After a long run, a lot of memory is allocated by my programme using QuickOPC.

This problem occurs when you instantiate the EasyDAClient class very often and do not clean up the instances correctly.
When an instance should not be used anymore call EasyDAClient.Dispose() to flag to the .NET GC (Garbage Collector) that the instance should be deleted. Alternatively, try to use one or few instances of EasyDAClient instead of several. But call EasyDAClient.Dispose() when the instances should be deleted.