity of the feedstock , for example more than 5 h − 1 . the reduction of the reaction temperature may markedly lower energy consumption in the operation , aid to reduce side reactions , and lower the rate of catalyst coking to thereby effectively prolong the service lifetime of the catalyst . the enhancement of the space velocity of the feedstock may enhance throughput per unit volume of the reactor . furthermore , the heat released by the reaction of dehydrating methanol compensates the heat taken up by the reaction of dehydrating ethanol so that a non - shell and tube type monostage adiabatic fixed bed reactor may be employed to carry out the reaction of dehydrating ethanol to ethylene . as a result , the difficulty relating to reactor enlargement is greatly reduced and energy consumption in the operation is further reduced . by the process according to the invention , higher conversion of ethanol , for example approximately 100 %, higher selectivity to ethylene , for example more than 96 %, and higher selectivity to dimethyl ether , for example more than 90 %, are achieved . the following examples are given for further illustrating the invention , but do not make limitation to the invention in any way . ten grams of γ - al 2 o 3 catalyst having a specific surface area of 200 m 2 / g and an alumina content of 99 . 7 wt . % were charged into a fixed bed reactor having an inner diameter of 22 mm , and then activated in a nitrogen flow at 550 ° c . for 2 h . after allowing the temperature inside the reactor to lower to reaction temperature , a feedstock consisting of methanol and ethanol ( mass ratio of methanol to ethanol = 2 : 1 ) was continuously fed into the reactor and allowed to react under the following conditions : reaction temperature = 360 ° c ., whsv of the feedstock = 1 . 5 h − 1 , and reaction pressure = 0 . 02 mpa ( gauge ). effluent of the reactor was analyzed , and it was found that conversion of ethanol was approximatively 100 %, selectivity to ethylene was 99 . 6 %, conversion of methanol was 78 . 1 %, and selectivity to dimethyl ether was 98 . 1 %. experiments were carried out following the procedure as described in the example 1 under the conditions as set forth in the table 1 below . the results are shown in the table 1 . ten grams of γ - al 2 o 3 catalyst having a specific surface area of 200 m 2 / g and an alumina content of 99 . 7 wt . % were charged into an adiabatic fixed bed reactor having an inner diameter of 22 mm , and then activated in a nitrogen flow at 550 ° c . for 2 h . after allowing the temperature inside the reactor to lower to reaction temperature , a feedstock consisting of methanol and ethanol ( mass ratio of methanol to ethanol = 1 : 2 ) was continuously fed into the reactor and allowed to react under the following conditions : temperature at reactor inlet = 400 ° c ., whsv of the feedstock = 3 . 6 h − 1 , and reaction pressure = 0 . 2 mpa ( gauge ). the temperature at reactor outlet was 328 ° c . effluent of the reactor was analyzed , and it was found that conversion of ethanol was approximatively 100 %, selectivity to ethylene was 95 . 7 %, conversion of methanol was 80 . 7 %, and selectivity to dimethyl ether was 93 . 2 %. 10 g of γ - al 2 o 3 catalyst having a specific surface area of 200 m 2 / g and an alumina content of 99 . 7 wt . % was charged into an adiabatic fixed bed reactor having an inner diameter of 22 mm , and then activated in a nitrogen flow at 550 ° c . for 2 h . after allowing the temperature inside the reactor to lower to reaction temperature , a feedstock consisting of methanol and ethanol ( mass ratio of methanol to ethanol = 4 : 1 ) was continuously fed into the reactor and allowed to react under the following conditions : temperature at reactor inlet = 360 ° c ., whsv of the feedstock = 4 h − 1 , and reaction pressure = 0 . 06 mpa ( gauge ). the temperature at reactor outlet was 362 ° c . effluent of the reactor was analyzed , and it was found that conversion of ethanol was approximatively 100 %, selectivity to ethylene was 97 . 7 %, conversion of methanol was 81 . 2 %, and selectivity to dimethyl ether was approximatively 100 %. 