(206d) Optimization of Water Use in Oil Refinery

In most process industries water is vital in many operations and is utilized for different purposes like product formulation, cooling, high-purity water makeup systems, general plant service water, waste conveyance/transfer, potable/sanitary service and fire protection [1]. However, processes and systems using water are being subjected to the increasing costs of wastewater treatment to meet the increasingly stringent environmental regulations, the growing demand for fresh water, the scarcity of good quality water around the world for different purposes, economic consideration, growing public concern for the quality of the environment, and restrictions on the expansion of water use at many sites [2]. All these factors have created a powerful economic driving force to rationalize the water use and have increased the need for better water management and wastewater minimization, i.e., it deals with a water/wastewater allocation-planning problem. It consists of finding the minimum amount of fresh water that each water-using process needs, together with the maximum amount of water effluent from these processes that can be reused in other processes [3].

There are two basic strategies for reducing water demand in such plants. One strategy consists of modifying individual process and utility units to reduce their inherent need for water. Examples include replacing water-cooling by air cooling, changing live steam by a reboiler in a stripping column, etc. The other basic strategy consists of seeking opportunities to use outlet water from one operation to satisfy the water requirement of another or the same operation. In some cases, the water may require some regeneration prior to re-use and/or recycle.

The industrial wastewater optimization is in many cases a tailor made problem. The main focus of this work is to present a general procedure to analyze the water/wastewater allocation-planning problem and its application in a real-life situation ? a Brazilian oil refinery.

Oil refineries consume a large amount of water, mainly for processing and cooling. The worldwide reference for specific water consumption in refineries is between 0.7 and 1.2 m³ of water per m³ of crude oil processed [4]. This means that an industry that refines nearly 27 thousands m³ of petroleum per day, as the case study, consumes about 27 million liters of water each day. The refinery considered in this study is located in a region with a high concentration of industrial activities and nearby to an urban agglomeration. Thus, the scarcity of good quality water and competition with human consumption are important reasons to develop this study.

Although the emphasis is to show results for this kind of industry, the systematic methodology developed is valid for any process plants, independent of their dimensionality.

It consists of specific steps: to evaluate the whole process in order to identify points of great consumption of water and wastewater generation; to classify the water by type of use and the wastewater by level and kind of contaminants; to identify all possible opportunity to apply the strategies for optimizing the water allocation in a plant, including rationalization techniques, process or operational conditions changes, water re-use, regeneration and re-use and regeneration and recycling; to optimize the final structure and finally to evaluate economically the solution presented.

In the traditional approaches, the study begins by treating the effluent before discharging back into the environment or reusing in the process. Thus, the aim is to mitigate the effects but the solutions obtained by these methods are often technically difficult and costly to implement.

An optimization study on water uses must start by analyzing the causes: the internal processes that use water. Internal processes and treatments necessary to adjust the water quality should be modeled, taking account the water quality after the treatments and also the costs of treatment. The optimized network results in minimizing the volume of freshwater required and, consequently, in reducing the effluent generated compared to the original network. It is the best solution under the consideration of technical, economic and ecological aspects.

Moreover, the study also includes water uses at utilities plant (cooling towers and steam generation) and for general uses (human uses, fire fighting and services), while studies in the literature only consider water uses in the production processes.

The optimization basic idea is to combine water reuse and specifics treatments in order to reduce water consumption per crude oil refined. The quality of water necessary for each process and the maximum tolerable concentration of contaminants associated with mathematical tools allow the water network optimization. For the case study, different alternatives of possible arrangements and treatment technologies are explored considering contaminants like salts, organics, sulfur and ammonia. The basic configuration is the usual one, which consists of use fresh water and end-of-pipe treatment. The proposed alternatives are schemes where wastewaters are allocated to reuse/recycling and according to quality of water necessary for the receiver process they could be regenerated. In this case end-of-pipe and decentralized treatments are considered.

The objective function of the model consists of minimizing the amount of water consumption and/or the minimum wastewater treatment capacity with minimum annual cost. The results show that the relationship between regeneration and fresh water costs, as the concentration of some pollutants after treatments, is the determinant factor for operation viability. The ideal situation is to reach the ?almost zero liquid discharge?. This aim is expected in the alternative that reuse and decentralized regeneration are utilized [5].

The results has proved that the water/wastewater minimization techniques used involve environmental benefits since they can effectively reduce overall fresh water demand and the overall effluent generated, resulting in lower costs of fresh water and effluent treatment costs. As a consequence of the strategy employed, the industry saves money, gains community trust and shows sensitivity to the environment.

References [1] R. M. Rosain, Chemical Engineering Progress, 89(4) (1993) 28. [2] M. E. Goldblatt, K. S. Eble and J. E. Feathers, Chemical Engineering Progress, 89(4) (1993) 22. [3] M. Bagajewicz and A. Koppol, AIChE Annual Meeting (2001) [4] P. Diepolder, Hydrocarbon Processing, 71 (10) (1992) 129. [5] M. Bagajewicz, Computers and Chemical Engineering, 24 (2000) 2093.