This paper presents a comparative thermodynamic and economic analysis of two kinds of organic Rankine cycles (ORCs) with pure and zeotropic mixtures for recovering waste heat from the exhaust gases of large diesel engines used in the offshore platforms of phase 12 of South Pars Gas on Persian Gulf. The mixtures of three hydrocarbons with two refrigerants in two cycle arrangements (simple ORC and ORC with internal heat exchanger) at different evaporation temperatures are investigated to optimize three indicators. The results showed that both the energy and exergy efficiencies are maximized at particular mass fractions of refrigerants. The ORC with mixture of R236ea/Cyclohexane (with a ratio of 0.6/0.4) has the best performance as its energy and exergy efficiency are 14.57% and 37.84%, respectively. These values are increased to 16.81% and 40.75%, respectively by adding IHE to system. The minimum amount of the specific investment cost for the most cases is achieved at the mass fractions of 0.1 and 0.5 and it is greater for the ORC with IHE. Also the payback period of investment is calculated for comparison of economic value of systems and it is observed that its amounts for the ORC with IHE are greater than simple one