100 g of zsm - 5 molecular sieve having a sio 2 / al 2 o 3 molar ratio of 40 was mixed with 60 g of a silica sol ( having a silica content of 30 wt . %), and then the mixture was extruded . the extrudates were dried at 180 ° c . for 6 h , and then calcined at 500 ° c . for 4 h , to give a zsm - 5 molecular sieve catalyst . 3 g of the prepared zsm - 5 molecular sieve catalyst was charged into a fixed bed reactor having an inner diameter of 18 mm , and then activated in a nitrogen flow at 550 ° c . for 2 h . after allowing the temperature inside the reactor to lower to reaction temperature , a feedstock consisting of methanol and ethanol ( mass ratio of methanol to ethanol = 2 : 1 ) was continuously fed into the reactor and allowed to react under the following conditions : reaction temperature = 250 ° c ., whsv of the feedstock = 3 h − 1 , and reaction pressure = 0 . 02 mpa ( gauge ). effluent of the reactor was analyzed , and it was found that conversion of ethanol was 99 . 2 %, selectivity to ethylene was 95 . 4 %, conversion of methanol was 78 . 1 %, and selectivity to dimethyl ether was 90 . 4 %. experiments were carried out following the procedure as described in the example 15 under the conditions as set forth in the table 2 below . the results are shown in the table 2 . 100 g of zsm - 5 molecular sieve having a sio 2 / al 2 o 3 molar ratio of 50 was mixed with 60 g of a silica sol ( having a silica content of 30 wt . %), and then the mixture was extruded . the extrudates were dried at 180 ° c . for 6 h , and then calcined at 500 ° c . for 4 h , to give a zsm - 5 molecular sieve catalyst . 3 g of the prepared zsm - 5 molecular sieve catalyst was charged into an adiabatic fixed bed reactor having an inner diameter of 18 mm , and then activated in a nitrogen flow at 550 ° c . for 2 h . after allowing the temperature inside the reactor to lower to reaction temperature , a feedstock consisting of methanol and ethanol ( mass ratio of methanol to ethanol = 2 : 1 ) was continuously fed into the reactor and allowed to react under the following conditions : temperature at reactor inlet = 360 ° c ., whsv of the feedstock = 3 h − 1 , and reaction pressure = 0 . 2 mpa ( gauge ). the temperature at reactor outlet was 280 ° c . effluent of the reactor was analyzed , and it was found that conversion of ethanol was approximatively 100 %, selectivity to ethylene was 91 . 3 %, conversion of methanol was 83 . 7 %, and selectivity to dimethyl ether was 90 . 8 %. 100 g of zsm - 5 molecular sieve having a sio 2 / al 2 o 3 molar ratio of 80 was mixed with 60 g of a silica sol ( having a silica content of 30 wt . %), and then the mixture was extruded . the extrudates were dried at 180 ° c . for 6 h , and then calcined at 500 ° c . for 4 h , to give a zsm - 5 molecular sieve catalyst . 3 g of the prepared zsm - 5 molecular sieve catalyst was charged into an adiabatic fixed bed reactor having an inner diameter of 18 mm , and then activated in a nitrogen flow at 550 ° c . for 2h . after allowing the temperature inside the reactor to lower to reaction temperature , a feedstock consisting of methanol and ethanol ( mass ratio of methanol to ethanol = 4 : 1 ) was continuously fed into the reactor and allowed to react under the following conditions : temperature at reactor inlet = 300 ° c ., whsv of the feedstock = 0 . 8 h − 1 , and reaction pressure = 0 . 06 mpa ( gauge ). the temperature at reactor outlet was 300 ° c . effluent of the reactor was analyzed , and it was found that conversion of ethanol was approximatively 100 %, selectivity to ethylene was 92 . 3 %, conversion of methanol was 84 . 2 %, and selectivity to dimethyl ether was 91 . 3 %. the patents , patent applications , non - patent literatures and testing methods cited in the specification are incorporated herein by reference . while the invention has been described with reference to exemplary embodiments , it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention . therefore , the invention is not limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention , but the invention will include all embodiments falling within the scope of the appended claims .