Excel Services service application

I created a new Excel Services service application, because I wanted it to have its own application pool and a service account that I could choose.

(The reason I did this is because I'm finding errors in the logs:" There was an error in communicating with Excel Calculation Services exception: An error has occurred. [Session:  User: xxxx]."
and
"ExcelService.PostProcessRequest: The server encountered a very serious error and will continue to function. Please log this as a bug. Exception was: System.TypeInitializationException: The type initializer for 'Microsoft.Office.Excel.Server.CalculationServer.ExcelServerApp' threw an exception. ---> System.IO.FileLoadException: Loading this assembly would produce a different grant set from other instances. (Exception from HRESULT: 0x80131401)     at Microsoft.Office.Server.Diagnostics.FirstChanceHandler.ExceptionFilter(Boolean fRethrowException, TryBlock tryBlock, FilterBlock filter, CatchBlock catchBlock, FinallyBlock finallyBlock)     at Microsoft.Office.Server.Diagnostics.ULS.SendWatsonOnExceptionTag(UInt32 tagID, ULSCatBase categoryID, String output, Boolean fRethrowException, TryBlock tryBlock, CatchBlock catchBlock, FinallyBlock finallyBlock)     at Microsoft.Office.Excel.Server.CalculationServer.ExcelServerApp..ctor()     at Microsoft.Office.Excel.Server.CalculationServer.ExcelServerApp..cctor()     --- End of inner exception stack trace ---     at Microsoft.Office.Excel.Server.CalculationServer.ExcelServiceBase.PostProcessRequest(CommandResult result, Exception ex) System.TypeInitializationException: The type initializer for 'Microsoft.Office.Excel.Server.CalculationServer.ExcelServerApp' threw an exception. ---> System.IO.FileLoadException: Loading this assembly would produce a different grant set from other instances. (Exception from HRESULT: 0x80131401)     at Microsoft.Office.Server.Diagnostics.FirstChanceHandler.ExceptionFilter(Boolean fRethrowException, TryBlock tryBlock, FilterBlock filter, CatchBlock catchBlock, FinallyBlock finallyBlock)     at Microsoft.Office.Server.Diagnostics.ULS.SendWatsonOnExceptionTag(UInt32 tagID, ULSCatBase categoryID, String output, Boolean fRethrowException, TryBlock tryBlock, CatchBlock catchBlock, FinallyBlock finallyBlock)     at Microsoft.Office.Excel.Server.CalculationServer.ExcelServerApp..ctor()     at Microsoft.Office.Excel.Server.CalculationServer.ExcelServerApp..cctor()     --- End of inner exception stack trace ---     at Microsoft.Office.Excel.Server.CalculationServer.ExcelServiceBase.PostProcessRequest(CommandResult result, Exception ex)"

I'm wondering if I can delete the old Excel Services service application?

Also any tips about these errors would be appreciated.

thanks!